Water is removed from foods under natural field conditions, by a variety of controlled dehydrations processes and during such common operations as cooking and baking. However, in modern food processing, the terms food dehydration and food concentration have acquired rather special meanings.
Grains in the field dry on the stalk by exposure to the sun. often sufficient degree of dryness is achieved (approximately 14% moisture) to require no further drying for effective preservation. This also is true of many plant seeds and spices, and is approached by certain fruits, such as dates and figs, that develop high sugar contents as they dry out on the tree. Centuries ago, humans learned to copy this natural sun drying process to dry fish and thin slices of meat by hanging the in the air and sun. where drying of these animal products took a long time, bacterial spoilage during the slow operation occurred, so the use of smoke and salt as further preservative agents in combination with drying gradually evolved.
Sun drying is still in use in many parts of the world, but although sun drying in some parts of the world and for certain products is the most economical kind of drying, it has several obvious disadvantages. Sun drying is dependent on the elements; it is slow and not suitable for many high quality products; it generally will not lower moisture content below about 15%, which is too high for storage stability of numerous food products; it requires considerable space; and the food being exposed is subject to contamination and losses from dust, insects, rodents, and bird droppings.
Efforts at artificial drying with heated air date back to the close of the eighteenth century. The term food dehydration refers generally to artificial drying under controlled conditions. However, in modern food processing the term does not refer to all processes that remove water from foods. For example, water is removed when potatoes are fired, cereals toasted, and steak broiled. But these operations do much more than simply remove water and are not considered as a form of food dehydration. Likewise, concentration processes that remove only part of the water from foods (e.g., in the preparation of syrups, evaporated milk, condensed soups) do not come under the currently accepted meaning of the term food dehydration.
In a strict sense, then, food dehydration refers to the nearly complete removal of water from foods under controlled conditions that cause minimum or ideally no other changes in the food properties. Such foods, depending on the item, commonly are dried to final moisture within the range of about 1-5%. Examples are dried milk and eggs, potato flakes, instant coffee, and orange juice crystals. Such products will have storage stability at room temperature of a year or longer. A major criterion of the quality of dehydrated foods is that when reconstituted by the addition of water they be very close to, or virtually indistinguishable from, the original food material used in their preparation. In food dehydration, the technological challenge is especially difficult since very low moisture levels for maximum product stability are not easily obtained with minimum change to food materials. Further, such optimization frequently can be approached only at the expense of increased drying costs. With sensitive foods, product quality and processing costs usually are correlated also in the case of concentration processes.
FOOD DEHYDRATION
Preservation is the principal reason but not the only reason for dehydrating foods. Foods may be dehydrated to decrease weight and bulk (Fig.10.1). Since orange juice contains approximately 12% solids, removal of all the water leaves one-eight the weight; that is, 237 ml (8fl oz.) of orange juice yield approximately 28 g (1 oz.) of solids. To reconstitute, 207 ml (7 fl. oz.) of water are added prior to consumption. In the case of juices, the volume of the powders is less than the original juices, although rarely are the powder decreased in volume to the same extent that they are reduced in weight. These reductions can result in lower shipping and container cost, but this is not always the case with dehydrated foods.
Some drying processes are chosen to retain the size and shape of the original food. Freeze-drying of large food pieces is such a process. The freeze-dried steak on the left in Fig. 10.2 has essentially the dame volume as the original steak. Savings may be made in shipping costs from reduced weight, but not in size of containers in this instance. Further, sometimes shipping costs are not based on weight but are based on volume. In such a case, freeze-dried steaks would not be cheaper to package or ship than their original counterparts.
A third reason for dehydration is the production of convenience items. Good examples of this are instant coffee and instant mashed potatoes. In both cases all brewing or cooking steps are completed before the products are dried. The consumer simply adds water and stirs or mixes. Regardless of the reasons for water removal, food dehydration processes are based on sound scientific principles.
Heat and Mass Transfer
Whatever method of drying is employed, food dehydration involves getting heat into the product and getting out moisture. These two processes are not always favored by the same operating conditions. For example, pressing food between two heated plates would give close contact and improve heat transfer into the food through the top and bottom surfaces but would interfere with the escape of free moisture. It might be better to use one bottom hot plate and get heating, and a free surface on top of the food to let out moisture. In food dehydration, we generally are interested in a maximum drying rate, and so every effort is made to speed heat and mass transfer rates. The following considerations are important in this regard.
Surface Area
Generally, food to be dehydrated is subdivided into small pieces or thin layers to speed heat and mass transfer. Subdivision speeds drying for two reasons. First, a larger surface area provides more surface in contact with the heating medium and more surface from which the moisture can escape. Second, smaller particles or thinner layers reduce the distance heat must travel to reach the surface and escape. Nearly all types of food driers ensure a large surface area of the food to be dried.
Temperature
The greater the temperature difference between the heating medium and the food, the greater will be the rate of heat transfer into the food; this provides the driving force for moisture removal. When the heating medium is air, temperature plays a second important role. As water is driven from the food as water vapor, it must be carried away or it will create a saturated atmosphere at the food’s surface, which will slow down the rate of subsequent water removal. The hotter the air, the more moisture it will hold before becoming saturated. Obviously, a greater volume of air also can take up more moisture than a lesser volume.
Air Velocity
Not only will heated air take up more moisture than cool air, but air in motion, that is, high-velocity air, will sweep it away from the drying food’s surface, preventing the moisture from creating a saturated atmosphere. Thus, clothes dry more rapidly on a windy day.
Humidity
When air is the drying medium, the drier the air, the more rapid is the rate of drying. Moist air is closer to saturation and so can absorb and hold less additional moisture than if it were dry.
But the dryness of the air also determines to how low a moisture content the food product can be dried. Dehydrated foods are hygroscopic. Each food has its own equilibrium relative humidity. This is the humidity at a given temperature at which the food will neither lose moisture to the atmosphere nor pick up moisture from the atmosphere. Below this atmospheric humidity level, food can be further, dried; above this humidity, it cannot, rather it picks up moisture from the atmosphere. The equilibrium relative humidity at different temperatures can be determined by determined by exposing the dried product to different humidity atmospheres in bell jars and weighing the product after several hours of exposure. The humidity at which the product neither loses nor gains moisture is the equilibrium relative humidity. Plots of such data yield water sorption isotherms (Fig. 10.3). We can see from Fig. 10.3 that at 100C and 40% relative humidity (RH), potato comes into equilibrium at 4% moisture; if we wish to dry it down to 2% moisture with 100C air, then this air must be at about 15% RH. Similar water sorption isotherms have been established for a wide variety of food products and can be found in appropriate references. But for a new product, a mixture of ingredients, such as a dehydrated soup, or a new variety of fruit or vegetable, it usually is necessary to experimentally determine the isotherms for the specific product. With this information, the best temperature and humidity of the drying air can be selected. Equilibrium relative humidity data also are important when we consider storage of dried product. If the food is packaged in a container that is not moisture-tight and is stored in an atmosphere above the dried foo’s equilibrium relative humidity, then the food will gradually pick up moisture and may cake or otherwise deteriorate.
Atmospheric Pressure and Vacuum
At a pressure of 1 atm (760 mm Hg) water boils at 100C. as the pressure is lowered, the boiling temperature decreases. At constant temperature, a decrease in pressure increases the rate of boiling. Thus, food in a heated vacuum chamber will lose moisture at a lower temperature or at a faster rate than it would in a chamber at atmospheric pressure. Lower drying temperature and shorter drying times are especially important in the case of heat-sensitive foods.
Evaporation and Temperature
As water evaporates from a surface, it cools the surface. This cooking is largely the result of absorption by the water of the latent heat of phase change from liquid to gas,
Figure; 10.3
That is, the heat of vaporization going from water to water vapor. This heat is takes from the drying air or the heating surface and from the hot food, and so the food piece or droplet is cooled.
The same thing happens with a wet bulb thermometer. A sling psychrometer consists of two identical thermometers, except that the bulb of one is immersed in a wet wick. If we sling this around in the air to speed evaporation, the temperature of the wet bulb thermometer drops compared to the dry bulb thermometer if the relative humidity of the air is less than 100%. The wet bulb continues to be lower than the dry bulb as long as the wet bulb can give off moisture to the atmosphere. If the wick becomes dry, the temperature cases to drop. If the air is at a high humidity, then the rate of evaporation and the amount of water vapor evaporated from the wet wick are less than that evaporated at lower humidity. The extent of the wet bulb temperature depression is a measure of relative humidity.
A particle or piece of solid food, or a droplet of liquid food, while it is being dehydrated acts as a wet bulb so long as it still contains free water. Regardless of the temperature of the drying air or heating surface, the temperature of the food will not be substantially higher than the temperature of a wet bulb so long as water is evaporating rapidly. Thus, in a spray drier the incoming air may be at 200C and the exit air at perhaps 120C, but a food particle while drying may be not higher than about 70C. As the moisture content of the food particle decreases and evaporation slows down, the particle rises in temperature. When there is virtually no freer water, the food particles rise in temperature to that of the incoming air, and the exit air also approaches that temperature if there are no other heat losses through the drier. Because most foods are heat sensitive, they generally are removed from high-temperature driers before they reach the maximum temperatures possible or are exposed to the highest temperatures for only a very short time.
Unless heated specifically for the purpose, foods are not sterile at the end of dehydration. Although a large proportion of the microbial load is killed during most drying operations, many bacterial spores are not. This becomes still more significant if the dehydration method is designed to be gentle to protect delicate foods. In freeze-drying, for example, comparatively few microorganisms are destroyed, and indeed freeze-drying has been used for many years as a method of preserving the viability of bacterial cultures. The no sterilizing aspects of food dehydration also apply to certain natural food enzymes that may survive drying conditions.
Time and Temperature
Since all important methods of food dehydration employ heat, and food constituents are sensitive to heat, compromises must be made between the maximum possible drying rate and maintenance of food quality. With few exceptions, drying processes that employ high temperatures for short times do less damage to food than drying processes employing lower temperatures for longer times. Thus, vegetable pieces dried in a properly designed oven in 4 h would retain greater quality than the same product sun-dried over 2 days. Several drying processes achieve dehydration in a matter of minutes or even less if the food is sufficiently subdivided.
Freeze-drying, which is discussed later in this chapter, may appear to contradict the high-temperature –sort-time principle, since drying may take 8 h or more and still produce excellent quality. However, in this case the product is dried directly from the frozen, state, and under such conditions, there is little deterioration.
Normal Drying Curve
When foods are dried, they do not lose water at a constant rate all the way down to bone dryness. As drying progresses, the rate of water removal under any set of fixed conditions drops off. This is seen in Fig. 10.4 for carrot dice. In practice, if 90% of a product’s water is removed in 4 h, it may require another 4 h to remove most of the remaining 10%. Since the removal rate becomes asymptotic, zero moisture is never reached under practical operating conditions. At the beginning of drying, and for some time thereafter, water generally continues to evaporate from a food piece at a rather constant rate, as if it were drying from a free surface. This is referred to as the constant rate period of drying, in Fig. 10.4 it extends of 4 h. This is followed by an inflection in the drying curve, which leads into the falling rate period of drying.
These changes during dehydration can be largely explained in terms of heat and mass transfer phenomena. A cube of food in the course of dehydration will lose moisture from its surfaces and gradually develop a thick-dried a thick dried layer, with remaining moisture largely confined to its center. From the center to the surface, a moisture gradient will be established. As a result, the outside dried layer will form an insulation barrier against rapid heat transfer into the food piece, especially since the evaporating water leaves air voids behind it. In addition to less driving force from decreased heat transfer, water remaining in the center has farther to travel to get out of the food piece than did surface moisture at the start of drying. Further, as the food dries, it approaches its normal equilibrium relative humidity. As it does, it begins to pick up molecules of water vapor from the drying atmosphere as fast as it loses them. When these rates are equal, drying ceases.
Figure; 10.4
These are not the only food changes that contribute to the shape of the typical drying curve, although they are major factors. The precise shape of the normal drying curve varies with different food materials, for different types of driers, and in response to varying drying conditions such as temperature, humidity, air velocity, direction of the air, thickness of the food, and other factors. But the drying of most food materials generally shows periods of constant and falling rate, and the removal of water below about 2% without damage to the product is exceedingly difficult.
Effects of Food Properties of Dehydration
The physical factors affecting heat and mass transfer such as temperature, humidity, air velocity, surface area, and the like are usually relatively easy to optimize and control, and largely determine drier design. Far subtler are the properties of food materials that may change during dehydration and affect drying rates and final product quality. In the case of food freezing, it was pointed out that various food properties affect heat transfer. The picture is more complex with regard to dehydration, since food raw material properties affect both heat and mass transfer, and both can have gross effects on characteristics of the dried products.
Constituent Orientation
Few foods approach homogeneity at the molecular level. A piece of meat, for example, will have lean and fat interlaced or marbled together. A piece of meat being dried will give up water at different rates in the regions of fat and lean, especially is the water must escape through a fat layer. This suggests that where fat occurs in layers, faster drying will occur if the meat is oriented relative to the source of heat so that moisture escape in a line parallel to the layers of fat rather than having to pass through them. The same principle applies to layers of muscle fibers. The rate of drying will differ depending on whether orientation with respect to the heat source encourages moisture to escape parallel with or transverse to the stratification of muscle fibers. Parallel escape generally gives faster drying.
Constituent orientation also applies in food emulsions. If in a food piece or droplet, water is emulsified in oil so that the oil is the continuous phase and coats the moisture droplets, then dehydration should be slower than if the emulsion is reversed and water is the continuous phase. Sometimes this can be controlled in a manufactured food being dried, but more often we must take what nature gives us.
Solute Concentration
Solutes in solution elevate the boiling point of water systems. This occurs in food dehydration processes. Foods high in sugar or other low-molecular-weight solutes dry more slowly than foods low in these soluble. What is more, the concentration of solutes becomes greater in the remaining water as drying progresses. This is another factor that slows drying and contributes to the falling rate period in the drying of many foods.
Binding of Water
Water escapes freely from a surface when its vapor pressure is greater than the vapor pressure of the atmosphere above it. But as a product dries and its free water is progressively removed, the vapor pressure of a unit area of the product decreases. This is because there is less remaining water per unit volume and per unit area and also because some of the water is held or bound by chemical and physical forces to solid constituents of the food.
Free water is easiest to remove and evaporates first. Additional water may be loosely held by forces of adsorption to food solids. More difficult to remove is water that enters into colloidal gels such as when starch, pectin, or other gums are present. Still more difficult is the removal of chemically bound water in the form of hydrates (e.g., glucose monohydrate or hydrates of inorganic salts). These phenomena also contribute to the flattening of normal drying curves with time.
Cellular Structure
Solid foods of natural tissue have a cellular structure with moisture between and within the cells. When the tissue is alive, the cell walls and membranes hold moisture within the cells. Such cells have turgor, rather than exhibiting leakage or bleeding.
When an animal or plant is killed, its cells become more permeable to moisture. when the tissue is blanched or cooked, the cells may become still more permeable to moisture. Generally, cooked vegetables, meat, or fish will dry more easily than their fresh counterparts, provided cooking dies not cause excessive toughening or shrinking.
Shrinkage, case Hardening, Thermoplasticity
Even dead cells retain varying degrees of elasticity and will stretch or shrink under stress. If the stress is excessive, then their elastic limit is exceeded and they will not return to their original shape on removal of the stress. One of the most obvious changes during dehydration of cellular, as well as noncellular foods is shrinkage.
If moisture was removed evenly throughout the mass of a perfectly elastic material under turgor, then the material would shrink in an even linear fashion with removal of moisture. This uniform shrinkage is rarely seen in food materials being dehydrated since the food pieces generally do not have perfect elasticity and water is not removed evenly throughout the food piece as it is dried. Different food materials exhibit different shrinkage patterns in the course of dehydration. Typical changes of vegetable dice during dehydration are indicated in Fig. 10.5. The original piece before drying is represented in Fig. 10.5a. The effect of surface shrinkage is seen in Fig. 10.5b., where the edges and corners gradually pull in giving the cube a more rounded appearance in the early stages of drying. Continued dehydration gradually removes water from deeper and deeper layers and finally from the center. This causes continued shrinkage toward the center and the concave cube appearance as in Fig. 10.5c.
Often with quick high-temperature drying of food pieces, the surface becomes dry and rigid long before the center has dried out. Then when the center dries and shrinks, it pulls away from the rigid surface layers causing internal splits, voids, and honeycomb effects. Such differences in shrinkage patterns can affect the bulk density of the dried product, that is, the weight per unit volume. Products dried rapidly have a rigid, less concave surface, and more internal shrinkage and air voids. Products dried slowly are more concave and dense.
Both kinds of dried product have advantages and disadvantages. For example, a less dense product will absorb water and reconstitute quicker, is more attractive and more closely resembles the original material, and may be psychologically more acceptable to consumers, who frequently interpret greater volume as more substance even through weight is the same. On the other hand, a less dense product is more expensive to package, ship, and store, and because of its air voids, may be more easily oxidized or otherwise have shortened storage stability. A less dense product often is favored when it is to be sold directly to consumers, ho value appearance and quick reconstitution of the product, whereas a denser product frequently is preferred by food manufacturers who purchase dehydrated ingredients for further processing and have reconstitution kettles and mixers; processors are likely to be less concerned with reconstitution rate than with container, shipping and storage costs.
A special condition related to shrinkage and sealing of the surface of a food piece is known as case hardening. This may occur when there is a very high surface temperature and unbalanced drying of the piece so that a dry kin forms quickly, before most of the internal moisture has had opportunity to migrate to the surface. The rather impermeable skin then traps much of the remaining water within the particle, and the drying rate drops off severely.
Case hardening is particularly common with foods that contain dissolved sugars and other solutes in high concentration. This can be explained from the various ways water may escape from a product during dehydration. Some of the water moves through cell walls and membranes of cellular foods y molecular diffusion. If the membranes are highly selective against solutes, the water will leave dissolved substances behind. Also, water may be heated to vapor within a food piece and escape as water vapor molecules free of solute. But food pieces and food purees being dried also contain voids, cracks, and pores of various diameters down to minute capillary size. Water in foods rises in these pores and capillaries, many of which lead to the food surface. Capillary water carries sugars, salts, and other materials in solution to the surface of food pieces during dehydration. Then at the surface, to water is evaporated and the solutes are deposited. This is what causes a sticky, sugary exudate on the surface of some fruits in the early stages of drying. This can seal off the surface pores and cracks, which also are shrinking during drying. The combined effects of shrinking and pore clogging from solutes contributes to case hardening. Where case hardening is a problem, it generally can be minimized by lower surface temperature to promote a more gradual drying throughout the food piece.
Many foods are thermoplastic, that is, they soften on heating. A cellular food, such as plant and animal tissue, has structure and some rigidity even at drying temperatures. A fruit or vegetable juice, on the other hand, lacks structure and is high in sugars and other materials that soften and melt at the drying temperature. Thus, if orange juice or a sugar syrup is dried on a pan or on a heated belt, even after all of the water has been removed the solids will be in thermoplastic tacky condition, giving the impression that they still contain moisture. They also will stick to the pan or belt and be difficult to remove. However, on cooling, the thermoplastic solids harden into a crystalline or amorphous glass form. In this more brittle condition they generally are more easily removed from the pan or belt. Most belt type driers are equipped with a cooling zone just prior to a scraper knife to facilitate removal of this type of material from the drier.
Porosity
Many drying techniques or treatments given to food before drying are aimed at making the structure more porous so as to facilitate mass transfer and thereby speed drying rate. But in some instances, even though potential mass transfer rates are increased by puffing or otherwise opening the structure, they drying rate is not increased. Porous sponge like structures are excellent insulating bodies and will slow down the rate of heat transfer into the food. The net result depends on whether the change in porosity has a greater effect on the rate of mass transfer or heat transfer in the particular food material and drying system.
Porosity may be developed by creating steam pressure within a product during drying. The escaping steam tends to puff such a product as in the case of the potato puffs of Fig. 10.6. Porosity can be developed also by whipping or foaming a food liquid or puree prior to drying. A stable foam that resists collapse during drying is then desired.
Figure; 10.6
Porosity can be developed in a vacuum drier by rapid escape of water vapor into the high vacuum, and by still other means.
Quite apart from its effect on drying rate, any process that retains or creates a highly porous structure does many of the other discussed in relation to internal voids. A porous product has the advantages of quick solubility or reconstitution and greater volume appearance, but the disadvantages of increased bulk and generally shorter storage stability because of increased surface exposure to air, light, and so on.
Chemical Changes
A great range of chemical changes can take place during food dehydration along with the physical changes already described, and these contribute to the final quality of both the dried items and their reconstituted counterparts in terms of food color, flavor, texture, viscosity, reconstitution rate, nutritional value, and storage stability. These change frequently are product specific, but a few major types occur in virtually all foods undergoing dehydration. The extent of these changes depends on the composition of the food and the severity of the drying method.
Browning reactions may be caused by enzymatic oxidations of polyphenols and other susceptible compounds if the oxidizing enzymes are not inactivated. Drying temperatures, because of the water evaporation cooling effect, often are not sufficient to inactivate these enzymes during drying, so it is common to pasteurize or blanch foods with heat or chemicals prior to drying. Caramelization of sugars and scorching of other materials if heat is excessive is another common type of browning. Highly important in food dehydration are nonenzymatic or Mallard browning products from the reaction of aldehydes and amino groups of sugars and proteins. Mallard-type browning, like, like other chemical reactions, is favored by high temperature and by high concentrations of reactive groups in the presence of some water. In the course of dehydration, reactive groups are concentrated. Mallard browning generally proceeds most rapidly during drying when the moisture content is decreased to the range of about 20-15%. As moisture content drops further, the rate of Mallard browning slows, so that in dried products below 2% moisture further color change from this kind of browning is minimally perceptible even on long-range storage. Drying systems or heating schedules generally are designed to dehydrate rapidly through the 20-15% moisture range so as to minimize time for Mallard browning at this optimum condition.
Another common consequence of dehydration is some loss in the ease of rehydration. Some of this is caused by physical shrinkage and distortion of cells and capillaries, but much also results from chemical or physicochemical changes at the colloidal level. Heat and the salt concentration effects from water removal can partially denature proteins, which cannot then fully reabsorb and bind water. Starches and gums also may be altered and become less hydrophilic. Sugars and salts escape from damaged cells into the water used to reconstitute dehydrated foods, resulting in loss of turgor. These and other chemical changes make reabsorption of water by dried products somewhat less than equal to the original water content and contribute to altered texture.
Still another common chemical change associated with dehydration is some loss of volatile flavor constituents. This invariably occurs to at least a slight degree. Complete preventions of flavor loss have as yet proven virtually impossible, and so methods of trapping and condensing the evolved vapors from the drier and adding them back to the dried product are sometimes employed. Additional techniques involve addition to dried products of essences and flavor preparations derived from other sources, as well as methods of minimizing flavor loss by incorporating gums and other materials into certain liquid foods prior to drying. Some of these materials have flavor fixative properties; others work by coating dried particles and providing a physical barrier against loss of volatile substances.
Optimization of Variables
In the design of food dehydration equipment, efforts are made to produce maximum drying rate with minimum drying rate with minimum product damage at the most economical drying cost. This requires a balancing of the various factors discussed so far; food dehydration is truly an area where the food scientist and engineer must work together to achieve optimum results.
Mathematical relationships exist between each of the major controllable drying variables and heat and mass transfer. Because of the peculiarities of food materials, optimum drying conditions are seldom the same for two different products. Engineering calculations based on model systems can go a long way toward selecting favorable drying conditions, but seldom are sufficient in themselves to accurately predict drying behavior. This is because food materials are highly variable in initial composition, in amounts of free and bound water, in shrinkage and solute migration patterns, and, most important, in how properties change throughout the drying operation. This is especially so in the falling rate period of the drying curve, where quality and economics are most affected. For these reasons, in selecting and optimizing a drying process, experimental tests with the food to be dried must always supplement engineering calculations based on less variable model systems.
Drying Methods and Equipment
There are several basic drying methods and a far greater number of modifications of the basic methods. The method of choice depends on the type of food to e dried, the quality level that must be achieved, and the cost that can be justified. Because orange juice crystals command a much higher price than starch, a processor can afford to use a more delicate and generally more expensive drying method to dehydrate orange juice, which needs a milder drying method since it is far more sensitive than starch.
Some of the more common drying, methods include drum drying, spray drying, vacuum shelf drying, vacuum belt drying, atmospheric belt drying, freeze drying, freeze-drying, fluidized-bed drying rotary drying, cabinet drying, kiln drying, tunnel drying, and others. Some of these methods are particularly suited to liquid foods and cannot handle solid food pieces; others are suitable for solid foods or mixtures containing food pieces.
One useful division of drier types separates them into air convection driers, drum suitability of more common drier types for liquid and solid foods. In air convection driers, heated air is put into intimate contact with the food material and supplies a major source of the heat for evaporation. If liquid, the food may be sprayed or poured into pans or on belts. Pieces may be supported in any number of ways. Although heated moving air is common to this group of driers, additional heat also may be supplied by heated tray or belt supports. Drum or roller driers are limited to use with purees, mashes, and liquid foods that can be applied as thin films. Vacuum driers may employ any degree of vacuum to lower the boiling point of water. Freeze-driers are special kinds of vacuum driers generally operated at extremely low internal pressures so as to sublime water vapor directly from ice without going through the liquid phase. This classification is not rigid, since many driers are combinations. Thus, we can place a drum drier in a vacuum chamber or low high-velocity heated air over the drum to speed drying; both practices are done commercially.
Drum or Roller Driers
In drum or roller drying, liquid foods, purees, pastes, and mashes are applied in a thin layer onto the surface of a revolving heated drum. The drum generally is heated from within by steam. Driers may have a single drum or a pair of drums (Fig. 10.14). The food may be applied between the nip where two drums come together, and then the clearance between the drums determines the thickness of the applied food layer; or the food can be applied to other area of drum. Food is applied continuously and the thin layer loses moisture. At a point on the drum or drums a scraper blade is positioned to peel the thin dried layer of food from the drums. The speed of the drums is so regulated that the layer of food will the dry. When it reaches the scraper blade, which also is referred to as a doctor blade. The layer of food is dried in one revolution of the drum and is scraped from the drum before that position of the drum returns to the point where more wet food is applied. Using steam under pressure in the drum, the temperature of the drum surface may be well above 100C, and often is held at about 150C. With a food layer thickness commonly lefts 2 mm, drying can be completed in 1 min or less, depending on the food material. Other features of drum driers include hoods above drums to withdraw moisture vapor and conveyors in troughs to receive and move dried product.
Typical products dried on drums include milk, potato mash, heat-tolerant purees such as tomato paste, and animal feeds. But drum drying has some inherent limitations that restrict the kinds of foods to which it is applicable. To achieve rapid drying, drum surface temperature must be high, usually above 120C. This gives products a more cooked flavor and color than when they are dried at a lower temperature. Drying temperature can, of course, be lowered by constructing the drums within a vacuum chamber (Fig. 10.14c, d), but this increases equipment and operating cost over atmospheric drum or spray drying.
A second limitation is the difficulty of providing zoned temperature control needed to vary the drying temperature profile. This is particularly important with thermoplastic food materials. Whereas dried milk and dried potato are easily scraped from the hot drum in brittle sheet form, this is not possible with many dried fruits, juices, and other products which tend to be sticky and semi molten when hot. Such products tend to crimp, roll up, and otherwise accumulate and stick to the doctor blade in taffy like mass.
This condition can be substantially improved by a cold zone to make the tacky material brittle just prior to the doctor blade. But zone-controlled chilling is not as easy to accomplish on a drum of limited diameter, and therefore limited arc, as it would be in perhaps 6 m of length of a horizontal drying belt 45 m long. One means of chilling is by directing a stream of cool air onto a segment of the product on the drum prior to the doctor blade. A system for doing this and providing additional zoned temperature control around the drum is shown in Fig. 10.15.
For relatively heat-resistant food products, drum drying is one of the least expensive dehydration methods. Drum dried foods generally have a somewhat more “cooked” character than the same materials spray dried; this drum dried milk is not up to beverage quality but is satisfactory as an ingredient in less delicately flavored manufactured foods. More gentle vacuum drum drying or zone-controlled drum drying increases dehydration costs.
Vacuum Driers
Vacuum dehydration methods are capable of producing the highest quality fried products, but costs of vacuum drying generally also are higher than other methods which do not employ vacuum. In vacuum drying, the temperature of the food and the rate of water removal are controlled by regulating the degree of vacuum and the intensity of heat input. Heat transfer to the food is largely by conduction and radiation. Vacuum drying methods usually can be controlled with a higher degree of accuracy than methods depending on air convection heating.
All vacuum drying systems have four essential elements: a vacuum chamber of heavy construction to withstand outside air pressures that may exceed internal pressure by as much as 9800 kg/m² (2000 lb./ft²); a heat supply; a device for producing and maintaining the vacuum; and components to collect water vapor as it is evaporated from the food. Typical arrangements of these elements are shown in Fig. 10.16.
The vacuum chamber generally contains shelves or other supports to hold the food; these shelves may be heated electrically or by circulating a heated fluid through them. The heated shelves are called platens. The platens convey heat to the food in contact with them by conduction, but where several platens are arranged one above another, they also radiate heat to the food on the platen below. In addition, special radiant heat sources such as infrared elements can be focused on the food to supplement the heat conducted from platen contact.
The device for producing and maintaining vacuum is outside the vacuum chamber and may be a mechanical vacuum pumps or a stream ejector. A steam ejector is kind of aspirator in which high-velocity steam jetting past an opening draws air and water vapor from the vacuum chamber by the same principle that makes an insect spray gun draw fluid from the can.
The means of collecting water vapor may be a cold wall condenser. It may be inside the vacuum chamber or outside the chamber but must come ahead of the vacuum pump so as to prevent water vapor from entering and fouling the pump. When a steam ejector is used to produce the vacuum, the same steam ejector can condense water vapor as it is drawn along with the air from the vacuum chamber, and so cold wall vapor condenser may not be needed except where a very high degree of efficiency is required. In Fig. 10.16, the system at the top employs steam ejectors connected to the vacuum chamber, the middle system uses refrigerated condenser and vacuum pumps; the lower system employs a refrigerated condenser and steam ejectors.
Atmospheric pressure at sea level is approximately 15 psi, or sufficient pressure to support a 30-in. column of mercury. This is also stated as 760 mm of mercury (Hg) or 760 tour. At 1 atom, pure water boils at 100C; at 250 mm Hg, pure water boils at 72C; at 50 mm Hg, pure water boils at 38C. High-vacuum dehydration operates at still lower pressures such as fractions of a millimeter of mercury. Freeze-drying generally will operate in the range of 2 mm to about 0.1 mm Hg.
Vacuum shelf Driers.
One of the simplest kinds of vacuum driers is the batch-type vacuum shelf drier (Fig. 10.17). if liquids such as concentrated fruit juices are dried above about 5 mm Hg, the juice boils and splatters, but in the range of about 3 mm Hg, and below, the dehydrated juice then retains the puffed spongy structure seen in Fig. 10.17. Since temperatures well below 40C can be used. In addition to quick solubility there is minimum flavor change or other kinds of heat damage. A vacuum shelf drier is also suitable for the dehydration of food pieces. In this case, the rigidity of the-solid food prevents major puffing, although there also is a tendency to minimize shrinkage.
Air Convection Driers
All air convection driers have some sort of insulated enclosure, a means of circulating air through the enclosure, and a means of heating this air. They also have various means of product support and special devices for collecting dried product; some have air driers to lower drying air humidity. Movement of air generally is controlled by fans, blower, and baffles. Air volume and velocity affect drying rate, but its statics pressure also is important since products being dried become very light and can be blown off trays or belts. Airflow patterns are complex when they encounter surfaces, and their velocities and pressures in contact with food are seldom comparable with measurements made on the main airstream, but such measurements usually can be correlated with drying behavior. Even if two driers have an air velocity of 5 m/sec (1000 ft./min), the surface of the food in the two driers, probably encounter different velocities when the driers are of different geometries.
The air may be heated by direct or indirect methods. In direct heating the air is in direct contact with a flame or combustion gases. In indirect heating the air is in contact with a hot surface, such as pipes or fins heated by steam, flame, or electricity. The important point is that indirect heating leaves the air uncontaminated. On the other hand, in direct heating the fuel is seldom completely oxidized to carbon dioxide and water. Incomplete combustion leaves gases and traces of soot, which are picked up by the air and can be transferred to the food product. Direct heating of air is also contributing small amounts of moisture to the air since moisture is a product of combustion, but this is usually insignificant except with very hygroscopic foods. These disadvantages are balanced by the generally lower cost of direct heating of air compared to indirect heating, and both methods are widely used in food dehydration.
Kiln Drier. one of the simplest kinds of air convection drier is the kiln drier Kiln driers of early design were two-story constructions. A furnace or burner on the lower floor generated heat, and warm air would rise through a slotted floor to the upper story. Foods such as apple slices would be spread out on the slotted floor and turned over periodically. This kind of drier will not reduce moisture to below about 10%. It is still in use for apple slices.
Cabinet, Tray, and Pan Drier. a step more advanced is the cabinet drier in which food may be loaded on trays or pans in comparatively thin layers up to a few centimeters. A typical construction for this type of drier is shown in Fig. 10.7. Fresh air enters the cabinet (B), is drawn by the fan through the heater coils (C), and is then blown across the food trays to exhaust (H). In this case, the air is heated by the indirect method. Screens filter out any dust that may be in the air. The air passes across and between the trays in this design. Other designs have perforated trays and the air may be directed up through these. In Fig 10.7, the air is exhausted to the atmosphere after one pass rather than being recirculated within the system. Recirculation is used to conserve heat energy by reusing part of the warm air. In recirculating designs, moist air, after evaporating water from the food, may have to be dried before being recirculated to prevent saturation and slowing down of subsequent drying. In such a case, this air could be dried by passing through a desiccant such as a bed of silica gel, or the moisture could be condensed out by passing through a desiccant such as a bed of silica gel, or the moisture could be condensed out by passing the moist air over cold plates or coils. But when the exhaust air is not dried for recirculation, then the exhaust vent should not be close to the fresh air intake area, otherwise the moist exhaust air will be drawn back through the drier and drying efficiency will be lost.
Cabinet, tray, and pan driers are usually for small-scale operations. They are comparatively inexpensive and easy to set in terms of drying conditions. They may run up to 25 trays high and operate with air temperatures of about 95C dry bulb and with air velocities of about 2.5-5 m/sec across the trays. They commonly are used to dry fruit and vegetables pieces, and depending on the food and the desired final moisture, drying time may be of the order of 10 or even 20 h.
Figure; 10.7
Tunnel and Continuous Belt Driers. For larger operations, tunnel driers with elongated cabinets, through which trays on carts pass, are used (Fig. 10.8). If drying time to the desired moisture is 10 h, each wheeled cart of trays will take 10 h to pass through the tunnel. When a dry cart emerges, it makes room to load another wet cart into the opposite end of the tunnel. Such an operation then becomes semicontinuous.
A main construction feature by which tunnel driers differ has to do with the direction of airflow relative to tray movement. In the direr shown in Fig. 10.8, wet food carts move from left to right. The drying air moves across the trays from right to left. This is a counter flow, or countercurrent, pattern in which the hottest and driest air contacts the nearly dry product, whereas the initial drying of entering carts gets cooler, moisture air that has cooled and picked up moisture going through the tunnel. This means that initial product temperature and moisture gradients will not be as great, and the product is less likely to undergo case hardening or other surface shrinkage, leaving wet centers. Further, lower final moisture can be reached because the driest product encounters the driest air. In contrast, concurrent flow tunnels have the incoming trays and incoming hottest driest air traveling in the same direction. In this case, rapid initial drying and slow final drying can cause case hardening and internal splits and porosity as centers finally dry, which sometimes is desirable in special products.
Just as carts of trays can be moved through a heated tunnel, so a continuous belt may be driven through a tunnel or oven enclosure. This approach is used in a continuous belt or conveyor drier, and a great number of designs are possible. Some of the more common features are uniform automatic feeding of product to the belt in a controlled thin layer, zoned heat and airflow control in different sections, tumbling over of product onto a second strand of belt, automatic collection of dried product, and, of course, continuous operation. The drying capacity of such driers generally is stated in terms of weight of product dried from one moisture level to another per square meter of belt surface per hour. This also can be expressed in terms of kilograms of water removed per square meter of belt surface per hour under defined operating conditions.
Belt Trough Drier. a special kind of air convection belt drier is the belt trough drier in which the belt forms a trough. The belt is usually of metal mesh, and heated air is blown up through the mesh. The belt moves continuously, keeping the food pieces in the trough in constant motion to continuously expose new surface. This speeds drying, and with air of about 135C, vegetable pieces may be dried to 7-5% moisture in about 1 h. Figure 10.8
But not all products may be dried this way since certain sizes and shapes do not readily tumble. Fragile apple wedges may break. Onion slices tend to separate and become entangled. Fruit pieces that exude sugar on drying tend to stick together and clump with the tumbling motion. These are but a few additional factors that must be considered in selecting a drier for a particular food.
Air Lift Drier. several types of pneumatic conveyor driers go a step beyond tumbling to expose more surface area of food particles. These generally are used to finish-dry materials that have been partially dried by other methods, usually to about 25% moisture, or at least sufficiently low so that the material becomes granular rather than having a tendency to clump and mat. One type of air lift drier is illustrated in Fig. 10.9. This might be used to finish-dry semi moist granules coming from a drum drier. such granules at about 25% moisture can be brought to about 6% moisture more efficiently in a heated airstream than on the drum. This is because the more difficult moisture to remove in this falling rate period of dehydration is more easily evaporated from suspended particles in intimate contact with the heating medium. The suspended particles when dry are separated from the air and collected in a cyclone-type separator, which is described in the subsection on spray driers.
Fluidized-Bed Drier. another type of pneumatic conveyor drier is the fluidized-bed drier. this is similar in principle and construction to the fluidized-bed freezer descried in chapter 9. In fluidized-bed drying (Fig. 10.10), heated air is blown up through the food particles with just enough force to suspend the particles in a gentle boiling motion. Semidry particles such as potato granules enter at the left and gradually migrate to the right, where they are discharged dry. Heated air is introduced through a porous plate that supports the bed of granules. The moist air is exhausted at the top. The process is continuous and the length of time particles remain in the drier can be regulated by the depth of the bed and other means. This type of drying can be used to dehydrate grains, peas, and other particulates.
Spray Driers. By far the most important kind of air convection drier is the spray drier. Spray driers turn out a greater tonnage of dehydrated food products than all other kinds of driers combined. There are various types of spray driers designed for specific food products. Spray driers are limited to food s that can be atomized, such as liquids and low-viscosity pasts and purees. Atmosphere into minute droplets results in drying in a matter of seconds with common inlet air temperatures of about 200C. Since evaporative cooling seldom permits particles to get warmer than about 80C and properly designed systems quickly remove the dried particles from heated zones, this method of dehydration can produce exceptionally high quality with many highly heat sensitive materials, including milk, eggs, and coffee.
Figure; 10.10
In typical spray drying, the liquid food is introduced as a fine spray or mist into a tower or chamber along with heated air. As the small droplets make intimate contact with the heated air, they flash off their moisture, become small particles, and drop to the bottom of the tower from where they are removed. The heated air, which has now become moist, is withdrawn from the tower by a blower or fan. The process is continuous in that liquid food continues to be pumped into the chamber and atomized, along with dry heated air to replace the moist air that is withdrawn, and the dried product is removed from the chamber as it descends.
The principle components of a spray drying system differ in construction depending on the product to be dried. In the case of milk, the system includes tanks for holding the liquid, a high-pressure pump for introducing the liquid into the tower, spray nozzles or a similar device for atomizing the milk, a heated air source with blower, a secondary collection vessel for accumulating product drawn from the tower, and means for exhausting the moistened air (Fig. 10.11).
The main purpose of the drying tower or chamber is to provide intimate mixing of heated air with finely dispersed droplets. In the various spray driers shown in Fig. 10.12, the heated air and the atomized droplets may enter the tower together at the top or bottom or may enter separately, the particles may be made to descend straight down or take a spiral path, and the chamber may be vertical or horizontal.
An in tunnel driers, introduction of droplets and air in the same direction results in quick initial drying and slower final drying; countercurrent streams may be favored for highly hygroscopic materials. Further, if a liquid product is introduced at the top of the tower, it descends through and out of the tower in one pass; if product is introduced at the bottom, it first ascends and then descends and its time in the drier can thus be made longer. This also is true if the droplets are given a spiral motion in the tower. A longer residence time may be desirable to bring the particles down to a lower moisture content or to permit particles to grow in size in the drier (longer residence time gives greater opportunity for dry particles to collide with less dry particles and form clusters).
Figure; 10.11 and 10.12
This is one way to carry out the instant zing process known as agglomeration, which yields clusters that have many voids, sink in water, and are therefore easier to dissolve than certain spray dried particles which are small in size, float on water, and are difficult to wet.
As important to dried product characteristics as the geometry and air pattern in the chamber is the nature of the atomization. Atomizers are of two main types: pressure spray nozzles and centrifugal spinning disks, or baskets. Spinning disks and baskets, from which deposited food throws out droplets, are favored where passage through a fine-hole pressure nozzle can damage the food, as might be the case in denaturing proteins of egg white. Viscous liquids and purees with fine pulp also may not be able to pass through a fine pressure nozzle but can be easily spun from a high-speed, rotating disk.
Small droplets promote quick drying, and uniform droplet size is necessary for even drying. Actually, the size and trajectory of the largest droplets determine drying time and as a consequence, the size of the drying chamber. No atomizers have yet been developed that produce all droplets of the same size, but the object of their design is to make droplet size as uniform as possible. If not uniform, the small droplets dry first and then over dry before the larger droplets have become dry. The droplet size determines the final died particle size; if dried particle size varies substantially, then settling and stratification of fines may occur in the final package. Particle size affects solubility rate large particles may sink and very fine ones generally float on water, making for uneven wetting and reconstitution of no uniform products. Further, very small droplets in an atomized distribution dry as minute fines. These are hard to recover as product from the drier since they tend to be lost with the exit air even if the collection system is made highly efficient.
During atomization, the angle of departure from a spray nozzle or the trajectory from a spinning disk also must be considered. As droplets descend through the drying chamber, they go from a liquid to a sticky condition and then the dryness. If they encounter the drier wall before they are dry, they stick and build up as a cake, become heat damaged, and are difficult to remove. Trajectory generally is designed to prevent or minimize wall contact in the early stages of drying.
The appearance, size, shape, density, and solubility of the final spray dried particle can be affected by nozzle pressure, shear, liquid viscosity, surface tension, nature of the solids, and so on. Generally, spray dried particles have a spherical shape (Fig. 10.13), which is the form assumed by free-floating liquid bodies. Sometimes if drying is extremely rapid, the droplets are dehydrated as they emerge from the atomizer before they have had time to form a spherical shape. Then the dried particles may be irregular or dumbbell shaped. When drying is appropriately controlled, water vapor escaping from droplets can be made to leave voids and hollows in the dried particles, which give lighter density but also more surface for possible oxidative deterioration.
Powder collectors may simply be zones in the conical base of the drying chamber from which product can be periodically removed. More commonly, collectors consist of secondary smaller conical structure known as cyclone separators (Fig. 10.11). The exit air from the drying chamber carries the dried particles into the cyclone separator, where the air acquires a whirling motion, throwing dried particles against the conical wall. The particles settle for easy removal while the nearly particle-free air exist at the top. Since the exit air is never entirely free of fine particles, another kind of collector may be employed above the cyclone. This is a bag collector or filter just preceding air exhaust to the atmosphere. Product fines remaining in the bag collector for long periods exposed to heated exiting air generally become heat damaged and represent lower quality product.
One type of spray drying foams liquid food, such as milk or coffee, before spraying in into the drier. the result is a faster drying rate from the expanded foamed-droplet surface area and lighter-density dried product. This is known as foam spraying drying.
It was stated that when particles are dry, they do not stick to the drier wall. An exception is thermoplastic substances such as juices high in sugar. Even when dry, these melt, stick, and build up on the wall. One kind of spray drier has a double wall and circulates cold water or cool air so as to chill the lower portion of the inner wall where dried juice particles would accumulate. Thus prevented from melting and fusing, these juices too may be spray dried and collected in particulate form.
Another type of spray drier has been developed specially to handle thermoplastic materials and other highly heat-sensitive foods. This is known as the BIRS spray drier. The BIRS drier uses countercurrent cool, dry air of about 30C and 3% RH. To give the droplets sprayed in at the top of the tower sufficient time to dry at this relatively low temperature, the drying tower is built exceptionally tall. It may be 67 m high and 15 m in diameter. As droplets descend, they dry in about 90 sec. products like orange, lemon, and tomato juices, otherwise difficult spray dry because of Thermoplasticity, can be dried this way. Because there is not rapid escape of steam from particles in this cool process, such particles are less puffed and denser than many conventionally spray dried products. Low temperature also favors flavor retention.
Continuous Vacuum Belt Drier.
Vacuum driers can be engineered for continuous operation. A diagram of a continuous vacuum belt drier is shown in Fig. 10.18. This drier is used commercially to dehydrate high 1uality citrus juice crystals, instant tea, and other delicate liquid foods.
The drier consists of a horizontal tank like chamber connecting to a vacuum producing, moisture condensing system. The chamber is about 17 m long and 3.7 m in diameter, within the chamber are mounted two revolving hollow drums. Around the drum on the right is heated with steam confined within it. This drum heats the belt passing over it by conduction. As the belt moves, it is further heated by infrared radiant elements. The drum to the left is cooled with cold water circulated within it and cools the belt passing over it. The liquid food in the form of a concentrate is pumped into a feed pan under the lower belt stand. An applicator realer dipping into the liquid continuously applies a thin coating of the food onto the lower surface of the moving belt. As the belt moves over the heating drum and past the radiant heaters, the food rapidly dries in the vacuum equivalent to about 2 min Hg. When the food reaches the cooling drum, it is down to about 2% moisture. At the bottom of the cooling drum is a doctor blade which scrapes the cooled, embrittled product in product into the collection vessel. The belt scraped free of product receives additional liquid food as it passes the applicator roller and the process repeats in continuous fashion.
Products dried with this equipment have a slightly puffed structure. If desired, a greater degree of puffing can be achieved. This has been done in the case of milk by pumping nitrogen gas under pressure into the milk prior to drying. Some of the gas goes into solution in the milk. Upon entering the vacuum chamber this gas comes out of solution violently and further puffs the milk as it is being dried.
Freeze Drying.
Freeze-drying has been developed to a highly advanced state. Much of the development work has been aimed at optimizing the process and equipment to reduce drying costs, which still may be two to five times greater per weight of water removed than other common drying methods. Freeze-drying can be used to dehydrate sensitive, high-value liquid foods such as coffee and juices, but it is especially suited to drying solid foods of high value such as strawberries, whole shrimp, chicken dice, mushroom slices, and sometimes food pieces as large as steaks and chops. These types of food, in addition to having delicate flavors and colors, have textural and appearance attributes that cannot be well preserved by any current drying method expect freeze-drying. A whole strawberry, for example, is soft, fragile, and almost all water. Any conventional drying method that employs heat would cause considerable shrinkage, distortion, and loss of natural strawberry texture. Upon reconstitution, such a dried strawberry would not have the natural color, flavor, or turgor and would be more like a strawberry preserve or jam. This can be largely prevented by drying from the solidly frozen state, so that in addition to low temperature, the frozen food has little chance to shrink or distort while giving up its moisture.
The principle behind freeze-drying is that under certain conditions of low vapor pressure, water can evaporate from ice without the ice melting. When a material can exist as a solid, a liquid, and a gas but goes directly from a solid to a gas without passing through the liquid phase, the material is said to sublime. Dry ice sublimes at atmospheric pressure and room temperature. Frozen water will sublime if the temperature is 0C or below and the frozen water is placed in a vacuum chamber at a pressure or 4.7 mm or less. Under such conditions the water will remain frozen, and water molecules will leave the ice block at a faster rate than water molecules from the surrounding atmosphere reenter the frozen block. Figure 10.19 is a diagrammatic illustration of a food piece being freeze-dried. Within the vacuum chamber, heat is applied to the frozen food to speed sublimation. If the vacuum is maintained sufficiently high, usually within a range of about 0.1-2 mm Hg, and the heat is controlled just short of melting the ice, moisture vapor will sublime at a near maximum rate. Sublimation takes place from the surface of the ice, and so as it continuous, the ice front recedes toward the center of the food piece; that is, the food dries from the surface inward. Finally, the last of the ice sublimes and the food is below 5% moisture. Since the frozen food remains rigid during sublimation, escaping water molecules leave voids behind them, resulting in a porous spogelike dried structure. Thus, freeze-dried foods reconstitute rapidly but also must be protected from ready absorption of atmospheric moisture and oxygen by proper packaging.
In Fig. 10.19, a heating plate is positioned above and below the food to increase the heat transfer rate, but an open space is left with expanded metal so as not to seal off escape of sublimed water molecules. Nevertheless, as drying progresses and the ice front recedes, the drying rate drops off for several reasons. The porous dried layer ahead of the receding ice layer acts as an effective insulator against further heat transfer and slows the rate of escape of water molecules subliming from the ice surface.
Figure; 10.19
But, in well-engineered freeze-drying systems, the growing porous dried layer generally interferes more with heat transfer than with water mass transfer. Some of the more practical means of increasing overall drying rates have therefore made use of energy sources with penetrating power, such as infrared and microwave radiations, to pass through dried food layers into the receding ice core.
A typical freeze-drying curve for asparagus is shown in Fig. 10.20, which also include plots of the temperatures of the heating plates and the food surface during the drying run. At the start of the drying operation, no moisture has yet been removed, and the frozen product is below -30C at its center and surface. The chamber is evacuated to a pressure of 150 µm and the heating platen is set at 120C. as drying progress and platen, but the receding ice core remains frozen, cooled by the latent heat of sublimation. The platen temperature now must be regulated to establish a delicate balance. Sufficient heat is needed to provide the driving force for rapid sublimation, but not so much as to melt the ice. If the ice was pure water, its melting point would be 0C, but since some of the ice is frozen with solute as a eutectic, the maximum ice temperature that can be tolerated is usually somewhat below -4C, depending on the particular food. As more and more ice sublimes, the dried shell surface temperature continues to rise, approaching the 120C platen. It then becomes necessary to gradually decreased the platen temperature to about 65C to prevent scorching of the dried food surface. As the platen temperature is decreased, heat transfer to the remaining ice core also drops off augmenting the insulating effect of the growing dried layer. The result is a further decrease in the drying rate. Ultimately, all of the ice is sublimed, the entire dried mass reaches the 65C temperature of the heating platen, and moisture is down to about 3%. This may require 8 h or longer. The dried product can now be removed from the vacuum chamber. However, the dried porous product is under high vacuum if the vacuum is broken by admitting air, the product would instantaneously absorb this at into its pores, resulting in impaired storage stability. Therefore, it is common practice to break the vacuum with inert nitrogen gas. The nitrogen-impregnated product and then packaged, also under nitrogen.
Today, food companies wishing to install freeze-drying equipment on a major scale must consider the process from an overall systems approach. This includes materials handling, the freezing operation, loading of drier trays the drying operation, high vacuum and condenser requirements, unloading of trays, packaging requirements and, of course, equipment, labor, and utility costs. Many equipment companies have designed total systems that can be custom engineered for a specific product and the needs of the manufacturer. It is common for such equipment companies, working with food manufacturers, to design and install entire freeze-drying plants. Seldom are two such plants quite the same. One type of plant layout is illustrated in Fig 10.21 also is sometimes advantageous to combine freeze-drying with air drying. Vegetables piece may be air dried to about 50% moisture and then freeze-dried down to 2 -3% moisture, as in the “Aire Freeze” process of the California vegetable concentrates company. This combination gives a high quality product at lower cost than will freeze-drying alone.
Atmospheric Drying of Foams
Vacuum drying methods, and freeze-drying in particular, can produce dehydrated foods of exceptional quality. With liquids and purees, nearly the same quality can be obtained at atmospheric pressure with less expensive equipment and operating costs. This has been done in some instanced by drying preformed liquid foods. As mentioned earlier, foaming is done to expose enormous surface area for quick moisture escape. This, in turn, can permit rapid atmospheric drying at somewhat reduced temperatures. In this type of drying, naturally foaming foods such as egg white are mechanically whipped to a foam density of about 0.3 g/cm³. foods that do not whip as readily, such as concentrated citrus juices, fruit purees, and tomato paste, are supplemented with low levels of an edible whipping agent belonging to such groups of materials as vegetable proteins, carbohydrate gums, or monoglyceride emulsifiers prior to being whipped. Stable foams are then cast in thin layers onto trays or belts and are dried by various heating schemes.
Figure 10.20 and 10.21
One such dehydration method is known as form-mat drying (Fig. 10.22). In one particular type of foam-mat drier, the foam is deposited on a perforated tray or belt support as a uniform layer approximately 3 mm thick. Just before the perforated support enters the heated oven, it is given a mild air blast from below. This forms small craters in the stiff foam which further expands the foam surface and increases the drying rate. At oven temperatures of about 80C, foam layers of many foods can be dried to about 2-3% moisture in approximately 12 min.
Another system casts similar stable foams on a non-perforated stainless steel belt at a uniform thickness of approximately 0.4 mm. The belt is heated from below by condensing steam and from above by high-velocity heated air. Product temperature is kept below 80C, and drying time is 1 min or less. The exceptionally rapid drying rates are due largely to the extreme thinness of the foam layers and to the method of heating by condensing steam.
Figure; 10.22
When steam condenses under the belt, it gives up both sensible heat and latent heat of condensation, which together provide a substantial driving force to evaporate food moisture. An illustration of the integrated equipment employed in this system is shown in Fig. 10.23
FOOD CONCENTRATION
Foods are concentrated for many of the same reasons that they are dehydrated. Concentration can be a form of preservation, but only for some foods. Concentration reduces weight and volume and results in immediate economic advantages. Nearly all liquid foods to b dehydrated are concentrated before they are dried because in the early stages of water removal, moisture can be more economically removed in highly efficient evaporators than in dehydration equipment. Further, increased viscosity from concentration often is needed to prevent liquids from running off drying surfaces or to facilitate foaming or puffing. Also, some concentrated foods are desirable components of diet in their own right. For example, concentration of fruit juices plus sugar yields jelly. Many concentrated foods, such as frozen orange juice concentrate and canned soups, are easily recognized because of the need to add water before they are consumed. However, maple syrup and butter are somewhat less obvious concentrated food. In the case of maple syrup, the dilute maple tree sap is concentrated from about 2% solids to 66% solids by boiling off water in open pans or kettles. In making butter, constituents of cream are concentrated from about 40% solids to 85% solids by breaking the fat emulsion and draining the buttermilk, which is largely water, from the churn. In these two cases, water removal is accompanied by other changes that we wish to achieve. As in dehydration, however, most food concentration aims at minimal alteration of food constituents.
The more common concentrated foods include evaporated and sweetened condensed milk, fruits and vegetables juices and nectars, sugar syrups and flavored syrups, jams and jellies, tomato paste, many types of fruit purees used by bakers, candy makers, other food manufacturers, and many more.
Preservative Effects
The level of water virtually all concentrated foods are in themselves more than enough to permit microbial growth. Yet although many concentrated foods such as nonacid fruit and vegetable purees may quickly undergo microbial spoilage unless additionally processed, such items as sugar syrups and jellies and jams are relatively immune to spoilage. The difference, of course, is in what is dissolved in the remaining water and what osmotic concentration is reached. Sugar and salt in concentrated solution have high osmotic concentration is reached. Sugar and salt in concentrated solution have high osmotic pressures. When these are sufficient to draw water from microbial cells or to prevent normal diffusion of water into these cells, a preservative condition exists. Heavy syrups and similar products will keep indefinitely without refrigeration even if exposed to microbial contamination, provided they are not diluted above a critical concentration by moisture pickup.
The critical concentration of sugar in water to prevent microbial growth will vary depending on the type of microorganisms and the presence of other food constituents, but usually 70% sucrose in solution will stop growth of all microorganisms in foods. Less than this concentration may be effective but for shorter periods of time, unless the foods contain acid or are refrigerated. Salt is highly preservative when its concentration is increased, and levels of 18-25% in solution generally will prevent ail growth of microorganisms in foods. Expect in the case of certain briny condiments, however, this level is rarely tolerated in foods. Removal of water by concentration also increases, the level of food acids in solution. This is particularly significant in concentrated fruit juices.
In the sugar industry, juice squeezed from sugar cane contains approximately 15% sucrose and is highly perishable. Evaporators are an essential part of the equipment in sugar processing plants and are used to remove most of the water from cane or beet juice prior to subsequent crystallization steps in the production of dry granulated sugar. However, concentrated syrups with sugar levels of approximately 70% are sold also as important items of commerce. These syrups, with consistencies similar to honey, are pumped into tank cars and delivered to storage tanks of bakeries and confectionery manufacturers. Preservation is quite satisfactory provided there is no moisture condensation from the air onto interior tank surfaces. Sugar in this from, already in solution and easy to pump, is more convenient and economical than granulated sugar to use in many manufacturing operations.
Reduced Weight and Volume
Whereas the preservative effects of food concentration are important, the principal reason for most food concentration is to reduce food weight and bulk. Tomato pulp, which is ground tomato minus the skins and seeds, has a solids content of only 6%, and so a 3.78-liter can would contain only 231 g of tomato solids (Table 10.2). Concentrated to 32% solids, the same can would contain 1.38 kg of tomato solids, six times the value of the original product. For a manufacturer needing tomato solids, such as a producer of soups, canned spaghetti, or frozen pizzas, the savings from concentration are enormous in cans, transportation costs, warehousing costs. Warehousing costs, and handling costs throughout operations. This is especially so since much of the U.S tomato crop is grown in the Sacramento Valley area of California and shipped to manufacturing plants in Chicago and eastern areas of the country. These savings can be even greater by eliminating small containers and shipping the aseptically packed concentrate in bulk (Fig. 10.24). For the same reasons, millions of tons of concentrated fruits, juices, restaurant operations, and in the home. Large quantities of concentrated buttermilk, whey, blood, yeast, and other food by-products, are used also in animal feeds by poultry and livestock growers.
Methods of Concentration
Solar Concentration
As in food dehydration, one of the simplest methods of evaporating water is with solar energy. This was done to derive salt from seawater from earliest times and
Is still practiced today in the United states in man-made lagoons. However, solar evaporation is very slow and is suitable only for concentrating salt solutions.
Open Kettles
Some foods can be satisfactorily concentrated in open kettles that are heated by steam. This is the case for some jellies and jams and for certain types of soups. However, high temperatures and long concentration times damage most foods. In addition, thickening and burn-on of product to the kettle wall gradually lower the efficiency of heat transfers and slow the concentration process. Kettles and pans are still widely used in the manufacture of maple syrup, but here high heat is desirable to produce color form caramelized sugar and to develop typical flavor.
Flash Evaporators
Subdividing the food material and bringing it into direct contact with the heating medium can markedly speed concentration. This is done in flash evaporators of the kind shown in Fig. 10.25. Clean steam superheated at about 150C is injected into food which is pumped into an evaporation tube where boiling occurs. The boiling mixture then enters a separator vessel in which the concentrated food is drawn off at the bottom and the steam plus water vapor from the food is evacuated through separate outlets. Because temperatures are high, foods that lose volatile flavor constituents will yield these to the exiting steam and water vapor. These can be separated from the vapor by essence-recovery equipment on the basis of different boiling points between the essences and water.
Thin-film Evaporators
In thin-film evaporators (Fig. 10.26), food is pumped into a vertical cylinder which has a rotating element that spreads the food into a thin layer on the cylinder wall. The cylinder wall of double jacket construction usually is heated by steam. Water is quickly flashed from the thin food layer and the concentrated food is simultaneously wiped from the cylinder wall. The concentrated food and water vapor are continuously discharged to an external separator, from which product is removed at the bottom and water vapor passes to a condenser. In some systems the water vapor temperature is raised by mechanical vapor recompression to yield steam for reuse to save energy. Product temperature may reach 85C or higher, but since residence time of the concentrating food in the heated cylinder may be less than a minute, heat damage is minimal.
Vacuum Evaporators
Heat-sensitive foods are most commonly concentrated in low-temperature vacuum evaporators. Thin-film evaporators frequently are operated under vacuum by connecting a vacuum pump or steam ejector to the condenser.
It is common to construct several vacuum vessels in series so that the food product moves from one vacuum chamber to the next and thereby becomes progressively more concentrated in stages. The successive stages are maintained at progressively higher degrees of vacuum, and the hot water vapor arising from the first stage is used to heat the second stage, the vapor from the second stage heats the third stage, and so on. In this way, maximum use of heat energy is obtained. Such a system, called a multiple effect vacuum evaporator (See Fig. 5.9), may be sizable and expensive. Systems employed in the grape juice industry continuously concentrate juice from an initial solids content of 15% to a final solids concentration of 72% at rates of 4500 gal of single strength juice per hour. Similar systems concentrate tomato juice from 6% solids to 30% solids at rates of 15,00 gal or more of single strength juice per hour. Use of energy saving mechanical vapor recompression is common.
Even with efficient vacuum evaporators where water may boil at 30C or slightly lower, some volatile flavor compounds are lost with the evaporating water vapor. These volatile essences can be recovered, or “stripped” from the water vapor and returned to the cool concentrated food as has been mentioned earlier. However, it is possible to concentrate foods at still lower temperatures and further minimize heat damage and volatile flavor loss; one method of doing so is known as freeze concentration.
Freeze Concentration
As discussed previously, when a solid or liquid food is freeze, all of its components do not freeze at one. First to freeze is some of the water which forms ice crystals in the mixture. The remaining unfrozen food solution is now higher in solids concentration.
It is possible, before the entire mixture freezes, to separate the initially formed ice crystals. One way of doing this is to centrifuge the partially frozen slush through a fine-mesh screen. The concentrated unfrozen food solution passes through the screen while the frozen water crystals are retained and can be discarded. Repeating this process several times on the concentrated unfrozen food solution can increase its final concentration several-fold. Freeze concentration has been known for many years and has been applied commercially to orange juice.
Ultrafiltration and Reverse Osmosis
Low-temperature separation and concentration processes employing perm-selective membranes are increasing being used in the food industry. These applications are largely dependent on membrane properties such as water permeability rate, solute and macromolecule rejection rates, and length of useful membrane life. Different membranes are required for different liquid foods. Synthetic membranes are manufactured from cellulose acetate, polyamide, and other materials, with considerable control over their physical and chemical properties.
Ultrafiltration membranes are generally “less tight” than reverse osmosis membranes; that is, they restrict macromolecules such as proteins but with moderate pressure allow smaller molecules such as sugars and salts to pass through. Reverse osmosis membranes are “tither,” and with greater pressure will permit the passage of water but hold back various sugars, salts, and larger molecules. In nature, osmosis involves the movement of water through perm-selective membrane from a region of higher concentration to a region of lower concentration. The region of lower concentration generally contains solutes in solution and has associated with it an osmotic pressure. It is possible to reverse the normal flow of water through the membrane by applying pressure on the solute side of the membrane in excess of the osmotic pressure. This is reverse osmosis.
Applied to food concentration, ultrafiltration and reverse osmosis process involve pumping liquid foods under pressure against perm-selective membranes in suitable support. Equipment may be similar to pressure filters in design. Filtrates passing through one membrane may be further modified by passing through a second tighter membrane. This is done in the processing of cheese whey. One may force the whey through a reverse osmosis membrane and remove much of the water, thus concentrating virtually all of the whey solids. The whey also may be forced through an ultrafiltration membrane first, which would concentrate the lactalbumin protein above the membrane. The filtrate then may be forced through a reverse osmosis membrane selected to retain and concentrate lactose but allowing lower-molecular-weight salts to be removed with the water. Not only are valuable food constituents concentrated in this process, but the water discharged is very low in organic matter (low biological oxygen demand), which decreases its pollution load. Such a process is illustrated in Fig. 10.27. Besides various applications in the dairy industry, these membrane processes are being used to concentrate fruit juices, coffee and tea extracts, egg white and whole egg, soy proteins, enzymes, and other materials.
Change During Concentration
Concentration processes that expose food to 100C or higher temperatures for prolonged periods can cause major changes in organoleptic and nutritional properties.
Figure; 10.27
Cooked flavors and darkening of color are two of the more common results. In addition to the desirability of controlled amounts of these changes in maple syrup, heat-induced reactions also characterize certain candies such as caramel. In caramel production, sugar-milk mixtures are intentionally concentrated at high temperature. With most other foods the lower the concentration temperature the better, since the reconstituted concentrated food should resemble as closely as possible the natural product. Even at the lowest temperature, however, concentration can cause other changes that are undesirable. Two such changes involve sugars and proteins.
All sugars have an upper limit of concentration in water beyond which they are not soluble. For example, at room temperature, sucrose is soluble to the extent of about 2 parts sugar in 1-part water. If water. If water is removed beyond this concentration level, the sugar crystalizes out. This can result in gritty, sugary jellies or jams. It also results in a condition known as “sandiness” in certain milk products when lactose crystallizes due to overconcentration. Since the amount of sugar that can be in solution decreases with decreasing temperature, a concentrated product may be smooth in texture at room temperature but become gritty or sandy when put into a refrigerator. This condition occurs in the manufacture of ice cream due to lactose crystallization during freezing if the concentration of lactose from concentrated milk ingredients in excessive.
As for effects on proteins, it has been pointed out that proteins can be easily denatured and precipitated from solution. One cause of denaturation can be high concentration of salts and minerals in solution with the protein. As protein-containing foods such as milk are concentrated, the levels of milk salts and minerals can become sufficiently high to partially denature the milk proteins and cause it to slowly gel. The gelling may not show up immediately but only after weeks or months of storage, as frequently occurs in cans of evaporated and certain other condensed milks. The gelation of concentrated milk and other protein aqueous foods is an extremely complex phenomenon and is affected by many variables in addition to degree of concentration.
Microbial destruction, another type of change that may occur during concentration, will be largely dependent on temperature. Concentration at a temperature of 100C of slightly above will kill many microorganisms but cannot be depended on to destroy bacterial spores. When the food contains acid, such as fruit juices, the kill will be greater, but again sterility is unlikely. On the other hand, when concentration is done under vacuum, many bacterial species not only survive the low temperatures but multiply in the concentrating equipment. It, therefore, is necessary to stop frequently and sanitize low-temperature evaporators, and where sterile concentrated foods are required, to resort to an additional preservation treatment.
INTERMEDIATE-MOISTURE FOODS
Water activity was briefly discussed in Chapter 7. In recent years, adjustment and control water activity to preserve semi moist foods has attracted increasing attention. Intermediate-moisture foods or semi moist foods, in one form or another, have been important items of diet for a very long time. Generally, they contain moderate levels of moisture, of the order of 20-50% by weight, which is less than is normally present in natural fruits, vegetables, or meats but more than is left in conventionally dehydrated products. In addition, intermediate-moisture foods contain sufficient dissolved solutes to decrease water activity below that required to support microbial growth. As a consequence, intermediate-moisture foods do not require refrigeration to prevent microbial deterioration. In the past there have been various kinds of intermediate-moisture foods: natural products such as honey; manufactured confectionery products high in sugar, plus jellies, jams, and bakery items such as fruit cakes; and partially dried products including figs, dates, jerky, pemmican, pepperoni, and the like. Sweetened be considered an intermediate-moisture food. In all of these products, preservation is partially from high osmotic pressure associated with the high concentration of solutes; in some, additional preservative effect is contributed by salt, acid, and other specific solutes.
Principles Underlying Technology
Essential to any discussion of intermediate-moisture foods is understanding water activity (A) and its relationship to food properties and stability. Water activity may be defined in a number of ways. Qualitatively, A is a measure of unbound, free water in a system available to support biological and chemical reactions. Water activity, not absolute water content, is what bacteria, enzymes, and chemical reactants encounter and are affected by at the microenvironment level in foods materials. Two foods with the same water content can have very different A values, depending on the degree to which the water is free or otherwise bound to food constituents.
Figure 10.28 is a representative water sorption isotherm for a given food at a given temperature. It shows what the final moisture content of the food will be when it reaches moisture equilibrium with atmospheres of different relative humidities. For example, at the temperature for which this sorption isotherm was established, this food will attain a moisture content of 20% at 75% RH. If this food were previously dehydrated to below 20% moisture and placed in an atmosphere of 75% RH, it would absorb moisture until it reached 20% Conversely, if this food were moistened to greater than 20% water and then placed at 75% RH, it would lose moisture until it reached the equilibrium value of 20%. Under such conditions some foods may reach moisture equilibrium in the very short time of a few hours, others may require days or even weeks. When a food is in moisture equilibrium with its environment, then the A of the food will be quantitatively equal to the RH divided by 100.
Water activity can be defined in still other terms in accordance with Raout’s law. Thus, “A” of a solution is quantitatively equal to the vapor pressure of the solution divided by the vapor pressure of pure water. This also is equal to the mole of water in the solution divided by the total number of moles present. Thus, a 1 mole solution of sucrose would contain 1 mole of sucrose and 55.5 moles of water (1000 g/18 g), and assuming it behaved as an “ideal” or “perfect” solution, would have an A value of 55.5/56.5 or 0.98. Such a solution would be quite dilute and if it constituted the water phase of a food would not of itself generally inhibit microbial growth.
Determining Water Activity
The foregoing relationship provide the means for measuring the A at various moisture contents and temperature. One method involves placing small proteins of the food in jars maintained at a fixed temperature and at different relative humidities with standard sulfuric acid or salt solutions. The samples are periodically weighed until they reach moisture equilibrium as indicated by no further gain or loss in weight until they reach moisture equilibrium as indicated by no further gain or loss in weight. The equilibrium moisture content of each portion is next plotted at its corresponding RH. This plot yields a moisture sorption isotherm for the specific food (at the temperature chosen) of the kind indicated in Fig. 10.28. From the resulting moisture sorption isotherm curve, the RH corresponding to each moisture content divided by 100 is equal to the A̫ at that moisture level.
In practice, the A̫ of an experimental food formulation can be readily determined instrumentally. in this case, a sample of the food is placed in a vessel of limited headspace at a chosen temperature. the vessel is provided with a sensitive hygrometer sensor not in contact with the food but connected to a potentiometric recorder. as the food exchanges moisture with the headspace, a curve of RH is traced. The A̫ then corresponds to the RH/100 at equilibrium. RH also can be measured with highly sensitive wet bulb and dry bulb temperature probes.
as stated earlier, A̫ is a measure of free or available water, to be distinguished from unavailable or bound water. these states of water, also bear a relationship to the characteristic sigmoid shapes of water sorption isotherm curves of various foods. Thus, according to theory, most of the water corresponding to the portion of the curve below its first inflection point (below 5% moisture in Fig. 10.28) is believed to the tightly corresponding to the region above this point and up to the second inflection point (about 20% moisture in Fig. 10.28) is thought to exist largely as multi molecular layers of water less tightly held to food constituent surfaces. Beyond this second inflection point, moisture generally is considered to be largely free water condensed in capillaries and interstices within the food. in this latter portion of the sorption isotherm curve, small changes in moisture content result in great changes in the A̫.
of the greatest importance with respect to intermediate-moisture foods is the effect of A̫ on microorganism growth. The A̫ values for growth of most food-associated bacteria, yeasts, and molds have received considerable study. The minimum A̫ below which most important food bacteria will not grow is about 0.90, depending on the specific bacterium. some halophilic bacteria may grow down to an A̫ of 0.75, and certain osmophilia yeasts even lower, but these seldom are important causes of food spoilage. Molds are more resistant to dryness than most bacteria and frequently will grow well on foods having an A̫ of about 0.80, and slow growth may appear after several months at room temperature on some foods even at an A̫ 70. At A̫ values below 0.65, molds growth is completely inhibited, but such low A̫ generally is not applicable in the fabrication of intermediate-moisture foods. this level would correspond to total moisture content well below 20% in many foods; such foods would lose chewiness and approach a truly dehydrated product. for most items, A̫ values between 0.70 and 0.85 are required for semi-moist texture. these levels are sufficiently low to inhibit common food-spoilage bacteria. where they are not sufficiently low for long-term inhibition of mold growth, an antimitotic such as potassium sorbate is included in the food formulation to augment the preservative effect.
although A̫ values for microbial inhibition commonly are cited in the literature to two or three decimal places, this should not convey the impression that an A̫ given as a minimum for growth of a particular microorganism is an absolute value. It can be influenced by such factors as the ph. temperature nutritional status in terms of microbial requirements, and the nature of specific solutes in the efficacy of a target A̫ to prevent microbial spoilage of a new intermediate-moisture formulation by running appropriate bacterial plate counts. bacteriological tests also are necessary from a public health standpoint.
when attempting to fabricate an intermediate-moisture food, one selects an appropriate target A̫ and then selects ingredients to provide solute concentrations to yield the desired A̫. The total solute concentration corresponding to any A̫ can be easily calculated from equations based on Roult's law provided the food'’ water phase behaves as an ideal solution.
As solutions become more concentrated and more complex, however, they fail to behave in ideal fashion; then calculations relating solute concentrations and A̫ become only approximations. for an A̫ of 0.995, for example, theory calls for a total solute concentration of 0.2811 moll. Sucrose and glycerol, which do not dissociate to yield two and three ions, respectively, also approach ideal behavior in such dilute solutions when the sum of the concentrations of their ions is considered, however, in concentrated solutions, solutes become more effective in lowering A̫ than would be predicted on the basis of ideal behavior. this is not due to suppression of ion dissociation, which in itself would be expected to contribute an opposite effect, but rather is thought to result from increased total hydration of large numbers of solute molecules. this also is so with solutes which do not dissociate such as sucrose and glycerol, favored ingredients in the formulation of intermediate-moisture foods. such phenomena make it necessary to augment mathematical calculation of A̫ by experimental measurement when attempting to establish intermediate-moisture food compositions.
much of what has been said thus far with regard to A̫ has had to do with microbial inhibition; however, A̫ affects many other properties of foods, including chemical reactivity and equilibria, enzymatic activity, flavor, texture, color, and stability of nutrients.
Products and Technology
Aside from semi-moist dog food, few intermediate moisture foods have been specifically developed for human consumption. However, several common foods meet the definition of intermediate moisture food. Jams, jellies, some processed/fermented meats, dried fruits, confections, bakery products, and snacks are common examples. The major concern with such foods is the control of moisture. Loss of moisture results in undesirable changes in texture, whereas pick up of moisture results in the potential for microbial growth. The major way in which changes in moisture are controlled is by packaging which inhibits moisture transfer.
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