The number of different food products and the operations and steps involved in their production are indeed very great. Further, each manufacturer introduces departures in methods and equipment from the traditional technology for that product, and processes are in a continual state of evolution. The food scientist would soon experience great frustration if there were not unifying principles and a systematic approach to the study of these operations.
The processes used by the food industry can be divided into common operations, called unit operations. Examples of unit operations common to many food products include cleaning, coating, concentrating, controlling, disintegrating, drying, evaporating, fermentation, forming, heating/cooling (heat exchange), materials handling, mixing, packaging, pumping, separating, and others. These operations are listed alphabetically, not in the order of their natural sequence or importance.
Most unit operations are utilized in the making of a variety of food products. Heat exchanging, or heating, for example, is used in the manufacture of liquid and dry food products, in such diverse operations as pasteurizing milk, sterilizing foods in cans, roasting peanuts, and baking bread.
Unit operations may include numerous different activities. The unit operation of mixing, for example, includes agitating, blending, diffusing, dispersing, emulsifying, homogenizing, kneading, stirring, whipping, and working. We may want to mix to beat in air, as in making an egg white foam, or to blend dry ingredients, as in preparing a ton of dry cake mix; or we may wish to mix to emulsify, as in the case of mayonnaise, or to homogenize to prevent fat separation in milk. We may wish to mix and develop a bread dough, which requires stretching and folding, referred to as kneading.
One of the key elements to food processing is the proper selection and combination of unit operations into more complex integrated processing systems. These operations and processes consume great quantities of energy.
COMMON UNIT OPERATIONS
Materials Handling
Materials handling includes such varied operations as hand and mechanical harvesting on the farm, refrigerated trucking of perishable produce, box car transportation of live cattle, and pneumatic conveying of flour from rail car to bakery storage bins.
Throughout such operations emphasis must be given to maintaining sanitary conditions, minimizing product losses (including weight loss of livestock), maintaining raw material quality (e.g., vitamin content and physical appearance), minimizing bacterial growth, and timing all transfers, and deliveries so as to minimize holdup time, which can be costly as well as detrimental to product quality.
The movement of produce from farm to processing plant and of raw materials through the plant may take many forms. Oranges, for example, are moved by truck trailers to juice plants, where they are graded and washed. There is a limit to the size the trucks may be and the length of time the fruit may be held since fruits and vegetables are alive, respire, and can cause the temperature of a batch to rise to the point where complete spoilage may occur.
Bulk dry sugar delivered to confectionery and other types of food plants is conveyed from the truck to storage bins by a pneumatic lift system. Storage must not be for a period of time nor at a temperature and humidity that will allow the sugar to cake. Transfer of sugar in the plant must avoid dusting and the buildup of static electricity to prevent possible explosion of the highly combustible sugar particles. This is also true I handling finely divided flour. The pneumatic conveying of spices is shown in this method of materials handling has the additional advantages of preventing loss of desirable volatiles from the spices, irritation to personnel, and flavor exchange between different spices.
The use of a wide variety of screw conveyors, bucket conveyors, belt conveyors, and vibratory conveyors in food plants need no elaboration here beyond the obvious recognition that conveying and handling equipment for eggs in the shell must be different than for less fragile products.
Cleaning
Foods by the nature of the way they are grown or produced on farms in open environments often require cleaning before use. Cleaning ranges from simple removal of dirt from egg shells with an abrasive brush to the complex removal of bacteria from a liquid food by passing it through a microporous membrane. Grains must be cleaned of stones before use. Cleaning can be accomplished with brushes, high-velocity air, steam, water, vacuum, magnetic attraction of metal contaminants, mechanical separation, and so on, depending on the product and the nature of the dirt.
The cleanliness of water used in the soft drink bottling industry must exceed many of the standards found adequate for drinking water. If a high degree of carbonation is to be achieved, then the water used in making the drink must be remarkably free of dust particles, colloidal particles, and certain inorganic salts, since these minimize carbon dioxide solubility and promote excessive escape of gas bubbles. To adequately clean this water may require that city water receive such additional treatments as controlled chemical flocculation of suspended matter, sand filtration, carbon purification, microfiltration, and de aeration. This is no longer the unit operation of cleaning but a total cleaning process.
Some cleaning methods are dictated by surface characteristics of the product. Because pineapples have an irregular surface, the scrubbing action of high-pressure water jets is used.
Just as different food materials require special cleaning, the surface of food processing equipment need thorough and frequent attention. The cleaning of equipment, as well as a facility’s walls and floors, must take into consideration the chemical and physical properties of both the surface to be cleaned and the type of soil. Many types of soil can be removed with mildly alkaline detergents, but strong alkali may be required for more tenacious-deposits and heavy deposits of fats and oils or built up protein deposits. Alkaline films and hard-water scales may require mildly acid detergents. Strong acids are highly corrosive to several metals, fabrics, wood, rubber, and concrete floors. Strong alkaline and neutral detergents find wide application in the food industry. Several are listed in Fig. 5.2, along with the properties that affect their cleaning efficiency. Food plant operators generally call on detergent manufacturers for expertise in establishing highly effective cleaning procedures since these further depend on detergent concentrations, temperatures of application, order of application where more than one cleaning aid is used, and other variables.
Separating
The unit operation of separating can involve separating a solid from a solid, as in the peeling of potatoes or the shelling of nuts; separating a solid from a liquid, as in the many types of filtration; or a liquid from a solid, as in pressing juice from a fruit. It might involve the separation of a liquid from a liquid, as I centrifuging oil from water, or removing a gas from a solid or a liquid, as in vacuum removal of air from canned food in vacuum canning.
One of the commonest forms of separating in the food industry is the hand sorting and grading of individual units as in the case of vegetables and fruit. However, because of the high cost of labor, mechanical and electronic sorting devices have been developed. Difference in color can be detected with a photocell and off-color products rejected. This can be done at enormous speeds with automatic rejection of discolored or moldy nuts or kernels of grain that flow past the photocell. In the case of peanuts to be made into peanut butter, each peanut individually passes through a light beam that activates a jet of air to blow the discolored peanuts from the main stream when an off-color changes the amount of reflected light. Light shining through eggs can detect blood.
Spots and automatically reject such eggs. Automatic separation according to size is easily accomplished by passing fruits or vegetables over different size screens, holes, or slits.
The skins of fruits and vegetables may be separated using a lye peeler (Fig. 5.3).
Peaches, apricots, and the like are passed through a heated lye solution. The ley or caustic softens the skin to where it can be easily slipped from the fruit by gentle action of mechanical fingers or by jets of water. Differences in the density of the fruit and skin can then be used to float away the removed skin.
To separate corn oil from corn kernels, the germ portion of the corn first is separated from the rest of the kernel by milling; then the oil is separated from the germ by applying high pressure to the germ in an oil press. Similarly, pressure is used to squeeze oil out of peanuts, soybeans, and cottonseeds. The last traces of oil can be removed from the pressed cake by the use of fat solvents. There them remains the separation of the oil from the solvent.
Crystallization is used to separate salt from sea water, or sugar from sugar cane juice. Here, evaporation of some of the water causes super saturation, and crystals form. Since crystals are quite pure, this is also considered a purification process. The crystals are then separated from the suspending liquid by centrifugation.
Newer methods of separation include several techniques involving manufactured membranes with porosities or permeability capable of separations and fractionations at the colloidal and macromolecular size level. Ultrafiltration uses membranes of such porosity that water and low-molecular-weight salts, acids, and bases pass through the membrane but larger protein and sugar molecules are retained. This selective separation process carried out at ambient temperature, avoids the heat.
Damage to sensitive food constituents that is often associated with water evaporation at high temperatures. Further, removal of acids and salts with the water prevents their concentration, which would otherwise be detrimental to sensitive retained solids.
Disintegrating
Operations which subdivide large pieces of food into smaller units or particles are classified as disintegrating. It may involve cutting, grinding, pulping, homogenizing, and so on. Although the dicing of vegetables is done on automatic machines, the cutting of meat still largely represents a time-consuming, hand-labor operation. This is because skill is required to separate specific cuts of meat, and the value of the cuts can be sharply reduced by a sloppy job. However, automatic knives with a “brain” are being researched and developed. In another application, cutting of bakery products can be done cleanly and precisely with fine jets of high-pressure, high-velocity water. Laser beams also can replace knives in some cutting applications.
Disintegrating by grinding, an in the preparation of hamburger or hash, always is associated with heating of the product due to friction created in the grinding process. This can be damaging to the food product. It can partially denature proteins or it can give burned flavors to ground coffee. Some kind of cooking is therefore required. In the case of meat, this sometimes is done by grinding the meat in frozen form. Dry ice can be added to the meat or other food to chill it. Dry ice is used rather than regular ice since regular ice would melt and water the food, but dry ice goes off as carbon dioxide and so does not change the composition of the food.
Homogenizing produces disintegration of fat globules in milk or cream from large globules and clusters into minute globules. The smaller fat globules then remain evenly distributed throughout the milk or cream with less tendency to coalesce and separate from the water phase of the milk. Disintegrating the fat globule is done by forcing the milk or cream under high pressure through a hole with very small opening. There are many ways to homogenize, including the use of ultrasonic energy to disintegrate fat globules or break up particles.
Pumping
One of the most common operations in the food industry is the moving of liquids and solids from one location or processing step to another by pumping. There are many kinds of pumps and the choice depends on the character of the food to be moved. One common type is a rotary gear pump (see external gear pump in Fig. 5.4). the inner gears rotate, sucking food into the pump housing and subsequently squeezing food out of the pump housing. For reasons of mechanical efficiency, with this type of pump, close clearances between the gears and housing are essential. Although such a pump would be effective for moving liquids and pastes, it would chew up chunk-type foods, reducing them to purees. Actually, pumps sometimes are used to do just this, but, generally, disintegration is a change that can best be controlled with specialized equipment other than pumps, and pumps should be chosen primarily for their pumping efficiency. A single screw pump is best for moving food with large pieces without disintegration. Such pumps are also called progressing cavity pumps and can be selected for large clearances of the cavities ds between the turning center rotor and the housing. The food is gently propelled from large clearance to large clearance by the screw like action of the turning rotor..
Food pieces such as corn kernels, grapes, and even small shrimp can be pumped without physical damage. In the gear-type pump these would be ground up.
An essential feature for all food pumps is ease of disassembly for thorough cleaning. Today’s sanitary stainless steel pumps in many cases can be disassembled in minutes with a single tool.
Mixing
Like pumps, there are scores of kinds of mixers, depending on the materials to be mixed. One may wish to mix solids with solids, liquids with liquids, liquids with solids, gases with liquids, and so on.
For simple mixing of dry ingredients such as the components that make up a baking powder, a conical blender is suitable. The bowl has a tumbling action and this may be continued for 10-20 min until the mixture is made homogenous.
If we are preparing a dry cake mix, we must cut the shortening into the flour, sugar, and other dry ingredients in order to produce a fluffy homogeneous dry mix. We may use a ribbon blender, which is a horizontal tough with one of several types of mixing elements rotating within it. The efficiency of mixing depends on the choice of the mixing element. Three types of ribbon like elements suitable for cutting in shortening are illustrated in Fig. 5.5.
For mixing solids into liquids to dissolve them, a propeller-type agitator mounted within a stainless steel vat is best. There are a great number of propeller, turbine, and paddle types available for this kind of mixing.
All types of mixers do some work on the material being mixed and produce some increase in temperature. It is often desirable to minimize this temperature rise. However, mixers are also chosen to do special kinds of work on heavy viscous materials while they are being mixed. These mixers may have arms that knead dough, or paddles and arms that work butter. These working mixers are designed with precise geometries to maximize efficiency and minimize energy requirements to achieve the mixing working operation.
Still other mixers are designed to beat air into a product while it is being mixed. The mixer-beater found in ice cream freezers is an example. As the ice cream mix is being frozen within the freezer bowl, the beating element, or dasher, turns within the bowl. It not only keeps the freezing mass moving to speed freezing and make freezing more uniform, but it also beats air into the product to give the desired volume increase, or overrun, necessary for proper texture.
Heat Exchanger
Heating
We heat foods for many different reasons. Many foods are heated to destroy microorganisms and preserve the food, for example, during pasteurization of milk and canning of vegetables. Others are heated to drive off moisture and develop flavors, as during the roasting of coffee and toasting of cereals. Still others are heated during normal cooking to make them more tender and more palatable. Some food ingredients, such as soybean meal, are heated to inactivate natural toxic substances. Foods are heated by conduction, convection, radiation, or a combination of these.
Most foods are sensitive to heat, and prolonged heating causes burned flavors, dark colors, and loss of nutritional value. Microorganisms are more sensitive to rapid heating than are chemical reactions. Rapid heating can, therefore, destroy microorganisms faster than it causes undesirable chemical reactions. Hence, it is desirable to heat and cool foods rapidly to maintain optimal quality. Rapid heating is facilitated if the food is given maximum contact with the heating source. This may be accomplished by dividing the food into thin layers in contact with heated plates as in the plate-type heat exchanger used to pasteurize milk. The milk flows across one side of the plates while hot water or steam heats the other side. The same equipment can be used for quick cooking with cold water or brine instead of hot water. This type of heater can be used only with liquid foods.
A jacketed tank or kettle with steam circulated in the jacket is another means of heating liquid foods. It will also heat foods with suspended solids like vegetable soup.
The soup is kept in motion with a mixer propeller of uniform heating and to minimize burning onto the kettle wall.
For sterilizing foods in cans and other containers, entirely different heaters are used. The containers must be heated to temperatures higher than the boiling point of water to achieve sterility in nonacid foods, and so large pressure cookers or retorts are used (Fig. 5.8). Steam under pressure is used to obtain the high temperature needed, and the retort is of heavy construction to withstand this pressure. Another type of retort employs mechanical agitation for better convection of heat within individual cans. The outside of the cans is heated by conduction from the steam.
For roasting coffee beans or nuts, many kinds of heaters have been used. In one type, the beans or nuts move from overhead hoppers into cylindrical vessels that turn and keep the beans in constant motion for even heating. The vessels may be heated within by circulating heated air, or with radiant heat from the vessel walls, the exterior of which can be heated by contact with hot air, gas flame, or steam. In some instances
This type of raster is replaced with tunnel ovens in which the coffee beans or nuts pass on moving belts or are vibrated beneath radiating infrared rods or bulbs. Whatever the method, precise control of temperature is essential for proper roasting.
Foods may be heated or cooked using toasters, direct injection of steam, direct contact with flame, electronic energy as in microwave cookers, and so on; all of these methods are currently used in the food industry. Suh processes as baking, frying, most food concentration, food dehydration, and various kinds of package closure all employ the unit operation of heating.
Cooling
While heating is the addition of heat energy to foods, cooking is the removal of heat energy. This may be done to the degree where food is chilled to refrigerator temperature, or beyond this range to where the food is frozen. Primarily, we refrigerate and freeze foods to prolong their keeping quality. But there are some foods that owe their entire character to the frozen state. A prime example is ice cream.
A great deal of milk and cream are cooled by passing them in thin layers through heat exchangers of the type shown in Fig 5.7, or by allowing the liquids to run down over the surface of a hinged leaf cooler. Within the leaves are pipes through which cold water or refrigerant is pumped.
Liquid egg, apple slices, and other fruits in 13.6-kg cans are commonly frozen solid in an air-blast freezer or sharp freezer room maintained at about -26C. The cans are spaced to allow the cold air, which is circulated by fans and blowers, to get between them and speed the freezing operation. Products in this form go mainly to bakeries for use after thawing.
There are many kinds of commercial air-blast freezers designed to freeze peas, beans, and other vegetables as individual pieces. In one type the peas are loaded on trays that are automatically moved upward through a cold air blast. After freezing, the peas are dislodged from the trays and conveyed under the pea hopper to receive additional product and the cycle is repeated. Freezing of canned or packaged foods may be done by direct immersion in a refrigerant. Here the cans may be agitated as they pass through the refrigerant within a cylindrical shell, or tube. Agitation increases the efficiency of heat transfer.
The value of quick freezing to food quality, which is discussed more fully in Chapter 9, has led to the use of liquid nitrogen with its extremely low temperature of 196C. Many food plants have installed large liquid nitrogen tanks and pump liquid-nitrogen to freezers where it is sprayed directly onto foods to be frozen. Delicate products such as mushrooms are frozen this way.
Evaporation
Evaporation in the food industry is used principally to concentrate foods by the removal of water. It is also used to recover desirable food volatiles and to remove undesirable volatiles.
The simplest kind of evaporation occurs when the sun evaporates water from sea water and leaves behind salt; this process is used commercially. Grapes and other fruits can be dried using the energy from the sun. another simple form of evaporation occurs when a heated kettle is used to boil water from a sugar syrup, as is common in some kinds of candy-making. However, this requires considerable energy in the form of heat for a long period of time, which would cause heat damage to such products as milk or orange juice if an attempt were made to concentrate them this way.
All liquids boil at lower temperatures under reduced pressure and this is the key to modern evaporation. If a heated kettle is enclosed and connected to a vacuum pump, one has a simple vacuum evaporators are used to remove water from sugar can press juice in the early stages of crystalline sugar production.
Evaporators differ widely in their design and can be connected in series as in the triple-stage system diagrammed in Fig. 5.9. In this device, progressively higher degrees of vacuum are maintained in the subsequent stages through which the liquid food passes. Regardless of design, however, a principal objective of vacuum evaporates is to remove water at temperatures low enough to avoid heat damage to the food. Multiple stage evaporators can easily remove water at 50C and some are designed to boil off water at temperatures as low as 21C.
Drying
In drying, the object also is to remove water with minimum damage to the food. Whereas evaporators will concentrate foods twofold or threefold, driers will take foods very close to total dryness in many cases less than 2% or 3% water. Driers are used to prepare such well-known products as dried milk powder and instant coffee. Although food traditionally has been dried to preserve it from spoilage and to reduce its weight and bulk, some foods are dried as convenience items and for their novelty appeal; an example has been freeze-dried fruits for cereals. Drying, as well as several of the other unit operations, is treated in detail in later chapters, and so only a few brief comments are needed here.
Insert Figure; 5.9 schematic for forward-feed triple effect evaporator. Courtesy of Hall and Hedrick,
Liquid foods such as milk and foods in chunk form like shrimp or steak may be dried. It is generally much easier to dry liquid foods because these are easier to subdivide, either as a spray or a film, and in a subdivided form, the moisture can be removed more quickly.
Subdivision of a liquid is the principle behind the widely used spray drier (Fig. 5.10). Liquid food such as milk, coffee, or eggs is pumped into the top of the large tower, at which point the liquid is atomized by a spray nozzle or equivalent device. At the same time, heated air is introduced to the tower. The heated air in contact with the fine droplets of food dries the droplets and dehydrated particles fall to the bottom of the tower and are drawn off into collectors. The moisture removed during drying is exhausted separately. Most commercially dried liquid foods are made this way.
Drying by subdividing food as a thin film is commonly done on a drum or roller drier of the kind illustrated in Fig. 5.11. The drum is heated by steam from within, and the applied layer of food flashes off its moisture on contact with the heated drum. The dried food is then mechanically scraped from the drum with long knives. Mashed potatoes, tomato puree, and several milk products are frequently dried this way.
Small food pieces such as peas and diced onions can be dried by moving them through a long tunnel-oven, and many types are in use. However, overheating and shrinkage in the course of water removal may give poor quality products in the case of particularly sensitive foods. For chunk foods a milder method is vacuum freeze-drying. There are many kinds of vacuum driers. In all types the food pieces first are frozen and then dehydrated under vacuum from the frozen state. The ice does not melt but under the conditions of high vacuum goes off directly as gaseous water vapor, a process known as sublimation. This very gentle kind of drying protects all food quality attributes such as texture, color, flavor, and nutrients. Freeze-drying is by no means restricted in its use to solid and particulate foods. Brewed coffee and quality juice products are dehydrated by freeze-drying.
Forming
Foods must often be formed into specific shapes. For example, hamburger patties are formed by gently compacting ground beef into a disk shape in various types of patty-making machines, which apply controlled pressure to the beef within an appropriate form. Excessive pressure is avoided or the hamburger will be overly tough after cooking. Uniform pressure is essential or patties will vary in weight. Pressure extrusion through dies of various shapes forms doughs into spaghetti and other pasta shapes for subsequent oven drying.
In the confectionery industry, besides pressure extrusion, candies are formed by depositing fondants, chocolate, and jellies into appropriate molds where they cool and harden. Here edible release agents may be sprayed on the mold to help separate the hardened confection. Other confections and food tables may be formed from powdered ingredients by the application of intense pressure in specially designed tableting machines. Sometimes, as in the case of malted milk tablets, an edible binding agent is required to hold the malted milk powder together. When the powders are high in certain sugars or other thermoplastic food constituents, an additional binding agent is not required. Here high pressure during tablet forming produces heat, melting some of the sugar or other thermoplastic material, which on cooling helps fuse the powdered mass together. This is one way to form fruit juice tablets from dehydrated fruit juice crystals. Some tablet forming machines also may employ additional heat beyond that generated by pressure.
Forming is an important unit operation in the breakfast cereal and snack food industries. The characteristic shapes of several popular breakfast cereals are the result of pressure extrusion through meticulously designed dies, together with adherence to precisely controlled operating conditions of temperature, pressure, dough consistency, cutoff, and other variables. One special kind of forming is known as extrusion cooking. In this case a formulated dough or mash is extruded under high pressure with or without supplemental heat. The heat, largely from pressure, causes gelatinization of starch and other cooking effects while the material is forced through the extruder. In some cases, pressure and temperature are so regulated that the food rises in temperature above that required to boil, water rapidly boils as pressure is relieved at the exit nozzle. This causes puffing of the formed piece. Such formed and puffed items may then receive additional oven drying. A versatile cooker extruder is diagrammed in Fig.
Further examples of forming include the shaping of butter and margarine bars, the pressing of cheese curd into various shapes, the many manipulations given to bread dough to produce variety breads, and the shaping of sausage products in natural and artificial flexible casings.
Packaging
Food is packaged for several purposes, including containment for shipping, dispensing, and unitizing into appropriate sizes, and improving the usefulness of the product. A primary reason is to protect it from microbial contamination, physical dirt, insect invasion, light, moisture pickup, flavor pickup, moisture loss, flavor loss, and physical abuse.
Foods are packaged in metal cans, glass and plastic bottles, paper and paperboard, a wide variety of plastic and metallic films, and combinations of these. Packaging is done by continuous automatic machines sometimes at speeds of more than 1000 units per minute. Many items formerly filled into rigid containers of metal and glass are being increasingly packaged in flexible and formable materials, and filling and capping machines are being joined by more sophisticated systems. Much of the consumer milk supply is packaged in paper cartons.
Supply is packaged in paper cartons. Containers are automatically formed from stacked paper flats, volumetrically filled, and sealed by passing the upper flaps through heated jaws, which melt the plastic coating and thus provide adhesion. In recent years, paper-board cartons which have been coated with special plastics which inhibit oxygen from entering the carton have become widely used for arrange juice and similar products.
Other machines form pouches from rolls of plastic him, fill them, and seal them. This is the way many popular snack food items are packaged. Still more complete systems (Fig. 5.13) form the container from roll stock film, fill the container to exact weight, draw a vacuum on the package to remove oxygen, back flush the package with inert nitrogen gas, seal the package, and finally stack the packages into cardboard cartons. This is the way some dessert powders and dehydrated soups are commonly packaged.
The container-forming step is not limited to the use of paper flats or films of various materials. Some food-packaging machines start with plastic resins in granular form, melt these, and blow-mold or otherwise form rigid semi rigid containers for immediate filling and sealing. Two advantages of such a system are the savings of space in food plants that otherwise would have to store great numbers of empty containers, and the in-line production of virtually sterile containers, since the heat to melt the plastic resins also kills microorganisms.
Controlling
With all of these and several additional unit operations combined into complex processing operations there have to be ways of measuring and controlling them to obtain the desired food product quality. Controlling may be considered a unit operation in itself. Its tools are valves, thermometers, scales, thermostats, and a wide variety of other components and instruments to measure and adjust such essential factors as temperature, pressure, fluid flow, acidity, specific gravity, weight, viscosity, humidity, time, liquid level, and so on.
Figure 5.14 shows a retort commonly used in the canning industry for processing food in metal cans. It is equipped to heat cans of food to the proper sterilization temperature, hold them for the required theme, and then cool them. It has controls for steam flow, steam pressure, air pressure, water temperature, water level, and holding time. These controls can be manually operated or can be designed for automatic operation. In modern food plants most instrumentation and controls are automatic, and the plant operator or supervisor directs the process form a remote panel board or integrates several processes under microprocessor-based computer control form a programmable console.
Overlapping Unit Operations
The division or grouping of food processing steps into unit operations is not perfect and there can be overlapping. For example, filtering bacteria out of beer might logically be considered cleaning or it might be considered separating. Moving milk to a cheese vat might be viewed as pumping or it might be considered materials handling. Milling grain to yield flour might be considered disintegrating followed by separating.
Overlapping does not detract from the value of the unit operations concept. This concept permits one to think in an orderly fashion. What is more, some food texts and most food equipment catalogs are divided by unit operations. One may have the problem of blending fragile stuffed olives into sausage meat emulsion with minimum breakage as in the manufacturer of certain table-ready meats, or of incorporating a whip improver into commercial liquid egg white without foaming the white. Applicable available equipment generally will not be found in reference sources under food commodity headings but will be grouped in the mixing sections of equipment and engineering references.
If one considers any total food process, such as the manufacture of bread or frozen orange juice concentrate, it is seen that the process is always series of unit operations performed in a logical sequence. In modern food processing these operations are so connected as to commonly permit smooth, continuous, automatically controlled production.
Energy Conservation
Many food processing unit operations require considerable amounts of energy. Thus, the cost of energy is a significant part of the cost of producing foods. This has focused attention on unit operations, equipment design, and overall processes from the stand point of optimizing energy use. There is now much interest in the analysis of heating and cooling processes and the recovery and reuse of heat units that formerly were skewered or vented to the atmosphere. Dehydration, concentration, freezing, sterilization, and other operations are being reevaluated in terms of times and temperatures, which affect energy expenditures but also product properties and safety. The energy requirements for materials handling and cleaning often are influenced by varietal type of fruits and vegetables and by agricultural practices before commodities reach processing plant. This also is true for the energy required in peeling, cutting, and disintegrating. The energy requirements for producing the papers, tinplate, aluminum, glass and plastics of modern packaging, as well as the various package forms, differ, but so do their protective properties, contributions to litter and waste, and recycle values. Preservation by methods that remove water affect product weight and volume and therefore the energy requirements of subsequent transportation and climate controlled storage. Often the energy required to produce processed foods industrially is for less than that needed to prepare similar foods at home, but careful analyses in this area have been few and the variables that can influence results are many.
There are countless simple measures to conserve energy throughout the food production chain that often are overlooked. These include common everyday practices such as increasing boiler and steam efficiency, optimizing refrigeration and space conditioning through improved temperature control and insulation, closing unnecessary building opening and reducing excessive ventilation rates, reducing lighting excessive to the task performed, using optimum-sized equipment, scheduling regular maintenance including periodic checks on sensors and controlling devices, and so on. Today it is common practice to employ energy conservation specialists and involve plant engineers in general energy management.
New Processes
New processing technologies are constantly being developed which increase the range of options within each unit operation. Major goals of food scientists and processing engineers are to develop new methods which improve quality or increase efficiency. Not surprisingly, there has been considerable research in this area. Newer processing methods that hold promise for improved products over the coming years include super-critical fluid extraction, ohm heating, and high hydrostatic pressure.
Supercritical fluid extraction uses gases such as carbon dioxide at high pressures to extract or separate food components. At high pressures carbon dioxide behaves as a liquid and can selectively dissolve portions of a food product. For example, supercritical fluid extraction is already commercially used to extract caffeine from coffee to produce a decaffeinated product. The advantage is that the extraction can be made highly selective and occurs under mild conditions which result in a high quality product. This process also has the advantage that the solvent (i.e. carbon dioxide) is not toxic and can be disposed of easily. Supercritical fluid processing can also be used to extract and concentrate delicate flavor compounds from biological materials such as spices and herbs. Research has further shown that supercritical extraction can be used to fractionate the fats from dairy products, lower their cholesterol content, and produce lipids with desirable functional characteristics. Large continuous systems are being developed which may lower the cost of such applications and increase efficiency.
Heat to destroy microorganisms can cause undesirable effects in food texture, flavor, and color. The more quickly heat can be applied and removed, the less detrimental are the changes that occur, and so it is not surprising that considerable effort has been directed at developing high-temperature-short time thermal processes.
When foods containing particulates such as beef in a stew are heated by conventional heat exchange systems, the liquid portion becomes over processed by the time the inner portion of the beef is sufficiently heated. One proposed solution to this problem is termed ohm heating. in ohm heating, the temperature of particulates in a conducting medium such as a salt brine is raised quickly. The food pumped between two electrodes which are charged with an alternating current similar to common household electricity. The temperature of both particles and liquid increases rapidly. Although this process was first applied to fruit juices, it is particularly useful for foods which consist of particles suspended in liquids like soups or stews.
A third new technology utilizes high hydrostatic pressure. Liquid foods such as fruit juices and beverages or particulate foods suspended in liquid are subjected to pressures as high as several thousand atmospheres. These high pressures can inactivate microorganisms and in some cases enzymatic activity. In a suggested application, foods are packaged in flexile pouches which are loaded into vessels capable of withstanding the high pressures. Water is then pumped into the vessel and all air removed. The inside the vessel is activated after the vessel is closed. The major advantage of such a system is that foods may be preserved without the input of large amounts of heat, thus improving their quality. The commercial feasibility of such a system, however, remains to be determined.
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