Both irradiation and microwave heating employ radiant energies which affect foods when their energy is absorbed, whereas ohm heating raises the temperature of foods by passing an electrical current through the food. Each requires special equipment to generate, control, and focus this energy. Each of these are relatively new technologies as applied to foods. Food irradiation is used primarily as a preservation method, but it also has potential as a more general unit operation to produce specific changes in food materials. Microwave energy, on the other hand, has been employed specially to produce rapid and unique heating effects, one application of which can be food preservation. Ohm heating is the newest and least used of the three technologies. Like microwave heating, ohm heating can preserve foods by the application of heat and has the ability to very rapidly heat foods with minimal destruction.
FOOD IRRADIATION
The discoveries of artificially produce radiations such as X-rays and radioactivity of natural materials date back to 1895-1896. Food irradiation studies are more recent, having begun in earnest shortly after World War ii. The impetus for this research came largely from intensive investigations of nuclear energy, which led to developments in the economic production of radioactive isotopes and to evolution of high-energy accelerators. In the period since 1945, food irradiation has been investigated intensively. Much of this work has had to do with the safety and wholesomeness of irradiation sterilized bacon, the first in a growing list of proposed products. This approval, which limited the types of energy sources and does that could by employed, was revoked in 1968. The safety of irradiated foods has continued to receive study since then, with several countries gradually adding to the list of approved irradiated foods. in 1983 the FDA approved irradiation as a means of controlling microorganisms oh spices, and in 1985 the FDA widened the allowed uses of irradiation to additional foods such as strawberries, poultry, ground beef, and pork.
In its early development, irradiation was thought of as a process to preserve foods for extended periods by sterilization, much as thermal processing does. However, this has proven to be impractical for many products because the amount of irradiation required to commercially sterilize foods causes its own form of deterioration. Freezing prior to irradiation can reduce the damage, but this makes the process excessively expensive.
More recent developments have focused on the use of lower doses of irradiation which are less damaging to the food yet have desirable effects. As currently practiced, irradiation is used for three purposes. First, it can be used as an alternative to chemical fumigation to control insects in foods such as spices and fruits and vegetables. The second use is to destroy vegetative cells of microorganisms including those that might cause human disease. This results in an increase in safety and shelf life.
Forms of Energy
There are several forms of radiant energy emitted from different sources. These belong to the electromagnetic spectrum of radiations and differ in wavelength, frequency, penetrating power, and the effects they have on biological systems. Some of these forms of radiant energy and their bactericidal effects are indicated in Table 11.1.
A light bulb emits visible energy. This is radiated from the bulb filament and travels in all directions; however, it can be focused and aimed at a target. Similarly, an infrared heat lamp contains a glowing element that radiates infrared energy. This can be directed at a steak, which absorbs the infrared energy and becomes warm or, indeed, cooked if the quantity of the absorbed energy is sufficiently high.
There are other forms of energy which produce neither light nor heat and cannot readily be detected by the human eye or sense of touch, like radio waves, ultraviolent light, or cosmic rays, but they are present nevertheless.
Certain types of energy radiations are emitted from the breakdown of atomic structure. Materials that undergo such as uranium are naturally radioactive; others can be made radioactive by high-energy bombardment of their atoms as in the case, of cobalt-60. Another form of energy is associated raise can be given virus degrees of acceleration and increased energy by passage through special electronic device.
Some of the kind of energy indicated above are used to a limited degree in food preservation. Ultraviolet light, specially within the wave length range of 200-280 nm, is employed to inactivate microorganisms on the surface of foods. The severe limitation here is the low degree of penetration of ultraviolet light into foods, restricting its usefulness to outermost surface of food or liquid’s that can be exposed in thin layers. Treatments of equipment surfaces, water, and air used in food plants are additional current application of ultraviolent light X-rays have greater penetrating power than ultraviolent and have received consideration as a means of preserving foods. however, X-rays cannot easily be focused, leading to low efficiency of use with current equipment. Thus, X-rays food application to date have been experimental rather than commercial.
Currently, when the terms food irradiation is used, it generally is understood to mean processing with a limited number of kinds of radiant energy that together are referred to as “ionizing” radiations. These are chosen because they have penetrating power but do not produce radioactivity in treated foods. they also do not produce significant heat in foods, and so the additional term cold sterilization has been applied to this kind of food preservations.
Ionizing Radiations and Sources
Natural radioactive elements and artificially induced radioactive isotopes, which can be produced in nuclear reactors, emit a variety of radiations and energy particles during radioactivity decay. Among these are alpha particles, which are really helium atoms minus two outer electrons; beta particles or rays, which are high-energy electrons also referred to as cathode rays; gamma rays or photons, which are a type of X-rays; and neutrons. These radiations have different penetrating power: Alpha particles will not even penetrate a sheet of paper, beta particles or electrons are more penetrating but can be stopped by a sheet of aluminum, and gamma rays are highly penetrating and will go through a block of lead if it is not too thick. Neutrons have great penetrating power and are of such high energy that they can alter atomic structure and make atoms that they strike radioactive. Such atoms, in turn, emit high-energy radiations.
The most suitable emissions for food irradiation have good penetrating power so that they will inactivate microorganisms and enzymes not only on the surface but deep within the food. On the other hand, high-energy emissions as neutrons would break down atomic structures in the food and make the food radioactive. Therefore, gamma rays and beta particles are those used most often.
Gamma and beta rays used in food irradiation may be derived from approved spent fuel elements after their use in a nuclear reactor. These fuel elements, which eventually develop fission fragments and other impurities, making them unsuitable for further use in a nuclear reactor, still possess intense radioactivity. Such spent fuel elements can be placed in an appropriately shielded and enclosed region, and the food brought into the path of their radiation. In early experimental food irradiation facilities, spent fuel elements were placed in shielded pits under about 5 m of water. Cans of food were lowered into vertical cylinders immersed in the water and surrounded by the fuel elements at the bottom of the pit. The containers were then held there for sufficient time to absorb an appropriate radiation dose. Current facilities are less cumbersome and make considerable use of artificially induced radioactive elements such as 60C for the radiating fuel. Where 60c is used, it is employed primarily as a gamma ray source, since beta particles may be more efficiently produced by electronic machines.
Units of Radiation
Various terms have been used to quantitatively express radiation intensity and radiation dosage:
· A roentgen of radiation is equivalent to the quality of radiation received in 1 h from a 1-g source of radium at a distance of 1 yd. this is also the quantity of radiation that will produce 2.08X109 ion pairs per cubic centimeter of dry air, or one electrostatic unit of charge of either sign per cubic centimeter of air under standard conditions of temperature and pressure.
· The energy required to produce ion pairs in air can also be expressed in terms of electron volts. Approximately 32.5 eve are required to produce one ion pair in air. An electron volt is the energy equivalent of 1.6X 10-19 J.
· A rad is a measure of ionizing energy absorbed. It has a quantitative equivalent of 10-5 J absorbed per gram of absorbing material.
· The rad and its multiples (krad, Mrad) are commonly used as units of absorbed radiation dose.
· Another unit, the Gray (Gy) equal 102 rads.
In irradiation processes the dose of radiation that a substrate receives is important. Different materials absorb radiation energy to different degrees.
A rad of radiation dosage represents the same amount of absorbed energy whether it comes from rays, particles, or a mixture of the two. The magnitude of a radioactive isotopes power source is expressed in terms of curies, which is a measure of disintegrations per second. The strength of gamma radiations is expressed in terms of roentgens. The intensity or energy level of beta particles emitted from a linear electron accelerator is defined in terms of joules or electron volts. However, the length of time food is exposed to such sources, and the absorption properties of the food (and its container), will determine the number of rads received by the food, which is the effective dosage that produces changes in the food’s microflora, enzymes, and other constituents.
Radiation Effects
Ionizing radiations penetrate food materials to varying degrees depending on the nature of the food and the characteristics of the radiations. Gamma rays have greater penetrating power than β particles. The efficacy of radiations in producing radiation effects, however, also is dependent upon their abilities to alter molecules and their ionizing potential, that is, their abilities to knock electrons out of atoms of the materials through which they pass. Beta particles generally have greater ability to produce ionizations in matter through which they pass than gamma rays. Electron beams of higher energy levels penetrate more and produce more altered molecules and total ionization along their traveled paths than lower-energy electron beams.
Just as neutrons possessed of extremely high energy can alter atomic nuclei so as to make them radioactive, there are energy levels beyond which gamma rays and electron beams may induce radioactivity in foods. these energy levels are far in excess of what is needed to alter molecules, produce ionization, and inactivate microorganisms in foods. they also are far in excess of the energy levels of such isotope sources as 60 Co and 137 Cs, or 1.6X 10-12 J electron beams, that in the past were considered safe by the FDA for irradiation processing of approved foods.
When ionizing radiations of moderate energy level pass through foods, there are collisions between the ionizing radiations and the food at molecular and atomic levels. Ion pair production results when the energy from these collisions is sufficient to dislodge an electron from an atomic orbit. Molecular changes occur when collisions provide sufficient energy to break chemical bonds between atoms; an important consequence of this is formation of free radicals.
Free radicals are parts of molecules, groups of atoms, or single atoms that possess an unpaired electron, and an unpaired electron configuration is an extremely unstable form. Free radicals, therefore, have a great tendency to react with one another and with other molecules to pair their odd electrons and attain stability.
The formations of ion pairs, free radicals, reaction of free radicals with other molecules, recombination of free radicals, and related physical and chemical phenomena provide the mechanisms by which microorganisms, enzymes, and food constituents are altered during irradiations.
Direct Effects
In the case of living cells and tissues, destructive effective effects and mutations from radiation were originally thought to be due primarily to direct contacts of high-energy rays and particles with vital centers of cells, much as a bullet hits a specific target. The same theory of action was extended to explain changes in nonliving materials and foods. thus, a change in the color or texture of a food would be due to direct collision of a gamma ray or high-energy beta particle with a specific pigment or protein molecule. Such direct hits unquestionably do occur, but their frequency of occurrence at a given radiation dose probably is not sufficient to explain the major of radiation effects in a given substrate.
Indirect Effects
Direct hits need not occur for radiation to affect living or nonliving substrates. Just as radiations colliding with a cell or specific food molecule would produce ion pairs and free radicals, much the same occurs when high-energy radiations pass through water. In this case, water molecules are altered to yield highly reactive hydrogen and hydroxyl radicals. These radicals can react with each other, with dissolved oxygen in the water, and with many other organic and inorganic molecules and ions that may form hydrogen peroxide,
OH + OH → H2O
Two hydrogen radicals produce hydrogen gas,
H + H → H2
A hydrogen radical plus dissolved oxygen yields a peroxide radical,
H + O2 → HO2
Two peroxide radicals produce hydrogen peroxide and oxygen.
HO2 + HO2 → H2O2 + O2
Hydrogen peroxide is a strong oxidizing agent and a biological poison. Hydroxyl and hydrogen radicals are strong oxidizing and reducing agents, respectively. They can enter also into reaction with organic materials and grossly alter molecular structure. Since living cells and food materials are mostly water, the activity imparted to this solvent by radiation constitutes a most important factor contributing to lethal and sub lethal changes in living cells and to alteration of food constituents.
A substrate receiving ionizing radiation probably will experience some direct effects and will certainly be affected by indirect effects.
In food irradiation preservation, the primary goal is to inactivate undesirable microorganisms and enzymes while producing minimum changes in other food constituents. Microorganisms and enzymes can be inactivated by direct hits from radiations as well as by indirect effects. Other food constituents, largely in aqueous solutions, are largely affected by indirect effects from free radicals produced during radiolysis of water. Therefore, attempts to minimize changes in foods during irradiation have been focused on limiting indirect effects.
Limiting indirect Effects. Efforts to limit the indirect effects of radiations have been largely directed at minimizing free radical formation from water and reaction of free radicals with food constituents. Three approaches that have had varying degrees of success, depending on the food material, illustrate this reasoning:
Irradiation in the frozen state. Free radicals are produced even in frozen water, though possibly to a lesser extent. The frozen state also hinders free radical diffusion and migration to food constituents beyond the site of free radical production. Thus, freezing can limit undesirable reactions.
Irradiation in a vacuum or under inert atmosphere. As indicated earlier, a hydrogen radical reaction with oxygen will produce a highly oxidative peroxide radical. Peroxide radicals produce hydrogen peroxide. By removing oxygen from the system, such reactions are minimized and food constituents are more protected. However, removal of oxygen and minimization of these reactions also has a protective effect on food microorganisms, limiting the benefits that can be obtained. There is also the problem of getting oxygen out of food systems.
Addition of free radical scavengers. Ascorbic acid is an example of a compound that has a great affinity for free radicals. Addition of ascorbic acid and certain other materials to food systems results in consumption of free radicals through reaction with these and a sparing of other sensitive pigments, flavor compounds, and food constituents. But a problem exists in incorporating such scavengers throughout non liquid foods.
Sensitivity to Ionizing Radiations
It is beyond the scope of this chapter to consider the many changes that occur within living systems and biological materials exposed to radiation. Ionizing radiations can alter the structures of organic and biochemical compounds essential to normal life when these radiations are received in high dosage. In foods, just as excessive amounts of heat deteriorate foods, excessive dosages of radiation adversely affect proteins, carbohydrates, fats, vitamins, pigments, flavors, enzymes, and so on. Excessive dosages also can change the protective properties of certain packaging materials such as plastic films and plastic or enamel interior can coatings. In this latter case, however, such dosages generally are in excess of sterilizing or pasteurizing requirements compatible with acceptable food quality.
Excessive dosage has meaning only in terms of specific substrates. As in the case of heat, different food materials vary greatly in sensitivity to ionizing radiations. Bacon, for example, can withstand a radiation dosage of 5.6 Mrad (5.6 Gy) and retain highly satisfactory organoleptic qualities. Such bacon is microbiologically sterile. Certain proteins, on the other hand, become highly disorganized at far lower doses, showing varying degrees of molecular uncoiling, unfolding, coagulation, molecular cleavage, and splitting out of amino acids, odorous compounds, and ammonia. Egg white is a particularly sensitive mixture of proteins and becomes thin and watery with a moderate radiation dose of 0.6 Mrad (6 k Gy). This dosage is insufficient to produce sterility if the egg contains spores of certain bacterial. Therefore, a higher dosage to ensure sterility would be impractical since it would make fresh egg quite unacceptable for most food uses. Many foods cannot be irradiation-sterilized for similar reasons. Some of these, however, are given improved keeping quality by irradiation pasteurization at lower doses.
Some of the more important overall effects and uses of different irradiation doses are indicated in Fig. 11.1. About 10,000 rads will inhibit sprouting of potatoes and slightly more will destroy insects. Several hundred thousand rads will kill yeasts and molds, and this level will pasteurize many foods. gross destruction of bacterial spores, producing food sterility, requires several million rads.
Dose-Determining Factors
When the purpose of irradiation is food preservation, the choice of dosage must take into account several factors. The more important of these include safety and wholesomeness of the treated food, resistance of the food to organoleptic quality damage, resistance of microorganisms, resistance of food enzymes, and cost. Safety and wholesomeness involve considerations beyond the absence of dangerous radioactivity and pathogens, which will be discussed later.
Figure 11.1
Resistance of Food
The extent to which irradiation affects the organoleptic quality of foods varies widely and depends, in complex ways, on the chemical composition and physical structure of foods. these differences in foods set the upper limits of radiation dose that produce foods consumers will accept. Much of the acceptability data on specific foods has been obtained in studies conducted with volunteer troops of the U.S. Army. Pork loin, chicken, bacon, and shrimp have withstood sterilizing does of 4.8 Mrad well. In some instances, off-flavors detected in newly irradiated items largely disappear on storage. Some vegetables also have tolerated 4.8 Mrad. Various fruits have withstood sterilizing does of 2.4 Mrad. More sensitive meats, fish, and fruits have been found quite acceptable at pasteurizing doses in the range of about 105-106 rads. These tolerances are reflected in the irradiation product and process specifications suggested in Table 11.2.
Resistance of Microorganisms
The most radiation-resistance microorganisms of consequence in foods is Clostridium botulinum. Some viruses and microorganisms are yet more radiation resistant but are easily controlled by mild heating prior to irradiation. Many conditions in foods can prevent growth and toxin formation by C. botulinum. Among these are pH 4.6 and below, aerobic conditions, extreme dryness of certain foods, refrigeration temperatures below 3C, and certain preservative chemicals. In foods where these conditions will not exist, C, botulinum must be assumed to be present and radiation dosage sufficient for its destruction employed.
As in the case for heat preservation, and based on similar logic, radiation dosages required to destroy spores of C. botulinum in various foods have been established. In the irradiation destruction curve shown in Fig. 11.2, DM is the radiation dose giving a 90%, reduction in population.
In beef substrate (above Ph. 4.6) the value of DM 0.4 is Mrad (4 kGy). It can be calculated that if 1 kg of beef contained a million botulinum spores and the food received a radiation dose of 12DM then there would be only one chance in a billion that a 1-kg can of such food would contain live spores (in calculating, one should not overlook the fact that rad is an amount of energy per gram of materials). A 12 DM dosage (12 X 0.4 Mrad) is 4.8 Mrad. Such a dosage provides a wide margin of safety.
For foods with Ph 4.6 and below, C. botulinum is not a problem, but other spoilage organisms must be inactivated. The most resistant of these has been found to have a DM of about 0.2 Mrad. For sterilization with a substantial margin of safety, a 12 DM dosage (equivalent to 2.4 Mrad) also may be employed.
Resistance of Enzymes
DMost food enzymes are more resistant to ionizing radiations than even spores of C, botulinum. Enzymes destruction curves comparable to bacterial destruction curves have been established. It has been found that DM values (radiation doses producing 90% reduction in enzyme activity) are of the order of 5 MraD
d
Four DM values would produce nearly total enzyme destruction, but such a dosage of 20 Mrad would be highly destructive to food constituents. For these reasons, irradiation alone is not suitable where substantial enzyme destruction is required for storage stability.
This problem has been resolved by the use of various combination processes. Enzymes are readily inactivated by blanching and by certain chemicals. Temperatures of about 70C for a few minutes are fairly effective. The combination of microorganism-destructive radiation doses plus such heat treatments are extremely effective.
Cost
Cost is another dose-determining factor. Higher doses are obtained by using stronger radiation sources or by exposing foods to less intense radiations for longer periods of time. Either practice increases processing costs. In the case of some foods, irradiation pasteurization may economically feasible, whereas irradiation sterilization would not be. More broadly speaking, for some foods, preservation by irradiation can be costlier than preservation by heat, refrigeration, or freezing. Where these methods are applicable, there would be little incentive to use irradiation.
On the other hand, irradiation is uniquely suited to certain applications. Low-dose-irradiation pasteurization has extended the normal storage life of refrigerated marine products, meats, fruits, and vegetables from a few days up to several weeks. This can influence marketing practices and safety. The potential of irradiation stabilization is indicated by the irradiated moist shrimp shown in Fig. 11.3, which were stored for a year at room temperature before being prepared as illustrated. Irradiation is being considered as a method to reduce the risk of pathogenic microorganisms in foods that are commonly contaminated. Poultry products and Salmonellas, or ground beef and certain types of Escherichia coli are examples. Low-dose irradiation reduces the numbers of these and other pathogens reducing the risk of food-borne disease. These processes are a form of pasteurization and not intended to make the products commercially sterile.
Figure 11.3
Controlling and Measuring Radiation
There are many similarities between radiation preservation and the principles discussed in Chapter 8. Like heat, radiations capable of cold sterilization can destroy microorganisms and inactivate many food enzymes, but they also can damage food constituents, and so radiation dose must be carefully controlled. As with heat, it is not just the intensity of the radiation source that is important but the amount of radiation that the food absorbs; thus, processing time is important. Radiation energy must be provided in such a manner that it reaches every particle of food within the mass or container. In the case of heat preservation, conduction and-natural convection help distribute heat throughout the container; in the case of cold sterilization by irradiation, with the exception of limited diffusion of free radicals (indirect effect), these processes do not occur. An adequate killing dose must be obtained by uniformly irradiating throughout the entire food mass.
Safety and Wholesomeness of Irradiated Foods
The complex question of safety and wholesomeness of irradiated foods has been investigated in the United States by the Office of the Surgeon General of the Department of Defense, the Nuclear Regulatory Commission, the U.S. Department of Agriculture, the Food and Drug Administration, and others. Several international groups have likewise studied the safety of food irradiation. In addition to safety from a microbiological standpoint, these studies have been concerned with (a) effects of irradiation treatments on the nutrient value of foods, (b) possible production of toxic substances from irradiation, (c) possible production of carcinogenic substances in irradiated foods, and (d) possible production of harmful radioactivity in irradiation treated foods. these particularly at the lower doses now being considered for pasteurization, insect control, and sprout inhibition.
Future for Food Irradiation
The use of irradiation for food in the United States must be specifically approved by the Food and Drug Administration on a food-by-food basis. No current approvals are in effect for food sterilization, but the FDA has approved several low-dose to kill insects, slow ripening, or inhibit sprouting. Irradiation of potatoes would be a specific example. Irradiation of poultry, pork, spices, herbs, and other seasonings to reduce microorganisms is also permitted. In approving these applications, the FDA not only indicates which foods can be irradiated, but it also sets the amount of radiation that can be applied. Current rules also require that all foods be labeled in a specific way so that consumers know that the food has been irradiated.
Despite these controls, irradiation of food is controversial in the United States and other parts of the world. In some places there is much less controversy. How important food irradiation will eventually become is difficult to assess. Much will depend on future policies of the FDA and similar agencies abroad, relative to safety and approval of specific foods so processes. The improved keeping qualities and microbiological safety of specific foods so processed. The improved keeping qualities and microbiological safety of irradiated foods could play a substantial role in international food exports and imports. To this end, international meetings have been held to consider the problems of drafting uniform guidelines and legislation pertaining to traffic in irradiated foods. These meetings have concluded that there is no toxicological hazard resulting from irradiation will become commonly used for producing shelf-stable commercially sterile foods in the near future. There was one commercial irradiation facility in operation in the United States in 1993.
MICROWAVE HEATING
Unlike ionizing radiations, microwave energy in food applications is used for its heating properties. Microwave energy is similar to the energy that carries radio and television programs and to the energy involved with radar.
Properties of Microwaves
Microwaves are electromagnetic waves of radiant energy, differing from such other electromagnetic radiations as light waves and radio waves only in wavelength and frequency. Microwaves fall between radio waves and infrared radiations, with wave-lengths in the range of about 25 million to 0.75 billion nanometers, which is equivalent to about 0.025-0.075 m. the wavelengths of radio waves and infrared radiations, in comparison, are measured in kilometers and micrometers, respectively (Table 11.1). wavelengths of electromagnetic radiations are inversely related to frequency, which is the rapidity with which the waveform occurs. Microwave wavelengths of about 0.025-0.75 m correspond to frequencies of about 20,000-400 MHz (1Hz = 1 cycle/sec). Because microwave frequencies are close to the frequencies of radio waves and overlap the radar range, they can interfere with communication processes, and so the use of specific microwave frequencies comes under the regulations of the Federal Communications Commission. For food applications the approved and most commonly used microwave frequencies are 2450 MHz and 915 MHz
Microwaves, like light, travel in straight lines. They are reflected by metals, pass through air and many, but not all, types of glass, paper, and plastic materials, and are absorbed by several food constituents including water. When they are reflected or pass through a material without absorption, they do not impart heat to the object. To the extent that they are absorbed, they heat the absorbing material. In heating the material, they lose electromagnetic energy. The terms loss factor and loss tangent are used to indicate the microwave energy “lost” in passing through, or being entirely absorbed by, various materials under defined conditions. Materials that are highly rapidly heated by microwaves. Loss factors for various substances are given in Table 11.3. Since foods differ in composition and in the physical distribution of components, foods vary in their heating patterns from microwave radiation.
The loss factor is also a measure of the degree of penetration of microwaves into materials. Since microwaves lose energy in the form of heat as they penetrate, the greater the loss factor; and the more heat that is produced, the shorter is the distance they can penetrate before all of their energy than 2450-MHz microwaves in certain materials, whereas the reverse is true in other materials; in some materials, loss is the same at both frequencies. Where depth of penetration is desired in a given material, one can choose the microwave frequency with the lower loss factor.
It can be shown that under similar conditions, by the time half of their incident energy is lost, 900 MHz microwaves will penetrate water to a depth of 76 mm, whereas 2450-MHz microwaves will penetrate to a depth of only about 10 mm.
Mechanism of Microwave Heating
Common alternating electric current reverses its direction 60 times a second. Microwaves do the same, but at frequencies corresponding to 915 or 2450 MHz Food and certain other materials contain molecules that act as dipoles, that is, they exhibit positive and negative charges at opposite ends of the molecule. Such molecules also are said to be polar. Water molecules are polar with the negative charge centered near the oxygen atom and the positive charge nearer the hydrogen atoms.
When microwaves pass into foods, water molecule and other polar molecules tend to align themselves with the electric field. But the electric field reverses 915 or 2450 million times per second. The molecules attempting to oscillate at such frequencies generate intermolecular friction, which quickly causes the food to heat. Quite the same phenomenon occurs in dielectric heating, which is like microwave heating but employs radiations in the frequency range of about 1-150 MHz Although microwaves generate heat within the food, components with different loss factors do not immediately heat up equally. However, as heat is generated, it also is conducted between food components, tending to equalize temperature. In liquid foods, the heat also is moved by convection. However, these secondary effects must not be confused with the prime mechanism of intermolecular friction, which occurs within the food at the sites of billions of molecules simultaneously.
Differences from Conventional Heating
In conventional heating, employing a direct flame heated air, infrared elements, direct contact with a hot plate, and so on, the heat source cause food molecules to react largely from the surface inward, so that successive layer’s heat in turn. This produces a temperature gradient which can burn the outside of a piece of food long before the temperature within has risen appreciably. This is why steak can be crusted on the outside but still be rare on the inside.
In contrast, microwaves penetrate food pieces up to several centimeters of thickness uniformly, setting all water molecules and other polar molecules in motion at the same time. Heat is not passed by conduction from the surface inward, but instead it is generated quickly and quite uniformly throughout the mass. The result is an internal boiling away of moisture, the steam also heats adjacent food solids by conduction. Incidentally, as long as there is free water being converted to steam, the temperature of the food piece does not rise much above the boiling point of water, except as the steam within the food may be under some pressure as it attempts to escape. As a result, there is virtually no surface browning or crusting from excessive surface heat. This is a limitation of microwaves in such operations as bread baking, meat cooking, and the like, where crusting or browned surfaces are desired. In such cases, if microwave heating is employed, it must be preceded, accompanied, or followed by a conventional kind of heating to produce such surfaces. On the other hand, low thermal gradient microwave heating lends itself to numerous special applications, as indicated in the list of food applications at the end of this section.
Microwave Generators and Equipment
The most commonly used type of microwave generator is an electronic device called a magnetron. The components of a magnetron are shown in Fig. 11.4. A magnetron is a kind of electron tube within a magnetic field which propagates high-frequency radiant energy. The power output of different size magnetrons is rated in kilowatts. A larger magnetron, or several smaller ones working together, will heat a given quantity of food to a given temperature in a shorter time than a smaller one. Also, it is well to recognize that since microwave energy heats only objects into which it is an absorbed, there is a relationship between food load and heating time to come to a boil as will 1 kg of water.
A simple microwave oven consists of a metal cabinet into which is inserted a magnetron (Fig. 11.5). The cabinet frequency is equipped with a metal “fan” to distribute the microwaves throughout the cabinet as they are reflected and bounce off the metal fan blades.
Figure 11.4 and Figure 11.5
The microwaves also are reflected and bounce between the metal cabinet walls. Food placed in such an oven (generally raised above the oven floor by a screen or rack through which microwaves can pass) is thus contacted by microwaves from all directions. This speeds heating time and facilitates steam escape. If the food is wrapped, the wrapper should be perforated, or otherwise allow for steam escape to prevent it from bursting. Microwave radiations are not dangerous when confined within properly designed equipment. Since microwaves can cause damage to the eyes and other tissues that may absorb them, safety engineering of microwave ovens and related equipment has evolved to a high degree. All microwave ovens have interlocks or equivalent devices that cut off the power supply when the oven door is opened.
More complex microwave tunnel ovens are equipped with an endless moving belt of low-loss material on which food is conveyed past magnetrons in continuous fashion. Such ovens generally are open at the inlet and outlet ends to receive and discharge the product. In such a case the microwaves are prevented from escaping through the open ends by providing trapping materials which absorb stray microwaves, by providing metal reflectors to turn would-be stray microwaves back into the oven chamber, and by other means.
It is also possible to heat liquid materials continuously with microwaves. In this case the liquid may be pumped through a coil of low-loss glass or like material placed within the microwave heating zone; or the magnetron(s) may be positioned surrounding a low-loss tube through which the liquid is pumped.
Microwave Food Applications
The current and potential uses of microwave heating in the food industry are many and are of growing importance. The following industrial applications listing by the Cryodry Corporation, a manufacturer of microwave heating systems, is highly illustrative.
1. Baking. Internal heating quickly achieves desired final temperature throughout the product. Microwaves can be combined with external heating by air or infrared to obtain crust.
2. Concentrating. Permits concentration of heat-sensitive solutions and slurries at relatively low temperatures in relatively short times.
3. Cooking. Microwaves cook relatively large piece without high-temperature gradients between surface and interior. Well suited for continuous cooking of meals for large-volume institutional feeding.
4. Curing. Effective for glue-line curing of laminates (as in packaging) without direct heating of the laminates themselves.
5. Drying. Microwaves selectively heat water with little direct heating of most solids. Drying is uniform throughout the product; preexisting moisture gradients are evened out. Drying is at relatively low temperatures; no part of the product need be hotter than the vaporizing temperature.
6. Enzyme Inactivation (Blanching). Rapid uniform heating to the inactivating temperature can control and terminate enzymatic reactions. Microwaves are especially adaptable to blanching of fruits and vegetables without leaching losses associated with hot water or steam. Also, does not overcook the outside before core enzymes are inactivated.
7. Finish-drying. When most of the water has been removed by conventional heating methods, microwaves remove the last traces of moisture from the interior of the product quickly, and without overheating the already dried material.
8. Freeze-drying. The ability of microwave energy to selectively heat ice crystals in matter makes it attractive for accelerating the final stages of freeze-drying.
9. Heating. Almost any heat transfer problem can benefit from the use of microwaves because of their ability to heat in depth without high-temperature gradients.
10. Pasteurizing. Microwaves heat a product rapidly and uniformly without the over-heating associated with external, high-temperature heating methods.
11. Precooking. Microwaves are well suited for precooking “heat and serve” items because there is no overcooking of the surface and cooking losses can be negligible. When the consumer reheats the food by conventional methods, the desired methods, the desired texture and appearance of conventionally cooked items can be imparted.
12. Puffing and foaming. Rapid internal heating by microwaves causes puffing or foaming when the rate of heat transfer is made greater than the rate of vapor transfers out the product interior. May be applied to the puffing of snack foods and other materials.
13. Solvent Removal. Many solvents other than water are efficiently vaporized by microwaves, permitting solvent removal at relatively low temperatures.
14. Sterilizing. Where adequate temperature may be reached (acid foods), quick, uniform come up time may permit high-temperature short time sterilization. Selective heating of moisture-containing microorganisms makes possible the sterilization of such materials as glass and plastic films, which are not themselves heated appreciably by microwaves. This application must be considered cautiously, since escaping steam temperatures generally are not sufficient to kill bacterial spores.
15. Tempering. Because the microwave heating effect is roughly proportional to moisture content, microwaves can equalize the moisture in a product that came from a process in a non-uniform condition.
16. Thawing. Controlled, rapid thawing of bulk items is possible due to substantial penetration of microwaves into frozen materials.
It must be recognized that several of the above applications may be achieved by other heating methods or combination processes. The choice of methods must then depend on relative product quality and cost.
OHMIC HEATING
Ohmic heating was briefly mentioned in Chapter 5. Ohmic heating is one of the newest methods of heating foods. it is often desirable to heat foods in a continuous system such as a heat exchanger rather than in batches as in a kettle or after sealing in a can. Continuous systems have the advantages that they produce less heat damage in the product, are more efficient, and they can be coupled to aseptic packaging systems. Continuous heating systems for fluid foods that contain small particles have been available for many years. However, it is much more difficult to safely heat liquids containing larger particles of food. Beef stew would be one example. This is because it is very difficult to determine if a given particle of food has received sufficient heat to be commercially sterile. This is especially critical for low-acid foods such as beef stew which might cause fatal food poisoning if under heated. Products tend to become over processed if conventional heat exchangers are used to add sufficient heat to particulate foods. this concern has hindered the development of aseptic packaging for foods containing particulates. Ohmic heating may overcome some of these difficulties and limitations.
Considerable heat is generated when an alternating electric current is passed through a conducting solution such as a salt brine. In Ohmic heating a low-frequency alternating current of 50 or 60 Hz is combined with special electrodes. Products in a conducting solution (nearly all polar food liquids are good conductors) are continuously passed between these electrodes, each of which raises the temperature. Figure 11.6 shows a diagram of such a system.
The major advantage of Ohmic heating is that the food particles do not experience a significant temperature gradient from their outside to inside. This means that they can be heated without the usual heat damage associated with excessive surface heating. This is because there are no direct heat transfer surfaces to degrade product. The solid pieces and the liquid are heated nearly simultaneously.
After heating, products can be cooled in continuous heat exchangers and then aseptically filled into pre-sterilized containers in a manner similar to conventional aseptic packaging. Both high-and low-acid products can be processed by this method.
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