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COST REDUCTION APPROACHES A viable dehydration and compression capability in the U.S. food industry would be of unquestioned value to the armed forces. The advantages offered by reductions in the weight and volume of food items, long-term shelf life without refrigeration, and ease of preparation, combined with high quality end products when rehydrated, justify the use of dehydrated and compressed foods for a variety of military missions. Currently, detailed systems cost analyses are being carried out on the dehydration and compression technology as a whole to provide accurate, timely information on the costs of each operation from raw material preparation to compression and packaging. While detailed figures are not available at this time for many missions, the ad- vantages of compressed-dried foods are so great that cost is only a minor consideration. However, reductions in the present costs would make it possible for the armed services to benefit from the advantages offered by dehydration and compression in a number of other situations. Lower costs would make it advantageous to use dehydrated and compressed foods for garrison feeding abroad and within the United States, par- ticularly when such factors as shipping costs, storage costs, and preparation costs (manpower, equipment, and associated costs) are introduced into the economic equation. An expansion in the military market for dehydrated and compressed foods would stimulate research, driving down the costs still further. If the costs were reduced sufficiently to make dehydrated and compressed foods more competitive in the civilian sector, the size of the market would expand enormously and the technology and the development of a sizable dehydrated and compressed food industryâof great benefit to both the civilian and military sectorsâwould very likely advance significantly. A large civilian market would guarantee a reliable, varied, competitively priced source of dehydrated and compressed foods for the armed forces. In the usual competition for budget dollars in the military sector, the funds available for research and development in the area of de- hydration and compression have been very limited. The civilian sector has not seen sufficient promise in the development of a commercial market, except in very specialized applications, to justify the diversion of research and development funds from areas with greater
8 and more immediate potential. Research and development efforts to produce higher quality compressed-dried foods probably will need continuing military support in the near term. DRYING Military and civilian effort over the past decade has focused on all steps of the freeze-dehydration and compression process. However, the freeze-drying step is by far the costliest part of the process since water must be sublimed at low temperatures and high vacuums. Opportunities to identify cost-reducing techniques can be found during raw material selection, preparation, and pretreatment. However, the most significant opportunities for cost reduction are: â¢ Partial drying prior to freeze-drying using more economical techniques for water removal while retaining equivalent product quality. â¢ Developing methods yielding controllable and uniformly distributed levels of moisture prior to compression. â¢ Removing moisture prior to freezing or freeze-drying by minimum cost methods such as convection or conduction drying at atmospheric pressure. â¢ Modifying the freeze-drying method so that the initial end point will leave enough water in the product to allow successful compression. â¢ Ensuring that water is evenly distributed throughout the pieces prior to compression. Research is needed on these approaches as well as on the transfer of laboratory findings to commercial practice for specific products. Of all the dehydration methods that have been found practical on a commercial scale to date (air-drying, vacuum-drying, freeze-drying, puff-drying), the one that has the least detrimental effect on quality, in most instances, is freeze-drying. Freeze-drying has proven to be a superior method of dehydration for most of the commonly used food items including green beans and other beans, peas, carrots, spinach, potatoes, beets, turnips, asparagus, parsnips, green and red peppers, okra, chives, corn, mushrooms, cottage cheese, coffee, cherries, blueberries, strawberries, and a variety of meat, poultry, and fish products. Some notable exceptions are milk, some fruits, and salad vegetables such as cucumbers, tomatoes, lettuce, cabbage, and celery. The main disadvantage of freeze-drying has been its cost. With the cost of dehydrated and compressed foods now at the point where a small drop in price can produce a sizable increase in the market potential, considerable attention is being given to hybrid schemes that take advantage of the positive effect of freeze-drying on the cellular structure of food but reduce the cost of the process by removing some of the moisture before freezing or freeze-drying. Each of the variety of such schemes raises questions and problems that must be resolved by further research and development on a product-by-product
basis. Especially important is the effect a particular scheme has on flavor, texture, color, and storage life. All of these factors must be balanced against the saving in cost. Until detailed systems cost analyses are available only broad areas of research for cost reduction can be identified. PARTIAL DRYING PRIOR TO FREEZE-DRYING A food can be partially dried before freezing in order to remove all or part of the unbound and loosely bound water (up to 50 percent of the total moisture content). This can be accomplished by the use of (a) air at temperatures under 93.3Â°C (200Â°F) to prevent the item from cooking, (b) liquids at temperatures that are high enough to boil out the water and usually result in some cooking of the item (as is done at the start of the sautg freeze-drying process), (c) radiant heating, (d) conduction heating (using heated trays or tumblers), or (e) a combination of these methods. When considering the economics of predrying, attention should be given to cheap sources of heat (areas of the country in which the climate is hot and dry, or areas in which there is geothermal power or cheap supplies of natural gas) and high altitude. It is assumed that civilian markets would be served first with these production capabilities. Raw material costs would be a prime consideration. A number of techniques used for the complete dehydration of vegetables and fruits for some purposes (for example, dry soup and gravy mixes and dry snack mixes) can also be used for the partial drying of vegetables, fruits, meats, and seafood that are to be freeze- dried and compressed. Such techniques include the following, either singly or in combination: (a) convection drying, (b) conduction heat drying, and (c) radiant heat drying. If any of the food items to be processed are wet as a result of washing, precooking, or any other pretreatment, a significant amount of energy can be saved by first centrifuging, shaking, blowing air through, or pressing the items to remove the excess liquid. Pressing can also be used to remove some of the unbound moisture in items such as bean curd and cottage cheese, but the use of this technique is limited since it will turn most foods into mush. Hot air can be used to partially predry most dehydrated and compressed food items, the degree of prehydration that can be done without degrading the final product varies with each item. Frozen starting materials are less suitable for hot air predrying. When the amount of predrying would result in a considerable saving in total energy use, it might be economically advantageous to locate processing plants in the arid Southwest where the air is warm and dry enough to use in food-dehydrating equipment without any further heating (Flink, 1977). Where the climate is right, the air for predrying might be heated with solar collectors (Food Engineering, 1978b, 1979a, 1979b). Processing plants might also be located where cheap supplies of natural gas are available. Using natural gas as a source of heat for
10 convection predrying is particularly advantageous. The products of combustion are clean enough to be used directly (mixed with whatever ambient air is required to produce the desired temperature) and, if the plant is large enough, costs can be cut by first putting the combustion products through a gas turbine to cogenerate electricity or produce process steam. Coal and oil can also be used as sources of energy for predrying but the products of combustion of these fuels are often too dirty to allow them to come in direct contact with foods. Heat exchangers must be used to transfer the heat from their combustion products to process air. Some of this energy could first be used to take care of process steam requirements or to cogenerate electricity. Any other source of energy could be analyzed, of course, including geothermal energy, wastes, and biomass and other forms of solar energy (Food Engineering, 1978a, 1980; Food Processing. 1980; Robe, 1980; Stinson, 1980; Webster and Robe, 1977). When conduction or radiation (tumblers, spikes, trays, or any of a variety of conveyor systems) (Levine, 1977) is the preferred source of heat for predrying, the heat can be provided by any of the fuels already mentioned, with or without the associated production of steam and electricity. Further economies might be realized by incorporating any of the many other heat recovery and conservation methods now in use in industry (Griffith et al., 1977; Levine, 1977). COMPRESSION The tendency of "fully" freeze-dried food items to fall apart on compression necessitates giving them some plasticity. Moisture appears to be the best plasticizing agent; however, to achieve the long-term storage stability required for many military applications, it is necessary to remove moisture used for plasticizing. Fully air-dried foods have not been found suitable for compression after drying. If water is added to the dry product, the additional energy and time required for the rehydration and subsequent dehydration increase the cost of the dehydrated and compressed food item, limiting the military market and making dehydrated and compressed foods less attractive to the civilian market. A lesser degree of dehydration can be used before compression. At the present time, vegetables, fruits, and other food items are being dehydrated to an average moisture content of 4 to 6 percent in order to attain a reasonably low moisture content at the center of each piece and a reasonably uniform moisture content from piece to piece. For optimal compression, however, the average moisture content must be increased to 15 to 20 percent. Considerable energy, time, and cost could be saved if a more controlled and uniform drying technique were developed that would allow the moisture content of a food item to be set at 15 to 20 percent before compression. Microwave energy can be used to equilibrate food pieces that have a low moisture content at the surface and a high moisture content at the
11 center. The use of microwave energy would make it unnecessary to draw down the average moisture content as much as is now being done prior to compression. Methods are needed to ensure an average moisture content of 15 to 20 percent. Commercially, the product can be rehydrated with mist, spray, or steam. Mist treatments can result in large variations in moisture content from piece to piece (sometimes more than 3:1) and sizable variations within each piece (sometimes as much as 3:1 unless an inordinate amount of time is allowed for equilibration) and can lead to uneven and sometimes unsatisfactory results (King et al., 1976). Overly wet surfaces can result in discoloration and other forms of degradation. Attempts have been made to avoid the disadvantages of spraying the food to plasticize it prior to compression by rewetting the food with humid air and steam under atmospheric conditions or in a vacuum chamber. Early results were not encouraging (Rahman et al., 1970; MacKenzie and Luyet, 1969; Pilsworth and Hoge, 1973). The rehydration rate was low, and attempts to increase it by increasing the partial pressure of the water vapor led to the same uneven and unsatisfactory results obtained by spraying. It was recognized early that a fundamentally better alternative to full freeze-drying and spray rewetting would be a partial or limited freeze-drying that would reduce the moisture content of the food item only to the level desired for compression. If this could be accom- plished satisfactorily, it would lower costs and produce more even results, two very important advantages. There are complications, however. Freeze-drying is not an equilibrium process in which the moisture in a food item remains uniformly distributed as the overall moisture content of the item is lowered. The moisture is locked in a core of ice that gradually shrinks in size, leaving behind a region that is relatively dry (King, 1970). When, in a conventional freeze-drying process, the overall moisture content of a food item drops to the level that is considered optimal for compression, the item usually ends up with a wet unstable core and a dry outer region and will not compress satisfactorily. Several methods for achieving limited freeze-drying have been explored to date, and some of them show promise. King et al. (1976) set up a freeze-drying apparatus in which they could carefully control the platen temperature and the partial pressure of the water vapor in the chamber during the entire dehydration process, and were able to freeze-dry 1cm cubes of beef so that they had a uniform moisture content of about 10 percent in 12 to 13 hours (as compared with a dehydration time of four hours by conventional freeze-drying and a moisture content of 2.5 percent). Subsequent work has resulted in a reduction of the dehydration time by one third (Zakarian and King, 1978). This approach to limited freeze-drying could be commercially useful, particularly if it were developed further and improved. It has the added advantage of allowing the use of available freeze-drying equipment for both limited and full freeze-drying after relatively minor modifications, making major new capital expenditures unnecessary for units already engaged in freeze-drying.
12 King et al. (1976) also investigated the use of dessicants to control the water vapor in a freeze-drier, since this might make it possible to eliminate most of the refrigeration and heating required. Calcium chloride offers an advantage of being inexpensive and readily available. Tests showed that it took approximately 8.5 hours for reliable limited freeze-drying and adequate equilibration, beef ended up with an average moisture content of 9.5 percent and turkey with an average moisture content of 11.0 percent. The ability of microwave energy to generate heat inside a food item uniquely suits it to limited freeze-drying. An economic analysis performed by Hammond (1967) and subsequently revised by Peltre et al. (1977) clearly demonstrates the economic advantages of microwave- assisted freeze-drying over conventional freeze-drying with radiant heat. A more recent economic analysis by Arsem (1980) shows that costs can be cut further by freeze-drying with a combination of microwave and radiant heating. Arsem has also demonstrated the incorporation of a microwave unit into a conventional freeze-dryer, circumventing the need for large capital expenditures when converting to a hybrid sys- tem. The resultant doubling of the throughput makes a hybrid system attractive. Most of the effort in this area supported by the Natick Laboratories, both in-house and at Worcester Polytechnic Institute, centers on such problems as corona discharge, nonuniform heating, impedance mismatch, and applicator efficiency (Peltre et al., 1977). Since solid entrainment may play an important role in microwave freeze-drying, it is important that this phenomenon be thoroughly investigated. Also essential are more basic data of the type de- veloped in university laboratories on dielectric, sorptive, and diffusive properties of frozen and dried foodstuffs. The attractive cost savings possible with microwaves have been an incentive to apply microwave heating commercially as a method for rapidly equilibrating partially freeze-dried foods prior to compression. A new technique in dehydration and compression processing has been the use of microwave heating in combination with vacuum-drying prior to compression. Rehydration of the final product requires about 20 minutes instead of 10. However, for most shipboard applications, this would be of little consequence. Results from a laboratory simulation in a large microwave oven containing the product in a stoppered flask connected to a vacuum source demonstrated that the moisture content of green beans could be reduced to 14 percent in 45 minutes. The product was directly compressable since it was warm and plasticized. Com- pression was followed by further vacuum-drying to the desired final moisture. The total dehydration cycle using the vacuum and microwave combination was about one hour. A commercial system has been produced by Gigovac of France. (Ref. A. Rahman, Personal Communication.) This continuous vacuum, microwave-heated dryer is 0.6m (2 ft) in diameter and is 2.7m (9 ft) long. Much more work is needed to evaluate drying time against rehydrated product quality of particular foods to make such rapid drying methods practical. Detailed systems cost analyses are needed to justify large capital outlays. Compression of partially dried foods by centrifugation could elimi- nate multiple handling steps currently employed in dehydration. In a
13 prototype study, a standard centrifuge tube was adapted by drilling a hole in its base and adding a false bottom. A stainless steel weight could be added to increase pressure during centrifugation. The final product was found to equal products compressed under 100 psi, which is sufficient for most products. This technique might be applied commercially by centrifically compressing foods in the can used for shipping. Finish-drying could be completed with the product already in the can. Container filling could be increased as much as 25 percent due to space saved between discs and between the product and the walls of the container. However, flexible packages are more desirable and represent a research need. Sulfite has long been a challenge to the food dehydration industry. It is not allowed in some European countries and Japan. U.S. industries would like to buy sulfite-free, preblanched products for freeze-drying, but current specifications may require the addition of sulfite. Frozen, blanched, sulfited and nonsulfited green beans were freeze-dried by the limited method, then microwave heated for approximately one minute prior to compression. The flavor was found to be quite similar for both treatments even after storing the products at 37.7Â°C (100Â°F) for six months. This demonstrates that foods to be used for freeze-drying and compression may not need prior sulfite treatment; however, addi- tional research is needed. COMMODITIES Carrots Objective measurements of the color of raw carrots are the best indicators of their subsequent quality after dehydration, compression, and rehydration (Hruzek, 1973). Rehydration characteristics are directly related to Gardner Color a and the a/b ratio. Rehydration rates of freeze-dried and compressed carrots are affected by the size of the pieces; the larger the size, the shorter the shed or block disintegration time and, hence, the rehydration time (Macpherson, 1973). Rehydration times increase with the degree of maturity and the sugar content (Bennet, 1976). Rushing (1975) found that the optimum conditions for compressing freeze-dried carrots without fragmentation were the following: 5 per- cent sugar, 7 percent moisture, and a temperature of 32.2Â°C (90Â°F). Curry (1974) found that the rehydration rate for dehydrated carrots increased when the carrots were cooked in salted water prior to de- hydration. Taking photomicrographs of the dehydrated carrot tissue with a scanning electron microscope, he noted the presence of sodium chloride crystals and concluded they were responsible for the increased rehydration rate because water has an affinity for crystalline salt. When the salt is not in crystalline form because of the presence of impurities, it has the opposite effect, decreasing the rehydration rate. The rehydration rate was highest when the salt in the cooking water was 1.2 percent by weight (0.2 molar salt solution); this salinity also resulted in a rehydrated product of exceptionally good quality (Burns, 1980).
14 Wisakowsky (1977), in his work on the optimum moisture (plasti- cization) level in carrots prior to compression, found that the use of microwave heating produced a more uniform moisture distribution, minimizing the cellular compaction noted by Curry. The quality of the final product was found to be best when the moisture level was 10 to 15 percent after dehydration. Microwave heating for 40 seconds prior to compression, at a compression pressure of 500 psi for 20 seconds, proved best. The compression and rehydration were not affected by the relative sizes of the core and outer wall of the carrots used. In tests on a number of carrot varieties at Texas A&M University, the Danvers 126 variety was found to be the best for the freeze-dried, compressed food market (Burns, 1980). This variety holds up well in the field before harvest, has a harvest period of almost six weeks, and the final product has a superior quality. Green Beans The color and fiber content of raw green beans are the best indicators of their potential quality after dehydration, compression and sub- sequent rehydration (Burns, 1980). A very green green bean with a lower fiber content gives the best results. Although the fiber around the bean facilitates rehydration, a small amount is sufficient for satisfactory results; if the bean is allowed to overmature even slightly before harvesting, it will develop too much fiber and become unacceptable to the consumer. Spinach Tests on various types of spinach (smooth, savoy, and semi-savoy) have indicated no discernible change in quality after freeze-drying, compression, and subsequent rehydration (Burns, 1980). A trained taste panel was unable to detect any differences in flavor between the final product and samples of the original that had been cooked without being processed. Cabbage Cabbage is one of the few vegetables that does not freeze-dry well. Freeze-drying increases the intercellular spacing, resulting in the absorption of so much water during rehydration that it leaves the cabbage with a mushy texture (Haralampu et al., 1976). Celery Celery can be dehydrated best by pretreating with glycerin and air- drying. Because of the high moisture content, the tissue is disrupted by the freezing process leaving crevices that fill with water during
15 rehydration. The result is a mushy texture. Air-drying produces a somewhat better product. Rahman found that air-dried celery could be improved significantly with the use of a water-displacing agent (glycerol, a polyglycerol, or gum, for instance) prior to the air- drying step (Food Processing, 1976). Celery treated with gums, particularly locust bean gum, has a some- what better flavor than celery that has been treated with glycerol or various polyglycerols (triglycerol, hexaglycerol, and decaglycerol). Beef, Pork, Poultry, and Seafood Freeze-drying is the best process found so far for dehydrating beef, pork, poultry, fish, and shrimp. Air-dried or vacuum-dried animal products do not rehydrate satisfactorily. Freeze-dried, the quality of the rehydrated product is very close to that of the original and, if desired, the item can be compressed after freeze-drying without significantly diminishing the quality of the final product. It is possible to freeze-dry (and compress) beef, pork, poultry, and seafood in a variety of forms. High quality cuts of beef and pork can be freeze-dried (and compressed) when they are sliced, cut in chunks for stews and other dishes, or ground for meatballs, patties, or sauces. They can be freeze-dried (and compressed) raw or after they have been cooked, but meatballs and patties require the use of binders if they are cooked first (see "Ground Meat" below). Poorer quality slices and chunks that contain gristle and connective tissue cannot be dehydrated satisfactorily by any method because the gristle and connective tissue become as hard as bone particles and do not rehydrate. These parts of a carcass can be used, however, if they are flaked or cut into small chunks and "restructured" before they are freeze-dried (see "Restruc- tured Meat Products" below). Poultry (raw or cooked) rehydrates very satisfactorily after it has been freeze-dried (with or without compression). The armed services usually prefer to use raw chicken that has been deboned and diced before freeze-drying. Fish that has been filleted (raw or cooked) rehydrates very satis- factorily after it has been freeze-dried (with or without compression). Shrimp (raw or cooked) can be processed whole. Ground Meat Ground beef in the form of meatballs and patties and ground pork in the form of patties and sausage links can be freeze-dried either raw or after cooking with equally good end results. If ground meat is cooked before being freeze-dried, it is necessary to use binders so that the pieces of meat hold together during the rehydration stage. Binders made from carbohydrates, proteins, and gums (alone or in combination) form matrices that keep the shape of the item intact without blocking the penetration of water.
16 Formulations and methods of preparation set down for the armed services (U.S. Army Natick Research and Development Laboratories, 1967, 1969) specify the use of pregelatinized corn meal as a binder for ground beef and ground pork that is to be deep fried before freeze- drying and compression. On rehydration with the correct amount of water, the end product has a very satisfactory flavor, texture, color, and shelf life. This product will overrehydrate if there is an excess of water. Freeze-dried ground beef is now being produced in West Germany on a limited commercial basis with no major problems in its production and utilization so far (Judge et al., 1981). Cost analysis by Judge et al. show differences of 5^/kg (12^/lb) between the delivered costs of freeze- dried (unrefrigerated) and fresh (refrigerated) ground sausage meat when transported over long distances, and differences of 2 to 4^/kg (5 to 10^/lb) between the delivered costs of freeze-dried (unrefrigerated) and frozen (refrigerated) ground sausage meat when transported over long distances, but these calculations are based on assumptions of capital costs and interest rates that are subject to change. Restructured Meat Products The poorer quality portions of a beef or pork carcass that contain gristle and connective tissue can be used for freeze-dried and com- pressed products if the meat is first cut into very small chunks or flaked and then "restructured." When this is done, the gristle and connective tissue are finely dispersed so the consumer does not notice that they have not rehydrated. There are other problems, however. Restructured meat tends to fracture or shatter easily after dehydration, indicating that additional work is needed on binders and processing procedures. The presence of fat also causes difficulties; the higher the fat content, the greater the likelihood that fat "smearing" will inhibit rehydration. There continues to be an excellent opportunity for research on air-drying and other non-freeze-drying methods for the production of compressed animal products. The use of reformed meat, partially or fully dried by means other than freeze-drying and then formulated to allow compression and quality rehydration, should be studied.