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SHELF LIFE PACKAGING Minimum weight and volume and convenience are key attributes of compressed freeze-dried foods. Appropriate packaging of compressed freeze-dried foods is essential to realize these attributes. To date these packaging needs have been met using rigid metal containers and by adapting existing flexible packaging materials to match the zero transmission properties of rigid metal containers. While the use of existing packages such as rigid containers guar- antees predictable container performance, the use of these packaging systems does not allow the savings of weight and volume of the dried and compressed foods to be realized, particularly when round cans are specified. The use of existing laminates may result in overpackaging or the need for additional internal liners to protect the barrier materials from physical damage. New packaging systems that meet military requirements for trans- portation, storage, and handling must be designed specifically for compressed freeze-dried products. The packaging systems must be designed along with the product during its development to obtain maximum savings and optimum performance. As with all products, the effect of oxygen concentration, composition, water activity, and light must be evaluated against quality with storage time and temperature. When requirements for levels of oxygen, water activity, and light are found, appropriate packaging materials can be selected and evaluated against the additional requirements for product protection from transportation, handling, and storage damage. Two areas of research have been identified. The first is the need for improved shelf life prediction methods. Analytical methods that can be correlated with flavor panel analysis are required since flavor is a critical quality factor. Second, analytical methods are needed that can be used to predict shelf life as a function of temperature, time, water activity, and oxygen concentration using a minimum number of storage test conditions. Many foods undergo an induction period when flavor changes little pro- gressing to a rapid loss or change in flavor analogous to lipid oxidation in which antioxidants are consumed after the induction period. 20
21 Compressed freeze-dried foods are similar to other military rations in that they are often designed for a specific operational use. Long storage life is highly desirable. Often large quantities of rations must be available on short notice. Since the industrial manufacturing capabilities and certain seasonal raw materials may not be available in time of need, ongoing research is required to find optimum methods for strategic product acquisition and storage. An item-by-item systems analysis can identify minimum cost strategies for strategic stockpiling of frozen raw materials, freeze-dried products or compressed freeze- dried products ready for issue. The availability of processing facilities should also be studied since freeze-dryer, vacuum-driers, and microwave heating equipment represents a costly capital investment which cannot be purchased off the shelf with a short delivery time. Storage conditions for strategic stockpiles of frozen or freeze- dried materials must be optimized to ensure maximum food quality at minimum cost. Storage at -18Â°C (0Â°F) or below should be considered in a systems analysis for both frozen and freeze-dried stockpiled materials. Moisture levels (10 to 15 percent) for compression can be maintained at -18Â°C (0Â°F), for example. It may be possible to achieve moisture equalization during storage. Packaging of dehydrated food is necessary to extend shelf life of the product after moisture removal and to prevent mechanical damage. The design of the package must ensure optimum levels of water activity and oxygen independently of the exterior storage environment. It must also provide protection from light, and act as a barrier to micro- organism, insect, and animal infestations even during abusive storage conditions. In addition to extending shelf life, the package must facilitate the transportation, distribution and use of the product, whether it be for civilian or military consumption. The consumers' decision to use dehydrated products is based on cost and quality. Military use is based on operational requirements. These requirements must be satisfied at minimum cost. Specifications for the production of dehydrated components of military rations reflect operational needs and cost. For example, the original military specifications for freeze-dried beef patties allowed a maximum of 14 to 17 percent fat. This was later increased to a maximum of 23 percent fat in the final product when research indicated that freeze-dried beef rehydrated as well at 23 percent fat as at 17 percent. By increasing the range of maximum fat content in the specifications, the military decreased the cost of the product with little sacrifice in operational performance. Of course, a higher fat content means less protein is available in a given weight of ration. Packaging specifications must be based on operational needs for the product and low oxygen levels in the meal, ready-to-eat fruit bars. While contractors thought that low oxygen concentrations could be achieved without an inner polyethylene bag, military specifications require this more expensive additional package. This example is included to point out the continuing need for packaging research in support of freeze dehydrated and compressed products. Minimum costs and maximum operational performance can only be achieved by a periodic review of operational needs and of available new packaging materials and systems.
22 STORAGE To ensure maximum storage stability, processing procedures no longer under the military's surveillance and control should be spelled out in the product specifications. This would include specifying the proper handling of raw material. The more a product is handled, the greater the potential for decrease in quality in terms of flavor, texture, nutrition, and functionality of the end product. Mishandling of raw materials can lead to unacceptable products after processing. If raw meat or poultry is mishandled, the fat begins to oxidize. Although the oxidation is organoleptically undetectable in the raw product, the mis- handled food becomes rancid after freeze-drying. Certain materials packaged together in the dry state are not com- patible when stored at slightly elevated oxygen levels. For example, the deterioration of military long-range patrol spaghetti and chili rations has been linked to high levels of tomato paste. The freeze- dried tomato paste and other dry ingredients maintain their quality longer when stored individually than when mixed. Another example of the problem of stability of dry blends is demonstrated by a complete pancake mix which included eggs, milk, and shortening. The complete mix, with regular flour, is quite stable. Buckwheat pancake mix without shortening was also quite stable. However, when shortening was added to the buckwheat mix to make it complete, it rapidly became rancid. This demonstrates the need for all dry materials to be mixed and tested for compatibility and shelf life before any new combination can be successfully marketed. Because the needs of the commercial marketplace are entirely differ- ent than those of the military, there is a need for research on the effect of storage of frozen foods to be used for dehydrated and com- pressed products. The formulation, preparation, and packaging of commercial frozen food is based on a desired shelf life of 18 months at -18Â°C (0Â°F). This is adequate for a seasonal supply of raw materials. At a commercial storage temperature of -23Â°C (-10Â°F), seasonal products have a shelf life of several years. This effort should be undertaken with military support. The military should study the need for a stockpile of frozen foods since price may depend on the shelf life and storage period of the food. Factors that can adversely affect the stability of food in cold storage are mishandling of the raw products and residual enzyme activity from inadequate blanching or microbial sources. The delay between har- vesting, and blanching and freezing can determine the storage life. Slight variations in handling and processing times could result in dramatic differences in quality over the longer storage periods necessary for military stockpiles. Storage temperature is also very crucial for extended storage of frozen foods. Cod stored at -8Â°C (18Â°F) reaches a just noticeable difference (JND) in flavor in approximately seven weeks, whereas cod handled in a similar manner and stored at -30Â°C (-22Â°F), as in the large European cold stores, will reach the JND in seven years. Products with high concentrations of electroyles such as salted butter, salami, and bacon have a shorter shelf life during frozen
23 storage than refrigerated storage (Olson, 1971). The cost of energy for storage of a military stockpile can determine to a great extent whether the food will be stored frozen or compressed and dehydrated. The use of microwave-assisted equilibration during the dehydration and compression process can provide a more satisfactory storage-stable product. Depending on the type of equipment used, 10 to 14 hours are usually needed for the dehydration of green beans. This time could be significantly reduced if the drying process were stopped when the product reached a moisture content of 10 to 30 percent in approximately 5 1/2 hours. Compression could take place at this moisture level if a 40- to 45-second microwave treatment is used to cause the moisture to diffuse from the center of the product to the relatively dry exterior, thus plasticizing the food. Compression and further drying by other techniques such as vacuum-drying would complete the process. The availability of costly microwave and vacuum equipment must be assessed. Theoretically, compressed foods are not exposed to as much oxygen as noncompressed foods in containers. The physical action of com- pression discharges oxygen from within and between the cells of the product. The resulting high-density product is less sensitive to the residual headspace gas. For example, the flavor of sulfited and nonsulfited freeze-dried compressed carrots and peas was found to be quite similar even after storage at 37.8Â°C (100Â°F) for six months. All of these products were packaged in flexible pouches under a vacuum of 101.1cm (28 in.) of mercury. While the use of an aluminum foil laminate in the pouch is believed to be equivalent to packaging in a full steel can with limited headspace, the military operational requirements for shipping, storage, and handling increase failure rates for existing flexible materials than steel cans. FOOD KINETICS A critical factor which does not receive much attention in shelf life studies is the lag phase. High-temperature/short-time accelerated storage studies do not provide information on actual storage con- ditions due to the absence of an induction or lag phase. Most methods of analysis are not sensitive enough to detect chemical changes during the induction period. Recent developments in inverse gas chromatography (IGC) may help detect the small changes taking place during the induction or lag phase. The degradation by hydrolysis of a mixture of lactose and aspirin was studied as a model. The rate of hydrolysis of this complex at low temperatures was found to be related to the moisture content since the hydrolytic action dominates. At high temperatures, the kinetics change drastically and the storage stability is less dependent on moisture content. This phenomenon has been well known by those who market compounds such as aspirin in pill form, but previously there was not a clear explanation for it. By using the technique of IGC, a reasonable answer was found. The basic principle of IGC is that a carrier gas is passed through a stationary phase (food). It is the reaction of the stationary phase
24 that is of interest in this type of chromatography instead of the carrier gas. The system was applied to the early low-moisture phase of water absorption by collagen which is present in most meat. By measuring the hydrogen bonding sites of collagen as a function of concentration, workers were able to form an alternative definition of bound water (Coelho et al., 1979). Hydrogen bonding as a thermodynamic parameter of a dehydrated food system is highly concentration dependentâthat is, the lower the moisture content, the more difficult it is to remove the water from the sample. By using IGC, the energy requirement for removal of water at low moisture levels can be measured. This technique could be used to study oxidation of food systems by using a continuous stream of moist carrier gas with a specific low oxygen concentration. With time, the response would be proportional to oxygen content. Changes in oxidation could be measured in the presence of light or an antioxidant. The oxygen sorption relation could be used to study the factors related to the lag phase in color changes in nonenzymatic browning or in changes in peroxide values. IGC could possibly be used to create drying curves for various foods by passing a carrier gas of known moisture through a food at a given temperature. Changes in the thermal conductivity of the outgoing gas over time could be measured, thus giving an estimation of the drying properties of the food. Changes in the shape and size of the food particles would allow a study of the relationship between physical factors and drying rates. With the increased knowledge of the kinetics of changes in low- moisture foods during storage, it is possible to simulate such changes by using sets of descriptive equations or models. Kinetic models are based on the process rate, which describes quality changes by the general equation: -dc/dt = f (EI...EN, FI...FN) (Saguy and Karel, 1980) In dealing with the problem of predicting the shelf life of a food, certain definitions are needed. The first definition needed is that of the quality of the food or F. Deriving this definition is very often the basic problem in dealing with shelf life. Depending on the food, its desired use, and the inspector, quality will always be a variable term. A standard index of quality must be agreed upon. This index may be the result of an organoleptic test, oxygen content, per- oxide value, moisture content, or another characteristic; but without a common index, the mathematics of a kinetic study is meaningless. The quality of food is known to be a function of the environment surrounding the food inside the package or E. This may be oxygen pressure, water vapor pressure, or presence of sulfur dioxide in the air. It also depends on the initial quality of the food and time. This interior environment is in a constant state of flux, depending on the oxygen content of the initial packaging. The food is constantly absorbing and emitting gases. This dynamic system is also dependent
25 on the environment outside the package, barrier properties of the package, and time. The barrier properties, which are very important in determining the shelf life of a low-moisture food, are at times very dependent on the internal and external conditions. For example, cellophane at high humidities will readily transmit oxygen, but if kept at low humidities on both sides, it is a very good oxygen barrier. Thus, optimum pack- aging lies somewhere between the rigid round steel can and perhaps the cellophane wrapper used to package dry split pea and bean soup mixes for the retail consumer. At this point, storage and operational needs must be clearly stated so that minimum-cost packaging systems can be tailored to provide a minimum-cost operational product.