Dehydration

Freeze Drying. The primary dehydration technology used for certain ration components is freeze drying (1,2). By maintaining the product frozen well below 32°F and by removing the water through sublimation, highly desirable features are retained in the dried product. Freeze-dried components include fruits, vegetables, meat patties, and entrée items. All are blanched or precooked prior to processing.

Large-scale freeze drying by industry is done on a batch process basis following procedures that optimize the drying rate without compromising product quality. Pre-frozen products spread out on trays placed on radiant heating platens within a vacuum chamber are slowly processed to sublime away the moisture. Typically, the product temperature is maintained around 0— 40°F and the chamber pressure is controlled to about 0.1-1.0 torr. As the process proceeds, the outer surface layer dries and the frozen ice core recedes. The energy for sublimation must transfer through the dried layer to the ice and the water vapor must migrate through the dry, porous layer to the surface. The transfer of this heat energy is driven by the difference between surface temperature and ice temperature and is affected by thermal conductivity of the porous layer. The migration of the moisture is driven by the difference between vapor pressure of the ice and the chamber pressure and is affected by the permeability of the porous layer. Typically, processors limit the slab thickness, adjust the platen temperature to maintain product surface temperature at or below 150°F, and try to achieve a suitable porosity in the product. Drying times are in the range of 10-15 h. The final product is packaged under vacuum or nitrogen in a water and oxygen impermeable material.

Reversible Compression. Because certain military rations must be compact, many freeze-dried components are reversibly compressed (3). A product that is "plastic" before compression recovers its original volume and textural properties upon rehydration. Such plasticity is achieved by spraying the freeze-dried material with enough moisture so that it has a uniform moisture content of about 12%. After compression, the product is redried and packaged in a protective container. Compression reduces the volume effectively, a ratio of 16:1 being achievable for green beans.

Microwave Freeze Drying. Microwave-assisted, freeze drying (MWFD) has been explored for producing products equivalent in quality to conventionally freeze dried products, but at lower cost. It has the advantage that the energy for sublimation is directly absorbed by the ice and that the remaining moisture can be continually redistributed throughout the sample. Control of microwave power and sample temperature is crucial, because the ice must not melt and the ice temperature must not be so low as to decrease the efficiency with which frozen water absorbs microwave radiation at 2,450 MHz. This advantage is evident in the decrease in drying time down to 5.5 h for green peas. Another advantage is the ability to dry the sample to a relatively uniform moisture level of 12%, which makes it directly compressible.

Centrifugal Fluidized-Bed Drying. Centrifugal fluidized-bed drying (CFBD) is another rapid drying technology. The fluidization is achieved by counterbalancing the centrifugal force exerted on diced or spherical samples contained in a rapidly rotating drum against the force of hot air streaming through small perforations in the drum wall. Air temperatures between 175-212°F are used. In 15 min, 50% of the moisture in green peas can be removed. Further drying would be inefficient and detrimental to quality. A combination of CFBD with MWFD could be used to optimize the dehydration process.

Thermoprocessing

Conventional Retorting. The primary thermoprocessing technology used in producing shelf stable tray-packed and pouch-packed ration components is retorting (4). By subjecting foods in these containers to high temperatures for specified times in steam-charged retort vessels, any contaminating disease-causing or spoilage-inducing microorganisms that might otherwise multiply during storage are destroyed. The minimum temperature-time requirement for the slowest heating point in a geometrically distinct container depends on the acidity of the product and on the heat resistance of the target microorganism. For most of the tray-packed and flexible pouch-packed components, the pH exceeds 4.5 and the targets are mesophilic spore-forming bacteria, which could grow between 50 and 105°F. It suffices then to ensure that the integrated temperature-time exposure is equivalent to 6 min at 250°F, which would reduce the population of the reference putrefying anaerobe, PA 3679, by 6 D (ie, six orders of magnitude) and which provides a large margin of safety for reducing Clostridium botulinum by about 24 D. For these products, the retorting temperature is set between 230 and 250°F, and the integrated lethality (ie, F0) of the process takes into account the change in thermal resistance of PA 3679 as the product goes through the heat-up, cook, and cool-down cycles.

Industrial thermoprocessing of such different items as potatoes in butter sauce, frankfurters in brine, beef slices in barbecue sauce, lasagna with meat sauce, and spice cake is currently done in batch mode using either a still or rotating retort. The heating medium could be pressurized steam or water heated with steam. Tray packs or flexible pouches, after being either vacuum sealed or hot filled and purged with steam and then sealed, are loaded into the retort vessel. Once the process is begun, the heat is transferred from the surface of the container into the food. The flat shape of the 2-in.-deep tray-pack container and the 0.5-in.-thick profile of the pouch, in principle, allow the heat to penetrate quickly and uniformly, which should produce a high-quality product. If the food has a significant fluidity, the heat penetration is aided by natural convection; if it is solid, the penetration is exclusively conductive. In a rotating retort, the agitation introduced further facilitates con-vective heat transfer and would lead to a faster, more uniform heat treatment.

Before the process for a particular product in a specific retort is filed with the USDA or FDA, verification must be obtained that the cycle to be used ensures attainment of commercial sterility at the slowest heating point in the slowest heating container. For this purpose, thermocouples are used to obtain heat penetration data and calculations are made to specify the cycle time and temperatures needed to achieve the minimum F0. Processors typically will add a safety factor to ensure that the processing is in excess of the requirement for microbial lethality. If an entirely new product or container has been developed, inoculation studies may be required.

Ration components so processed are not only safe, but show high retention of nutrients and score high in ratings of color, flavor, and textural attributes. However, some products might be overprocessed due to wide variation in treatment from container to container. Techniques are now being investigated to validate, noninvasively, that each container received a treatment that is greater than the minimum requirement and less than the maximum set as a high-quality limit. One promising technique uses a optically read, bar-coded label with a patch that becomes darker in color as the integrated temperature-time exposure increases.

Aseptic Processing. Another thermoprocessing technology that is receiving attention for possible future use is aseptic processing. It is a high-temperature, short-time exposure process that sterilizes the food product before it goes into a container. In this process, a fluid product with relatively large particulates, such as a stew with chicken cubes, potato dices, and carrot dices, is pumped through a scraped surface heat exchanger, then through a holding tube maintained at the specified high temperature, then through a rapid cool down system, and finally into a sterile filler unit utilizing surface-sterilized packaging material. Temperatures of 260-280°F can be used, and residence times of about 3 min in the holding tube are all that is needed to ensure that the center of the particulate has received an adequate lethal treatment. This technology has several inherent advantages including high quality and nutrient retention, lower overall energy input, continuous processing, and the option to package the product in any size container without the need for changing and verifying process parameters.

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