Along with establishing consistently satisfactory materials and obtaining defect-free closure seals, the subject of retorting, the commercial sterilization step, has received considerable attention. During the early exploratory phases, enough studies, including inoculated packs, were performed so that apprehension over sterility was relieved and reliable cooking procedures were established. Then, as experience was accumulated and as productivity as well as quality became important for successful commercialization, retorting received more attention. For example, process times for retort pouches were established at 25 to 40% of those required for equivalent canned items; however, the total heat exposure in commercial processes was actually 70 to 83% of cans (26).

For determining thermoprocessing conditions (ie, cook time and temperature), the general and mathematical techniques for calculating cook times for traditional cylindrical cans based on heat-penetration data have been found to be generally suitable for retort pouches. The lethality levels, known as F0, adequate for commercial cans were also found to be suitable for pouched products. Comparisons of the various process determination techniques and listings of precautions to note during their application to pouches (such as difficulty in locating the cold spot or occasional occurrence of nonlinear heating curves) have been published (12,19,30).

For successful retort pouch thermoprocesses, the following factors need to be closely controlled (19,26,31,32):

1. Pouch fill weight and residual gases

2. Pouch thickness during processing

3. Control and reproducibility of the entire cook cycle: come-up, process, and cool

4. The retort rack or other device that controls thickness during processing, and that must not adversely affect heat medium flow or temperature uniformity in the retort

5. Overriding or total pressure

A variety of processing techniques and equipment designs have been used with retort pouches, and each has its proponents. Still cooks under water with superimposed air pressure was the initial reliable procedure and is still used in some instances for some products. Pouches are positioned usually horizontally onto perforated pocketed trays and accumulated on carts that are wheeled into horizontal retort chambers. Most systems are designed so that rotation of the cart is possible. To conserve energy and reduce come-up time, the processing water is preheated to a high temperature in a separate tank and then fed into the retort chamber after loading. Processes are now computer con trolled (33). Steam-air retorts are now proven, accepted, and more prevalent. They are basically the same design except that an air tank replaces the hot water preheating tank.

Other retort designs include a continuous cooker-cooler (trade-named Hydrolock) using steam-air as the heating medium, and a Universal Convenience Food Sterilizer (34), where pouches in retort carts are exposed to horizontally flowing hot water. Water is pumped from a hot water storage tank into each rectangular tanklike cart (as opposed to the entire chamber) fitted with a water flow distribution plate configured to assure uniform lateral water flow distribution.

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