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water, normally at or above 117°C (242°F). The time and temperature required for sterilization must be sufficient to kill Clostridium botulinum spores that, when viable, can grow in an anaerobic (nonoxygen) atmosphere and produce lethal toxins. These spores must be held for 32 min at 110°C (230°F) to insure total destruction. Many low pH (high acidity) products such as certain fruits, vinegar-packed foods, and highly acid formulated foods prevent C. botulinum spores from growing. Many of these foods do not have to be sterilized at the temperatures required for high pH (low acidity). However, near neutral or high pH vacuum-packed products, such as seafood, are particularly vulnerable to anaerobic spore growth and sterilization must be insured.

To ensure a margin of safety, the sterilization requirement for canned fish is that the geometric center of a can or pouch must be held for 32 min at 116°C (240°F) (14). Each food and each different geometric form of container requires a different total processing time to accomplish sterilization. These processing times, determined by thermal death time laboratory studies, are mandatory for each processing company that is canning hermetically sealed food. Each batch of canned food must be coded and retort processing records kept to prove that the product was sterilized.

It should be emphasized that anaerobic conditions often prevail in a canned food even though an incomplete or no vacuum is drawn on the can prior to sealing. This is a result of the subsequent oxidation of components in a food by the remaining oxygen in the air.

Another precaution that must be practiced by processors is the venting of a steam retort prior to beginning the official retorting time. This is to prevent air pockets from insulating some of the cans so that there is nonuniform temperature in the retort. Of course, hydrostatic retorts that heat containers in a column of water kept above normal atmospheric boiling temperature by hydrostatic water pressure do not have a problem with entrapped air.

Effects of Heating. Fresh or frozen seafood being cooked for a meal should be heated for the minimum time required to improve the texture and the taste for the consumer. The normally dangerous microorganisms, those known as public health disease organisms are destroyed at relatively low temperatures. As shown in Table 9, the most heat resistant of the group, thermophiles, have an optimum growth at 50-60°C (122-150°F). Hence, a seafood is normally safe to eat if the geometric center has been raised above 150°F when it is actually pasteurized.

Table 9. Optimum Growth Temperature Range for Bacterial Group

Bacterial group

Optimum temperature range

Psychotrophs

14-20°C (58-68°F)

Mesophiles

30-37°C (86-98°F)

Faculative thermophiles

38-46°C (100-115°F)

Thermophiles

50-66°C (122-150°F)

Overcooking causes heat degradation of nutrients, oxidation of vitamins and oils, and leaching of water-soluble minerals and proteins. The retention of B vitamins, zinc, and iron is particularly important for populations that consume fish as the major source of meat in their diet. In addition, overcooking causes too much water to be released and the drying effect causes flesh to become tough, thus nullifying the desired effect of texture improvement.

Refrigeration and Freezing Technology

As has already been stressed, the most important factor in handling fresh fish is to lower the temperature to just above freezing as soon as it is removed from the water. Because the condition of the harvested fish and the subsequent handling determines the shelf life of a fresh fish, it is not possible to state exact times that a seafood can be held in ice or under refrigeration and be considered a high-quality food. This is shown in the wide range of shelf life that has been published in the literature (Fig. 12). In general, fish with a high oil content or enzyme activity have greatly reduced shelf life and are often of marginal quality after a few days.

High-quality seafood, when frozen properly soon after being removed from the water, is often far superior to fresh fish available on the market. This is due to the fact that all microbial action is stopped and enzyme action is significantly reduced in frozen fish. However, the initial quality of the fish being frozen, the rate of freezing, the temperature at which the frozen seafood is held, and the uniformity of the freezing temperature are all important to maintaining a high-quality product. It is surprising how many people involved in the seafood chain do not understand some of these basic factors in insuring the high quality of a seafood. Thus the constant challenge of those involved in seafood technology is the continual education and reeducation of everyone involved in the commercial seafood chain.

Freezing Seafood. During the freezing of seafood, structural changes take place in the cells and cell walls as well as in components that are between the cells. Many of these adverse changes are caused by water crystals that expand and rupture the cell walls. This allows liquid within the cells to leak out when the flesh is thawed. Hence, free liquid, called drip, exudes from seafood when it is thawed. This loss of free water reduces the water content of the seafood causing economic loss to the seller and greatly reduces the fresh qualities of the thawed product.

As seafood is cooled above and below approximately 28°F in a constant-temperature environment there is a near linear relationship between the temperature decrease and the time. However, as the water in the flesh begins to freeze, there is a long period of time during which the temperature remains almost constant. This period is a critical range for freezing and is caused by heat (heat of fusion) being removed from the fish to freeze water rather than to lower the temperature of the flesh. This relationship is shown in Figure 13.

The longer a product remains in the critical zone, the larger the ice crystals formed in the cells will be. When

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