Preservation Of Living Tissue

The genetically controlled taste, color, nutritive value, cell structure and conformation of fresh foods are usually best at harvest. Protection of these qualities is achieved by retarding or inhibiting the detrimental action of microbes, enzymes, and other chemical degradation while maintaining the integrity of the membranes, enzyme systems, and gross structure of the food. Specifically, the following must be accomplished:

1. The growth rate of microbes normally associated with the product at harvest or during handling and transport must be inhibited or reduced.

2. Production of deteriorative enzymes or enzyme systems responsible for softening, color loss, or flavor changes must be inhibited or their rate of action must be reduced. Enzymes may be associated with the product or with incidental microbial contamination.

3. Loss of moisture and deterioration by chemical contamination must be prevented. Volatile organic compounds generated through respiratory action or present in storage often must be removed.

4. Higher life forms, insects, mites, and so on, must be inactivated or eliminated.

5. Bruising and various forms of mechanical damage must be prevented.

Processing operations required for the distribution of living tissue depend on the following factors: inherent storage potential of the tissue (this can be improved through genetic engineering, eg, by improving skin strength, resistance to microbes, or reducing respiration rate), desired storage life, degree of handling and shipping required, and intended use of the tissue (eg, further processing, food service, or retail consumption).

The intensity of processing operations is limited by the tissue itself in terms of temperature range, water activity, respiratory gas composition, mechanical stresses, and concentrations of chemicals. Temperature has the greatest effect on shelf life, because activation energies (and hence reaction rates) for microbial growth and enzyme activity are two to five times greater than those of most deteriorative chemical reactions. Except for certain fruits and vegetables, storage should be to 0°C without ice formation (Table 4). When shipping distances are great, expensive and highly perishable commodities are routinely shipped by air freight. Many fruits and vegetables are stored in atmospheres with controlled or modified 02, C02, or C2H4 compositions.

A clear distinction is now made between processing whole, intact living tissue and minimally processed products that have arisen largely due to consumer demand in developed countries for freshlike, high-quality convenience foods. Minimally processed products include living tissues that have been cut, peeled, or shredded and generally require refrigeration in combination with other treatments, such as pH control, antioxidant addition, or chlorinated water dips, to extend the shelf life of the products. The application of "hurdle" technology to ensure safety and stability refers to synergistically applying several preserving treatments in series to minimize or eliminate microbial and enzymatic activity while preserving the freshlike nature of the products.

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