FREEZING

Frozen fish products

The variety of species, processes, methods of presentation and packaging available provide scope for the preparation of numerous frozen fish products. These products, however, can be separated into two main groups; products intended for direct consumption and products intended for further processing.

Products for direct consumption

Individually quick frozen (IQF) products are frozen as single units which need not be thawed for sub-division or perhaps even for cooking purposes. IQF single fillets and shrimp are two products of this type.

The demand for IQF products has increased with the upsurge in the number of low temperature "freezer" cabinets both in catering establishments and in the home. IQF freezing allows for the purchase of a frozen product in bulk and the selection from storage of only sufficient quantities to meet immediate requirements.

Other products such as blocks of fish and fish portions usually packaged in cartons are also produced for direct consumption without the need for reprocessing. The consumer will purchase this type of product from the retailer, still in the frozen state, and either cook it in the frozen state or thaw it for immediate consumption.

The production of products for direct consumption may not yet be appropriate in many developing countries. This type of product requires the provision of an extensive network of refrigerated storage and transport. This facility, which is popularly known as the "cold chain", may not be developed enough to enable this system to operate.

Products for further processing

These products are produced for two purposes:

1. Frozen in bulk and thawed after storing, to be used as newly caught, unfrozen fish.
2. Frozen in bulk and after storage, further processed without thawing so that it may be presented as a retail pack.

Products frozen in bulk can be unprocessed, such as blocks of whole fish frozen in contact freezers. Blocks of frozen fish may weigh up to 50 kg; they are usually glazed or wrapped after freezing and are then stored until required for further processing.

In some cases, fish are bulk frozen, stored and finally thawed all in one place. This is usual when there is a short seasonal fishery and fish are preserved for processing over a longer period. Bulk frozen fish may also be distributed in the frozen state. This enables the fish to be sold to a larger home market and also allows the product to be exported. In this case there are additional requirements for low temperature transport and a more extensive cold chain.

Fish frozen in bulk may also be fully processed before freezing and only the skinless, boneless portion used. One particular process of this type worth special mentioning is the production of frozen fillet blocks. A frozen fillet block is a regular shaped block of fish flesh frozen in a horizontal plate freezer within a treated cardboard carton and a metal retaining frame The filling process ensures that there are no voids in the block. After freezing, the blocks are stored in bulk and at a later date cut into smaller portions of different shapes. The fish portions may then be packaged and sold in this form or they may be coated with a flour batter and breadcrumbs. Coated fish portions should be returned to the freezer and rehardened before packaging and storing.

The type of frozen fish product and the form in which it is produced in a particular country may well depend on the extent of the cold chain as well as on the demands of the consumer. It therefore seems likely that in most developing countries a bulk freezing process will be the initial development. This will enable the industry to cater for seasonal variations and allow a wider distribution of the fish catch. Other frozen products will follow later when the industry develops and the cold chain is extended.

Time-Temperature Tolerance

As in the case of iced fish the storage life for frozen fish varies considerably.

Practical storage life for fish. From IIR Guide to Refrigerated Storage

From the table the importance of low temperature storage is clearly illustrated. It is, however, not only the length of storage life which is of importance, but the higher quality at any given moment during storage.

A number of scientific works have shown the importance of low temperature storage and for frozen foods the Time-Temperature Tolerance concept was introduced very early. The corner stones of the TTT theory are:

• There is, for every frozen product, a relationship between storage temperature and the time it takes at this temperature for the product to undergo a certain amount of quality change.
• Changes during storage and distribution at different temperatures are cumulative and irreversible over the entire storage period and sequence is without influence on the size of the accumulated total quality change.

The storage life based on one or more of the chemical, biochemical and physical changes can be defined in many ways. A common definition is High Quality Life - HQL HQL is defined as the elapsed time between freezing of High Quality product and the moment when 70 percent of experienced tasters are able to distinguish the product from the control stored at very low temperature.

Other definitions of storage life are also used. Regardless of the definition the accumulated quality loss can be integrated from plots of 1/HQL against time, independent of the order of the exposures to different temperatures.

The distribution in this case includes 106 days at -30°C, 30 days at -25°C and 14 days at -18°C. The total quality loss during the distribution of this particular fish fillet is 61 percent. There is, in other words, 39 percent of the original quality left for the consumer. It is important to note that if the, storage and distribution had been carried out at -18°C, the corresponding quality loss would have been obtained in 60 days. By keeping the product at -30°C during the main part of the distribution, it has been possible to more than double the storage life for the same quality level.

As indicated above it must however, be observed that the quality changes in fish products are not only influenced by the storage temperature. Among the factors which are important are the original quality of the raw material, the processing method and the packaging material and method used for the final product. Those three factors are usually defined as the PPP factors - Product-Processing-Packaging.

Source :

"So Easy to Preserve", 5th ed. 2006. Bulletin 989, Cooperative Extension Service, The University of Georgia, Athens. Revised by Elizabeth L. Andress. Ph.D. and Judy A. Harrison, Ph.D., Extension Foods Specialists.

Updated on : Feb 2015