Meat and fish

Quick freezing is extensively used to preserve a wide range of raw and cooked meat and fish. Freezing and frozen storage does not significantly affect the nutritional value of meat and fish proteins. However, as pointed out above, on thawing frozen meat and fish substantial amounts of intra- and extra-cellular fluids and their associated water-soluble proteins and other nutrients may be lost ('drip-loss'). The volume of drip-loss on thawing of meat and fish is highly variable, usually of the order of 2-10% of wet weight but in exceptional circumstances up to 15% of the weight of the product may be lost.

Many factors influence the amount of drip loss and not all are related to freezing:

  • Variables influencing the raw material - The age, the species and the variety of the animal may have important effects. Additional factors may include the diet fed to the animal, the method of slaughter, and the pre- and post-slaughter handling.
  • Water-binding chemicals - A variety of chemicals are often used as additives to meat and fish before freezing, e.g. polyphosphates. These chemicals penetrate muscle fibres and they associate with proteins where they serve to protect the texture and succulence of meat and fish and to reduce drip loss.
  • Freezing and frozen storage - The rate of freezing, the temperature of frozen storage and temperature fluctuations during storage.
  • Thawing - The rate of thawing from the frozen state and the holding temperature before cooking.

In the frozen state meat and fish are generally less susceptible to oxidative spoilage than are vegetables and fruits and they are not subjected to the equivalent of blanching. On prolonged storage, however, oxidation may lead to significant chemical changes and loss of labile vitamins. The poly-unsaturated fatty acids in meat and fish are particularly susceptible to oxidation. As with vegetables and fruits, it is the products of fatty acid oxidation that give rise to characteristic 'off' and rancid flavours and aromas. The recommended storage lives of frozen meat and fish products are chosen to be within the period before 'off' and rancid flavours and aroma are detectable. In general, those meat and fish products that contain a larger amount of poly-unsaturated fatty acids are least stable and have shorter storage lives. For example, oily fish have a typical frozen shelf life in the region of 6-9 months at -18°C whereas white fish have a frozen shelf life of 12-24 months. Equivalent cuts of pork and beef have frozen shelf lives of 10-12 months and 18-24 months respectively (International Institute of Refrigeration, 1986).

A particular nutritional advantage of fish, and especially of oily fish, is as a dietary source of long chain n-3 poly-unsaturated fatty acids (docosahexanoic acid and eicosapentanoic acid; DHA and EPA respectively). Intake of these fatty acids has been implicated in many health benefits and as noted above they are particularly susceptible to oxidation. Several recent studies have been carried out to determine the effects of freezing and frozen storage on their levels in fish. A significant reduction in the total n-3 PUFA content was reported in saithe (a lean fish) fillets stored at -20°C for six months (Dulavik et al, 1998). Similarly, levels of total n-3 PUFA were reduced in salmon fillets stored at -20°C (Refsgaard et al, 1998) and levels of DHA and EPA were reduced in sardine and mackerel fillets stored for 24 months (Rougerou and Person, 1991). In contrast to these reports of PUFA loss, Polvi et al (1991) found no difference in total n-3 PUFA levels when salmon fillets were stored at the relatively high temperature of -12°C for three months. Xing et al (1993) also failed to see any losses of DHA and EPA in mackerel and cod fillets stored frozen at -20°C for 28 weeks.

As with many aspects of nutrient stability, the extent of n-3 PUFA loss from frozen fish by oxidation will depend on several factors, e.g. access of oxygen to the muscle, handling before freezing and the type of muscle (dark fish muscle suffers higher rates of iron-catalysed oxidation than does white muscle). Although loss of nutritionally important n-3 PUFAs from frozen fish may undoubtedly occur on prolonged frozen storage, in practice this is not likely to be a serious cause for concern. The threshold for sensory detection of rancidity is very low and therefore if frozen fish are consumed within the recommended period of storage, significant proportions of their original content of n-3 PUFAs will not have been lost to oxidation.

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