Chilled and frozen storage 217 The type of fatty acid present in the meat is therefore of major impor- tance in determining its oxidative stability. Beef and mutton tallow, both very hard fats, which contain few unsaturated fatty acids are much more stable than lard, a softer fat, which contains a large quantity of unsaturated fatty acids. The effect of fatty acid composition can, however, be much more subtle. Lea(1936)observed that feeding an ounce of cod liver oil per day to pigs increased the susceptibility of the fat to oxidation by a factor of 5, lthough the change in the iodine value of the fat was negligible. This result may be compared with the studies of Dahl (1957) where an increase in the linoleic acid content of pig fat from 7% to 15.6%, which increased the iodine number by 10 units, only reduced the oxidative stability by one quarter. The reason for the difference is that the quantity of polyunsatu- rated fatty acids from the cod liver oil that was stored in the pig fat was small. However, since these fatty acids contained 4, 5 and 6 double bonds, the initiation of oxidation was much easier than in the fat where the high iodine number was produced by an increase in a fatty acid with only 2 double bonds. The relative susceptibly of oleic, linoleic and linolenic acid with 1, 2 and 3 double bonds respectively, to oxidation is 1: 12: 100(Kuhn nd m 1031. 2 Natural antioxidants In view of the ease with which fat oxidation takes place one might enquire whether it occurs in the living animal, and if not, why not? The answer is that it does but only to a small extent. Nor does it occur in meat imme diately after slaughter. The reason is the presence in the animal tissues of antioxidants which prevent the peroxide breakdown products from catalysing the oxidation. The major antioxidant in meat is alpha-tocopherol or vitamin E. Deficiency of vitamin E in many animals leads to the oxida- tion of the adipose tissue, which turns yellow(Dam, 1957). Such material from pigs would of course not pass inspection, but even meat having a lowered alpha-tocopherol content would be less stable in frozen storage The situation is well documented in the case of the turkey, which has low levels of alpha-tocopherol. The injection of alpha-tocopherol into turkeys decreased the thiobarbituric acid value. a measure of oxidation of the frozen carcass, and improved the flavour(Webb et al., 1972) Fatty acid composition and the antioxidant status of the tissue are the main factors affecting oxidation, which are fixed in the animal before slaughter. The fatty acid composition of the diet readily changes the fatty acid composition of pig fat, but has little effect in ruminants. The antioxi- dant levels of the tissues are not greatly affected by changes in the quan ity in the diet, since little of the added alpha-tocopherol finds its way to the fat. However, prolonged low levels of alpha-tocopherol in the diet can reduce the quantity in the animal Feeding high levels of polyunsaturated fatty acids can also reduce the bodys stores of alpha-tocopherol because of the extra quantity needed to prevent oxidation. The alpha-tocopherolThe type of fatty acid present in the meat is therefore of major impor￾tance in determining its oxidative stability. Beef and mutton tallow, both very hard fats, which contain few unsaturated fatty acids are much more stable than lard, a softer fat, which contains a large quantity of unsaturated fatty acids.The effect of fatty acid composition can, however, be much more subtle. Lea (1936) observed that feeding an ounce of cod liver oil per day to pigs increased the susceptibility of the fat to oxidation by a factor of 5, although the change in the iodine value of the fat was negligible. This result may be compared with the studies of Dahl (1957) where an increase in the linoleic acid content of pig fat from 7% to 15.6%, which increased the iodine number by 10 units, only reduced the oxidative stability by one quarter. The reason for the difference is that the quantity of polyunsatu￾rated fatty acids from the cod liver oil that was stored in the pig fat was small. However, since these fatty acids contained 4, 5 and 6 double bonds, the initiation of oxidation was much easier than in the fat where the high iodine number was produced by an increase in a fatty acid with only 2 double bonds. The relative susceptibly of oleic, linoleic and linolenic acids, with 1, 2 and 3 double bonds respectively, to oxidation is 1 : 12 : 100 (Kuhn and Meyer, 1929). 10.3.1.2 Natural antioxidants In view of the ease with which fat oxidation takes place one might enquire whether it occurs in the living animal, and if not, why not? The answer is that it does but only to a small extent. Nor does it occur in meat imme￾diately after slaughter. The reason is the presence in the animal tissues of antioxidants which prevent the peroxide breakdown products from catalysing the oxidation. The major antioxidant in meat is alpha-tocopherol or vitamin E. Deficiency of vitamin E in many animals leads to the oxida￾tion of the adipose tissue, which turns yellow (Dam, 1957). Such material from pigs would of course not pass inspection, but even meat having a lowered alpha-tocopherol content would be less stable in frozen storage. The situation is well documented in the case of the turkey, which has low levels of alpha-tocopherol. The injection of alpha-tocopherol into turkeys decreased the thiobarbituric acid value, a measure of oxidation of the frozen carcass, and improved the flavour (Webb et al., 1972). Fatty acid composition and the antioxidant status of the tissue are the main factors affecting oxidation, which are fixed in the animal before slaughter. The fatty acid composition of the diet readily changes the fatty acid composition of pig fat, but has little effect in ruminants. The antioxi￾dant levels of the tissues are not greatly affected by changes in the quan￾tity in the diet, since little of the added alpha-tocopherol finds its way to the fat. However, prolonged low levels of alpha-tocopherol in the diet can reduce the quantity in the animal. Feeding high levels of polyunsaturated fatty acids can also reduce the body’s stores of alpha-tocopherol because of the extra quantity needed to prevent oxidation. The alpha-tocopherol Chilled and frozen storage 217