Nutritional effects of autoxidized fats in animal diets

Abstract

I .Beef fat oxidized to a peroxide value of 109 ,umoles/g (218 m-equiv./kg), with a reduction in iodine value of 3'4 units, was incorporated at a 5% level in the diet of day-old chicks (diet 2) and stored for 8 weeks at room temperature during feeding. Similar diets containing fresh fat (diet I, peroxide value zero) or oxidized fat in which the peroxide had been largely destroyed by heating (diet 3, peroxide value 2 ,umoles/g) were also used. The diets contained adequate but not excessive levels of all vitamins, including stabilized vitamin A, except that for half of the chicks the supplement of stabilized vitamin E was omitted. 2. No further oxidation of the dietary lipid occurred during storage, and the initially high peroxide value in diet z decreased rapidly. The natural vitamin E decreased by more than 50% in diet 2, but remained unaffected in diets I and 3. 3. The chicks grew normally, with no difference in weight gain or feed conversion between the groups, the only differences attributable to the diets being marginally 1owervitaminAlevels accumulated in the livers of the birds on diet 2 and ajustdetectablyhigher liver weight in the birds on diet 3. 4. One of the twelve chicks receiving oxidized fat (diet 2) without synthetic vitamin E developed encephalomalacia. There was no other suggestion of performance being inferior as a consequence of the absence of the vitamin E supplement. 5. No difference could be detected in the flavour of the chickens, either freshly roasted or reheated.In recent years the inclusion of beef tallow at levels up to about 5 yo in commercial poultry rations has become common practice. Numerous reports claim that this amount of fat eases the compounding and the feeding of diets; although reports are conflicting as to whether or not it improves the growth rate of chicks (Baldini & Rosenberg, 1957;Donaldson, 1962), it has been shown that properly balanced rations containing levels of animal fat up to 33.8% support normal growth in chicks (Donaldson, Combs, Romoser & Supplee, 1957).There is, however, some controversy as to how the quality of the tallow used can be characterized. In the past this has frequently been done by the determination of free fatty acids and peroxide value, with rejection of materials that gave values above certain limits. In a previous paper (Lea, Parr, L'Estrange & Carpenter, 1964) it was pointedout that thereseemed to be noevidence that free fattyacidsas such were harmful, and that the 'peroxide' values permitted were only a small fraction of those that had been demonstrated in laboratory experiments to be harmful. It was shown (Lea et al. 1964) that beef fat oxidized to a peroxide value of 93 pmoles/g, which is a level much higher than is permitted in commercial practice, and incorporated at a level of 5 yo * Paper no. I: Br.