Chemical and nutritional changes in stored herring meal

That the protein of foods can suffer loss of nutritive value as a result of overheating during processing is well known, and Carpenter, Ellinger, Munro & Rolfe (1957) have reported the effects of heating under various conditions on the nutritive value and available lysine of the protein of fish meal. It has been shown also that interaction between proteins and reducing sugars can occur during storage in the 'dry' state, even under such mild conditions as a few days at 37" and 70 yo atmospheric relative humidity (Lea & Hannan, I950), leading to a reduction in the availability of a number of aminoacids and particularly of lysine (Henry & Kon, 1950). Recently, a further possible cause of loss in the nutritive value of the protein of foods containing highly unsaturated lipids has been disclosed by the experiments of Tappel (1955) on 'model' systems containing linoleic acid or cod-liver oil, emulsified with protein and an oxidative catalyst (haemoglobin, haemin or cytochrome c). On shaking such a mixture in air at 37" for 1-2 days extensive interaction of the protein with the oxidizing fat occurred, leading to the formation of yellow-brown products and, ultimately, of insoluble dark brown copolymers of high oxygen and nitrogen content. The total amino-acids recoverable from the copolymers, even after hydrolysis with concentrated hydrochloric acid, was of the order of 16 yo less than expected from their nitrogen content, indicating an appreciable destruction of amino-acids. Such changes must be accompanied by a considerable fall in the nutritive value of the protein. The present investigation has been made to determine how far similar changes may occur during the storage of an actual feeding product. Herring meal has been chosen as the test material, since it is valued as a source of high-quality protein but also contains a considerable quantity of highly unsaturated fat. The oil in the meal was found to oxidize rapidly but, despite these changes, comparative feeding tests with chicks on meal stored for 24 months in air at 25" and in nitrogen at 20" showed no more serious reduction in the value of the meal as a source of supplementary protein than could be accounted for by the observed very slight (4%) fall in available lysine. After 12 months at 25" the loss of available lysine had risen to about 9 yo. Almquist (1956) and Meade (1956) have recently reported reductions in the percentage of extractable oil as a result of storage or of 'curing'. The present experiments,