Chemical and nutritional changes in stored herring meal. 2.

In the first paper of this series (Lea, Parr & Carpenter, 1958) we reported the results of experiments on the effect of storage on the nutritive value of herring meal as a source of protein for chicks. Over a period of 24 months’ storage in air there was extensive oxidation of the lipids, but only a very small drop in ‘available’ lysine (lysine with a chemically reactive E-NH, group) and no significant fall in feeding value for chicks under the experimental conditions used. A further test has now been made with material stored for up to 12 months. The feeding trial was designed only to measure the value of proteins as supplements to diets under conditions in which lysine was the limiting amino-acid, At the same time, the opportunity has been taken to ascertain whether a meal in which extensive oxidation had occurred would be more or less likely to cause tainting of the meat and eggs than a fresh meal. The earlier experiments had shown a small but definite fall (9 yo in 12 months) in the ‘available’ lysine content of meal containing oil and stored in air, a fall that did not occur either with defatted meal exposed to air or with meal containing oil and stored in nitrogen. A reasonable hypothesis to explain this result would be that of a reaction between oxidation products of the oil, particularly carbonylic compounds (which could be either volatile fragments of the fatty-acid chains or non-volatile glycerides) and the 6-NH, groups of the lysine side-chains in the protein molecules. Further evidence for or against this view has been sought in several ways. In one approach propanal, known to be one of the volatile carbonylic oxidation products of fish oils, was caused to react with fish muscle in the ‘dry’ state and the effects on free amino nitrogen, available lysine and nutritive value were measured. Attempts were made also to measure the interaction between oxidized fat and protein by procedures less open to question than the usual determination of decrease in solvent-extractable fat. However, all of our experimentally stored and oxidized meals were still much superior as a source of protein, and specifically of lysine, to commercial meals that had been allowed to overheat badly during bulk storage. Another type of experiment was therefore made in which herring meal, either pre-oxidized or unoxidized, was heated in the absence of oxygen under conditions that might be expected to develop in the