A Spectrophotometric Study of the Changes inPeroxide Value of Spray-Dried Whole Milk Powder during Storage

Of the fatty acids present in milk (1), oleic and linoleic acids, being unsaturated, are probably most concerned in the oxidative changes that result in the production of oxidized flavor.It is thought by some that phospholipid material usually associated with the butterfat may also play an important rSle in oxidative changes.Butterfat displays a certain initial resistance to oxidation.It then passes through a period during which there is little or no absorption of oxygen.This period is termed "the period of induction" and when it is over, the fat will absorb oxygen at an increasingly rapid rate.During .theinduction period combination with oxygen is slow and the resulting deterioration in flavor is usually slight.The phenomenon of the induction period is considered to be due to the action of traces of protective substances known as antiokidants or inhibitors, which delay oxidation and prevent the reaction between oxygen and the fat from attaining its full velocity.Natural fats contain traces of non-fatty compounds.These substances lengthen the induction period but in time lose their protective action and are themselves oxidized to a non-antioxygenic form.A perio0 of active oxygen absorption follows the period of induction.Linoleic acid has an extremely rapid rate of oxygen absorption but there is only 3-4 per cent of it present in butterfat.The oleic acid oxygen absorption rate is decidedly slower than the linoleic acid, but butterfat contains 25-40 per cent of oleic acid.The process of fat deterioration resulting from oxidation is not well understood, but it is thought that several reactions occur simultaneously.The peroxides formed are an intermediate product of fat oxidation and are by no means the end product of the reaction.Further products of fat oxidation are thought to be aldehydes, ketones, keto hydroxy compounds and epihydrin aldehyde.Glycerol disappears practically as fast as it is formed, being oxidized immediately on formation.