In regard to the stability of the fat-soluble A toward heat and reagents, there has existed much difference of opinion. McCollum and Davis (15) observed that egg yolks which had been hard boiled were still a good source of this vitamin, and also that the melting of butter in a hot water funnel and its filtration during one or two hours, followed by solution in ether, filtration and subsequent evaporation of the ether did not appreciably lower the content of fat-soluble A. They, therefore, reported that the substance is fairly stable at high temperatures. In testing their butter fats and egg yolk fats, they employed in their feeding experiments 3 to 5 per cent of one or the other as the sole source of fat-soluble A, and judged from their findings that no extensive destruction could have taken place.

Osborne and Mendel (16) passed live steam through butter fat for two and a half hours, and found that 18 per cent of this fat in a diet served to support normal growth, when there was no other source of fat-soluble A. Since they used such very high fat intake they could have destroyed at least four-fifths of the substance and still had enough left to insure a successful growth experiment. Mendel has recently stated that butter fat heated for many hours to 96° C. would still suffice as a source of fat-soluble A when 0.25 gram per day was included in the diet of the rat. This may be roughly estimated to form about 2.5 per cent of the food mixture. Their data, therefore, support the view that fat-soluble A is a relatively stable substance at high temperatures.

Steenbock, Boutwell and Kent (17) took issue with the statements recorded in the preceding paragraph. They stated: "When butter fat is heated for 4 hours at 100°, so much of its vitamine may be destroyed that with our basal rations, using rats as the experimental animal, no demonstrable amounts of the vitamine remained." Steenbock and Boutwell (18) have, however, recently reported that plant tissues such as yellow maize, chard, carrots, sweet potatoes, and squash, may be autoclaved at fifteen pounds pressure, without causing any noticeable destruction of fat-soluble A.

295. Fat-Soluble A Is Easily Destroyed By Oxidation

Hopkins (19) and also Drummond (20) have shown that oxidation readily destroys the activity of fat-soluble A. It may, therefore, be accepted that the substance is stable toward high temperatures in the absence of oxygen.