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While the chemists have not yet been able to isolate and identify the various vitamins they have succeeded in demonstrating many of the properties of these substances and it is the knowledge of these properties that has enabled us to produce concentrates and conduct tests. Another practical consideration involved in this matter of properties lies in the effect of cooking and commercial methods of food preparation, for not only must we learn where the vitamin resides but how to prevent injury or destruction in our utilization of the source.

The properties of the vitamins may therefore be grouped under two heads: first chemical properties and second physiological properties.

I. Chemical Properties of Vitamin "A"

a. This dietary factor's presence in butter fat and egg yolk fat indicates its solubility in the fat and it would naturally follow that the fat solvents would suffice to remove it with the fats when food sources are treated with such a reagent. Experience has shown however that while ether extraction applied to butter or egg yolk removes the vitamin with the fat this process fails when it is applied to vegetable sources such as cotton seed, corn germ, spinach, lettuce, etc. Neither does the cold or hot press method of oil extraction liberate the vitamin with the oil. Recent experiments by Osborne and Mendel, to which we have previously referred, have shown that preliminary treatment of vegetable sources with alcohol seems to loosen the bond between the source and the vitamin and that when this binding is once loosened subsequent ether extraction will take the vitamin out.

That the binding is not difficult to break is shown by the fact that when vegetables are eaten as a source of vitamin the body is able to separate the complex. It is further evident that the body does separate this complex and stores it in animal fat from the experiments with cow feeds and feeding. Milk for example is rich or poor in vitamin according to the supply of the latter in the food given to the cow. The only logical conclusion to be drawn from this observation is that the cow does not synthesize this factor but splits it off from the food source and then, since it is fat soluble, is able to mobilize it in the butter fat of the milk or to a more limited extent in the body fat. This observation as to the dependence of milk content upon food has been confirmed in the case of nursing mothers and suggests the need of especial attention to the diet of the mother during the lactating period.

b. It has been generally assumed that the "A" vitamin is comparatively stable to heat. Sherman, MacLeod and Kramer state that "dry heating at a temperature of 100°C. with free access of air, only very slowly destroyed fat soluble vitamin." Osborne and Mendel reported that butter fat treated with steam for two hours and a half did not appear to have lost its value as a source of this vitamin. Drummond's earlier work with fish oils and whale oils seemed to confirm this conclusion. Sherman and his co-workers cited above put it this way: "The results thus far obtained emphasize the importance of taking full account of the time as well as the temperature of heating, and of the initial concentration of the vitamin in the food, as well as of the opportunity for previous storage of the vitamin by the test animal."

More recent work by Steenbock and his co-workers in America shows that these earlier results are incorrect in the case of butter fat and that twelve hours exposure of butter fat to 100°C. may, under certain conditions, destroy the efficiency of that substance as a source of the vitamin. Drummond and other English workers have confirmed Steenbock in later experiments. Their work has shown that the presence or absence of oxygen is a factor, which may determine the extent of destruction of the vitamin. Heat alone is of very limited effect but when sources are heated in the presence of oxygen destruction of the A vitamin may be very rapid. Drummond attributes the absence of the A vitamin in lard to the oxidation that takes place in the commercial rendering of this product. We must conclude therefore that while the vitamin may be destroyed by continuous exposure to a temperature of 100°C. the effect is largely determined by the nature of the process and the way the vitamin is held in the source. Cooking of vegetables therefore will not as a rule result in appreciable destruction of this factor.

c. The process of hydrogenation used in hardening fats appears to completely destroy the vitamin, hence the many lard substitutes now in use must in general be considered "A" vitamin-free regardless of the content of "A" in the fats from which they are derived unless they have been made by blending instead of hydrogenation.

d. Acids and alkalies have apparently little effect on this particular vitamin.

It may be well to state here however that owing to variability in behavior with variation in conditions it is dangerous to draw too general conclusions and until a given source has actually been investigated under specific cooking conditions one should not rely too strongly on analogies based on comparative experiments. This statement applies to all vitamins and presents one of the live subjects of investigation for the cooking schools and the food factories.

e. Little has been learned further about the chemistry of this substance. Butter fat, nitrogen free and phosphorus free is shown to carry the vitamin and it is therefore assumed that the vitamin lacks these elements. It has been claimed that it may be removed from butter fat by prolonged extraction with water but this has not been confirmed by more recent experimenters. Steenbock was the first to call attention to the association of the A vitamin with yellow pigment in plant and animal sources. Butter, egg yolk, carrots, yellow corn contain it while white corn and white roots are less rich in this vitamin. This observation suggested the chemical relation between the vitamin and carotin. It has however been shown by Palmer and others that carotin is not vitamin A. This association of the pigment with the vitamin is therefore apparently a coincidence and this clue has failed as yet to throw light on the chemical nature of vitamin A.

Tags: Vitamins