Stability of Vitamin C,and Absence of Ascorbic Acid Oxidase in Citrous Fruits and Milk

Abstract

It has been shown recently that in certain plants such as Hubbard Squash and Summer Squash, a powerful enzyme is present which oxidizes rapidly vitamin C 1 2 although these plants contain almost none of this vitamin. Similar is the case with cucumbers. Statements have appeared more recently that “plant tissues which contain ascorbic acid apparently also contain an ascorbic acid oxidizing enzyme,’ and that the partial destruction of vitamin C in cow's milk is also brought about by ascorbic acid oxidase. The present writer could not find ascorbic acid oxidase in mammalian tissue, 3 and Roe and Barnum 4 found in human and rat blood cells and plasma an enzyme which reduces the reversibly oxidized form of ascorbic acid, thus having just the opposite effect from the ascorbic acid oxidase of plant tissues.

The aim of the present work was to find out whether the vitamin C content of juices of citrous fruits, which are excellent sources of the vitamin, is exposed to the destructive action of the ascorbic acid oxidase. In other words, whether these juices contain the oxidase.

Table I shows that the vitamin C content of the juices of oranges, tangerines, lemons and grapefruits is not much affected when kept for 5 hours at 38° as compared to control samples which were placed in a refrigerator at 6° for the same amount of time. The slight decrease in reducing power does not necessarily indicate an irreversible oxidation. The results show, however, that there is no ascorbic oxidase in these fruit juices and that the vitamin keeps fairly well even at 38°. It is well known that the pH of these fruit juices is a stabilizing factor of vitamin C. Samples of orange juice which have been adjusted to pH 6.5 with ammonium hydroxide or CaCO₃, however, kept equally well and no evidence of enzyme action could be noticed at this pH either.