The present investigation was undertaken to see as a preliminary research the stability of ASA in aqueous solution under the various conditions. The following results were obtained.
It was found that ASA was remarkably stable below pH 3 and decreased rapidly above pH 8, especially was almost decomposed at pH 10 or 11. It seemed most reasonable to conclude, on the other hand, that DHA is most stable at about pH 3.
Though the presence of oxygen accelerated the autoxidation of ASA, it seemed that the effect of pH on the stability of ASA was stronger than that of oxygen.
Photoxidation of ASA to DHA by light illumination was recognized stronger in alkalic solutions than in acidic solutions. But DHA was little decomposed in every pH solution.
It was found that the activity of ASA oxidase was remarkably recognized between 35゜ and 45゜ and the maximum value was obtained at 40゜. The optimum pH of ASA oxidase was 5.5 and the activity was little recognized below pH 3 or above pH 8. The photoxidation of ASA, moreover, may be independent of the presence of ASA oxidase.
The oxidation of ASA oxidase was considerably protected in the final concentration of 0.5% sodium chloride. This concentration does not interfere with the daily cooking of diets. It seems, therefore, that addition of sodium chloride to diets is useful for the protection of the oxidation of ASA.