The catalytic oxidation of ascorbic acid in the presence of tea infusions

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Abstract
Tea infusions contained constituents capable of catalysing the aerobic oxidation of ascorbic acid in the presence of cyanide. Evidence was produced which suggested that these constituents were closely associated with, or chemically related to, the tannin fraction of the tea extracts. A commercial sample of gallic acid showed some catalytic activity while d-catechin, tannic acid, catechol, quinol and pyrogallol were almost inactive. The rate of the catalytic reaction was studied in the presence of varying concs. of tea and of ascorbic acid and at different pH values; it decreased (ca. 95%) between pH 7 and 5. Milk, egg albumin, cysteine and, to some extent, dimedone inhibited the catalytic action, while NaCl and KI accelerated it, Na pyrophosphate, glycine, 8-hydroxyquinoline and Na urate did not appreciably influence the velocity of oxidation. Benzoyl and acetyl derivatives obtained from the extracts, although not themselves active, became so on hydrolysis with alkali at 100[degree]. The alkaline treatment of tea infusion at 100[degree] under anaerobic conditions increased their catalytic activity. The active constituents of tea infusions were slowly decomposed in alkaline medium by air. The active constituents formed inactive complexes with boric acid. The possible mechanisms involved in this catalytic oxidation of ascorbic acid were discussed. The presence or formation of catalytic substances of this nature in vegetables and fruits may have to be considered as an additional factor in the stability of vit. C in vegetables and fruits.

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