Contributions of Grape Phenols to Oxygen Absorption and Browning of Wines

¹ Department of Viticulture and Enology, University of California, Davis.

The reaction of oxygen with wine in its normal acidic condition was in proportion to the total phenol content if the wine samples differed considerably in phenol content, but the reaction was fairly slow and did not reach an end-point in a convenient length of time even at an elevated temperature. Maximum oxygen consumption by wines made very alkaline was rapid, and reproducible values were obtained with a Warburg technique. These values were of the order of 200-600 cc O₂/l for red table wines and 50-100 cc O₂/l for white table wines. The oxygen consumption per unit phenol of wines or different purified phenol fractions under alkaline conditions was not constant, indicating differences related to the relative content of specific phenolic substances. All phenolic fractions tested did, however, absorb considerable oxygen, and white wines took up relatively more oxygen per unit phenol than did red wine.

Studies of both enzymatic and non-enzymatic browning indicated that the catechin fraction was most important in browning. The catechin fraction's oxidation products appear to be capable of accounting for the color of most white wines, whether very light or a dark amber. An analytical technique using the vanillin versus Folin-Ciocalteu assays has also been proposed as a criterion of a wine's tendency to brown.

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