random oxidation, also known as sporadic post-bottling oxidation, describes the premature browning that occurs in some white wines some months after bottling. The problem is common enough for some industry figures to refer to it as the ‘new cork taint’. Wines are protected against oxidation through the addition of sulfur dioxide at bottling but if the level of free sulfur dioxide falls too low, the wine is unprotected, and browning can occur. The main explanation is oxygen transfer through the cork, which seems to be highly variable. However, some scientists suspect that random oxidation may be caused by as yet poorly understood chemical reactions independent of the closure. It has been suggested that the addition of the antioxidant ascorbic acid just before bottling to keep white wines fresh may have the paradoxical effect of rendering the added sulfur dioxide less effective, and making some wines susceptible to oxidation. Worse still, when the level of free sulfur dioxide in the bottle becomes too low, ascorbic acid becomes a pro-oxidant, increasing the degree of oxidation. Another proposed cause is poor procedure or intermittent failure on the bottling line, allowing some wines to have much higher levels of dissolved oxygen from the outset (see total package oxygen). Random oxidation is mainly a problem with white wines: while oxygen ingress through the closure will certainly damage red wines, they are more resistant to oxidation because of their high phenolic content. Oxidation is also more likely to be spotted in white wines because of the dramatic colour change that accompanies it even though this change in colour is usually the last step in premature oxidation.
