New Insights into the Chemistry Involved in Aroma Development during Wine Bottle Aging: Slow Redox Processes and Chemical Equilibrium Shifts

This chapter summarizes some recent findings about the occurrence and formation of oxidation and reduction related off-odors during wine aging in the bottle. Concerning oxidation, different wines are able to produce-per the same amount of consumed oxygen-, quite different amounts of oxygen-related carbonyls. Those diverse carbonyl formation rates are related to wine metal content (iron, copper and manganese) and also to the wine content of precursor amino acids. With a new analytical strategy for measuring bonded and free forms of carbonyls, it is demonstrated that part of the oxidation-related aldehydes are already present in the wine in the form of bisulfite-complexes, so that part of the increase in carbonyl levels noted upon wine aging in the bottle, is just the simple consequence of the reversion of the bisulfite complex upon oxidation of sulfite to sulfate. A similar analytical strategy for measuring free and bonded forms of hydrogen sulfide and small mercaptans has revealed that the storage of wine under strict anaerobic conditions creates eventually the conditions lowering wine redox potential- under which odorless mercaptan species are transformed into odor-active forms. Only in some whites and ros wines, a slight "de novo" formation of mercaptans has been observed, suggesting that in reds, formation of mercaptans is more the consequence of the reduction of some unknown form of metal cation complexed to the previously existent mercaptan than the direct reduction of sulfur itself.