Issue link: http://winesandvines.uberflip.com/i/70679
WI ne MAKI n G The o-Diphenol Cascade—Red Wine's Defining Reaction HO R + OH + O2 elucidated the mechanism of oxidative po- lymerization in all its bizarre aspects.2 HO R He explained quite clearly why, contrary to all common sense, a young wine challenged with oxygen behaves homeopathically, increasing its reductive strength. He also showed why oxygen uptake is so strongly temperature-dependent, a critical morsel of cellar knowledge. The basics Grape skins are rich in a special type of phenols called flavonoids. Some of these are red-colored anthocyanins, but most are tannin building blocks that contain adjacent diphenols. When ortho-diphe- nols see oxygen, magic happens (see graphic above). OH + H2 O Without going into the details of Single- ton's explanation, the phenolic ring basi- cally goes Rambo, hungrily searching out a nearby molecule ("R") to attach to the ring in any unoccupied position. Choices include another diphenol, an anthocya- nins pigment, a sulfide or a protein frag- ment—usually from lees. Once this bond is formed, the diphenol is recreated and can react again. There's one important difference. Be- cause the R-substitution stabilizes its ion- ization, the phenolate is more stable, thus more favored, and its equilibrium pH shifts from 9.0 to 8.5. As a result, there is now one phenolate molecule in 50,000 instead of 100,000, so the reaction occurs twice as fast. In effect, the introduction of oxygen HO Where R= OH Anthocyanin H2 S- containing amino acids and peptides S, mercaptans has made the wine hungrier for oxygen. Instead of oxidation, we get increased re- ductive strength.3 For those who claim there is no scien- tific support for homeopathy, it's time to pay attention. Since the reactive diphenol is recre- ated and now reacts faster, the cascade repeats again and again, resulting in a complex of random linkages. The shape of the resulting polymer is thus depen- dent on the concentration of the various reactants and their relative affinity for the reaction. In effect, oxygen acts like a wire whisk, and the tannins, like egg whites, firm up into a rich, light structure simi- lar to a meringue. 56 Wines & Vines APRiL 201 1