Sulfur dioxide and grapes

Recent research by Considine and Foyer has revealed unexpected effects of sulfur dioxide on the chemical composition of grapes, including phenolics, flavonoids, and anthocyanins. Just as plant leaves can assimilate atmospheric sulfur (sulfur dioxide, hydrogen sulfide), so, too, can grape berries. As little as 2–3 mg/l sulfur dioxide (applied to the berries after harvest) caused massive changes to gene expression and synthesis of phenolic acids and other plant metabolites. This has implications for sulfur-containing compounds including glutathione, other thiols (See mercaptans), and amino acids, and conjugated flavour precursors, as well as secondary effects on redox-sensitive wine qualities, through effects on primary fermentation. Possibilities include increased levels of glutathione and an altered balance of glutathione and conjugated precursors.