Chemistry of Wine

Abstract

Wines are alcoholic drinks obtained from the fermentation of grapes. Their composition is determined by the composition of the grape, which depends on genetic characters, vine growing conditions, and grape ripeness at harvest, and by wine-making practices, which involve a series of successive operations, the sequence of which varies considerably depending on the wine type. In white wine making, the first step (usually after crushing) is pressing. This separates the solid parts (i.e., skins, seeds, and eventually stems) from the juice, and the juice is then fermented separately. In red wine making, fermentation is achieved on the whole must obtained after crushing, and pressing is performed only after the maceration phase. Maceration enables extraction of constituents present in the skins and seeds into the fermenting must, including not only the red pigments, but also tannins, volatile compounds and aroma precursors, and plant cell wall polysaccharides. Changes taking place during winemaking involve both biochemical and chemical processes. The former result from yeast and bacterial metabolism during alcoholic and malolactic fermentation, and from the action of various enzymes originating from grape, yeast, and other microorganisms, or added as process aids. Biochemical processes take place mostly in the early stages of the process, while chemical reactions continue throughout wine aging. Composition changes due to yeast and bacterial primary metabolism, the chemistry of aroma compounds, and the chemistry of phenolic compounds in wine are reviewed.