Chemistry of alcoholic beverages

Growth and Harvest
After blooming and fruit formation, the grape berry continues to grow until the middle or the end of August, but remains green and hard. The acid content is high, while the sugar content is low. As ripening proceeds, the berry color changes to yellow-green or blue-red. The sugar content rises abruptly, while both the acid and water contents drop. The harvest (picking the berry clusters from the vines) is performed as nearly as possible when the grape is fully ripe, about the middle of September until the end of November, or it may be delayed until the grapes are overripe. In the USA and Europe, machines are being increasingly used for this very laborious harvesting, e. g., grape harvesters. However, they cannot sort the grapes according to the degree of ripeness. Terms which relate to the time of harvest include “vorlese”, early harvest, “normallese”, normal harvest, and “spaetlese”, late harvest. The latter term, when applied to German wines, identifies excellent, top quality wines. Particularly well-developed grapes of the best cultivars from selected locations are picked separately and processed into a wine called “Auslese”. When the grapes are left on the vine stock, they become overripe and dry – this provides the raisins or dried berries for “Beerenauslese”, “Trockenbeerenauslese”, or “Ausbruch” wine (fortified wine). In some districts, such as Tyrol and Trentino, the grapes are spread on straw or on reed mats to obtain shrivelled berries – this provides the so-called straw wines, Grapes that are botrytised (a state of “dry rot” caused by the mold Botrytis cynerea, the noble rot) have a high sugar content and a must of superior quality, consequently also producing a superior, fortified wine.
Fermentation.
Wine fermentation may occur spontaneously due to the presence of various desirable wine yeasts and wild yeasts found on the surface of grapes. Fermentation can also be conducted after must pasteurization by inoculation of the must with a pure culture of a selected strain of wine yeast. Wild yeasts include Saccharomyces apiculatus and exiguus, while the pure selected yeasts are derived from Saccharomyces cerevisiae var. ellipsoides or pastorianus. The pure wine yeast possesses various desirable fermentation properties. High fermenting strains are used to give high alcohol wines (up to 145 g/l) and those which are resistant to tannin and high alcohol levels are used in red wine fermentation. Other types of yeast are “sulfite yeast” with little sensitivity to sulfurous acid (sulfur dioxide solutions), “cold fermentation yeasts”, which are active at low temperatures and, finally, special yeasts for sparkling wines, which are able to form a dense, coarse-grained cloudiness that is readily removed from the wine. The desired yeasts (5–10 g of dried yeast per hectoliter of must) are added to must held in fermenters (vats made of oak, or chromiumnickel steel tanks lined with glass, enamel or plastic). The must is then fermented slowly for up to 21 days below 20 ◦C for white wines or 20–24 ◦C for red wines. The course of fermentation is influenced by sulfurous acid: 100 mg/l SO2 delay the start of fermentation by 3 days, 200 mg/l SO2 by 3 weeks.