The major wine producing countries have laws that define wine as the product of the alcoholic fermentation by S. cerevisiae of the fresh grape juices. There is, of course, one exception,—Botrytis cinera, a plant pathogen, during their fermentation first send their hyphae into the skin of the grapes causing water loss, resulting in shriveled grapes with unusually high sugar concentration.
This is next allowed to ferment by S. cerevisiae and the so—called French Pourifure noble or the German Edelfaule is formed. The differences between the wines depend on the variety of the grape, the local soil and climate, the properties of the yeasts and the details of the production method.
The fermentation is traditionally carried out in open vats but fermenters giving a greater control of conditions are now widely used, especially for white wines. The yeast primarily responsible for alcoholic fermentation is Saccharomyces cerevisiae. The sugar rich grape juice and other nutrients permit rapid yeast growth which soon results in anaerobic conditions. Ethanol is produced more and more during fermentation until alcohol concentration of 10-12% (v/v) is reached.
Depending on how much sugars remains unutilized, a sweet, medium or dry wine results. After the first phase of fermentation, wine is transferred to wooden casks for maturation and storage. Microbiological activity continues at a reduced level during maturation to form blander lactic acid for rising of pH. This reaction is very desirable where the grape juice is strongly acidic.
The production of cider from apple juices and perry from the juices of pears follow the same techniques and organism for fermentation. In some tropical countries like India, palm wines, made from the sap of a variety of palms, are of considerable nutritional and social importance. The Mexican equivalent of palm wine pulque, is also very important
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