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The role of yeast in winemaking is the most important element that distinguishes wine from grape juice. In the absence of oxygen, yeast converts the sugars of wine grapes into alcohol and carbon dioxide through the process of fermentation.〔Jeff Cox ''"From Vines to Wines: The Complete Guide to Growing Grapes and Making Your Own Wine"'' pgs 133–136 Storey Publishing 1999 ISBN 1-58017-105-2〕 The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness.〔D. Bird ''"Understanding Wine Technology"'' pg 67–73 DBQA Publishing 2005 ISBN 1-891267-91-4〕 Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients, and the wine has not yet reached dryness this is considered a stuck fermentation.〔J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pgs 778–780 Oxford University Press 2006 ISBN 0198609906〕 The most common yeast associated with winemaking is ''Saccharomyces cerevisiae'' which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulfur dioxide as well as its ability to thrive in normal wine pH between 2.8 and 4. Despite its widespread use which often includes deliberate inoculation from cultured stock, ''S.cerevisiae'' is rarely the only yeast species involved in a fermentation. Grapes brought in from harvest are usually teeming with a variety of "wild yeast" from the ''Kloeckera'' and ''Candida'' genera. These yeasts often begin the fermentation process almost as soon as the grapes are picked when the weight of the clusters in the harvest bins begin to crush the grapes, releasing the sugar-rich must.〔K. Fugelsang, C. Edwards ''Wine Microbiology'' Second Edition pgs 3–28 Springer Science and Business Media , New York (2010) ISBN 0387333495〕 While additions of sulfur dioxide (often added at the crusher) may limit some of the wild yeast activities, these yeasts will usually die out once the alcohol level reaches about 5% due to the toxicity of alcohol on the yeast cells physiology while the more alcohol tolerant ''Saccharomyces'' species take over. In addition to ''S. cerevisiae'', Saccharomyces bayanus is a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such as ports and varieties such as Zinfandel and Syrah harvested at high Brix sugar levels. Another common yeast involved in wine production is ''Brettanomyces'' whose presence in a wine may be viewed by different winemakers as either a wine fault or in limited quantities as an added note of complexity.〔B. Zoecklein, K. Fugelsang, B. Gump, F. Nury ''Wine Analysis and Production'' pgs 281–290 Kluwer Academic Publishers, New York (1999) ISBN 0834217015〕 ==History== For most of the history of wine, winemakers did not know the mechanism that somehow converted sugary grape juice into alcoholic wine. They could observe the fermentation process which was often described as "boiling", "seething" or the wine being "troubled" due to release of carbon dioxide that gave the wine a frothy, bubbling appearance. This history is preserved in the etymology of the word "yeast" itself which essentially means "to boil".〔〔Douglas Harper ''"(Yeast )"'' Online Etymology Dictionary Accessed: May 31st, 2012〕 In the mid-19th century, the French scientist Louis Pasteur was tasked by the French government to study what made some wines spoil. His work, which would later lead to Pasteur being considered one of the "Fathers of Microbiology", would uncover the connection between microscopic yeast cells and the process of the fermentation. It was Pasteur who discovered that yeast converted sugars in the must into alcohol and carbon dioxide, though the exact mechanisms of how the yeast would accomplish this task was not discovered till the 20th century with the Embden–Meyerhof–Parnas pathway.〔J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pgs 267 & 508 Oxford University Press 2006 ISBN 0198609906〕 The yeast species commonly known as ''Saccharomyces cerevisiae'' was first identified in late 19th century enology text as ''Saccharomyces ellipsoideus'' due to the elliptical (as opposed to circular) shape of the cells. Throughout the 20th century, more than 700 different strains of ''Saccharomyces cerevisiae'' were identified. The difference between the vast majority of these strains are mostly minor, though individual winemakers will develop a preference for particular strains when making certain wines or working with particular grape varieties. Some of these difference include the "vigor" or speed of fermentation, temperature tolerance, the production of volatile sulfur compounds (such as hydrogen sulfide) and other compounds that may influence the aroma of the wine.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yeast in winemaking」の詳細全文を読む スポンサード リンク
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