WINE CHEMISTRY

Chemical and Biochemical Aspects of Winemaking

Biochemistry of Alcoholic Fermentation

Alcoholic fermentation is the anaerobic transformation of sugars, mainly glucose and fructose, into ethanol and carbon dioxide. This process, which is carried out by yeast and also by some bacteria such as Zymomonas mobilis, can be summarized by this overall reaction.

C6H12O6 Hexoses →2 CH 3CH2OH Ethanol + 2CO2 Carbon dioxide

However, alcoholic fermentation is fortunately a much more complex process. At the same time as this overall reaction proceeds, a lot of other biochemical, chemical and physicochemical processes take place, making it possible to turn the grape juice into wine. Besides ethanol, several other compounds are produced through out alcoholic fermentation such as higher alcohols, esters, glycerol, succinic acid, diacetyl, acetoin and 2,3-butanediol. Simultaneously, some compounds of grape juice are also transformed by yeast metabolism. Without the production of these other substances, wine would have little organoleptic interest. Atthestartofthewinemakingprocess,severalspeciesofyeastmaybepresentin the grape juice. This biodiversity depends on several factors such as grape variety,the ripening stage at harvest,the antifungal treatments,the climatic conditions of the year, the development of grey rot or other fungal plagues and the viticultural practices (Sapis-Domerq 1980; Pretorius et al. 1999). However, other factors are also important. All contact of grapes and must during harvest, transport and, in particular winery operations significantly influence the final distribution of yeasts at the beginning of alcoholic fermentation (Constant´ıetal.1997;MortimerandPolsinelli1999). Different yeast species participate in spontaneous alcoholic fermentation even when sulphurdioxide is present (Constant´ıet al. 1998;Beltran et al. 2002).Usually Kloeckera, Hanseniaspora and Candida predominate in the early stages of alcoholic fermentation. Later, Pichia and Metschnikowia prevail in the middle stages. Finally, during the latter stages of fermentation, Saccharomyces cerevisiae is the predominantyeastbecauseofitsgreaterresistancetohighethanolconcentration(Fleet1993; Fleet and Heard 1993). Some other yeast, such as Torulaspora, Kluyveromyces, Schizosacchaomyces, Zygosaccharomyces and Brettanomyces may also be present during alcoholic fermentation and even in the wine itself, which may cause some organoleptic defects. (Peynaudand Domercq 1959; Rib´ereau-Gayonet al. 2000a). Evidently, the succession of these different yeast species throughout alcoholic fermentation influences the final composition of wine in a way that, depending on which yeasts have grown, may be positive in some cases or negative in others (Chatonnet et al. 1995; Rib´ereau-Gayon et al. 2000a). To prevent undesirable yeasts developing, wineries add sulphur dioxide to the grape juice and inoculate selected strains of dry yeasts (Saccharomyces cerevisiae). Sulphur dioxide has a drastic selective effect on yeast development. As Saccharomyces cerevisiae is more resistant to Sulphur dioxide than most other yeasts, using this additive favours its development(Lafon-Lafourcadeand Peynaud1974; Romano and Suzzi 1993). On the other hand, the inoculation of selected dry yeasts greatly increases the initial population of Saccharomyces cerevisiae. Nowadays, most wineries inoculate selected dry yeast in order to guarantee alcoholic fermentation without any deviation. However, other wineries, especially traditional wine cellars, continue to use spontaneous alcoholic fermentation because they believe it gives their wines greater complexity.