58
of the yeast flora (Pretorius et al. 1999). Other yeast genera present on grapes
include Candida, Torulopsis, Zygosaccharomyces, Rhodotorula, Pichia,
Metschnikowia and Cryptococcus (Barnett et al. 1972; Rosini et al. 1982; Moore
et al. 1988). There are several other yeasts present in the vineyard which affect wine
fermentation: Hansenula, Sporobolomyces and Kluyveromyces (Davenport 1974).
Different species of yeast react differently towards different levels of ethanol.
S. cerevisiae has the highest tolerance level towards ethanol among all the other
yeasts involved in wine fermentation (Gao and Fleet 1988; Dittrich 1991).
Temperature has also been found to affect ethanol tolerance among yeasts. Higher
temperatures tend to decrease ethanol tolerance among yeasts. Some of the nonSaccharomyces yeasts present in the vineyard have also been found to produce ethanol: Torulaspora delbrueckii and Saccharomycodes ludwigii (Ciani and Maccarelli
1998). There are several other metabolites being produced by yeasts apart from
ethanol. Other metabolites include octanoic and decanoic acids and medium-chain
fatty acids which act by interfering with plasma integrity (Alexandre and Charpentier
1998; Bisson 1999).
4.4.6.2 Manufacturing Process
A schematic representation of wine manufacturing is shown in Fig. 4.6. Winemaking
starts with the collection and crushing of grapes. For white wines, the grape juice
can be separated away from the skins and clarified using any of the methods: cold
settling, filtration or centrifugation. The juice is then transferred to a big barrel or
fermentation tank and the alcoholic fermentation is carried out by yeasts primitive
to the juice or by inoculation of a selected S. cerevisiae strain as starter culture. The
fermentation is generally carried out for about 1 to 2 weeks at temperatures around
10–18 °C. Upon consumption of available sugars such as glucose and fructose present in grape juice, the wine is then considered “dry” and separated away from the
yeast and grape lees (sediment).
Red wine production is slightly different than white wines. The crushed skins of
grapes are left for fermentation to allow colour extraction. Like white wines, the
alcoholic fermentation takes place either through the action of indigenous yeasts or
by direct inoculation of a starter culture. During the fermentation the grape material
tends to float to the top of the vat forming a “cap”. To better enable extraction of red
pigments and to influence wine flavour, winemakers typically punch down the cap
or pump juice from the bottom over the cap. After a suitable period of time, the wine
is separated from the grape skins and the fermentation is completed in another vessel. As described for white wines, the red wine is now “dry” and devoid of the main
juice sugars.
Once the wine has been taken through the alcoholic and, if desired, the malolactic fermentation, it is often stored in tanks or barrels to allow flavour development.
The residence time for storage is primarily determined by the style of wine and
winemaker choice. Often white wines are not stored for long periods of time, while
R. Kumari et al.
of the yeast flora (Pretorius et al. 1999). Other yeast genera present on grapes
include Candida, Torulopsis, Zygosaccharomyces, Rhodotorula, Pichia,
Metschnikowia and Cryptococcus (Barnett et al. 1972; Rosini et al. 1982; Moore
et al. 1988). There are several other yeasts present in the vineyard which affect wine
fermentation: Hansenula, Sporobolomyces and Kluyveromyces (Davenport 1974).
Different species of yeast react differently towards different levels of ethanol.
S. cerevisiae has the highest tolerance level towards ethanol among all the other
yeasts involved in wine fermentation (Gao and Fleet 1988; Dittrich 1991).
Temperature has also been found to affect ethanol tolerance among yeasts. Higher
temperatures tend to decrease ethanol tolerance among yeasts. Some of the nonSaccharomyces yeasts present in the vineyard have also been found to produce ethanol: Torulaspora delbrueckii and Saccharomycodes ludwigii (Ciani and Maccarelli
1998). There are several other metabolites being produced by yeasts apart from
ethanol. Other metabolites include octanoic and decanoic acids and medium-chain
fatty acids which act by interfering with plasma integrity (Alexandre and Charpentier
1998; Bisson 1999).
4.4.6.2 Manufacturing Process
A schematic representation of wine manufacturing is shown in Fig. 4.6. Winemaking
starts with the collection and crushing of grapes. For white wines, the grape juice
can be separated away from the skins and clarified using any of the methods: cold
settling, filtration or centrifugation. The juice is then transferred to a big barrel or
fermentation tank and the alcoholic fermentation is carried out by yeasts primitive
to the juice or by inoculation of a selected S. cerevisiae strain as starter culture. The
fermentation is generally carried out for about 1 to 2 weeks at temperatures around
10–18 °C. Upon consumption of available sugars such as glucose and fructose present in grape juice, the wine is then considered “dry” and separated away from the
yeast and grape lees (sediment).
Red wine production is slightly different than white wines. The crushed skins of
grapes are left for fermentation to allow colour extraction. Like white wines, the
alcoholic fermentation takes place either through the action of indigenous yeasts or
by direct inoculation of a starter culture. During the fermentation the grape material
tends to float to the top of the vat forming a “cap”. To better enable extraction of red
pigments and to influence wine flavour, winemakers typically punch down the cap
or pump juice from the bottom over the cap. After a suitable period of time, the wine
is separated from the grape skins and the fermentation is completed in another vessel. As described for white wines, the red wine is now “dry” and devoid of the main
juice sugars.
Once the wine has been taken through the alcoholic and, if desired, the malolactic fermentation, it is often stored in tanks or barrels to allow flavour development.
The residence time for storage is primarily determined by the style of wine and
winemaker choice. Often white wines are not stored for long periods of time, while
R. Kumari et al.