1 Biochemistry of Alcoholic Fermentation 19
4.Nutrient deficiencies: The lack of some nutrients in the grape juice can cause
serious problems during fermentation. Nitrogen, vitamins, minerals, etc. may
be deficient in grape juice. For that reason yeast activators are usually added
in wineries. The standard activators are made up of ammonium salts (phos-
phate and/or sulphate), thiamine and their application is certainly very useful.
However, as previously mentioned, the nitrogen dose must be selected carefully,
taking into account the initial concentration of easily assimilated nitrogen and
the potential alcoholic content of the grape juice. Adding nitrogen will be more
effective if it is done two or more times and is combined with aeration. The
first dose should be added when fermentation starts, the second in the middle of
quasi-stationary phase and finally the thirdat the end of this stage. During recent
years a new generation of yeast activators has appeared on the market. These
new activators are prepared from yeast and provide several other interesting sub-
stances such as sterols, UFA, minerals, panthothenic acid, etc.These activators
are very useful during the latter stages of fermentation.5.Presence of anti-fungal substances: Sometimes grape juice can contain pesticide
residues which can seriously affect alcoholic fermentation. To avoid this, the
vineyard’s treatments must be closely inspected.6.Presence of medium chain fatty acids: The presence of MCFA can decrease yeast
viability and even stop alcoholic fermentation. This problem is more prevalent
in white winemaking because fermentation is usually carried out at low temper-
atures and without any aeration. Yeast hulls have been very useful for avoiding
this problem. Yeast hulls adsorb MCFA from the media and provide sterols and
UFA to the yeasts. Yeast hulls can be used as preventives (20 g/hl) or as curatives
(40–50 g/hl) of stuck and sluggish fermentations.7.Antagonism between microorganisms: The different microorganisms present in
grape juice compete for nutrients.Saccharomyces cerevisiaedevelopment is
usually encouraged through LSA inoculation and by adding sulphur dioxide.
However, sometimes autochthonous yeast or even bacteria can grow and cause
organoleptic deviations and even stuck and sluggish fermentations (Edwards
et al. 1999; Gerland 2000; Gao et al. 2002). This problem is greater when the
pH of grape juice is high because the antiseptic effect of sulphur dioxide is less
effective (Beech et al. 1979).All these causes are possible and can prevent alcoholic fermentation from devel-
oping correctly. However, it is usually a synergistic combination of some of these
causes which brings about stuck and sluggish fermentation. If these causes are elim-
inated, then stuck and sluggish fermentations are almost non-existent. Nevertheless,
if a fermentation tank presents problems, winemakers must act as soon as possi-
ble. Abundant aeration and adding yeast hulls may solve the problem. Finally, if
fermentation stops, the yeast must be reinoculated. The choice of the yeast and
the way it is preadapted to ethanol are the key to the success of the inoculum.
The use of commercial yeast with a high resistance to ethanol and the use of the
classical protocols for adapting yeast to the champagnization process are highly
recommended.