Agriculture Reference
In-Depth Information
can be found on the grape, their presence in wine is dictated by their ability to sur-
vive the relatively harsh conditions of fermentation. Whilst fermentation commenc-
es with the indigenous yeast species, it is
Saccharomycecs cerevisiae
that completes
the transformation of carbohydrate to ethanol. The evolution of yeast species during
fermentation occurs in a reasonably predictable manner. Initial fermentative activ-
ity is mainly due to yeasts belonging to the genera
Candida
,
Pichia
, and
Kloeckera
Hanseniapora
(particularly
Kloeckera apiculata
, or its perfect form
Hanseniaspora
uvarum
) which may grow to levels of around 106-107 cfu/mL. However, as fer-
mentation proceeds and the alcohol content of the wine increases, these yeasts be-
gin to die and are replaced by strains of
Saccharomyces cerevisiae
which possess
greater ethanol tolerance with cell concentrations for this organisms typically over
108 cfu/mL (Fleet
2003
) during fermentation.
The growth of non-
Saccharomyces
yeasts will affect the chemical composition
of the juice, and thus influence the final sensory quality of the wine. Firstly, these
yeasts produce flavour and aroma compounds as a direct result of their metabolic
activity. Secondly, the changes to the chemical composition of the juice as a result
of their growth will influence the growth and metabolism of the succeeding gen-
erations of
S. cerevisiae
. For example, strains of
Kloeckera apiculata
can produce
large quantities of acetic acid and ethyl acetate which impact the sensorial qualities
of the wine and inhibit growth of
Saccharomyces
. Additionally, the growth of the
non-
Saccharomyces
yeasts will reduce the amount of nutrients (carbohydrates, vita-
mins, trace elements etc) available for growth of Saccharomyces thereby adversely
impacting growth of the desirable species.
Temperature Requirements for Growth and Fermentation
Management
The growth and biomass production of yeasts during the initial stages of alcoholic
fermentation is reliant upon the typical parameters that beset growth of any mi-
croorganism, i.e. adequate nutrient supply, suitable pH and optimum temperature.
Frequently deficiencies of nutrients can be overcome somewhat by the addition of
fermentation aids such as diammonium phosphate, yeast derived cell membranes
and vitamins where specific food regulations permit the use of these supplements.
In regards to winemaking decisions, the most easily controlled parameter that exerts
a significant impact upon the growth and outcomes of fermentation is the regulation
of temperature.
Saccharomyces cerevisiae
has a wide range of temperature in which
it actively metabolises thus the manipulation of temperature during fermentation
is important to enhance the final sensory qualities of the finished wine. Low tem-
peratures of 10-15 °C support good growth on the non-Saccharomyces yeasts and
growth of these organisms will increase the amounts of aroma-active compounds
such as esters, higher alcohols and acetic acid. Temperature fermentations around
15 to 18 °C are favoured for the fermentation of white juices to preserve the more
delicate varietal characters in these wines. During the fermentation of red grapes,
