Optimization of nutritional and
environmental factors
Of the various nitrogenous
compounds evaluated, ammonium
sulphate induced the maximum alcohol
level of 11.5% (v/v) exhibiting a
fermentation efficiency of 91% (Fig. 2).
This corroborates the earlier findings27
which have indicated that fermentations
are benefited by the addition of
ammonium salts. Of the various metal
salts supplemented, potassium di
hydrogen phosphate and MgSO4 produced
the high alcoholic yields of 11 and 10.7%
v/v, respectively revealing fermentation
efficiencies of 87 and 86% (Fig. 3). These
results are in agreement with the earlier
studies, which suggest that potassium and
magnesium are required for optimum
growth of yeast though growth and
fermentation were possible on
unsupplemented molasses28. Magnesium
and potassium are regarded as bulk
cations establishing the required ionic
environment of the yeast cell29.
Of the various amino acids, the
supplementation of proline brought out
the highest fermentation efficiency
producing the maximum alcohol content
of 10.8% (Fig. 4), whereas among various
exogenous vitamins, biotin proved to be
most favourable for the production of
highest alcohol yield of 11.2 % with a
fermentation efficiency of 89% (Fig. 5).
Yeasts have no known direct requirement
for amino acids. However, fermentation
requires amino acids as a catalyst in
synthesizing nitrogen into the free
ammonium state which is required by
yeasts. The yeast strain used in the
present study though produced highest
alcohol content of (10%) at temperatures
of 25 and 30oC showing 83% efficiency, it
also worked very well at elevated
temperatures of 35 and 40oC, where it
produced 9% alcohol with 75%
fermentation efficiency (Fig. 6). Most of
the yeast strains grow best at a
temperature less than 35oC, the species
capable of growing at a temperature more
than 37oC are very rare and are very
important for alcohol production as the
temperature of the fermentation tanks
exceeds 40oC during summers because of
the exothermic nature of fermentation
process30. Changes in membrane fluidity
of the mesophilic yeast lead to a retarded
or no growth higher temperatures. The
yeast strains able to adapt to these changes
are able to survive at higher temperatures
and are thermotolerant31.
Amongst various concentrations
of sugar in various batches used for wine
production, the batch with 10% sugar
revealed the highest fermentation
efficiency of 90% followed by 88%
observed in the batch with 15% initial
sugar concentration. The efficiency of
alcohol production decreased with an
increase in initial sugar concentration
probably due to increased osmotic
pressure of the medium or overloading
of the cells because of high concentration
of substrate. The maximum alcohol
concentration which could be achieved in
the study was 12% in the batch having
25% initial sugar concentration (Fig. 7).
High substrate concentrations inhibit the
growth of yeast cells as a result of high
osmotic pressure and low water activity
leading to the dehydration of the yeast
cells. On the other hand yeasts able to
adapt to these changes in osmotic pressure
are osmotolerant