Yeast culture is prepared in the laboratory and
propagated in a series of fermenters, each about 10 times
larger than the previous one. The feed is inoculated with
about 10% by volume of yeast (Saccharomyces cerevisiae)
inoculum. This is an anaerobic process carried out under
controlled conditions of temperature and pH wherein
reducing sugars are broken down to ethyl alcohol and
carbon dioxide. The reaction is exothermic. To maintain
the temperature between 25 and 32 1C plate heat exchangers
are used; alternatively some units spray cooling water
on the fermenter walls. Fermentation can be carried out in
either batch or continuous mode (CPCB, 2003). Fermentation
time for batch operation is typically 24–36 h with an
efficiency of about 95%. Continuous operation, involving
higher sugar concentration and an osmotolerant variety of
yeast, is faster (16–24 h fermentation time) but the
efficiency is marginally lower (T.R. Sreekrishnan, pers.
comm.). The resulting broth contains 6–8% alcohol. The
sludge (mainly yeast cells) is separated by settling and