In a 20 day incubation experiments very little of the starch had
been removed but starch removal increased in the 25-day and 30-day
incubations. Yeast has two types of α-glucoamylases (EC:3.2.1.3) [20]
but metabolism of starch only became apparent after most of the other
useable organic carbon sources had begun to run out but yeast would
not have been able to make full use of starch because it lacks a full
range of amylases such as β-glucoamylases. This is an interesting result
because gas production experiments showed that CO2 production
ceased after 7 days and cultures were dormant for a long time before
starch breakdown commenced after 20 days incubation. When the
experiments were started with 10 ml yeast inoculum about 95% of
total sugar and 70% protein were removed after a 25 day incubation
but yeast produced large amounts of CO2 only during the first few
days of the incubation (Figures 5 and 6). CO2 gas production was high
after 2 or 3 days but then rapidly declined to zero due to the rapid
digestion of easily metabolized organic compounds. Mobilization of
starch only became apparent after more than 20 days incubation but
it was interesting that little or no CO2 was produced as a result of this
late mobilization of starch. Glucose would have been the breakdown
product of starch hydrolysis by the α-glucoamylases but no significant
amounts of CO2 was being produced by conversion of pyruvic acid into
alcohol and CO2.