Optimal substrate feeding policy for fed-batch cultures of S. cerevisiae expressing bifunctional fusion protein displaying amylolytic activities

Abstract

The genetically modified Saccharomyces cerevisiae strain (YPB-G) which secretes a bifunctional fusion protein that contains both the Bacillus subtilis α-amylase and the Aspergillus awamori glucoamylase activities was used for the direct conversion of starch into ethanol. Starch was added instantaneously to the reactor at various discrete time instants (pulse feeding), or at a constant flow rate in either equal or unequal sub-intervals (intermittent feeding). Experiments with intermittent feeding of starch yielded poor biomass and ethanol yields. Pulse experiments were initiated with starch concentrations of 0, 10, 20, 40 and 50 g/l, and then single, two or three feedings were made. Optimal feeding policy was found to depend heavily on initial conditions. Ethanol yields increased from 0.335 to 0.499 (g ethanol)/(g substrate) upon decreasing the initial starch concentration from 50 to 10 g/l and increasing the number of low starch containing pulses. Starch degradation rates were slower and fermentation times were longer for experiments initiated with minimal amounts (0 and 10 g/l) of starch.