Fed-batch production of baker's yeast using millet (Pennisetum typhoides) flour hydrolysate as the carbon source

A fermentation medium based on millet (Pennisetum typhoides) flour hydrolysate and a four-phase feeding strategy for fed-batch production of baker's yeast, Saccharomyces cerevisiae, are presented, Millet flour was prepared by dry-milling and sieving of whole grain, A 25% (w/v) flour mash was liquefied with a thermostable 1,4-alpha-D-glucanohydrolase (EC 3.2.1.1) in the presence of 100 ppm Ca2+, at 80 degrees C, pH 6.1-6.3, for 1 h, The liquefied mash was saccharified with 1,4-alpha-D-glucan glucohydrolase (EC 3.2.1.3) at 55 degrees C, pH 5.5, for 2 h, An average of 75% of the flour was hydrolysed and about 82% of the hydrolysate was glucose, The feeding profile, which was based on a model with desired specific growth rate range of 0.18-0.23 h(-1), biomass yield coefficient of 0.5 g g(-1) and feed substrate concentration of 200 g L(-1), was implemented manually using the millet flour hydrolysate in test experiments and glucose feed in control experiments, The fermentation off-gas was analyzed on-line by mass spectrometry for the calculation of carbon dioxide production rate, oxygen up-take rate and the respiratory quotient. Off-line determination of biomass, ethanol and glucose were done, respectively, by dry weight, gas chromatography and spectrophotometry. Cell mass concentrations of 49.9-51.9 g L(-1) were achieved in all experiments within 27 h of which the last 15 h were in the fed-batch mode, The average biomass yields for the millet flour and glucose media were 0.48 and 0.49 g g(-1), respectively, No significant differences were observed between the dough-leavening activities of the products of the test and the control media and a commercial preparation of instant active dry yeast, Millet flour hydrolysate was established to be a satisfactory low cost replacement for glucose in the production of baking quality yeast.