Culture of Saccharomyces cerevisiae on hydrolyzed waste cassava starch for production of baking-quality yeast

Abstract

A fermentation medium based on waste cassava starch hydrolysate and a four-phase feeding strategy for a fed-batch culture of Baker's yeast Saccharomyces cerevisiae are presented. Cassava starch isolated from the wastewater produced in processing of cassava mash into gari was liquefied with a thermostable 1.4-α- d-glucanohydrolase (EC 3.2.1.1) in the presence of 100 ppm Ca 2+ at 80°C and pH 6.1–6.3 for one h. The liquefied material was saccharified with 1.4-α- d-glucan glucohydrolase (EC 3.2.1.3) at 55°C and pH 5.5 for two h. Over 98% of the starch was hydrolyzed; about 80.7% of the hydrolysate was glucose. The fermentation feeding profile which was based on a desired specific growth rare range of 0.18–0.23 h −1, a biomass yield coefficient of 0.5 g g −1, and a feed substrate concentration of 200 g l −1 was implemented manually using the cassava hydrolysate feed in test experiments and glucose feed in control experiments. The fermentation off-gas was analyzed on-line by mass spectrometry for the calculation of the oxygen uptake rate, the carbon dioxide evolution rate, and the respiratory quotient. Off-line determinations of biomass, ethanol, and glucose were done, respectively, by dry weight, gas chromatography, and spectrophotometry. Cell mass concentrations of 50–58 g l −1 were achieved in all experiments within 28 h of which the last 15 h were in the fed-batch mode. The average biomass yields for the cassava and glucose media were identical at 0.49 g g −1. No significant differences were observed between the leavening activities of the products of the test, the control media, and a commerical preparation of instant active dry yeast. Waste cassava starch hydrolysate was established as a suitable low cost replacement for glucose in the production of baking-quality yeast.