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Fed-batch production of baker's yeast using millet (Pennisetum typhoides) flour hydrolysate as the carbon source
Authors:A O Ejiofor  Y Chisti  M Moo-Young
Affiliation:(1) Department of Chemical engineering, University of Waterloo, N2L 3G1 Waterloo, Ontario, Canada;(2) Present address: Department of Applied Microbiology & Brewing, Nnamdi Azikiwe University, PMB 5025, Awka, Anambra State, Nigeria
Abstract: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-agr-d-glucanohydrolase (EC 3.2.1.1) in the presence of 100 ppm Ca2+, at 80°C, pH 6.1–6.3, for 1 h. The liquefied mash was saccharified with 1,4-agr-d-glucan glucohydrolase (EC 3.2.1.3) at 55°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 fedbatch 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.Nomenclature Cox Dissolved oxygen concentration (mg L–1) - CPR Carbon dioxide production rate (mmol h–1) - Cs0 Glucose concentration in the feed (g L–1) - Cs Substrate concentration in the fermenter (g L–1) - Cs.crit Critical substrate concentration (g L–1) - E Ethanol concentration (g L–1) - Fs Substrate flow rate (g h–1) - i Sample number (–) - Ke Constant in Equation 6 (g L–1) - Ko Constant in Equation 7 (mg L–1) - Ks Constant in Equation 5 (g L–1) - m Specific maintenance term (h–1) - OUR Oxygen up-take rate (mmol h–1) - qox Specific oxygen up-take rate (h–1) - qox.max Maximum specific oxygen up-take rate (h–1) - qp Specific product formation rate (h–1) - qs Specific substrate up-take rate (g g–1 h–1) - qs.max Maximum specific substrate up-take rate (g g–1 h–1) - RQ Respiratory quotient (–) - S Total substrate in the fermenter at timet (g) - S0 Substrate mass fraction in the feed (g g–1) - t Fermentation time (h) - V Instantaneous volume of the broth in the fermenter (L) - V0 Starting volume in the fermenter (L) - Vsi Volume of samplei (L) - x Biomass concentration in the fermenter (g L–1) - X0 Total amount of initial biomass (g) - Xt Total amount of biomass at timet (g) - Yp/s Product yield coefficient on substrate (–) - Yx/e Biomass yield coefficient on ethanol (–) - Yx/s Biomass yield coefficient on substrate (–)Greek letters agr Moles of carbon per mole of yeast (–) - beta Moles of hydrogen atom per mole of yeast (–) - gamma Moles of oxygen atom per mole of yeast (–) - delta Moles of nitrogen atom per mole of yeast (–) - mgr Specific growth rate (h–1) - mgrcrit Critical specific growth rate (h–1) - mgrE Specific ethanol up-take rate (h–1) - mgrmax.E Maximum specific ethanol up-take rate (h–1)
Keywords:Millet  Pennisetum typhoides  liquefaction  saccharification  baker's yeast  Saccharomyces cerevisiae  fermentation
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