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--
d-glucanohydrolase (EC 3.2.1.1) in the presence of 100 ppm Ca
2+, at 80°C, pH 6.1–6.3, for 1 h. The liquefied mash was saccharified with 1,4--
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
C
ox
Dissolved oxygen concentration (mg L
–1)
-
CPR
Carbon dioxide production rate (mmol h
–1)
-
C
s0
Glucose concentration in the feed (g L
–1)
-
C
s
Substrate concentration in the fermenter (g L
–1)
-
C
s.crit
Critical substrate concentration (g L
–1)
-
E
Ethanol concentration (g L
–1)
-
F
s
Substrate flow rate (g h
–1)
-
i
Sample number (–)
-
K
e
Constant in Equation 6 (g L
–1)
-
K
o
Constant in Equation 7 (mg L
–1)
-
K
s
Constant in Equation 5 (g L
–1)
-
m
Specific maintenance term (h
–1)
-
OUR
Oxygen up-take rate (mmol h
–1)
-
q
ox
Specific oxygen up-take rate (h
–1)
-
q
ox.max
Maximum specific oxygen up-take rate (h
–1)
-
q
p
Specific product formation rate (h
–1)
-
q
s
Specific substrate up-take rate (g g
–1 h
–1)
-
q
s.max
Maximum specific substrate up-take rate (g g
–1 h
–1)
-
RQ
Respiratory quotient (–)
-
S
Total substrate in the fermenter at time
t (g)
-
S
0
Substrate mass fraction in the feed (g g
–1)
-
t
Fermentation time (h)
-
V
Instantaneous volume of the broth in the fermenter (L)
-
V
0
Starting volume in the fermenter (L)
-
V
si
Volume of sample
i (L)
-
x
Biomass concentration in the fermenter (g L
–1)
-
X
0
Total amount of initial biomass (g)
-
X
t
Total amount of biomass at time
t (g)
-
Y
p/s
Product yield coefficient on substrate (–)
-
Y
x/e
Biomass yield coefficient on ethanol (–)
-
Y
x/s
Biomass yield coefficient on substrate (–)
Greek letters
Moles of carbon per mole of yeast (–)
-
Moles of hydrogen atom per mole of yeast (–)
-
Moles of oxygen atom per mole of yeast (–)
-
Moles of nitrogen atom per mole of yeast (–)
-
Specific growth rate (h
–1)
-
crit
Critical specific growth rate (h
–1)
-
E
Specific ethanol up-take rate (h
–1)
-
max.E
Maximum specific ethanol up-take rate (h
–1)
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