Yeast cells (Saccharomyces cerevisiae, CH1.64O0.52N0.16) were grown in sucrose g

Question

Yeast cells (Saccharomyces cerevisiae, CH1.64O0.52N0.16) were grown in sucrose growth substrate using ammonia (NH3) as the nitrogen source. Yeast cells contain 5% ash. The process is represented by the following general equation:

CwHxOyNz + a*O2 + b*HgOhNi c*CHON + d*CO2 + e*H2O

or Substrate + oxygen + nitrogen source biomass + carbon dioxide + water

The respiratory quotient, RQ (CO2 generated /O2 consumed) for this test is 1.5.

(i) Using elemental mass balance, solve the stoichiometric unknowns

(ii) Calculate the yeast cell yield or mass of biomass generated per unit mass of substrate.

(iii) Calculate the maximum cell yield that could be generated based on the given substrates. Table 1.

Yeast Cell Growth Test Growth Substrate S

Sucrose (C12H22O11) 4.0

Saccharomyces cerevisiae 4.12

Explanation / Answer

Substrate + Ammonia + Oxygen ------> Biomass + Carbon dioxide + Water

C6H12O6 + NH3 + O2 ------------> CH1.79O0.56N0.17 + CO2 + H2O + C2H6O

Molecular weight of substrate = 180

Molecular weight of biomass = 12 x 1 + 1 x 1.79 + 16 x 0.56 + 14 x 0.17 = 25.13

Degree of reduction of substrate S = 4 x 1 + 1 x 12 + (-2 x 6) = 4

Degree of reduction of biomass B = 4 x 1 + 1 x 1.79 + (-2 x 0.56) + (-3 x 0.17) = 4.16

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