31. The main reason for the production of antibiotics in fed-batch reactors is

A. the presence of precursors is often toxic to the cells
B. higher yields when cells enter the stationary phase
C. higher yields when cell growth slows
D. all of the above

32. Fed-batch reactors are used to produce vinegar because

A. it can maintain low ethanol concentrations
B. it can maintain low acetic acid concentrations
C. acetic acid bacteria tend to ferment at high ethanol concentrations
D. all of the above

33. When a culture of fermenting yeast becomes metabolically uncoupled, ethanol

A. becomes a secondary metabolite
B. production becomes growth associated
C. production becomes non-growth associated
D. becomes a growth nutrient

34. Mixing profiles closest to plug flow are observed in

A. continuous packed bed reactor
B. stirred tank reactor with biomass recycles
C. continuous fluidized bed bioreactor
D. all of these

35. On a glucose medium, the growth of an organism is characterized by the following Monod model and stoichiometric parameters μ_m = 0.3 h^-1, Ks =0.05 g.l^-1 and Yxs = 0.3 g.g^-1 When this organism is grown in a 2 liter continuous on a medium containing 10 g.l^-1 of glucose added at 1 l.h^-1, the steady-state concentration of biomass in the reactor will be

A. 0 g.l^-1
B. 0.4 g.l^-1
C. 0.5 g.l^-1
D. 10 g.l^-1