CBE 654 : BIOREACTOR ENGINEERING SEM 2 SESSION 2013/14ASSIGNMENT 2 : QUESTION

(SUBMISSION DATELINE 4pm FRIDAY 20/6/2014)

(a) State the assumptions that need to be fulfilled by a batch fermentation system in order to be able to use the following relationship:

μmax .t=( S0+K s+( X0

Y )S0+( X 0

Y ) ) ln( XX 0)−( K s

S0+( X0

Y )) ln(Y .S0+X0−XY .S0

)

where μmax is the maximum specific growth rate of the microorganism

t is time

S0 is the initial concentration of the limiting substrate

K s is the the value of the limiting substrate concentration which results

in a specific growth rate of half the maximum value

Y is the yield

X is the concentration of biomass at time t

X 0 is the initial concentration of the biomass

(1 Mark)

(b) A type of yeast for use as dairy cow food supplement is grown in a batch fermenter of working volume 200 m3 on a medium based on cheese whey. The limiting substrate in the medium is a carbohydrate whose initial concentration in the cheese whey is 15 kgm-3

.

If this substrate is available in abundance ( ie not limiting), the yeast has an average doubling time of 2 hours, and this changes to 4 hours when this carbohydrate is available only in a concentration of 5 kgm-3 .

Average production of yeast biomass is 1kg (dry wt.) for every 2kg of this carbohydrate used.

If this fermentation is innoculated with an innoculum whose size gives an initial concentration of yeast in the production medium of 0.5kgm-3 and the fermenter needs 4 hours to empty, to clean, to fill and to sterilize, and to cool down, get an estimate of the fermentation time needed to give the maximum productivity.

(3 Marks)