Sugar and Fermentation Industries Questions and Answers

Answer: A

Correct answer: a mould, most often Aspergillus niger

Almost all commercial citric acid comes from submerged culture of Aspergillus niger on a sugar substrate, under conditions of deliberate metal-ion deficiency that make the mould excrete citrate rather than metabolise it. The process displaced extraction from citrus fruit entirely, being far cheaper and independent of the harvest.

Answer: B

The first two are correct. The third is exactly the difficulty, since oxygen transfer per unit volume falls as the vessel grows and the power input must rise sharply to compensate. Vessel geometry is chosen with this in mind.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the geometry. Oxygen is sparingly soluble in water and must be transferred continuously from bubbles to broth, and that transfer depends on the interfacial area per unit volume, which falls as the vessel grows because volume rises faster than surface. Larger fermenters therefore need more agitation power and higher air rates per unit volume, and oxygen transfer rather than biology sets the practical limit on size.

Answer: D

Correct answer: a contaminant would outgrow the culture

A fermentation medium is a rich growth substrate at a comfortable temperature, so any organism entering it will grow - and a contaminant with a shorter generation time will overtake the production strain and consume the substrate for nothing. The whole vessel is then lost. Sterilising the medium, the air and the vessel is therefore a precondition rather than a precaution.

Answer: A

Correct answer: True

A fermentation broth is mostly water containing a low concentration of product mixed with cells, unconsumed substrate and metabolites, so recovering a pure product from it takes several separation steps - and for a high-value product such as an antibiotic the downstream train commonly accounts for more than half the total cost. That is why product concentration in the broth matters as much as yield.

Answer: B

The first two are correct and explain why batch persists. The third is wrong, since a continuous fermenter typically runs at a lower product concentration in exchange for a higher productivity. Volumetric productivity is the figure it improves.