When can washout occur in a bioprocess?

Correct answer: When cells are washed out too much in continuous culture

The direct answer is A: cells can be washed out when too many leave a continuous culture. In a continuous bioreactor, fresh medium enters while an equal volume of culture leaves. The dilution rate is the flow rate divided by the culture volume, often written as the reciprocal of the residence time. If the dilution rate becomes greater than the cells’ effective growth rate, cells cannot reproduce quickly enough to replace the cells leaving. Their concentration falls greatly, and the culture may be lost; this is washout. Option A is correct because it describes this loss of cells from the system. Option B is wrong because giving a fermenter a clean name has no biological meaning. Option C is wrong because washout does not guarantee product purity. Option D is wrong because freezing the medium is a different event and usually stops growth. The exam cue is: continuous culture plus dilution rate higher than growth rate leads to washout.