Why is product purity checked in a bioprocess?

Correct answer: To ensure a safe and usable product

The direct answer is B, to ensure a safe and usable product. Product purity testing is a quality-control step in which the final or intermediate material is examined for unwanted substances, contamination and unacceptable variation. Step by step: a bioprocess may contain cells, proteins, media components, residual chemicals or other by-products; a purity test detects whether such impurities remain; the result is compared with required quality and safety limits; only material meeting the requirements should be accepted for its intended use. Option B is correct because a sufficiently pure product is more likely to be safe, effective and suitable for use. Option A, changing colour, is not the purpose of purity testing; colour may sometimes be an observation, but it does not define purity by itself. Option C, enlarging the fermenter, concerns equipment scale, not product quality. Option D, boiling the medium, is a processing operation and may even damage some products; it is not the reason for testing purity. Purity is only one part of quality control, but it is an essential one. Memory cue: purity testing asks ‘what unwanted material is left?’