In brewing science, which enzymatic strategy is specifically practiced to reduce the buttery off-flavour (diacetyl) in beer, thereby improving flavour stability and overall sensory quality?

Introduction:
Beer can develop a buttery or butterscotch off flavour primarily due to diacetyl, a vicinal diketone formed as a by-product of yeast metabolism. Sensory thresholds for diacetyl are low, so even small amounts can mask hop aroma and malt nuances. Breweries therefore employ targeted biochemical and process controls to drive diacetyl down. One direct strategy is enzymatic reduction using diacetyl reductase.

Given Data / Assumptions:

• Off flavour in question: diacetyl, perceived as buttery.
• Goal: reduce diacetyl concentration post-fermentation or during maturation.
• Candidate enzymes listed include unrelated hydrolases and nucleases.

Concept / Approach:
Diacetyl is chemically reduced to acetoin and then to 2,3-butanediol, both of which have much higher sensory thresholds and neutral flavour. Diacetyl reductase catalyzes this reduction in the presence of suitable cofactors, lowering the active carbonyl content that drives buttery notes.

Step-by-Step Solution:

Verification / Alternative check:
Brewers also rely on yeast rest at warm temperatures to encourage endogenous yeast enzymes to reduce diacetyl. However, when process time is constrained or yeast activity is insufficient, adding or leveraging diacetyl reductase is a direct biochemical solution.

Why Other Options Are Wrong:

• Hesperidinase: Targets citrus flavonoid hesperidin, used in juice debittering, not beer diacetyl.
• RNase: Degrades RNA; irrelevant to carbonyl off flavours.
• Invertase: Hydrolyzes sucrose to glucose and fructose; no effect on diacetyl removal.

Common Pitfalls:
Assuming general filtration or pasteurization removes diacetyl is incorrect; diacetyl is small and heat stable. Over-aeration at this stage may cause oxidation, worsening flavour stability. Always manage oxygen exposure and confirm diacetyl via sensory or chemical tests.

Final Answer:
diacetyl reductase