Subtopic: Water activity, sorption and desorption isotherms
Question Statement:
A fat rich food product remains most stable in the water activity (a_w) range of
Options:
(A) a_w < 0.1
(B) 0.1 < a_w < 0.2
(C) 0.3 < a_w < 0.4
(D) 0.5 < a_w < 0.6
Official Answer Key:
C
Step-by-Step Solution & Explanation
Food stability against deterioration reactions plotted versus water activity shows a characteristic 'stability minimum' close to the monolayer moisture value, which for most fat-rich foods falls in the a_w ≈ 0.3–0.4 band. Below this range (very low a_w), lipid (fat) oxidation actually *accelerates* because the protective monolayer of bound water is lost; above it, microbial and enzymatic/browning reactions accelerate. The minimum overall deterioration rate — and hence greatest stability — for a fat-rich product is therefore in the 0.3–0.4 range. Answer: (C).
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