Discussion
Statement I: Water activity and moisture content are interchangeable measures.
Statement II: Microbial growth is governed by the availability of water rather than by its total amount.
In the light of the above statements, choose the correct answer.
Answer: D
Correct answer: Statement I is incorrect but Statement II is correct
Statement II is sound: microorganisms need water that is thermodynamically available, and water activity \( a_w \) measures exactly that, being the ratio of the vapour pressure over the food to that over pure water.
Statement I contradicts it. Two products at the same moisture content can have quite different water activities, because binding to sugars, salts and the solid matrix removes water from availability. This is the whole basis of intermediate-moisture foods, which are microbiologically stable at moisture contents that would spoil an unformulated product. Most bacteria stop below \( a_w = 0.90 \) and moulds below about 0.70, thresholds expressed in activity because moisture content alone does not predict them.
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