Discussion
Which of these is NOT true of water activity?
Answer: A
Correct answer: It is a percentage of the dry matter
Water activity is not a percentage of anything. It is a dimensionless ratio of vapour pressures, running from 0 to 1, and it measures availability rather than amount.
The other three statements are correct. It does range between zero and one; it does govern microbial growth, with most bacteria halted below 0.90 and moulds below about 0.70; and at equilibrium it equals the relative humidity of the surrounding air expressed as a fraction, which is what makes it measurable with a humidity sensor in a sealed headspace. Percentage of dry matter describes dry basis moisture content, an entirely different quantity.
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