Question

Water activity below which bacterial growth usually does NOT occur is

• A. 0.75
• B. 0.70
• C. 0.65
• D. 0.60

Hint:

Minimum $a_w$ limits: Most bacteria $\approx 0.91$, S. aureus $\approx 0.86$, Halophilic bacteria $\approx 0.75$, Osmophilic yeasts $\approx 0.60$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Microbial groups exhibit distinct minimum thermodynamic water activity ($a_w$) thresholds required for cellular metabolic growth.

Step 2: Detailed Explanation:
Minimum water activity thresholds for microbial proliferation are:
- Most normal foodborne bacteria: $a_w \approx 0.90\text{--}0.91$
- Staphylococcus aureus (aerobic): $a_w \approx 0.86$
- Halophilic bacteria: $a_w \approx 0.75$
Below $a_w = 0.75$, no known bacteria can proliferate (only xerophilic moulds down to $a_w \approx 0.65$ and osmophilic yeasts down to $a_w \approx 0.60$ can grow).

Step 3: Final Answer:
Therefore, water activity below which all bacterial growth ceases is 0.75.