Question Bank • QB_DFE_EXP_018 MCQ • 1 Mark

Section 7: Dairy and Food Engineering

Heat and Mass Transfer

Subtopic: Water activity, sorption and desorption isotherms

Question Statement:

Most food spoilage bacteria cease to proliferate when the water activity ($a_w$) of the food is reduced below approximately:

Options:

  • (A) 0.91
  • (B) 0.80
  • (C) 0.60
  • (D) 0.30

Answer Key:

A

Step-by-Step Solution & Explanation

Minimum water activity requirements for microbial growth: • Most pathogenic and spoilage bacteria: $a_w < 0.90 - 0.91$ (with *Staphylococcus aureus* capable of growing down to $0.85$ under aerobic conditions). • Yeasts: $a_w < 0.88$ (osmophilic yeasts down to $0.60$). • Molds: $a_w < 0.80$ (xerophilic molds down to $0.65$).
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