Question

The ratio of the vapour pressure on the aqueous solution to that of pure water at the same temperature is known as:

• A. Moisture content
• B. Rehydration
• C. Unbound water
• D. Water activity

Hint:

Water activity ($a_w$) controls microbial growth, not moisture content. Always remember: $a_w = \frac{P}{P_0}$.

Answer 1

The Correct Option is D

Concept: Water activity is a very important concept in food science, especially in determining the shelf life, microbial stability, and quality of food products. It is defined as the ratio of the vapour pressure of water present in a food system to the vapour pressure of pure water at the same temperature.

Step 1: Understanding vapour pressure.
Vapour pressure is the pressure exerted by water vapour in equilibrium with its liquid phase. In pure water, this pressure depends only on temperature. However, in food systems, dissolved solutes (like salts, sugars) reduce the vapour pressure.

Step 2: Defining water activity mathematically.
Water activity ($a_w$) is given by: \[ a_w = \frac{P}{P_0} \] where:
• $P$ = vapour pressure of water in the solution (food system)
• $P_0$ = vapour pressure of pure water at the same temperature

Step 3: Interpreting the definition.
This ratio indicates how “available” water is for microbial growth and chemical reactions. Lower $a_w$ means less available water.

Step 4: Evaluating options.
• Moisture content — Total amount of water present, not its availability.
• Rehydration — Process of adding water back to a dried product.
• Unbound water — Refers to free water but not defined as a ratio.
• Water activity — Correct definition as given in the question. Final Conclusion:
The ratio described is the definition of water activity. Hence, the correct answer is option (4).