Question

Potato are sliced, dried and then rehydrated. Corresponding water activity during adsorption will be ________ than that during desorption. 
 

• A. Higher
• B. Lower
• C. Equal
• D. Not related

Hint:

In food moisture isotherms, Desorption (drying) always holds onto water more tightly than Adsorption (wetting). Consequently, at any given moisture level: \[ a_{w, \text{adsorption}} < a_{w, \text{desorption}} \]

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A moisture sorption isotherm describes the relationship between water activity ($a_w$) and equilibrium moisture content of a food at a constant temperature.
The path of drying (desorption) is often different from the path of rehydration (adsorption), a phenomenon known as hysteresis.
Detailed Explanation:
Moisture sorption hysteresis refers to the divergence between the adsorption and desorption isotherms.
- At a given moisture content, the desorption curve always lies above the adsorption curve.
- This means that for a specific moisture content, the corresponding water activity during adsorption (rehydration) is lower than the water activity during desorption (drying).
- Theories explaining this include the "ink-bottle" capillary theory, where pores empty differently than they fill, and macromolecular conformation changes during drying that reduce available binding sites upon rehydration.
Therefore, rehydrated potato slices will exhibit a lower water activity ($a_w$) at a given moisture content compared to the same slices during the drying process.

Step 2: Final Answer:
Due to hysteresis, the water activity during adsorption is lower than that during desorption.