Question

In what direction will the water activity of a liquid food change (higher, lower, or virtually unaltered) if the following changes occur, keeping other factors constant? In each case give an explanation.

1. Addition of sodium chloride

2. Addition of native starch (granules)

3. Heating the food with starch and cooling again to the original temperature without loss of water

4. Enzymatic hydrolysis of the protein present

5. Emulsifying oil into it

6. Freezing part of the water

Answer 1

(1) Lowers the water activity.

NaCl (aq.) -----------> Na⁺ (aq.) + Cl ^- (aq.)

When NaCl is added to water it gets dissociated completely into its ions.The ions are stabilised by the hydration due to polar nature of water molecules. Hence molefraction water is appeared as if it was decreased which results in lowering of activity of water.

(2) Lowers the activity of water.

Similar to the (1) When starch is added to water, intermolecular hydrogen bonds can form inbetween water and starch molecules. Hence activity of water is decreased.

(3) Lowers the activity of water.

While heating starch, certain amount of water has been absorbed by starch to get cooked. Hence molefraction of water has been decreased and thus its activity.

(4) Lowers the activity of water

Enzymatic hydrolysis of protein consumes certain amount of water and hence decrease them mole fraction of water.

(5) Virtually unaltered.

Emulsifying oil can not change the amount of water.