A possible mechanism 
(Illustrated below)
 
Good!!
This is, indeed, a reasonable hypothesis, which explains all the observed data concerning the pH changes:
The food degradation and vacuole movement along the cell takes about 12 minutes. pH increase is first noted 9 min after vacuole formation (see right panel). Indeed some proteins  have such a limited half-life.

Does this hypothesis also explain the changes in the size of the food vacuoles?

Examine the next possibility