Anybody got info on manipulating pH and water activity in foods?

I'm doing a lab project on meat and need to change the pH and water activity to desired levels.



1. I can use ascorbic acid to decrease the pH but how do I know how much to add to decrease to a specific value eg. pH 4.5?



2. In terms of water activity, I was told to use either starch or whey. But again, to decrease to a value of 0.9 and 0.8, how much do I need to put?



Thanks! :)Anybody got info on manipulating pH and water activity in foods?
use the Pearson's chi square test