what material can we use as an indicator of acid?in other words,what material can we use to test a food to see if acid is present?

what is the relationship between the concentration of acid in a liquid and the amount of carbon dioxide produced?

You can use litmus paper to test for acidity if the food is in liquid form.

If you have stated the question properly (and completely) there is no relationship between acidity and the amount of CO2 produced. It takes another material besides acid to make CO2 (unless we are talking about carbonic acid, H2CO3).