You are trying to find out if the amount of digestive juices affects how fast a piece of bread is digested. This means that the amount of digestive juices should be different in both setups. Say, we use 10ml of digestive juice on bread 1 and 20ml of digestive juice on bread 2. We can then compare how much of the bread have digested in x hours' time.

Everything else should remain constant (i.e. amount of bread, duration, location) so that we know for sure that our variable (the amount of digestive juice) is what resulted in the speed at which the bread is digested.