I need to make a model of the digestive system. It would be really nice to actually show it in action, such as by breaking down bread into glucose or something similar. Is this feasible with a small budget and non-dangerous chemicals? If so, how? This should be as similar too the real digestive system as possible.
Amylase would be worth a try. The enzyme breaks down bread (starch), and you can buy it relatively cheaply online or probably at a chemist. Amylase is secreted in your saliva and in your stomach, so it would be most realistic if it was added in your model at those two points.
Generally speaking and as suggested by Rory, mechanical force probably plays a role in most instances (crustless bread may be different):
I was hoping to post more about this but the websites of one of the institutes involved in developing the model gut doesn't provide much accessible information on first read.
Maybe this still gives you something of a starting point: