Saliva is slightly alkaline. Enzymes in saliva convert starch into glucose:
The stomach produces hydrochloric acid. It kills many harmful microorganisms that might have been swallowed along with the food. The enzymes in the stomach work best in acidic conditions – at a low pH.
After it has been in the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions - but the food is acidic after being in the stomach. Bile is a substance produced by the liver and stored in the gall bladder.
Bile is secreted into the small intestine where it has two effects: