Salivary amylase is a digestive enzyme that is produced in the salivary glands (primarily in the parotid glands). Enzymes are molecules that affect the rate of a reaction without being used up by the reaction. This particular enzyme promotes the breakdown of starches into simpler sugars which can be absorbed by the body.
Salivary amylase, like most other enzymes, is a protein. Its action is most effective at a pH of 6.5 - 7.0. Under conditions more acidic or basic than that, the protein folding changes, which causes the molecule to become less effective as an enzyme. Typically the pH in the stomach is around 1.0 to 2.0, which is extremely acidic. This level of acidity causes the salivary amylase's protein structure to denature and change shape. Consequently salivary amylase does not function once it enters the stomach.
Salivary amylase which is an enzyme that catalyzes the breakdown of starch into simple sugar works at an optimal ph of between 5.2-7.0. In the gastric juice, the ph of the stomach is closer to 1.5-1.6. The highly acidic ph will hinder the enzyme from forming enzyme-substrate complexes with starch due to the fact that its active site will lose its shape as it becomes denatured. Enzyme action can be visualized as a bell-shaped curve, where the high point is when the enzyme is functioning at the proper conditions of ph. If you go below or above that optimal value for ph, the enyzme's effectiveness decreases.