Saliva contains an enzyme called amylase--which begins the process of starch digestion in the mouth. Starch is a polysaccharide consisting entirely of monomers known as glucose sugars. Starch is an energy-- storage polysaccharide and when it is hydrolyzed to glucose, its energy will be available for use by cells. In the mouth, amylase converts starch into a dissaccharide known as malt sugar, consisting of two glucose molecules. Later on, starch will be completely digested in the small intestine to single glucose monomers.
The enzyme amylase is a protein that acts as a catalyst which speeds up the digestion of starch. In order to work well, enzymes require specific conditions including pH and temperature within a specific range. If the environment of an enzyme is not optimal, the enzyme may lose its functional shape in a process called denaturation and it will be unable to catalyze a chemical reaction at all, or not as well as if it were in an optimal environment.
When saliva is added to various test tubes each with a different pH, along with starch, the purpose of the test is to find out at which pH the enzyme amylase (in saliva) works best.
Fehling's solution can be used as a test for the presence of monosaccharides like glucose or even the dissacharide maltose. The test tube containing starch, saliva and blue Fehling's solution will be heated and if glucose is present, a red copper oxide will form as a precipitate in the bottom of the test tube.
Benedict's solution which is blue, can be used to indicate the presence of sugar. Each test tube will have a different pH value, saliva and starch will be added and each will be heated. If glucose is present, the solution will change color in a specific order depending on the amount of glucose in the test tube. The color green (indicates a small amount of sugar), a little more is the color yellow, followed by orange and finally brick-red (indicating a large amount of sugar).
Either of these chemical indicators can be used in this experiment. By the presence of a red precipitate (Fehling's solution) or a color change (Benedict's solution) one could demonstrate at which pH enzyme amylase works best.