Question

In the Salivary-amylase test for starch, you have observed a blue color when tested with solution A. What is A? Why doesn’t cellulose produce blue color with A?

Answer 1

Salivary amylase enzyme secreted by the salivary gland and it is also called a ptyalin. We can test the presence of starch in saliva by starch test and iodine test. Salivary- amylase enzymes help to digest the starch in the mouth about 30% and rest of the starch about $70\%$ digest in the small intestine. There are three salivary glands associated with the secretion of the amylase enzyme.

Complete answer:
Salivary amylase is an enzyme that catalyzes the hydrolysis of starch into simple sugars in the mouth. Here, we observed the blue color change when salivary amylase tested with solution A. Solution A represents the Iodine solution which detects the presence of starch in saliva.
Starch is a polymer of glucose and occurs in many plants as storage foods. It is a polyglucan monosaccharide and is formed as an end product of photosynthesis. It is stored inside chloroplast or special leucoplasts called amyloplasts. Starch consists of two components amylose and amylopectin:
a. Amylose: It constitutes 15-$20\%$ of starch. It is more soluble in water. Amylose gives blue color with a dilute iodine solution . Amylose is in the form of a continuous straight but helically arranged chain where each turn contains about six glucose units.
b. Amylopectin: It constitutes about 80-$85\%$ of starch. It is insoluble in water but can absorb and swell up. It gives a reddish yellow color with iodine solution. Amylopectin is a highly branched structure formed by the polymerization of alpha-D glucose units.
Cellulose does not give the blue color because it is arranged in a close antiparallel fashion, unbranched and linearly arranged. It is a very stable insoluble compound.

Note:
Only the amylose responsible for the blue color change, not amylopectin. Salivary amylase production decreases with age. It also plays a role in protecting the oral cavity. It has a short active contact time with starch. There are three main classes of amylase:
Alpha – amylase
Beta – amylase
Gamma – amylase
Each of them acts on a different molecule of carbohydrate.