What is the purpose of the starch hydrolysis test and how is it read?

• A. To detect amylase production; iodine reveals clear zones around colonies where starch was degraded.
• B. To detect gelatinase activity.
• C. To detect proteolysis of casein.
• D. To detect lactose fermentation.

Answer: (A)

It checks whether a microbe makes amylase to hydrolyze starch in the surrounding medium. If amylase is produced, the starch around the colony is broken down. After incubation, you flood the plate with iodine; iodine binds to intact starch and turns dark. Where starch has been degraded by amylase, there is a clear, colorless halo because no starch remains for the iodine to bind. A visible clear zone around the colony therefore indicates amylase activity (positive result), while a uniform dark area means no starch hydrolysis occurred (negative result).

Starch is a polymer of glucose, and amylase cleaves the glycosidic bonds to release smaller sugars, which iodine cannot bind, producing the halo. The other options refer to different tests: gelatinase tests for gelatin breakdown, casein proteolysis shows as clearing in milk, and lactose fermentation is judged by acid or gas production rather than iodine staining.