The Starch Agar PlateTest or Amylase Agar Plate Test: Atlas page 76
The starch agar is supplied to you as a deep. Please melt, pour, cool, and streak a shared plate as described previously. Incubate the plate up-side-down in the incubator at 37 degrees C overnight. Then evaluate the plate or store it in the refrigerator until read.
EVALUATION: Obtain a bottle of Iodine Solution (Gram's Iodine is OK, but a stronger solution is better) and flood the plate (cover the entire surface of the agar just barely). The iodine will react with the starch in the agar and turn purple-brown-black. A completely purple-brown-black plate (except where the microbe is actually growing) is NEGATIVE for Starch Hydrolysis and Amylase production.
A POSITIVE STARCH HYDROLYSIS PLATE WILL SHOW CLEAR OR YELLOW AGAR "AROUND" THE MICROBIAL GROWTH AREA. It will appear as if the microbe has "sucked" the starch out of the agar... IF YOUR UNKNOWN PRODUCES THE ENZYME AMYLASE, IT WILL REMOVE THE STARCH FROM THE AGAR IN AN AREA SURROUNDING THE GROWTH STREAK... do not be confused by a "yellow" cleared area exactly where the microbial growth occurs - that clearing of the agar is NOT starch hydrolysis.
Use your Atlas on page 76-77 for further information on this test...