Observation of the Optical & Chemical Properties of Starch Granules

Starch granules in fresh and processed foods are routinely identified by a variety of chemical and microscopic methods. In the teaching laboratory, the use of cross-polarized light in a microscope (monocular or binocular) quickly demonstrates the presence of starch granules by their characteristic birefringence, often called a Maltese cross. In the present study, we first identified raw potato starch granules by their birefringence, followed by three overlapping chemical methods to confirm the presence (or absence) of starch. First, Lugol's iodine solution, a starch indicator, immediately stained the granules black. Second, amylase digested the small starch granules within 30–60 minutes. Third, glucose test strips demonstrated the presence of glucose that resulted from the enzymatic breakdown of starch granules by amylase. Fourth, we also confirmed that the starch granules were broken down by the enzyme amylase by observing the “digested” product under crossed polarization and noting the disappearance of the birefringent effect. Although the demonstration of starch has long been a staple in teaching laboratories, we have combined the optical and chemical demonstrations of starch for the same lab, with each serving as a confirmation for the other.