Digestion of Starch Granules from Maize, Potato and Wheat by Larvae of the the Yellow Mealworm, Tenebrio molitor and the Mexican Bean Weevil, Zabrotes subfasciatus

Scanning electron microscopy images were taken of starch granules from different sources following exposure in vivo and in vitro to gut alpha-amylases isolated from Tenebrio molitor L. (Coleoptera: Tenebrionidae) and Zabrotes subfasciatus Boheman (Coleoptera: Bruchidae). One alpha-amylase was isolated from whole larval midguts of T. molitor using non-denaturing SDS-PAGE, while two other alpha-amylase fractions were isolated from whole larval midguts of Z. subfasciatus using hydrophobic interaction chromatography., Digested starch granules from larvae fed on maize, potato or wheat were isolated from midgut contents. Combinations of starch granules with isolated alpha-amylases from both species showed similar patterns of granule degradation. In vitro enzymatic degradation of maize starch granules by the three different alpha-amylase fractions began by creating small holes and crater-like areas on the surface of the granules. Over time, these holes increased in number and area resulting in extensive degradation of the granule structure. Granules from potato did not show formation of pits and craters on their surface, but presented extensive erosion in their interior. For all types of starch, as soon as the interior of the starch granule was reached, the inner layers of amylose and amylopectin were differentially hydrolyzed, resulting in a striated pattern. These data support the hypothesis that the pattern of starch degradation depends more on the granule type than on the alpha-amylase involved.