Starch Degradation in Kiwifruit: In vivo and In vitro Ultrastructural Studies

Abstract

Kiwifruit were examined at harvest and during postharvest ripening to quantify structural changes to starch granules as net starch degradation occurred. The changes were compared with those found in in vitro studies utilizing granules prepared from fruit at harvest. In in vitro studies starch granules were incubated with various cocktails of starch-degrading enzymes and examined by scanning and transmission electron microscope (TEM) at sequential points during enzyme incubation. Only α-amylase, or α-amylase in combination with other enzymes, was able to degrade the native granules. A combination of α-amylase and α-glucosidase resulted in less degradation than with α-amylase alone. Crude extracts from kiwifruit or pea leaf resulted in minimal degradation under the conditions tested. In all experiments α-amylase attack began by creating channels to the center of the granule, which was then followed by radial degradation from the center outward. An external shell was generally all that remained. This pattern was not detected in vivo. Here degradation appeared to be from the outside in. Often individual granules within a plastid appeared to resist degradation when adjacent ones were being hydrolyzed. Measurements of granule diameter, both in vivo and with extracted granules, also indicated that large granules may be preferentially degraded after fruit harvest. These results are discussed in relation to control of starch degradation.