Comparison of Factors Involved in Starch Degradation in Barley Germination Under Laboratory and Malting Conditions

摘要

Barley has long been the model plant for the physiological studies of starch degradation and germination. The majority of barley germination research was and is conducted using various laboratory germination methods. The extrapolation of knowledge obtained in laboratory barley germination studies to malting is commonplace but could prove problematic because of the differences between the two techniques. Grains were laboratory-germinated (LG) or micromalted (MM) and sampled daily from 0 to 5 days after imbibition/steeping. alpha-Amylase and beta-amylase activities and protein levels along with starch, osmolyte concentration (OC), and sugar (glucose, sucrose, fructose, maltose, maltotriose, maltotetraose, maltopentaose, maltohexaose, and maltoheptaose) concentrations were determined. Two alpha-amylase isoforms were detected in LG and MM grains. In LG grains, an approximately 50-kDa isoform was the predominant isoform early in germination and, as germination proceeded, was degraded to an approximately 40-kDa isoform. In MM grains, the 40-kDa isoform was the predominant form, and levels increased as germination proceeded. beta-Amylase activity remained constant throughout both LG and MM treatments. However, LG grain alpha-amylase underwent more proteolytic processing than MM grain beta-amylase. Maltose, maltotriose, maltotetraose, maltopentaose, maltohexaose, total sugars, and OC all accumulated one day later in LG grains. The imbibition/steeping time was the critical step in determining the accumulation patterns, and the shorter imbibition/steeping time commonly used in laboratory germination experiments appears to have caused the one-day delay in sugar and OC accumulation. However, it did not explain the protein processing differences. Thus, intrinsic differences between the LG and MM procedures that cause differences in sugar accumulation and protein processing must be accounted for when trying to extrapolate data obtained using laboratory germination procedures to malting.