Submitted to: Sugarbeet Research and Extension Reports
Publication Type: Proceedings
Publication Acceptance Date: 1/30/2001
Publication Date: N/A
Citation: Interpretive Summary: Sucrose metabolism in postharvest sugarbeet roots is a costly problem for the sugarbeet industry. Sugarbeet roots contain enzymes capable of degrading sucrose. These enzymes are present and active in harvested roots and cause substantial sucrose loss during sugarbeet root storage and the initial stages of processing. Three major enzyme activities acid invertase, alkaline invertase and sucrose synthase are present in postharvest sugarbeet roots. The activities of these enzymes were determined in postharvest sugarbeet roots after prolonged storage or storage under unfavorable conditions. Sucrose synthase was the major sucrolytic activity under all storage conditions tested. Alkaline invertase activity was present at significantly lower levels than sucrose synthase activity. Acid invertase activity was barely detectable. Few changes in enzyme activity were observed even after prolonged storage or storage at elevated temperatures. The capacity of sucrose synthase and acid invertase to degrade sucrose under typical storage and processing conditions was also determined. Sucrose synthase and acid invertase enzymes retained a portion of their activity at temperatures typically encountered during storage and processing. Inactivation of these enzymes required a temperature of 65 degrees C or greater. Both enzymes exhibited activity at pH values typical of sugarbeet processing. These results suggest that sucrose loss by sucrose synthase and acid invertase is possible during postharvest storage and processing of sugarbeet roots.
Technical Abstract: The activities of the major sucrolytic enzymes in postharvest sugarbeet roots were determined after prolonged storage or storage under unfavorable conditions. Soluble acid invertase, alkaline invertase and sucrose synthase activities were measured in field grown sugarbeet roots after storage at 6, 12 or 21 deg C. Sucrose synthase was the major sucrolytic activity under all storage temperatures and durations tested. Alkaline invertase was present at significantly lower levels, while soluble acid invertase activity was barely detectable. Only alkaline invertase exhibited a change in activity that was consistent over all storage temperatures studied. The effect of temperature and pH on the activities of two sucrose synthase isoforms and the major soluble acid invertase of sugarbeet roots was also determined. All three enzymes retained a portion of their activity at the cold temperatures typical of storage. At 5 deg C, sucrose synthase I, sucrose synthase II and acid invertase retained respectively, 8, 14, and 16% of their activity relative to their activity at 35 deg C. Sucrose synthase II and acid invertase were completely inactive at temperatures of 60 deg C or greater. Sucrose synthase I was inactive at temperatures of 65 deg C or greater. The optimum temperature for sucrose synthase I and sucrose synthase II activities were 50 and 45- 50 deg C. The optimum temperature for acid invertase activity was 35 deg C. Sucrose synthase I and sucrose synthase II were active in the pH range of 5.0 to 8.0 and 5.5 to 7.5, respectively. Acid invertase exhibited a plateau of activity at pH 5.0 to 5.5 and its activity increased 7.5 fold with a decrease in pH from 5.0 to 3.0.