Submitted to: American Society of Sugarbeet Technologists
Publication Type: Proceedings
Publication Acceptance Date: 3/21/2001
Publication Date: N/A
Interpretive Summary: Sugarbeet roots contain a number of enzymes capable of degrading sucrose. These enzymes are present at harvest and cause a significant loss of sucrose during postharvest storage and processing of roots. Although the role of individual sucrose degrading enzymes is unknown, sucrose synthase and acid invertase have been implicated in postharvest sucrose loss. Two isoenzymes of sucrose synthase have been identified in sugarbeet roots. Two soluble acid invertase isoenzymes have also been identified, although nearly all acid invertase activity in sugarbeet roots is due to a single acid invertase isoenzyme. The activities of the two sucrose synthase isoenzymes and the major acid invertase isoenzyme of sugarbeet roots were determined with respect to temperature and pH to assess their ability to degrade sucrose at conditions typical of postharvest storage and processing. All three enzymes were capable of degrading sucrose at the cold temperatures encountered during storage. All three enzymes were also capable of degrading sucrose at the conditions likely to be encountered during the early stages of processing.
Technical Abstract: The enzymatic conversion of sucrose to invert sugars is responsible for a significant loss of sucrose during postharvest storage and processing. Sucrose catabolism causes sucrose loss directly by the degradation of sucrose and indirectly by the formation of invert sugars that increase the loss of sugar to molasses. The enzymes acid invertase and sucrose synthase have been implicated in postharvest sucrose loss. Some of the biochemical properties of the major soluble acid invertase (AcInv) and two sucrose synthase isoenzymes (SucSynI and SucSynII) were determined to assess their ability to degrade sucrose under conditions typical of storage and processing. Temperature activity profiles revealed that all three enzymes are capable of degrading sucrose at the low temperatures that occur in storage. AcInv, SucSynI and SucSynII retain 16, 8 and 14% of their activity at 5 deg C. Complete inactivation of these enzymes required temperature of 65 deg C or greater. AcInv and SucSynII were completely inactivated at 60 deg C; SucSynI was completely inactivated at 65 deg C. The sucrose synthase isoenzymes exhibited a surprising degree of heat stability, with a temperature optimum for activity of 45 to 50 deg C. AcInv exhibited a plateau of activity at pH 5.0 to 5.5 and was activated 7.5 fold with a decrease in pH from 5.0 to 3.0, possibly due to the loss of an inhibitor. SucSynI and SucSynII exhibited maximum activity at pH 6.5 and 7.0 respectively. These studies suggest that potential sucrose loss by these enzymes during sugarbeet root processing can be minimized by control of temperature and pH conditions.