Development of an efficient bioprocess for turanose production by sucrose isomerisation reaction of amylosucrase

Turanose, a structural isomer of sucrose, is a non-cariogenic and low-calorigenic disaccharide. It has a promising potential to be a high-quality functional sweetener, but there has been no competitive way to produce turanose until now. In this study, a recombinant amylosucrase, originating from Neisseria polysaccharea (NpAS), was utilised to efficiently convert sucrose to turanose. With 2.5 M sucrose and 400 U/l of NpAS, the production yield of turanose maximally reached to 56.2% at 35 °C after a 120 h reaction. Main transglycosylating reaction pathway of NpAS clearly shifted from α-(1,4)-glucan synthesis to turanose production by simply increasing sucrose substrate concentration. Purification, by a preparative recycling HPLC, effectively separated turanose from the reaction mixture with a high purity of 94.7% and a high recovery yield of 97.5%. Chemical structure of the purified turanose was confirmed by NMR analysis, and three tautomers, namely 3-O-α-d-glucopyranosyl-α-d-fructofuranose, 3-O-α-d-glucopyranosyl-β-d-fructofuranose, and 3-O-α-d-glucopyranosyl-β-d-fructopyranose, were identified with a ratio of approximately 1:2:2.