Study on Improvement of Sweetness of Steviol Bisglycosides: Selective Enzymic Transglucosylation of the 13-O-Glycosyl Moiety

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Based on studies on the acceptor specificity of the transglucosylation with cyclodextringlucosyltransferase (CGT-ase) as well as on the structure-sweetness relationship of steviol glycosides, selective elongation of the 13-O-glucosyl moiety of steviol bisglycosides was done to improve sweetness. The β-D-glucosyl ester group at the 19-carboxylic acid of rubusoside and stevioside was chemically replaced by β-D-galactosyl group as a blocker against the glucosylation and both the galactosyl esters were regioselectively 1, 4-α-glucosylated on the 13-glycosyl moiety with CGT-ase using soluble starch as a donor. Great improvement in sweetness was observed for the products which have three or four glucosyl units at the 13-position, while the products with five or six glucosyl units had less sweetness than the starting materials. It was also revealed that the sweetness of the products of the stevioside series were better than the corresponding isomers of the rubusoside series.
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