Stability of stevia sweeteners (stevia diterpene-glycosides from Stevia rebaudiana BERTONI) in raw soy sauce was examined. Complete degradation of stevioside, the predominant component of stevia sweeteners, was observed in raw soy sauce after overnight at 45°; however, there were no significant changes of the other components, rebaudioside-A and rebaudioside-C under the same conditions. In heat-treated raw soy sauce (100°, 10 min) stevioside was not affected under the above conditions. By silica-gel column chromatography, the degraded product [1] of stevioside was isolated from the raw soy sauce with stevia sweetener incubated overnight at 45°. The product [2] was also isolated from alkaline hydrolyzate of the product [1] by silica-gel column chromatography as the reference of [1].
By the criteria of thin-layer chromatography, and
13C-NMR and FT-IR spectroscopy, the structures of [1] and [2] were confirmed to be β-D-glycosyl ester of 13-O-β-D-glycosyl-steviol (rubsoside) and 13-O-β-D-glycosyl-steviol (steviol-monoside), respectively. These results indicate that β-1, 2-glycosyl linkage of sophorosyl residue in stevioside molecule was hydrolyzed by specified glucosidase in raw soy sauce to yield rubsoside.
As to the failure of the enzyme to attack rebaudioside-A and rebaudioside-C, it seemed likely that (a) the glucosidase could not hydrolyze β-1, 2-glycosyl linkage in rebaudioside-A molecule as a result of steric hindrance of the β-1, 3-glycosyl linkage attached to sophorosyl residue in rebaudioside-A molecule, and (b) rebaudioside-C has no β-1, 2-glycosyl linkage in the molecule
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