Prediction method for determining the carbon stable isotopic composition of berry sugars in the original must of Chardonnay wines

•A sugar isolation method was developed for δ13C measurements of 33 Chardonnay grapes.•Berry sugar δ13C values depend on the δ13C values and content of monosaccharides.•δ13C values of wine ethanol are consistently lower than those of berry sugars.•Carbon isotope discrimination aids prediction of sugar δ13C values of original musts.•Sugar δ13C values of original musts provide information on wine-sugar sources.