Variations in residual sugar composition have been observed during Jerusalem artichoke extract fermentations by using Saccharomyces diastaticus NCYC 625, a flocculating yeast strain. In batch cultures, these differences were due to the inulin polymer size distribution of the extracts: measurements of enzymatic activities on different polymerized substrates have shown that the hydrolysis and ft fermentation yield decreased when the fructose/glucose ratio of the extract increased. Inulin hydrolysis appealed to be the limiting factor of the fermentation rate. A comparison of continuous and batch cultures with the same extract showed that fermentability differences were related to the structure and size of the yeast flocs. This led to an hydrolysis selectivity of the inulin polymers according to their size: the chemostat culture in which the flee average size was larger gave longer chained residual sugars.