Influences of structures of galactooligosaccharides and fructooligosaccharides on the fermentation in vitro by human intestinal microbiota

•In vitro fermentation study of pure components of GOS and FOS was carried out.•A relatively high selectivity toward bifidobacteria and higher PI values with GOS.•GOS with β-(1→6) linkages had higher PI values than those of β-(1→4) linkages.•Higher values of lactobacilli were obtained with pure components of FOS.•Structure–prebiotic function relationship of GOS and FOS was obtained. A structure–function study was carried out to expand the knowledge base of the effects of carbohydrate glycosidic linkage, monosaccharide composition and degree of polymerization (DP) on the selectivity of fermentation by mixed culture. Galactooligosaccharides (GOS) with β-(1→4)- and β-(1→6)-linkages were prepared from the transgalactosylation of β-D-galactosidases from Bacillus circulans and Aspergillus oryzae with lactose as substrate, respectively, whereas fructooligosaccharides (FOS) with varied DP from three to five were commercially obtained. Pure single components of GOS and FOS were purified and characterized by mass spectrometry with an electrospray ionization source and nuclear magnetic resonance spectroscopy, and a prebiotic index (PI) was calculated for each oligosaccharide. The results demonstrated that GOS showed a relatively high selective stimulation toward bifidobacteria as well as higher PI value compared with FOS, and GOS with β-(1→6)-linkage exhibited relatively higher PI value than GOS with β-(1→4)-linkage, whereas higher value toward lactobacilli was observed for FOS. Additionally, with the exception of GOS with β-(1→6)-linkages, all investigated oligosaccharides gave similar PI values at 24 h, although significant variations were found for the growth of different genus bacteria in mixed cultures.