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Galacto-oligosaccharides (GOS) are used in infant formula to replace the health effects of human milk oligosaccharides, which appear to be dependent upon the structure of the individual oligosaccharides present. However, a comprehensive overview of the structure-specific effects is still limited as a result of the high structural complexity of GOS. In this study, porous graphitic carbon (PGC) was used as the stationary phase during ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS). This approach resulted in the recognition of more than 100 different GOS structures in one single run, including reducing and non-reducing GOS isomers. Using nuclear magnetic resonance-validated structures of GOS trisaccharides, we discovered MS fragmentation rules to distinguish reducing isomers with a mono- and disubstituted terminal glucose by UHPLC-PGC-MS. UHPLC-PGC-MS enabled effective recognition of structural features of individual GOS components in complex GOS preparations and during, e.g., biological conversion reactions. Hence, this study lays the groundwork for future research into structure-specific health effects of GOS.

Scope Next to galacto‐oligosaccharides (GOS), starch‐derived isomalto‐oligosaccharide preparation (IMO) and isomalto/malto‐polysaccharides (IMMP) could potentially be used as prebiotics in infant formulas. However, it remains largely unknown how the specific molecular structures of these non‐digestible carbohydrates (NDCs) impact fermentability and immune responses in infants. Methods and Results In vitro fermentation of GOS, IMO and IMMP using infant fecal inoculum of 2‐ and 8‐week‐old infants shows that only GOS and IMO are fermented by infant fecal microbiota. The degradation of GOS and IMO coincides with an increase in Bifidobacterium and production of acetate and lactate, which is more pronounced with GOS. Individual isomers with an (1↔1)‐linkage or di‐substituted reducing terminal glucose residue are more resistant to fermentation. GOS, IMO, and IMMP fermentation digesta attenuates cytokine profiles in immature dendritic cells (DCs), but the extent is dependent on the infants age and NDC structure. Conclusion The IMO preparation, containing reducing and non‐reducing isomers, shows similar fermentation patterns as GOS in fecal microbiota of 2‐week‐old infants. Knowledge obtained on the substrate specificities of infant fecal microbiota and the subsequent regulatory effects of GOS, IMO and IMMP on DC responses might contribute to the design of tailored NDC mixtures for infants of different age groups., The in vitro fermentation of galacto‐oligosaccharide (GOS), isomalto‐oligosaccharide preparation (IMO) and isomalto/malto‐polysaccharides (IMMP) by pooled fecal inocula of 2‐ and 8‐week‐old infants shows that next to the size, the highly variable structure of oligosaccharides shows a huge impact on the fermentability of non‐digestible carbohydrates (NDCs) by infant fecal microbiota. GOS, IMO as well as IMMP fermentation digesta attenuated cytokine profiles in immature dendritic cells of which the extent is dependent on the infants age and NDC structure.