Impact of pectin and alginate gel particle size and concentration on in vitro gut fermentation

Fermentation by human gut microbiota of dietary fibres (DF) is influenced by the particle size and concentration of these fibres. However, previous studies have faced challenges in investigating these effects in real food systems due to the complex composition and heterogeneous structure of most DF. Here we describe particle size and concentration effects on the fermentation of polysaccharide gel particles, used as examples of defined and homogeneous model DFs. Gel particles of calcium-induced gelling polysaccharides, alginate and pectin, were prepared using dripping (> 1 mm) and emulsion-based (< 1 mm) methods. These particles were directly introduced into an in vitro fermentation system with a human faecal inoculum to simulate interactions with the gut microbiota. Throughout the fermentation, gas production, short-chain fatty acids, ammonia and unfermented substrate contents were measured. The results showed that pectin fermented faster than alginate in both gel and solution forms, with gelled forms of both polysaccharides fermenting more slowly than their solutions. However, particle size and concentration had minimal effects on fermentation outcomes for particles formed by the dripping method. Notably, small pectin particles (∼0.5 mm) fermented similarly to solutions, suggesting a size threshold below which gel particle fermentation is equivalent to that of solutions. This work highlights the impact of gel particle size on in vitro gut fermentation and provides insights for designing food gels with desired fermentation properties. • Gelled particles (>1mm) of alginate and pectin fermented more slowly than solutions • Pectin fermented faster than alginate in both gel and solution forms • Size and concentration had little effect on fermentation for particles of 1-4mm • Pectin particles of ∼0.5 mm fermented similarly to solutions.