Depolymerized non-digestible sulfated algal polysaccharides produced by hydrothermal treatment with enhanced bacterial fermentation characteristics

Hydrothermal treatment was used to depolymerize sulfated polysaccharide extracted from Gracilaria chouae (GCP1) with an average molecular weight (Mw) of 1.79 × 103 kDa to produce three fractions GCP2, GCP3, and GCP4 with a Mw of 20.84, 8.25, and <5 kDa, respectively. While the chemical composition and monosaccharide profile of the three depolymerized GCPs were not significantly affected by the hydrothermal treatment. However, they exhibited lower apparent viscosity, storage modulus, and loss modulus. All four GCPs were shown to be nondigestible during simulated gastrointestinal digestion. During a 24-h in vitro fermentation using human fecal bacteria as inoculum, GCPs with lower Mw produced more short chain fatty acids (SCFAs), and lower branched chain fatty acids. Having the lowest Mw, GCP4 exhibited the best fermentation characteristics by inhibiting the growth of protein-fermentative bacteria including Bacteroides, Parabacteroides, Lachnoclostridium, and Phascolarctobacterium which have harmful effect and increased the relative abundance of butyrate-producing bacteria including Anaerostipes, Eubacterium, Roseburia, Ruminococcus and Lachnospira which are beneficial to gut health. The present study suggested that hydrothermal treatment could be used to prepare low Mw non-digestible polysaccharides that enhance the growth of beneficial bacteria for human gut health.