Chemical structure and molecular weight influence the in vitro fermentability of polysaccharide extracts from the edible seaweeds Himathalia elongata and Gigartina pistillata

Polysaccharides from the brown seaweed Himanthalia elongata and the red one Gigartina pistillata have been extracted by sequential fractionation based on the solubility in water (F-H2O), hydrochloride acid (F-HCl) and potassium hydroxide (F-KOH), remaining a residue (RES). Their structures have been studied through Fourier- transform infrared spectroscopy (FTIR) and molecular weight by molecular exclusion (HPSEC). Sugar compo- sition and sulphate content were also determined. F-H2O and F-HCl from Himanthalia elongata were rich in fucoidans with an estimated Mw of 926 and 430 × 103 g/mol. Laminarans appeared in the mentioned fractions and possibly xylofucoglycuronans and xylomannans are in F-H2O. F-HCl was poorly fermented with low pro- duction of total short chain fatty acids (SCFA) and two poor or non-fermentable low-molecular weight poly- uronans (68 and 4 × 103 g/mol) appeared in F-KOH. Carrageenans extracted from Gigartina pistillata presented low or non-fermentability. Xylofucoglycuronans and xylomannans, possibly presented in RES and F-H2O from Himathalia, and laminarans in the last mentioned fraction, seem to be more fermentable. Carragenophytes red seaweeds do not seem to be fermentable whereas brown algae could have major potential fermentability, probably due to the presence of laminarans, xylofucoglycuronans and/or xylomannans.