Effect of in vitro fermentation using human faecal inoculum on the water-holding capacity of dietary fibre.

The water-holding capacities (WHC) of four sources of fibre were measured using dialysis membranes and osmotic-suction pressures of 45, 89 and 178 mosmol/1 (1, 2 and 4 atm). At all pressures, pectin had the highest WHC, followed by cabbage (Brassica oleracea) and lucerne (Medicago sativa) and then cellulose. A suction pressure of 89 mosmol/1 (2 atm) was used in the subsequent fermentation study since it had the lowest standard error of the mean and most closely approximated physiological conditions. The four fibres were anaerobically fermented in vitro with human faecal inoculum for 24 h. The WHC of the fermentation residues were measured. The potential water-holding capacity (PWHC), a function of the extent of fermentability and the WHC of the fermentation residues, was highest for lucerne, followed by cellulose, then cabbage and, finally, pectin. Only the PWHC values ranked the four fibres in the same order as in vivo values. It was concluded that the ethanol-insoluble residues containing unfermented fibre organic matter and microbial organic matter, both of which hold water, should be used to calculate PWHC and to predict the effect of fibre on rate of passage and faecal mass in humans.