In vitro digestion methods to characterize the physicochemical properties of diets varying in dietary fibre source and content

Co-products from food and agro industries - barley hulls (BH), pectin residue (PR), sugar beet pulp (SBP) and potato pulp (PP) - were collected, dried (PR, SBP, PP) and milled to pass a 1-, 2- and 4-mm screen. A total of 48 diets originating from these sources with four intended dietary fibre (DF) levels (225, 300, 375 and 450 g/kg DM) and three particle sizes (1, 2, 4 mm) were formulated by substituting wheat and barley in a wheat and barley-based diet (W-B diet). The chemical composition of wheat and barley, the four co-products and the corresponding diets were analysed. Swelling and water binding capacity (WBC) of the experimental diets were studied using a conventional analytical method and two in vitro digestion models intended to simulate the conditions in the stomach and the foregut (FG; stomach and small intestine). All co-products had lower starch (2-217 g/kg DM) but higher total NSP (460-743 g/kg DM) and DF (577-887 g/kg DM) contents than barley and wheat. Soluble DF (SDF) of total DF varied from 10.6% in BH to 40.3% in SBP. Substituting wheat and barley with the fibre-rich co-products increased swelling and WBC, least for the diets with added BH and most for the diets with added PP. The in vitro FG simulation exerted the highest value for both swelling and WBC, followed by the conventional analytical method and the in vitro stomach simulation. There were marked differences in swelling and WBC between diet particle sizes for the conventional analytical method (P < 0.05) and the in vitro FG simulation (P < 0.01), but not for the in vitro stomach simulation (P >= 0.1). Simulation with low vs. high incubation pH (2.5 vs. 4.0) in the pepsin hydrolysis procedure and long vs. short incubation time (4 vs. 2 h) gave rise to higher swelling and WBC values. In conclusion, substituting wheat and barley with fibre-rich co-products resulted in higher swelling and WBC irrespective of the method used whereas the relative increase was influenced by the content of SDF. In vitro FG simulation seems to be a better reflection of the physicochemical properties of DF in the gastrointestinal tract.