Particle size of straw and gelation of pectin influence gastric mixing and emptying in pigs

Physicochemical properties of fibres can strongly impact gastric processes such as emptying and sieving. This study evaluated the influence of particle size of insoluble fibres, and gelation of soluble fibres when added to insoluble fibres, on gastric emptying of digesta phases from the proximal and distal stomach of pigs. Twenty-four boars (51.6 ± 4.90 kg) were assigned to one of four diets, containing either 150 g/kg coarse or finely milled wheat straw (median particle area of 5.4 vs 0.3 mm²), or 270 g/kg wheat bran without or with the addition of 100 g/kg low-methylated pectin. Tracers were used to quantify the mean retention time (MRT) of digesta liquids (Co-EDTA), fine solids (TiO₂), and fibrous particles (Chromium-mordanted fibres). For all diets, digesta pH was lower in the distal stomach than in the proximal stomach (-1.1 - 2.1 units; P < 0.05). In the proximal stomach, particle size reduction of straw tended to decrease digesta pH (-0.8 units; P = 0.072), reduced the MRT of fine solids (-117 min; P = 0.009) and the separation between fine solids and liquids (-88 min; P = 0.030). When particle size of straw was reduced, the MRT of liquids was no longer greater in the proximal stomach compared with the distal stomach (P > 0.10), while in both regions, the MRT of fibrous particles (-213 - 238 min; P < 0.05) and the difference between fibrous particles and fine solids were reduced (-96 - 181 min; P < 0.05). Accordingly, sieving of nutrients, such as starch and non-starch polysaccharides was reduced. In the proximal stomach, the greater water holding capacity and resistance to deformation conferred by the addition of pectin decreased the MRT of fine solids (-138 min; P = 0.003), and fibrous particles (-227 min; P < 0.001), reducing the difference between fine solids and liquids (-148 min; P < 0.001), and between fibrous particles and fine solids (-89 min; P < 0.001). In the distal stomach, pectin addition reduced the MRT of fibrous particles (-203 min; P = 0.007), and the difference between fibrous particles and fine solids (-154 min; P < 0.001). Concomitantly, sieving of nutrients across stomach regions was reduced. In conclusion, particle size reduction of straw and pectin addition accelerated the emptying of fine and coarse solids, and reduced sieving of digesta phases and nutrients in the proximal and distal stomach of pigs.