Predicting the viscosity of digesta from the physical characteristics of particle suspensions using existing rheological models

The measurement of the viscosity of digesta is complicated by settling and compositional changes that accompany digestion. The current work determined whether the apparent and relative viscosities (η a andη r ) of digesta could be accurately determined from the actual and maximum solid volume fractions (ϕ andϕ max , respectively) using the Maron–Pierce equation. The rheological properties of digesta from the small intestine of six pigs were determined at a shear rate of 1 s−1 at 37°C. A series of suspensions of plant fibre in a Newtonian liquid (70% aqueous fructose) were made at viscosities similar to pig digesta by adjustingϕ . The relationships between the apparent and relative viscosities (η a andη r ) and the plant fibre properties; aspect ratio (AR) andϕ andϕ max were then determined for digesta and the suspensions. The ARs for the digesta and plant fibre particles were determined using image analysis of scanning electron micrographs andη a from rheometric flow curves at 37°C,ϕ from image analysis and gas pycnometry, andϕ max from AR and suspension viscosity. Theη r of pig digesta and the test suspensions calculated using the Maron–Pierce equation were, with the exception of two outliers, in proportion withη a determined using a rheometer, indicating thatη r could be successfully predicted from the Maron–Pierce equation.