Rheological properties of digesta suggest little radial or axial mixing in the forestomach of the tammar (Macropus eugenii) and the parma (Macropus parma) wallaby

We examined the physical properties of digesta from the proximal (sacciform) and distal (tubiform) regions of the forestomach of tammar and parma wallabies maintained on a rye-grass sward. The digesta exhibited high viscosity, which, in conjunction with low flow rates calculated from published retention times of particulate marker in the stomach, results in a low potential for macrofluid mixing during onflow of food by displacement. The pseudoplastic nature of the viscosity profile and very low "flow behavior index" also indicate that macrofluid mixing around haustrae would be very localized. These findings indicate that the uniform mixing of whole digesta required for continuously stirred tank reactors (CSTRs) is unlikely. Voidage, the proportion of digesta not occupied by solid material, and "consistency index" were greater in the digesta from the proximal than in the distal regions of the forestomach. We conclude that the mixing conditions in wallaby stomachs are better described as a partially emptying batch reactor (PEBR) than a CSTR series and that lower permeabilities in conjunction with higher viscosities decrease axial mixing and thus increase efficiency in PEBRs.