Effects of acid and alkali concentration on in vitro measurement of wheat viscosity

In vitro viscosity (IVV) determination of wheat and wheat based diets, is a relatively fast method and is now being used as a rapid indirect method for the prediction of nutritive value for poultry. Investigations were carried out on the effect of varying hydrochloric acid (HCl) and sodium hydrogen carbonate (NaHCO3) concentrations on the pH of the slurry and the effects on IVV. IVV measurements were found to be extremely sensitive to HCl concentration, with viscosity increasing as HCl concentration increased. Wheat varieties were observed to change rank order with reference to IVV subject to pH conditions in some cases. The pH of the initial slurry decreased from 4.9 to 1.8 with a rise in concentration of HCl from 0.05 to 0.25 M and that of the final slurry, after NaHCO3 addition, from about 7.7 to 6.7. IVV was not affected by NaHCO3 concentration. As pH in the proventriculus and gizzard can be variable (between 1 and 5) these in vitro results suggest that gizzard pH could have a large effect on in vivo viscosity especially as the concentration of NaHCO3 does not appear to affect the final IVV. Hence, the relevance of IVV as a predictor of in vivo viscosity and apparent metabolisable energy (AME) would seem to be highly dependent on other dietary factors.