Heat stability and flow behaviour of buffalo milk added with corn starch

In this study, heat stability and flow behaviour of buffalo skim milk and its concentrate added with corn starch and then gelatinized at 85 °C were evaluated with regard to level of corn starch (1 and 2%) and gelatinization time (10–80 min). Heat stability measured in terms of the heat coagulation time (HCT) at 130 °C of the starch–milk mixtures declined with increasing gelatinization time, the decline being slight in case of 1% starch but significant (P<0.01) with 2% starch. The apparent viscosity (ηa, shear rate 500 s−1) of the heat-treated, i.e. gelatinized un-concentrated milk–starch mixtures was distinctly higher than that of heat-treated milk alone, but increasing gelatinization time from 10 to 80 min resulted in a relatively small rise in viscosity. However, in case of skim milk concentrate added with both 1 and 2% starch, the increase in viscosity induced by gelatinization beyond 10 min was significant (P<0.01). The skim milk–starch systems showed a power law behaviour. An increase followed by a decrease in the consistency coefficient (K) with increasing gelatinization time was noticed in the un-concentrated system, whereas the concentrate–starch mixtures exhibited steadily increasing K with increasing gelatinization time at 85 °C. The flow behaviour index (n) of the mixtures was in the shear thinning range, but this characteristic was more prominent in un-concentrated system, the value of n decreasing with increasing gelatinization time. The heat treatment of milk–starch mixtures resulting into gelatinization-induced increase in viscosity was conceived to be due to water uptake by starch granules accompanied by increased concentration of the non-starch phase leading to rapid aggregation (coagulation of colloidal casein micelles during subsequent heating at 130 °C) and thus causing the HCT to decline. The HCT was directly correlated with pH, and negatively with apparent viscosity and consistency coefficient, the former correlation being highly significant (P<0.01) in the milk-based system, while the latter correlations were so in the concentrate-based-one.