On the kinetics of heat-induced deamidation and breakdown of caseinate

Caseinate (dissolved in a milk salt solution) was heated at temperatures between 110 and 145°C for 0–120 min. Five different concentrations of caseinate were studied ranging from 1–5% (w/w) solutions. The heated solutions were analysed for ammonia and for non-protein nitrogen content (NPN). The increase in ammonia concentration was taken as a measure for deamidation of the protein. It was established that the order of the reaction with respect to concentration (the ‘true’ order) was 1, as determined from the dependence of deamidation on initial concentration of caseinate. The order with respect to time could not be clearly established as the extent of deamidation was not higher than some 30%. However, the order with respect to time tended towards 2 rather than 1, which may be an indication for inhibition of deamidation as the reaction proceeds. The temperature dependence of deamidation was characterized from the Eyring equation; the activation enthalpy was found to be 92 kJ mol−1, the activation entropy −70 J mol−1 K−1. These results suggest a bimolecular reaction, in accordance with mechanisms of deamidation described in literature. The increase in NPN appeared to be much higher than that in ammonia. Ammonia (a part of the NPN fraction) amounted to only 10–15% of the NPN fraction. The NPN fraction increased considerably with heating intensity, for instance, to as high as 20% of total nitrogen (i.e. protein) after heating for 90 min at 140°C. This suggests considerable heat-induced protein breakdown resulting in small peptides and amino acids, or other low molecular weight nitrogen-containing breakdown products. The order with respect to concentration tended towards 1, while the order with respect to time tended again towards 2. The activation enthalpy for NPN formation was found to be 107 kJ mol−1 and the activation entropy −37 J mol−1 K−1.