Rennet casein curds were dried in a pilot-scale fluidized bed drier at air-inlet temperatures of 80, 90, 110, 120 or 140 degreesC. As the drier air-inlet temperature was increased, there was a significant increase in hydration time and a significant decrease in hydrated viscosity and viscosity stability following hydration of the dried rennet caseins in 0.4% (w/w) disodium orthophosphate (DSP). The influence of air-inlet temperature during drying on composition, the extent of Maillard reaction, the resolution of the proteins on urea-polyacrylamide gel electrophoresis (PAGE) and cation-exchange fast protein liquid chromatography (FPLC), the dialysability of calcium and phosphorous from a 6 M urea dispersion at pH 4.6, and the particle-size parameters of milled rennet caseins were determined. Relationships between all these latter parameters and the hydration behaviours of the rennet caseins in DSP solution were also studied. Results suggest that the increase in drier air-inlet temperature resulted in a reduction in the mean particle size of milled casein, which was significantly correlated to the changes in hydration behaviour. Hydration times increased significantly and hydrated viscosities decreased significantly with increasing levels of fluorescent Maillard reaction products (MRPs) and with increasing levels of nondialysable calcium and phosphorous in the caseins. It appears that the increase in air-inlet temperature during drying reduced the solubility of colloidal calcium phosphate (CCP), possibly through binding of CCP to the increased levels of MRPs. This change in CCP solubility led to increased difficulty in sequestering CCP from the curd when the pilot-scale rennet caseins were dispersed in the DSP solution, leading to the changes in hydration behaviour.
