A kinetic analysis of crystallization of a milk fat model system

Crystallization behavior of milk fat, pure triacylglycerol (TAG) fraction of milk fat and three blends of 30, 40 and 50% of high-melting fraction (MDP=47.5°C) in low-melting fraction (MDP=16.5°C) of milk fat were studied by turbidimetry and nuclear magnetic resonance (NMR). Nucleation behavior was similar for these systems. In all cases, curves of log of induction time vs. temperature were continuous, which means that when samples were crystallized from the melt, only one polymorphic form was obtained at all temperatures. X-ray patterns verified formation of the β′ form. The calculated activation free energies of nucleation were quite low, but close to the ones published for other fats systems, such as hydrogenated sunflower oil. Only a low supercooling was needed to start crystallization. When isothermal crystallization was studied by NMR, the same behavior was also found for all samples. For low supercooling (crystallization temperatures above 25°C), curves had sigmoidal shapes with a period of induction of crystallization at the beginning. For high supercooling (temperature below 25°C), no induction times were observed as crystallization occurred rapidly. An intermediate plateau in SFC was clearly visible in all curves. The step in this plateau decreased steadily both in rate and height as T_c was increased. The curves were interpreted with the Avrami kinetic model, and parameters K_n and n were calculated. In all cases, the best fit was obtained by setting n to 3. K_n was, on average, 10 times lower above 25°C.