Isothermal crystallization of hydrogenated sunflower oil:: II.: Growth and solid fat content

Isothermal crystallization of sunflower seed oil hydrogenated under two different conditions was studied by means of pulse nuclear magnetic resonance (pNMR) and optical microscopy. Solid fat content (SFC) curves showed two different shapes depending on supercooling. When supercooling was high, hyperbolic curves were round, whereas with low supercooling sigmoidal curves were obtained. Curves were interpreted with the modified Avrami equation. Photographs of the crystals were taken from the beginning of crystallization, every 15 s until 15 min and every 5 min until 60 min. Samples which exhibited hyperbolic curves showed a slight increase in crystal number, and crystals were needle-shaped in all cases. Samples which had sigmoidal crystallization curves showed a marked increase in crystal number with time, and crystals were spherical in shape. Crystallization behavior was also in agreement with the chemical composition of the samples. Samples which had the highest content of high-melting triacylglycerols (especially trielaidin) showed only hyperbolic curves. Supercooling is a very important parameter that defines the way nucleation occurs. Depending on the initial number of nuclei, two different growth mechanisms were found: a uniform linear growth of the nuclei for a small initial number (sigmoidal curves) and an aggregate of the nuclei for a high initial number (hyperbolic curves).