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

Isothermal crystallization of sunflower seed oil hydrogenated under two dif ferent conditions was studied by means of pulse nuclear magnetic resonance (pNMR) and optical microscopy. Solid fat content (SFC) curves showed two di fferent shapes depending on supercooling. When supercooling was high, hyper bolic curves were round, whereas with low supercooling sigmoidal curves wer e obtained. Curves were interpreted with the modified Avrami equation. Phot ographs of the crystals were taken from the beginning of crystallization, e very 15 s until 15 min and every 5 min until 60 min. Samples which exhibite d hyperbolic curves showed a slight increase in crystal number, and crystal s were needle-shaped in all cases. Samples which had sigmoidal crystallizat ion curves showed a marked increase in crystal number with time, and crysta ls 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 on ly hyperbolic curves. Supercooling is a very important parameter that defin es the way nucleation occurs. Depending on the initial number of nuclei, tw o different growth mechanisms were found: a uniform linear growth of the nu clei for a small initial number (sigmoidal curves) and an aggregate of the nuclei for a high initial number (hyperbolic curves).