The relationship between crystallization behavior, microstructure and mechanical properties in a palm oil-based shortening.

LITWINENKO J.W., ROJAS A.M., GERSCHENSON L.N. Y MARANGONI A.G.

Kuala Lumpur, Malaysia.

Conferencia; The 2001 Porim International Palm Oil Congress (PIOPC), Chemistry & Technology Conference, Malaysian Palm Oil Board.; 2001

International Palm Oil Congress (PIOPC).

In this study, the effects of supercooling and storage time and temperature on the microstructure and macrocopic rheological properties of a soybean and palm oil-based vegetable shortening were examined. NMR profiles indicated a maximum solid fat content (SFC) of approximately 21% at 5ºC, with the end of melt occurring in the temperature range of 45º to 50ºC. Crytallization kinetics experiments were studied using the Avrami model, and suggesting instantaneous nucleation with rod-like growth at lower crystallization temperatures (approx. 5ºC) and sporadic nucleation with spherulitic growth at higher crystallization temperatures (around 30ºC). A peak of melting temperature of 42.7ºC was determined by differential scanning calorimetry (DSC). Apparent free energy of nucleation were calculated from induction times using the Fisher-Turnbull equation. DSC crystallization experiments suggested the occurrence of the alpha-polymorph upon cooling (>5ºC/min). and the beta prime-polymorphic form at lower cooling rates (<1ºC/min). Recrystallization peaks were observed upon melting of crystallized fat in the early stages of the crystallization proces, indicative of the presence of the alpha form. Stored samples did not diplay this behaviour, suggesting that alpha to beta-prime polymorphic transitions occurred during storage. Powder X-ray diffraction results confirmed  the presence of the polymorphic forms suggested by DSC. Polarized light microscopy confirmed the morphologies predicted by the Avrami model, and image analysis by the particle counting method demostrated the dependence of the fractal dimension (D) on the degree of suppercooling. Fractal dimensions, which describe the spatial distribution of mass or degree of order, decreased from 2.46 to 2.04 with a decrease in crystallization temperature from 27ºC to 5ºC. Crystallizations at 22ºC employing cooling rates of 1ºC/min yielded a high fractal dimension of 2.53, whereas a high cooling rate of 15ºC/min resulted in a D of 2.26, demostrating the dependence of D on cooling rate. These same treatments followed by 72 h of storage at 22ºc resulted in D values of 2.60 and 2.27 respectively. Rheological experiment confirmed that the storage and loss moduli were independent of frequency within the linear viscoelastic region. Higher G´ and breaking force values were observed for crystallizations at higher degrees of supercooling.