Isothermal crystallization kinetics of refined palm oil

Abstract

The induction times for the crystallization, under isothermal conditions, of refined, bleached, and deodorized palm oil from the melt were studied by viscometry. At temperatures below 295 K, the crystallization of palm oil was observed to occur in a two-stage process. This two-stage process was caused by the fractionation of palm oil, most probably into the stearin and olein fractions. At temperatures higher than 295 K, only a single-stage crystallization process was observed. As seen under polarized light microscopy, spherical crystals were initially formed from the first fraction at temperatures from 287 to 293 K. The diameters of these spherical crystals decreased as the temperature increased. After that, needle-shaped crystals were formed from the second fraction and continued to grow from the surface of these spherical crystals until the spherical crystals were fully enclosed, i.e., the cocrystallization of two polymorphs was observed. At temperatures higher than 293 K, the needleshaped crystals formed from a mixture of the two fractions were found to be the only polymorphs developed with the onset of crystallization. X-ray diffraction results showed that for temperatures below 295 K, the spherical crystals formed from the first fraction were in α form, whereas the needle-like crystals that nucleated later from the second fraction were in β′ form. β′ crystals were the only polymorphs formed for temperatures above 295 K. The results obtained were in good agreement with the discontinuity observed in the induction time vs. temperature curve. Activation free energies for nucleation were calculated according to the Fisher-Turnbull equation for the various polymorphic forms. Viscometry was observed to be a sensitive method for characterizing the overall crystallization process. This technique is suitable for induction time studies of palm oil crystallization, especially at lower temperatures and with viscous oil.

Image analysis of palm oil crystallisation as observed by hot stage microscopy

“…A similar discontinuity with temperature to that seen by Chen et al [10] is observed. They suggested that the discontinuity observed in the induction data was caused by a change in the polymorphic forms present and that below temperatures of 298 K two palm oil polymorphs, α and β' crystallised separately.…”

“…The spherulitic/needle morphologies were also observed by Kawamura [2] at 30 °C (who tentatively ascribed them to the β polymorph) and Chen et al [10] at 28 °C. Chen et al, however, identified such crystals as belonging to the β' polymorph using XRD.…”

“…However, the α core must then transform to a higher polymorph at some point as it melts at similar temperatures to other β' crystals. As mentioned by Chen et al [10], this type of observation may not be observable by XRD which mostly probes the surface regions of crystals.…”

“…Some β forms were also found at temperatures below 0 °C, which were presumed to be due to the low melting fractions. Isothermal XRD experiments (of particular relevance to this study) were performed by Chen et al [10] from 287 to 295 K.…”

Crystallization Kinetics Of Coconut Oil Based On Gompertz Model

“…An increase in µ does not suddently occur after T Cr isothermal achieved, as indicated by the SFC baseline plot which is still flat until a certain period of time. If at any point SFC drastically changes from the baseline, according to Chen et al (2002), the point reflected the occurance of crystallization induction time, and improvement degree of SFC after that point illustrates the amount of µ. At low v c , µ has a high value.…”