Time-resolved crystallization of palm oil, studied with WAXS, SAXS and USAXS

Abstract: For some time, the use of palm oil in food products has become highly controversial. However, it’s known that the crystallization behavior of palm oil is rather specific and very useful for many applications. In order to look for good alternatives, the crystallization behavior of palm oil must be fully understood. Lately, it has been shown that the polymorphic behavior of a fat by itself is not sufficient to explain the difference in final properties. Therefore, larger scale arrangements, like nanoplatelets, have become more of interest. This study combines the power of different length scales of X-ray scattering techniques to study the crystallization properties from basic polymorphism (WAXS), over longitudinal arrangements (SAXS) to nanoplatelet and nanoplatelet aggregates size (USAXS) [figure 1].

The set-up includes palm oil crystallized following different temperature protocols (cooled at 1°C/min or 20°C/min to 20 or 25°C), selected based on differences in kinetics and polymorphic transitions. The samples were analyzed during cooling and 1h isothermal time using synchrotron X-ray scattering (BM26 & ID02 ESRF, France). From this time-resolved study it can be concluded that changes in polymorphism, longitudinal arrangements and nanoplatelet formation differ based on the crystallization conditions, but that the changes are related. Contrarily, it was also found that using different temperature protocols resulted in the same polymorphic end state (WAXS), but different nanoplatelet and aggregate sizes (SAXS and USAXS). Figure 2 illustrates intensity as function of scattering vector q for USAXS, SAXS and WAXS for one sample. Transitions over time are illustrated by changing colors (blue to red).

This research contributes to the deepening of knowledge about the structural organization of the fat crystal network and is a step forward in the use of the still novel USAXS technique for food products. It also points out the need for techniques to visualize nanoplatelets in situ.