The application of Raman spectroscopy for unravelling the fat crystal network of palm oil on the microscale

Palm oil (PO), consisting of equal amounts of saturated and unsaturated fatty acids, is semi-solid at room temperature. Depending on the crystallization conditions applied, the microstructure of the fat crystal network varies. The goal of applying Raman spectroscopy was to study the spatial distribution of the triacylglycerol molecules on the microscale.

PO was cooled at 1°C/min or 20°C/min and crystallized statically at 20°C or 25°C. WAXS & SAXS were used to study the polymorphic forms over time, while PLM visualized the formation of the fat crystal network on the microscale. For Raman spectroscopy, the profiles of PO and PPP were compared, as PPP was found to be the main triacylglycerol present in the solid fraction of PO. From the PO profile, the band at 1127 cm-1 was selected as representative for saturated fat, whereas 1654 cm-1 was found to represent the oil (Figure 1). In every crystallized sample, an area of 80 µm x 80 µm was mapped and the resulting heatmaps at 1127 cm-1 and 1654 cm-1 were calculated.

In all cases a polymorphic transition from α-2L towards β’-2L were encountered. Nonetheless, PLM showed the formation of a dense network at 20°C, whereas at 25°C separate flocs were formed surrounded by oil. The crystal flocs contain a dense center that is formed at the start of crystallization when the α-2L polymorph is present. Later on, the center is surrounded by a fanning crown.

Raman spectroscopy did not show any spatial features at 20°C. Contrarily, at 25°C, the mappings clearly revealed that the triacylglycerol content in the dense center is more saturated compared to the surrounding crown. Raman spectroscopy enables to link the different microstructures formed to differences in the spatial distribution of the triacylglycerols.