On the fractional crystallization of palm olein: Solid solutions and eutectic solidification

In this contribution, the nature of particular crystal growth behavior observed in industrial dry fractionation of palm olein is further elucidated. Refined palm olein was prepared with three different concentrations of tripalmitoyl-glycerol (0.6%, 0.8% and 1%) and crystallized under stirring at 13, 15 and 17 °C to study the effect of internal seeding with tripalmitoyl-glycerol (PPP) on bulk crystallization properties. Notably at 15 °C, the increase in the solid fat content of the crystal suspension as function of time was heavily promoted by higher PPP-contents. The resulting crystals displayed a broad melting peak between 28 and 40 °C in DSC-measurements, which suggested the presence of a solid solution in a metastable form, mainly consisting of PPP and dipalmitoyl-oleoylglycerol (POP). SAXS-measurements supported this thesis by revealing long spacings with varying length, while short spacings obtained in WAXD consistently indicated the orthorhombic subcell of a β′-polymorph. The data suggest that the internal seeding effect of PPP in palm olein, as applied in industrial crystallization, in fact relies on substantial miscibility with POP, rather than on a heterogeneous nucleation effect resulting in two separate crystalline phases. The miscibility of PPP and POP then would result in a facilitated alignment of POP-molecules in the direction of the methyl end (or lamellar) plane and higher growth rates. Secondly, a peculiar change in crystal morphology could be consistently linked with the occurrence of a sharp melting peak at 24 °C in DSC, as well as with a substantial increase of the viscosity of the crystal suspension. This morphological shift concurred with an increased incorporation of palmitoyl-oleoyl-stearoylglycerol in the lattice, possibly giving rise to eutectic crystallization in lamellar crystal structures instead of the commonly observed dendritic crystals. The clarification of both phenomena provides a valuable, profound insight in the crystal growth step in the processing of edible oils.