Time-resolved SAXS/WAXS study towards understanding the effect of triglyceride composition on polymorphic transition

Abstract: The crystallization kinetics of structuring fats - generally complex triglyceride (TAG) mixtures of various TAG melting groups - are characterized by diverse aspects ranging from polymorphic transition to fractional crystallization. The co- or subsequent crystallization of different fractions is determined by the mixture's phase behavior, which is directly linked to the mixture's composition. Events such as de-mixing of immiscible solid phases can be superimposed by the polymorphic transition of single fractions or even triggered by such. This contribution presens the results of a time-resolved SAXS/WAXS study on model fat blends containing varying amounts of long-chain saturated TAGs (H3) and mono-unsaturated TAGs (H2U). The effect of composition was studied by modifying the ratio of H3 to H2U TAGs (0.4 vs. 0.8), and, further, by the variation between stearic and palmitic-based H3 TAGs (SSS vs. PPP). The SAXS and WAXS patterns were acquired upon cooling at different rates (1, 5, 10 K min^-1) and during the isothermal hold at 25 °C (298 K). The crystallization kinetics were followed by evaluating the 1st order reflex of the SAXS region, from which a mean d-spacing per time frame was determined. The WAXS region allowed general phase identification. The data reveal different kinetic pathways depending on the molecular makeup of the H3 fraction (i.e., either stearic or palmitic). The mainly palmitic-based blends show the crystallization of α transitioning into (co-existing) β' and β crystals. The blends with added stearic-based H3 show a longer persisting α phase that transitions into β even before a β' phase can be identified. Via relating the fat blends' composition to kinetic pathways, this research offers a step towards understanding phase separation upon polymorphic transition and how it can be influenced by TAG chain length, concentration, and cooling rate.
This contribution builds on a previous conference paper (Seilert, J.; Floter, E. (2022, May 2). Relating polymorphic transition and triglyceride composition. AOCS Annual Meeting & Expo).