Polymorphic Structures and Transitions of Triglycerides and Complex Fats: A Guide to X-Ray Scattering Approaches

Abstract

This contribution provides a comprehensive review of nanostructural aspects in pure triglyceride compared with more complex, real fat systems. Alongside this journey on understanding the crystallization of triglycerides, we provide a practical guide on using X-ray scattering to its fullest extent. Data from public-domain literature of monoacid and mixed-acid saturated and mono-unsaturated triglycerides are reviewed in great depth. On the nanoscale, lamellar stacking and hydrocarbon tilt angles are discussed, and on the molecular scale, trends in the chain packing density and concomitant geometry changes are explained. Useful tools to evaluate lab-scale and synchrotron X-ray scattering data are presented, including (i) electron density profile calculations, decomposing the lamellar repeat distance into the bilayer and monolayer contributions, (ii) refined estimations of the chain tilt angle in the bilayer region, (iii) crystallite size and strain investigations, and (iv) the determination of the area per hydrocarbon chain. Further, the review summarizes representative crystallographic data over the last decades, including extensive lists of long- and short-spacing data. Generalizing the introduced model-free methods to real fat systems makes it possible to also analyze complex phase transitions and their solid fat content as derived from the wide-angle scattering. The main aim of this contribution is to provide a robust set of X-ray scattering methods for a detailed evaluation of polymorphic states and phase transitions supporting research efforts at the interface of academia and industry.