FT–IR studies on polymorphism of fats: molecular structures and interactions

Abstract FT–IR analyses have been made on polymorphic structures of food fats, employing newly developed techniques such as attenuated total reflection (ATR), micro-probe polarized, oblique transmission, reflection absorption spectroscopy (RAS), etc. Two fat crystals, 1,2-dipalmitoyl-3-myristoyl-sn-glycerol (PPM) and 1,3-distearoyl-2-oleoyl-sn-glycerol (SOS) were focused on: PPM is a β′-stable fat and SOS is the major component of cocoa butter. The stearoyl chains in SOS were fully deuterated, so that the FT–IR spectra of the oleoyl and stearoyl chains were differentiated. As for β′1 form of PPM, the conformations of three acyl chains with respect to the glycerol group and the inclination of the acyl chains against the O⊥ subcell axes and the lamellar plane were observed. In five polymorphs of SOS, it was found that the conformational ordering of stearoyl chains took place in a less stable form, γ form, whereas the ordering of oleoyl chains occurred in a more stable form, β form. These results indicate that the FT–IR spectroscopic analyses are sensitive to molecular-level structures of the polymorphic forms of fats.