Polymorphic behavior was studied for series of 1, 3-di (saturated acyl) -2-oleoylglycerols in which the saturated acyl groups were palmitoyl (POP), stearoyl (SOS), arachidoyl (AOA) and behenoyl (BOB). The purity of the samples employed was 90.4% (POP), 91.3% (SOS) 83.2% (AOA), and 71.5% (BOB). Attention was directed to the identification of an independent polymorph by subjecting the sample to two thermal treatments : transformation in a crystalline state at different temperatures after chilling the melt at 0°C, and solidification of more stable polymorphs after rapidly melting the less stable forms. The long and short spacing spectra, examined by X-ray diffractometry and the melting point were used to determine the polymorph.
Five independent polymorphs were found to occur frequently in the four triacylglycerols examined : α, γ, pseudo-β', β2 and β1 at ambient temperatures. The melting points increased in the order described above, being α the lowest and β1, the highest. In addition, sub α appeared in SOS, AOA and BOB as a less stable form than α, where as β' was observed in POP, AOA and BOB with stability intermediate between α and γ. All the above polymorphs exhibited distinctively different short spacing spectra and melting points. The solid-state transformation occurred in a sequential way from (sub α) α to β1, implying the nature of the polymorphism to be monotropic. The chain length structure was a double chain for α and β' of the four triglycerides examined, and for pseudo-β', singly of POP. Triple chain length structure was found for γ, β2 and β1 in all the glycerides and pseudo-β' in SOS, AOA and BOB. Although rather complicated, the present findings showed good agreement with data in previous reports which have long been contradictory to each other. Taking the X-ray data and previous reports into account, the new nomenclature of the polymorphs discussed above was explained. Finally, the authors consider that the V-VI transformation of cocoa butter, responsible for the blooming phenomena of confectionery fats, may possibly be caused by polymorphic transformation from β2 to β1 of the solid fat fractions of cocoa butter.
