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', β
2 and β
1 at ambient temperatures. The melting points increased in the order described above, being α the lowest and β
1, 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 β
1, 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, β
2 and β
1 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 β
2 to β
1 of the solid fat fractions of cocoa butter.
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