Effect of minor components and temperature profiles on polymorphism in milk fat

Time-resolved synchrotron X-ray diffraction techniques were used to study the effect of cooling rates and temperature on the crystallization dynamics and polymorphism of anhydrous milk fat (AMF) and milk fat triacylglycerols (MFTs). The crystallization of AMF at fast cooling rates and low final temperatures proceeded through the metastable phase alpha and resulted in the formation of a mixture of phases beta' and beta. Slow cooling rates and high final temperatures resulted in the formation of phase beta' only, with no formation of phase alpha nor phase beta. Moreover, in the absence of polar lipids, MFTs had a decreased tendency to form the alpha and beta phases. The formation of the beta phase was largely dependent on the initial amount of alpha phase formed. At high cooling rates and low crystallization temperatures, polar lipids may have initiated the crystallization process together with a high melting fraction of milk fat in the a phase. In time, this metastable phase transformed to the beta phase. When no a phase was initially formed (high temperatures and/or slow cooling rates), no beta phase was formed either. Small-angle X-ray diffraction could be used to monitor compositional variation of crystals over time, and demonstrated that minor polar lipids present in the milk fat delayed the onset of crystallization and reduced the rate of crystal growth.