Abstract: In this study, anhydrous milk fat was fractionated by stepwise melting into high-melting fraction (S40), middle-melting fraction (L40 and L30) and low-melting fraction (L20). Fatty acid (FA) and triacylglycerol (TAG) composition of each fraction were analyzed by gas chromatography-mass spectrometry (GC-MS). The crystal form and the thermodynamic profile were evaluated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. From the results, S40 was found to be rich in saturated long-chain fatty acids but lack short-chain fatty acids and unsaturated fatty acids, while the opposite was true for L20. L40 and L30 were between L20 and S40. The heat map showed that high-melting triacylglycerols (TAGs) were concentrated in S40, whereas low-melting TAGs predominated in L20. L40 and L30 were rich in middle-melting TAGs. There were striking differences in the thermodynamic profiles of the milk fat fractions, whose crystallization temperature (TC) and melting temperature (TM) followed the decreasing order of S40 > L40 > L30 > L20 (P < 0.05). When the the TC or TM was higher, the phase transition enthalpy was higher. Moreover, the β’ crystal was dominant in milk fat (MF), L40 and S40, but not in L20 or L30. Crystallization of MF, S40 and L40 showed the formation of 2L (40.76 and 13.49 ?) and 3L (29.44 ?) lamellar structures, whereas L30 and L20 showed the formation of only 2L. In summary, higher degree of fat saturation leads to stronger crystal stability and larger enthalpy during the crystallization-melting process, which is suitable for the preparation of hard dairy products. In contrast, lower degree of fat saturation is suitable for preparing soft dairy products. This research provides a theoretical basis for the different requirements of milk fat production.

Key words: milk fat, dry fractionation, chemical composition, thermodynamics, crystal form