Relations between crystallisation mechanisms and microstructure of milk fat

Using differential scanning calorimetry, synchrotron time resolve X-ray diffraction and pulsed nuclear magnetic resonance, crystallisation mechanisms of milk fat were elucidated. Under the same crystallisation conditions, the microstructure of the milk fat was analysed with confocal laser scanning microscopy and oscillatory rheology. The milk fat was cooled without agitation to 20 °C at two different cooling rates, i.e., 0.1 and 10 °C min−1. Thereafter, the isothermal crystallisation at 20 °C was monitored. Faster cooling resulted in a two-step crystallisation, and a microstructure that comprised smaller and more uniform crystals than was the case with slower cooling. Consequently, the final texture of the faster cooled milk fat was firmer, i.e., higher complex modulus, than that of the slower cooled milk. X-ray diffraction showed that the two-step crystallisation involved a polymorphic transition from α to β′ phase. Furthermore, it was demonstrated that oscillatory rheology is very useful for detecting polymorphic transition during crystallisation of anhydrous milk fat.