Coupling of time-resolved synchrotron X-ray diffraction and DSC to elucidate the crystallisation properties and polymorphism of triglycerides in milk fat globules

Crystallisation properties and polymorphism of triacylglycerols (TG) in natural milk fat globules are studied with an apparatus permitting the coupling of time-resolved synchrotron X-ray diffraction (XRD) and high-sensitivity differential scanning calorimetry (DSC) for cooling rates 0.1 < Rcooling < 1000 ◦C·min−1. We showed that the fat phase of dairy products displays a complex polymorphism. For the first time, six different types of crystals were identified, several of them in coexistence, and their time- and temperature-dependent evolutions were quantitatively monitored. They correspond to lamellar structures with double-chain length (2L: 40.5–48 Å) and triple-chain length (3L: 54–72 Å) organisations of TG. Depending on the cooling rate, at least five crystalline subcell species were observed at wide angles: α and sub-α, two β’ and one β. All these crystalline structures coexist with a liquid phase even at low temperatures. Thermal events recorded by DSC were related to the structural information on the organisation of TG obtained by XRD. Moreover, we showed that crystallisation in fat globules induces a higher disorder and a smaller size of crystals than in bulk, i.e. in anhydrous milk fat. Furthermore, XRD permitted the identification of different crystallisation behaviour in natural milk fat globules with different sizes, which could be implicated in the manufacture of dairy products involving tempering periods in the technological process (e.g. butter, ice cream, whipped products). The increased knowledge of milk fat crystallisation in waterdispersed and highly complex food products might have a certain value for technical applications as well as for the development of new processes and dairy products.