Crystalline structures formed in cream and anhydrous milk fat at 4 °C

Crystallization and polymorphic evolutions of triacylglycerols (TG), at 4 degreesC, in cream and anhydrous milk fat (AMF) samples were studied using high-sensitivity differential scanning calorimetry (DSC), time-resolved synchrotron X-ray diffraction (XRD) and density measurement, and compared as a function of time after quenching from 60 to 4 degreesC. The evolution of the two crystalline structures initially formed in both samples, corresponding to a metastable 2L (47 Angstrom) and a 3L (70.5 Angstrom) lamellar organization with hexagonal chain packings (alpha form), were monitored in isothermal conditions at 4 degreesC as a function of time t. For t > 15 min, short spacings showed the formation of a beta' packing of the chains in coexistence with the alpha form. In AMF, the occurrence of some beta form was also recorded. For long spacings, a 2L (39 Angstrom) structure was formed in cream with the 3L (70.5 Angstrom) structure still being preponderant after conditioning for 30 min at 4 degreesC. In AMF samples, a TG polymorphic transition 3L (70.5 Angstrom) --> 2L (39 Angstrom) + 3L (66 Angstrom), correlated with an exothermic peak recorded simultaneously by DSC, was clearly visible. At t > 100 h conditioning at 4 degreesC, the same crystalline structures were found in cream and AMF with the coexistence of at least a 2L (40.5 Angstrom) and a 3L (54.2 Angstrom) longitudinal organization, associated with alpha, beta'(2), beta'(1) and beta packings of the chains. The melting behaviors of cream and AMF were monitored on heating at 2 degreesC. min(-1) by coupled XRD as a function of temperature and DSC experiments. The absence of further structural evolution and polymorphic transition until final melting demonstrates the relative stability of the structure formed. The evolution of cream density after quenching to 4 degreesC revealed two transitions, the amplitudes, rates and duration of which coincided with DSC and XRD experiments. All data are in agreement in demonstrating that two transitions, a fast and a slow one, occur in dairy products after fast cooling to 4 degreesC.