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

Abstract

Crystallization and polymorphic evolutions of triacylglycerols (TG), at 4 o C, in cream and anhydrous milk fat (AMF) samples were studied using high-sensitivity differential scanning cal- orimetry (DSC), time-resolved synchrotron X-ray diffraction (XRD) and density measurement, and compared as a function of time after quenching from 60 to 4 o C. The evolution of the two crystalline structures initially formed in both samples, corresponding to a metastable 2L (47 A) and a 3L (70.5 A) lamellar organization with hexagonal chain packings (α form), were monitored in isothermal condi- tions at 4 o C as a function of time t. For t > 15 min, short spacings showed the formation of a β' pack- ing of the chains in coexistence with the α form. In AMF, the occurrence of some β form was also recorded. For long spacings, a 2L (39 A) structure was formed in cream with the 3L (70.5 A) structure still being preponderant after conditioning for 30 min at 4 o C. In AMF samples, a TG polymorphic transition 3L (70.5 A) → 2L (39 A) + 3L (66 A), correlated with an exothermic peak recorded simul- taneously by DSC, was clearly visible. At t > 100 h conditioning at 4 o C, the same crystalline struc- tures were found in cream and AMF with the coexistence of at least a 2L (40.5 A) and a 3L (54.2 A) longitudinal organization, associated with α, β'2 , β'1 and β packings of the chains. The melting be- haviors of cream and AMF were monitored on heating at 2 o C.min -1 by coupled XRD as a function of temperature and DSC experiments. The absence of further structural evolution and polymorphic tran- sition until final melting demonstrates the relative stability of the structure formed. The evolution of cream density after quenching to 4 o C 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 o C.