Thermal and Structural Behavior of Milk Fat

Abstract

Crystallization of triacylglycerols (TG) within milk fat globules of creams is studied with an instrument coupling time-resolved synchrotron X-ray diffraction (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) at cooling rates of -3 and -1 degrees C/min from 60 to -10 degrees C and compared to that of the anhydrous milk fat (AMF). Simultaneous thermal analysis permits correlation of the formation of the different crystalline species monitored by XRDT to the DSC events. Under the above cooling conditions, milk fat TG sequentially crystallize, within the globules, from about 19 degrees C, in three different lamellar structures with double-chain length (2L) stackings of 47 and 42 A and a triple-chain length (3L) stacking of 71 A, all of alpha type, which are correlated to two or three overlapped exothermic peaks recorded by DSC. Compared to what is observed for AMF, TG crystallization in emulsion (i) favors sub-alpha formation at low temperature and (ii) induces layer stacking defects in 3L crystals. Subsequent heating at 2 degrees C/min shows numerous structural rearrangements before final melting, confirming that (i) cooling at -1 degrees C/min leads to the formation of unstable crystalline varieties in the dispersed state and (ii) a monotropic transition alpha-->beta' takes place. Similar behavior is observed for cooling at -3 degrees C/min and subsequent heating. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate, between >1000 and 0.15 degrees C/min, and stabilization time at 4 degrees C is given. Depending on the cooling rate, at least five crystalline subcell species are observed at wide angles, alpha and sub-alpha, two beta' and one beta. At small angles, at least six lamellar stackings are identified, three 3L and three 2L. However, a single subcell packing (e.g., alpha) might correspond to several longitudinal chain stackings, demonstrating the usefulness of the small-angle XRD technique. Reconstituted emulsions homogenized under different pressures are used to determine the influence of droplet size on crystallization. The decrease of droplet size induces (i) a higher supercooling/supersaturation and (ii) a higher disorder and/or a smaller size of TG crystals within the emulsion droplets. At the supramolecular scale, polarized light microscopy shows that various cooling rates applied in situ using a temperature-controlled stage directly influence crystal sizes and their type of organization within milk fat globules. The faster the cooling rate, the smaller the size of the crystals within the globules.

Factors governing partial coalescence in oil-in-water emulsions

“…In general, the higher the cooling rate, the larger the undercooling before nucleation occurs and the smaller the crystals within the globule [22]. The subsequent t,T-program and storage temperature will then determine the final size of the crystals.…”

“…The type of globule depends not only on the properties of the fat [17] but also on the actual temperature [37] and the temperature history [20,22].…”

“…Small pieces of growing crystals defuse away from the surface and initiate new crystal formation. Furthermore, the extent of undercooling can vary between a few and 30 K depending on the cooling rate [22], the size of the emulsion droplets [22] and the composition of the oil phase and of the oil-water interfacial layer [23].…”

“…The time during which the globules are close together is much longer and the film between the globules is thinner, leading to a higher rate of partial coalescence. • Larger crystals can be formed in larger globules [22,44] which, firstly, which lead to greater protrusion distances [2,3] and secondly to a larger pore size in the network. The greater protrusion distance involves a higher coalescence rate and the larger pore size makes the liquid oil more readily available as a lubricating agent for the creation of a permanent junction [1].…”

Effects of Addition of a Palmitic Sucrose Ester on Low-Trans-Fat Blends Crystallization in Bulk and in Oil-in-Water Emulsions

“…A higher supercooling was always necessary for crystallizing the fat phase in emulsions than in bulk systems 19,20 . For HMF, no solids were detected at temperatures of 28°C or higher.…”