Thermal and structural behavior of milk fat 2. Crystalline forms obtained by slow cooling of cream

Milk fat crystallization was studied using X-ray diffraction as a function of temperature (XRDT) and differential scanning calorimetry (DSC) analysis considering crystals formed during slow cooling of natural milk fat globules of cream. During cooling at /dT/dt/ = 0.15 degreesC/min from 55 to -8 degreesC, the crystalline varieties formed in fat globules by triacyglycerols (TGs) correspond to two double-chain-length organizations (2L) of 46.5 and 40 Angstrom and to two triple-chain-length stackings (3L) of 71.3 and 65 Angstrom. Nucleation occurs in the alpha form; then the alpha + beta' polymorphic forms coexist until the end of the cooling. The four crystalline varieties start to form within a 10 degreesC range, from about 21 degreesC, preventing separation of overlapped peaks by DSC recording. In a second step, the sample of cream was heated at 2 degreesC/min in the range -8 to +60 degreesC to follow the melting behavior of the crystals. XRDT measurements show the progressive transformations of the crystalline varieties correlated with endotherms and exotherms recorded by DSC. The 40-Angstrom structure takes advantage of the melting of the other species to grow until its melting. The comparison made with anhydrous milk fat behavior under the same conditions shows that crystallization is different in emulsion and in bulk. (C) 2001 Academic Press.