26-6 |
Crystallization kinetics and structure of milkfat fractions |
A. CISNEROS and A. G. Marangoni. Dept. of Food Science, Univ. of Guelph, Food Science Bldg., Guelph, ON N1H 2W1, Canada Milkfat possesses a variable and complex chemical composition, thus milkfat applications are limited by its nature. By fractionating milkfat, its chemical composition can be better controlled. The objective was to study the crystallization behavior and crystal habit (i.e. polymorphism and microstructure) of milkfat fractions under static isothermal crystallization conditions. A multi-step solvent fractionation was carried out to obtain three fractions with different physicochemical properties, namely high (HMF), medium (MMF), and low melting fractions (LMF). Crystallization kinetics were studied at 5, 15, and 25C by measuring the solid fat content (SFC) as a function of time by pulsed nuclear magnetic resonance. Microstructure was imaged by polarized light microscopy and fractal dimensions were calculated from the obtained micrographs by a box counting method. Polymorphism was studied by synchrotron x-ray diffraction. At all temperatures, HMF was the main fraction responsible for nucleation and the initial stages of crystal growth. MMF and LMF decreased the SFC of the mixtures and promoted two-step crystallizations. The two-step crystallizations were due to secondary crystallization and/or polymorphic transformations from the a form to a more stable b¢ form. High concentrations of LMF lead to lower degrees of space occupancy, i.e., a lower fractal dimension, and induced formation of a more aggregated microstructure. MMF displayed an ambivalent behavior: in the presence of HMF and at high degrees of undercooling (i.e., 5 and 15C) it behaved as a high melting fraction, but in the presence of LMF and/or at low degrees of undercooling (i.e., 25C) it behaved as a low melting fraction. By understanding the crystallization kinetics and crystal habit of the fractions and their blends, it is possible to control the crystallization behavior, shelf-life stability, and sensory characteristics of milkfat based products. New milk fat based products (i.e. cold spreads, sprayable butter oil, confectionery coatings for frozen desserts) can be designed to suit specific uses.
Session 26, Dairy Foods: General developments in dairy technology
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