Effect of processing conditions on microstructure of milk fat fraction/sunflower oil blends

Abstract

The effect of processing conditions on the crystallization of blends of a high-melting milk fat fraction and sunflower oil was investigated. Two cooling rates were selected for all studies: 0.1°C/min (slow rate) and 5.5°C/min (fast rate). Blends were crystallized in two conditions: (i) with agitation in an 80-mL crystallizer (dynamic), and (ii) on a microscope slide without agitation (static). The selected crystallization temperatures were 25, 30, and 35°C for both cooling rates. Photographs of the development of crystals with time were taken in both static and dynamic conditions, and the crystal size distribution was determined at the moment that the laser signal reached its peak. Photographs showed that when samples were cooled slowly, crystals had a more regular boundary, appeared to be more densely arranged, and were larger. In dynamic conditions, crystal sizes were smaller and the background contained numerous small crystals, which were not found in statically crystallized samples. All images showed that crystals were not single crystals, but grew by accretion.The formation of a solid from a solution or a melt is a complicated process in which molecules must first come in contact, orient, and then interact to form highly ordered structures known as nuclei. For a natural lipid to crystallize, it must be supersaturated or supercooled to provide a driving force for both crystallization steps (1). Nucleus formation can be encouraged by stirring, or the nucleation process can be circumvented by seeding the supercooled liquid with tiny crystals of the type ultimately desired. Following nucleation, enlargement of these nuclei (crystal growth) progresses at a rate dependent on operating parameters such as temperature, agitation rate, and remaining composition in the liquid phase (2).Differential thermal analysis, X-ray diffraction, and IR spectroscopy are techniques often used to characterize the morphological form of crystals. Although polarizing microscopy already has found wide application in the food industry, the technique has recently started to gain attention in polymorphism and crystallization studies of fats. Light microscopy proved to be an excellent method for the study of early events in the crystallization of fats (3). It is also an increasingly used technique for studying the microstructure and composition of food systems in relation to their physical properties and processing behavior (4). In many applications of edible fats, however, the morphology and number of glyceride crystals determine the suitability of the fat for a given purpose. For example, the morphology of TAG crystals is related to the possibility of network formation to give a plastic fat. The β′-polymorph forms thin needles, and, because of their shape, only the β´-crystals are effective in forming networks, as each needle in a disordered array will touch adjacent needles (5). Early microscopic studies of the polymorphic forms of single-acid TAG have shown that the crystal forms exhibit a wide range of microscopic appearances. More rec...

Palm-based diacylglycerol fat dry fractionation: effect of crystallisation temperature, cooling rate and agitation speed on physical and chemical properties of fractions

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Effect of sucrose esters on the crystallization behavior of bulk oil systems

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