The smoothness and perceived quality of an ice cream depends in large part on the small size of ice crystals in the product. Understanding the mechanisms responsible for producing the disc-shaped crystals found in ice cream will greatly aid manufacturers in predicting how processing and formulation changes will affect their product. Because ice cream mix is opaque, it has not yet been possible to observe ice crystallization in ice cream in situ. Studies to date, therefore, have used analogues or have related observed effects to a hypothesized mechanism. Still, some elements of the crystallization mechanism are well accepted. Because of the large supercooling at the freezer wall, ice nucleates there before being swept into the bulk of the freezer. In the bulk, heat and mass transfer cause some crystals to melt and others to grow. By the time the ice cream reaches the freezer exit, the ice crystals have become small, rounded discs.
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...
The purpose of this work was to investigate iciness perception and other sensory textural attributes of ice cream due to ice and fat structures and mix viscosity. Two studies were carried out varying processing conditions and mix formulation. In the 1st study, ice creams were collected at -3, -5, and -7.5 °C draw temperatures. These ice creams contained 0%, 0.1%, or 0.2% emulsifier, an 80:20 blend of mono- and diglycerides: polysorbate 80. In the 2nd study, ice creams were collected at -3 °C draw temperature and contained 0%, 0.2%, or 0.4% stabilizer, a blend of guar gum, locust bean gum, and carrageenan. Multiple linear regressions were used to determine relationships between ice crystal size, destabilized fat, and sensory iciness. In the ice and fat structure study, an inverse correlation was found between fat destabilization and sensory iciness. Ice creams with no difference in ice crystal size were perceived to be less icy with increasing amounts of destabilized fat. Destabilized fat correlated inversely with drip-through rate and sensory greasiness. In the ice cream mix viscosity study, an inverse correlation was found between mix viscosity and sensory iciness. Ice creams with no difference in ice crystal size were perceived to be less icy when formulated with higher mix viscosity. A positive correlation was found between mix viscosity and sensory greasiness. These results indicate that fat structures and mix viscosity have significant effects on ice cream microstructure and sensory texture including the reduction of iciness perception.
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