Nucleation food crystallization

From the general point of view, ultrasound has advantages regarding the measurement of dairy product quality in that it may be implemented inline, non-invasively and even using non-contacting techniques such as laser excitation and detection (Mulet et al, 1999). Ultrasound can also be safe, hygienic and economic in implementation, all characteristics desirable for any technique for the measurement of food quality (Povey, 1997a). Moreover, it can reveal aspects of the quality of dairy products which are not measurable by current techniques. An example is the extraordinary capability of ultrasound to detect crystal nucleation (Povey et al., 2001 Hindle et al., 2002). 

In foods something similar occurs during the crystallization of fats. Natural fats have a wide compositional range, implying that several different kinds of crystals have to be formed. This needs undercooling, but as soon as the first crystals have formed, other crystals nucleate on the existing ones. This must be due to epitaxy, since the various crystals are very similar in lattice structure. It is indeed observed that the undercooling needed is only by about 2 K. 

In Figure 4.2, crystallization of sucrose can only occur at conditions of temperature and composition that fall between the solubility and glass transition temperature lines. On either side of this crystallization envelope, there is either no thermodynamic driving force (dilute system) for crystallization or nucleation is constrained by kinetic effects due to limited molecular mobility. Thus, processing conditions must be controlled so that the system falls within the crystallization envelope to ensure crystallization. Furthermore, the point within the crystallization envelope at which crystallization occurs can significandy affect the nature of the crystalline phase in the food, and thus affect the material properties. 

It is this variability, often uncontrolled, that leads to problems in controlling nucleation in food systems. Even in relatively pure systems, like carefully controlled supersaturated sugar systems, nucleation may occur over a very wide time span. For example, van Hook and Frulla (1952) found that identical supersaturated sucrose samples held at the same temperature nucleated over a very wide time span, with some crystallizing as quickly as 3 hours while other samples took over a day to nucleate. 

In most food systems, a wide variety of ingredients are used to provide the desired textural and sensory characteristics. Thus, crystallization nearly always occurs in complex systems where phase behavior may be difficult to ascertain accurately. Furthermore, the specific interactions among ingredients can lead to significant inhibition of nucleation. Because of these often complex interactions, it is frequently difficult to predict nucleation behavior. 

Controlling lipid crystallization in foods has proven to be a technical challenge over the years. Despite a considerable amount of study, controlling the complex interactions between the various lipid components during crystallization remains essentially an empirical process of studying the effects of various operating parameters on crystal formation. Further work on the fundamental principles of lipid nucleation, growth, and polymorphic transformation is needed to truly control crystallization of lipids in foods. 

Currently, dry fractionation of anhydrous milkfat is performed by two conventional systems—Tirtiaux and De Smet (both from Belgium)—which are bulk crystallization processes. The widely used Tirtiaux dry fractionation process enables one-step or up to hve-step fractionation of anhydrous butter oil at any temperature, ranging from 50°C to 2°C (37, 110-113). The milkfat fractions thus obtained can be used as such or the fractions can be blended in various proportions for use as ingredients in various food-fat formulations. The major shortcoming inherent in this system is the long residence time (8-12 h) for nucleation and crystal growth. 

Fat is mainly composed of triacylglycerol (TAG) molecules. These TAG are polymorphic, i.e., they can crystallize under several crystalline forms (2). To get a stable food at the end requires the fat to be crystallized in the stable polymorphic form to avoid possible further transformation. Cocoa butter does not form in the stable Form V by simple cooling. It needs a well-controlled thermal path, performed under shearing conditions, called tempering. This specific temperature program is necessary to first nucleate enough crystals and then only keep the Form V crystals and melt the other metastable ones. Shear is also required during this process to get sufficiently rapid and intense nucleation of the stable forms. 

A way to preserve vegetables vs. time is first to blanch them and afterwards to cool them in order to transform available liquid water into ice. This process is called "freezing" and has been established as an excellent method for preserving high quality in foods (Reid, 1983). Nucleation (formation of ice crystals) and crystal growth are the two major thermal events of the... 

Next we come to phase transitions. Chapter 14 mentions the various phase transitions that may occur, such as crystallization, gas bubble formation, or separation of a polymer solution in two layers it then treats the nucleation phenomena that often initiate phase transitions. Chapter 15 discusses crystallization, a complicated phase transition of great importance in foods. It includes sections on crystallization of water, sugars, and triacylglycerols. Chapter 16 introduces glass transitions and the various changes that can occur upon freezing of aqueous systems. 

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