Crystallization, fats crystal formation

When TAGs in the liquid state are mixed, no changes in heat or volume are observed (Walstra et al., 1994). However, ideal behavior is not observed in the solid phase of milk fat (Timms, 1984 Walstra et al., 1994). As a result, the melting curve of milk fat does not equal the sum of its component TAGs (Walstra et al., 1994). Mulder (1953) proposed the theory of mixed crystal formation to explain the complex crystallization behavior of milk fat. Mixed or compound crystals contain more than one molecular species (Rossell, 1967 Mulder and Walstra, 1974). Mixed crystals form in natural fats, like milk fat, which are complex mixtures of TAGs (Mulder, 1953 Sherbon 1974 Walstra and van Beresteyn, 1975b Timms, 1980 ... 

Although the a-polymorph is meta-stable, it can have a relatively long lifetime in milk fat at low temperatures compared to other fats (Walstra et al., 1994). The a-crystal may be stabilized by the formation of compound crystals in milk fat (Walstra et al., 1999). A consequence of both polymorphism and mixed crystal formation in milk fat is that the material is rarely at equilibrium (Walstra et al., 1994). 

Tietz and Hartel (2000) studied the effects of removing or adding minor components naturally present in milk fat on the crystallization of milk fat-cocoa butter blends. They suggested that at low concentrations, minor lipids act as sites for nucleation and promote the rate of crystallisation and at higher concentrations inhibit crystallisation. They concluded that the presence of minor lipids, at the concentrations naturally occurring in milk fat, were sufficient to affect crystallisation rates, chocolate microstructure and fat bloom formation in chocolate. 

An awareness of crystal packing characteristics and polymorphism helps one to understand incompatibility problems of different fats. Crystal formation has specific demands, and individual crystals in mixed systems each consists of only one species ofTAG. However, surfactants and other molecules can act as impurities and interrupt crystal growth. Different TAG are considered compatible when they co-crystallize as separate crystals under the same conditions without the formation of a eutectic. 

The oil used for mayonnaise manufacture must remain liquid even during storage at refrigerator temperatures. If some portion of the oil solidifies, the fat crystals disrupt the surface film and the emulsion breaks. The oil is usually winterized, i.e., it is held at a low temperature for 1 or 2 days and the solid fraction is removed. Polyglycerol esters may also be added to the oil to inhibit fat crystal formation to prevent this problem. 

Initially, growth of crystals from the interior of fat droplets most likely follows a similar mechanism as was observed in the bulk. In the case of the PSCO system, fast nucleation of fine crystals within the bulk phase was followed by growth of larger crystals outward and growth at the interface (Fig. 3). In the lard system, slow growth of fine crystals was followed later by slower formation of larger crystals (Fig. 4). It was at sites where these larger crystal formations protruded to the exterior of the droplet that partial coalescence was observed by microscopy. 

A great deal of information about polymorphic changes has been obtained from studying pure triacylglycerols (TAG). However, in commercial fats, especially if they contain several ingredients, it may be more difficult to predict the likelihood of p crystal formation to occur. Factors that influence the stability of the p poly-... 

From his data on four kinds of fish, Dyer (21,22) suggested that fatty fishes are more stable in frozen storage than lean fishes. However, more recent data indicate that species differences may sometimes be more important than fat content (2,43,44). For example, the pattern of ice crystal formation differs between Alaska pollack and yellowtail muscles (34) and this may influence stability. 

The three commonly observed fat crystal forms are the a, ( >, and ( > forms. The ( ) form, with small and needle-like crystals which form smooth and fine-grained structures, is the most desired form in shortening and margarine applications. Oil composition plays an important role in crystal formation. Unmodified soybean oil has a tendency to form -crystals but the hydrogenated soybean oil can be crystallized in the ( /-form. Controlled crystallization (under defined conditions of temperature, time and mixing) and tempering is used to manipulate or stabilize the crystal forms to achieve products with the desired functional properties. 

A crystal nucleus is the smallest crystal that can exist in a solution and is dependent on the concentration and temperature. Spontaneous nucleation rarely occurs in fats. Instead heterogeneous nucleation occurs on solid particles or on the walls of the container. Once the crystals are formed, fragments that drop-off may either redissolve or act as nuclei for further crystal formation. " ... 

The added shear causes controlled partial coalescence (enhanced by the adsorbed emulsifier), causing air bubbles ( 50-150 im) to be trapped in clumped fat globules ( 0.5-0.8pm droplets) and also ice crystal formation ( 25-50pm). 

Cryo-SEM (examination at temperatures below - 80°C) is suitable for high-fat foods and foods of plant origin such as cooked vegetables, which could be altered by other methods involving dehydration. Rapid freezing of the samples is essential for the prevention of artifacts arising from ice-crystal formation. 

Gum arabic is used as an emulsifier and stabilizer, e. g., in baked products. It retards sugar crystallization and fat separation in confectionery products and large ice crystal formation in ice creams, and can be used as a foam stabilizer in bever ages. Gum arabic is also applied as a flavor fixative in the production of encapsulated, powdered... 

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