Crystallization, fats crystal size

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

During experiments the authors controlled crystallization temperature with an accuracy of 0.2°C/min. For cooling rates, a difference of2°C/min may be significant depending on the temperature range and type of fat. Agitation rates that differ by 50 rpm produce significant differences in crystal size. 

The growth rate of fat crystals can be found by measuring the particle size distribution as a function of time, and calculating growth rate as the rate of increase of maximum crystal size (van Putte and Bakker, 1987). 

Many interrelated factors influence the texture of plastic fats. Fatty acid and glyceride composition are basic factors in establishing the properties of a fat. These factors, in turn, are related to solid fat content, crystal size and shape, and polymorphic behavior. Once the crystal network is formed, mechanical treatment and temperature history may influence the texture. 

In the case of lipid fractionation, however, a different crystal size distribution is desired. As the fat crystals are to be separated from the liquid phase, uniform crystals of distinct size and shape are needed for the most efficient separation. For the most efficient separation by filtration, reasonably large (200 to 300 pm) crystals of fairly uniform size (narrow distribution of sizes) are needed. Fractionation technologies carefully control nucleation and growth to produce this uniform distribution of crystals to enhance filtration and separation of the high-melting stearin phase from the low-melting olein phase. 

Crystallization control. Lecithin can control crystallization in various food systems. In foods containing sugars or fats, the presence of as little as 0.5% lecithin can produce altered crystal sizes and stmctures that can have positive effects on product texture and viscosity. This is important in cookie fillings, butter-containing maple syrups, ice cream toppings, and similar products (7). 

These characteristics are related to a number of variable factors. These are temperature, concentration of the disperse phase or solid fat content, crystal size, crystal size distribution, crystal shape, interparticle forces of van der Waals type and mechanical treatment (2). 

The texture of fat is influenced by a number of factors, including fatty acid and glyceride composition, solid fat content, crystal size and shape, nature of die crystal network, polymorphism, mechanical treatment, and temperature history (1). Many of these factors are interrelated, making it difficult to establish the independent effect of each. It is well recognized diat different fats preferentially occur in either P or p form (Table 1). 

Stabilizers are macro-molecules which prevent the formation of large ice crystals. The average ice crystal size should be between 40-50 pm. Stabilizers increase viscosity, improve the smoothness and resistance to melting. The average content depends on the kind of stabilizer, but it is typically between 0.01% and 0.2%. The use of stabilizers corresponds to the content of fat and solids. Stabilizers are important when the fat content is below 12% or the total content of solids is below 40%. 

In some processes (Table 13.2), only a few crystals are desired with proper size distribution and shape for efficient separation. Refining of sugars, fractionation of fats, and freeze concentration are examples of processes where control of crystallization is necessary to provide efficient separation. In both types of products and processes, control of the crystal size distribution is important to efficient operation and production of a high-quality product. 

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