Crystallization dairy products

A good compilation of the functions of fats in various food products is available (26). Some functions are quite subtle, eg, fats lend sheen, color, color development, and crystallinity. One of the principal roles is that of texture modification which includes viscosity, tenderness (shortening), control of ice crystals, elasticity, and flakiness, as in puff pastry. Fats also contribute to moisture retention, flavor in cultured dairy products, and heat transfer in deep fried foods. For the new technology of microwave cooking, fats assist in the distribution of the heating patterns of microwave cooking. 

Crystallization of milk fat largely determines the physical stability of the fat globule and the consistency of high-fat dairy products, but crystal behavior is also complicated by the wide range of different triglycerides. 

By pre-treating milk with lactase, all adults can enjoy milk and a whole range of other lactose-free dairy products can be made such as ice cream and yogurt. In the production of ice cream, lactose hydrolysis can also be used to improve certain properties such as the texture, sweetness and tendency to crystallize. The increased sweetness is also advantageous in the manufacture of flavored milk products because less sugar needs to be added. 

The tendency of lactose to form supersaturated solutions that do not crystallize readily causes problems in many dairy products unless adequate controls are exercised. The problems are due primarily to the formation of large crystals, which cause sandiness, or to the formation of a lactose glass, which leads to hygroscopicity and caking (Figure 2.9). 

The water sorption characteristics of dairy products (like those of most other foodstuffs) are governed by their non-fat constituents (principally lactose and proteins). However, in many milk and whey products, the situation is complicated by structural transformations and/or solute crystallization. 

The sorption behaviour of a number of dairy products is known (Kinsella and Fox, 1986). Generally, whey powders exhibit sigmoidal sorption isotherms, although the characteristics of the isotherm are influenced by the composition and history of the sample. Examples of sorption isotherms for whey protein concentrate (WPC), dialysed WPC and its dialysate (principally lactose) are shown in Figure 7.13. At low aw values, sorption is due mainly to the proteins present. A sharp decrease is observed in the sorption isotherm of lactose at aw values between 0.35 and 0.50 (e.g. Figure 7.13). This sudden decrease in water sorption can be explained by the crystallization of amorphous lactose in the a-form, which contains one mole of water of crystallization per mole. Above aw values of about 0.6, water sorption is principally influenced by small molecular weight components (Figure 7.13). 

Cooling solutions to below their freezing point results in the formation of ice. If solutions of sugars are cooled rapidly, non-equilibrium ice formation occurs. This is the most common form of ice in frozen dairy products (e.g. ice-cream). Rapid freezing of ice-cream mixes results in the freeze concentration of lactose and other sugars, resulting in supersaturated solutions if the temperature is too low to permit crystallization. The rapid cooling of lactose results in the formation of a supersaturated, freeze-concentrated amorphous matrix. 

Detailed studies on the growth rates of the individual faces of a-lactose crystals have appreciably increased our understanding of the crystallization process. All the habits of lactose crystals found in dairy products are crystallographically equivalent to the tomahawk form different relative growth rates on the crystal faces account for the var-... 

In dairy products, crystallization is more complex. The impurities (e.g., other milk components), as far as lactose is concerned, may interfere with the crystalline habit. As a result, the crystals tend to be irregularly shaped and clumped, instead of yielding the characteristic crystals obtained from simple lactose solutions. In some instances, the impurities may inhibit the formation of nuclei and thus retard or prevent lactose crystallization (Nickerson 1962). 

The crystallization principles previously discussed are applied in processing dairy products, such as sweetened condensed milk, instant milk powder, stabilized whey powders, lactose, and ice cream. 

The crystallization behavior of milk fat is complex, owing, in large part, to its complicated composition. By manipulating composition and crystallization conditions, milk fat and dairy products with unique structures and mechanical properties can be designed. Understanding the relationships between composition, crystallization, structure, rheology and texture is a powerful tool in this regard. 

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). 

It is already clear, however, that ultrasound can provide unique information regarding crystallization processes and as the technology and underlying data interpretation models improve it is likely to make a very significant contribution to improving the quality of dairy products. In particular, better understanding of the interaction of ultrasound fields with proteins is required. 

Seeding is a commonly used procedure to prevent the slow crystallization of lactose and the resulting sandiness in some dairy products. Finely ground lactose crystals are introduced into the concentrated product, and these provide numerous crystal nuclei. Many small crystals are formed rapidly therefore, there is no opportunity for crystals to slowly grow in the supersaturated solution until they would become noticeable in the mouth. 

The second column shows models of two dairy products milk and molten butter. Milk consists mainly of a watery solution with about 4% by volume of fat (oil). This is dispersed in the form of droplets with a diameter of a few micrometres. This is an example of an oil-in-water or OAV emulsion. Molten butter is a mirror of this system, with water dispersed as droplets of a few micrometres. Molten butter is a water-in-oil or W/O emulsion. When it is cooled the fat crystallizes partly, and we get a three-phase structure butter. 

Direct measurement of thermal fat crystal properties NMR imaging of dairy products Acidification of pressure-treated milk Characterization of semi-solid state in milk fat Analysis of Maillard products in milk... 

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