Lactose crystallization

The growth of lactose crystals in ice cream results in a serious texture defect known as sandiness. It is usually caused by temperature fluctuations, high serum solids in the mix, and high cabinet storage temperatures. The ratio of serum solids and moisture must be controlled. Hydrocolloids have some effect on controlling sandiness, but not as much as proper handling techniques and formulation. 

The two main assumptions underlying the derivation of Eq. (5) are (1) thermodynamic equilibrium and (2) conditions of constant temperature and pressure. These assumptions, especially assumption number 1, however, are often violated in food systems. Most foods are nonequilibrium systems. The complex nature of food systems (i.e., multicomponent and multiphase) lends itself readily to conditions of nonequilibrium. Many food systems, such as baked products, are not in equilibrium because they experience various physical, chemical, and microbiological changes over time. Other food products, such as butter (a water-in-oil emulsion) and mayonnaise (an oil-in-water emulsion), are produced as nonequilibrium systems, stabilized by the use of emulsifying agents. Some food products violate the assumption of equilibrium because they exhibit hysteresis (the final c/w value is dependent on the path taken, e.g., desorption or adsorption) or delayed crystallization (i.e., lactose crystallization in ice cream and powdered milk). In the case of hysteresis, the final c/w value should be independent of the path taken and should only be dependent on temperature, pressure, and composition (i.e.,... 

Another established application in the dairy industry is the hydrolysis of lactose in milk and whey by lactases. Diminished digestibility problems, increased sweetness and prevention of lactose-crystal formation are the results. The lactose hydrolysis is worked out as a case later in this chapter (section 3.6). 

Larhrib H, Martin GP, Prime D, Marriott C. Characterization and deposition studies of engineered lactose crystals with potential for use as a carrier for aerosolized salbutamol sulfate from dry powder inhalers. Eur J Pharma Sci 2003 19(4) 211-221. 

Partly delactosed whey is produced by concentrating cheese whey or casein whey sufficiently to exceed the solubility limit of lactose, followed by cooling, seeding with lactose crystals and removal of the crystalline lactose. The resulting liquor fraction is recovered and dryed. 

Problems arising from the crystallization of lactose in milk and whey powders may also be avoided or controlled by pre-crystallizing the lactose. Essentially, this involves adding finely divided lactose powder which acts as nuclei on which the supersaturated lactose crystallizes. Addition of 0.5 kg of finely ground lactose to the amount of concentrated product (whole milk, skim milk or whey) containing 1 tonne of lactose will induce the formation of c. 106 crystals ml-1, about 95% of which will have dimensions less than 10/an and 100% less than 15 /an, i.e. too small to cause textural defects. 

Figure 2.12 Effect of added lactic acid (--) and degree of lactose crystallization (-) on the...

Ice-cream. Crystallization of lactose in ice-cream causes a sandy texture. In freshly hardened ice-cream, the equilibrium mixture of a- and /1-lactose is in the glass state and is stable as long as the temperature remains low and constant. During the freezing of ice-cream, the lactose solution passes through the labile zone so rapidly and at such a low temperature that limited lactose crystallization occurs. 

If ice-cream is warmed or the temperature fluctuates, some ice will melt, and an infinite variety of lactose concentrations will emerge, some of which will be in the labile zone where spontaneous crystallization occurs while others will be in the metastable zone where crystallization can occur if suitable nuclei, e.g. lactose crystals, are present. At the low temperature, crystallization pressure is low and extensive crystallization usually does not occur. However, the nuclei formed act as seed for further crystallization... 

In frozen milk products, lactose crystallization causes instability of the casein system. On freezing, supersaturated solutions of lactose are formed e.g. in concentrated milk at -8°C, 25% of the water is unfrozen and contains 80 g lactose per 100 g, whereas the solubility of lactose at — 8°C is only about 7%. During storage at low temperatures, lactose crystallizes slowly as a monohydrate and consequently the amount of free water in the product is reduced. 

The formation of supersaturated lactose solutions inhibits freezing, and consequently stabilizes the concentration of solutes in solution. However, when lactose crystallizes, water freezes and the concentration of other solutes increases markedly (Table 2.4). 

Any factor that accelerates the crystallization of lactose shortens the storage life of the product. At very low temperatures (below — 23°C), neither lactose crystallization nor casein flocculation occurs, even after long periods. Enzymatic hydrolysis of lactose by /S-galactosidase before freezing retards or prevents lactose crystallization and casein precipitation in proportion to the extent of the hydrolysis (Figure 2.14). 

Tumerman, L., Fram, H. and Comely, K.W. (1954) The effect of lactose crystallization on protein stability in frozen concentrated milk. J. Dairy Sci., 37, 830-9. 

A phase can be defined as a domain bounded by a closed surface in which parameters such as composition, temperature, pressure and refractive index are constant but change abruptly at the interface. The principal phases in milk are its serum and fat and the most important interfaces are air/serum and fat/serum. If present, air bubbles, and ice, fat or lactose crystals will also constitute phases. Forces acting on molecules or particles in the bulk of a phase differ from those at an interface since the former are attracted equally in all directions while those at an interface experience a net attraction towards the bulk phase (Figure 11.6). 

Even though liquid whey has been successfully commercialized in the form of alcoholic and nonalcoholic beverages, these are still a rarity in most countries. Most whey is converted to whey solids as ingredients for human food or animal feeds by traditional processes such as spray drying, roller drying, concentration to semisolid feed blocks, or production of sweetened condensed whey. Jelen (1979) reported other traditionally established processes including lactose crystallization from untreated or modified whey, production of heat-denatured whey protein concentrate, or recovery of milk fat from whey cheese in whey butter. ... 

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-... 

Growth studies of broken crystals, as well as studies of the individual faces, have shown that lactose crystals grow only in one direction... 

Figure 6.4. Lactose crystal forms. (A) A variety of a-hydrate crystals from pure solution. (B) /3-Lactose crystals from pure solution. (C) a-Hydrate crystals in sweetened condensed milk. (D) a-Hydrate crystals in sandy ice cream. (E) a-Hydrate crystals in frozen condensed milk. (From Nickerson 1974.)...

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). 

Various marine and vegetable gums are currently in wide use in ice cream formulations. Shown to inhibit the formation of lactose crystal nuclei, they have been the principal factor responsible for the reduced incidence of sandiness in ice cream in recent years (Nickerson 1962). 

Riboflavin also may adsorbed on growing lactose crystals and alter the crystalline habit. Since it is naturally present in the whey from which lactose hydrate is made and is present in all dairy foods, its influence on lactose crystallization may be of special interest. Adsorption is dependent upon concentration of riboflavin in solution, on degree of lactose supersaturation and on temperature (Leviton 1943, 1944 Michaels and Van Krevald 1966). No adsorption occurs below a certain minimum (critical) concentration of riboflavin (2.5 uglml), but adsorption increases linearly with riboflavin concentration above this critical level. Increasing the temperature of crystallization results in reduced riboflavin adsorption. Adsorption is favored at lower supersat-... 

When lactose crystallization occurs above 93.5°C, the crystals formed are anhydrous and have a specific rotation of [a]2D = +35.0° and a melting point of 252.2°C. They are composed of anhydrous /3-lactose,... 

Thus, the overall process of lactose crystallization can be summarized by the reaction ... 

Other systems make use of the sticking tendency of acid whey. Partially dried whey powder coats the inner wall of the drying chamber, whence it falls when the crystalline lactose content of the powder becomes high. However, sticking of the product on the hot metal surfaces can be a problem unless sufficient moisture is present so that lactose crystallization proceeds to the point where the powder no longer adheres to the equipment (Pallansch 1973). 

These researchers found no correlation between hygroscopicity of the powders and degree of lactose crystallization, but there was a direct correlation between hygroscopicity and the amount of lactic acid in the powder. Acid whey permeate dried readily, however, without many of the problems encountered during the drying of whole whey. 

The control of lactose crystallization in sweetened condensed milks has been reviewed extensively (Webb 1970 Hall and Hedrick 1971 Hun-... 

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