Lactose crystallized water

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). 

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.,... 

Figure 2.6 Initial solubility of a-lactose and -lactose, final solubility at equilibrium (line 1), and supersaturation by a factor 1.6 and 2.1 (a-lactose excluding water of crystallization). (Modified from Walstra and Jenness, 1984.)...

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. 

Forms of Anhydrous a-Lactose. The water of crystallization may be removed from a-hydrate crystals under various conditions to produce different types of anhydrous lactose. 

The primary function of stabilizers in ice cream is to bind water and provide added viscosity to limit ice and lactose crystal growth, especially during storage under temperature fluctuation conditions. Stabilizers also assist in aerating the mix during freezing and improve body, texture, and melting properties in the frozen product. 

The physical stability of the casein micelle system is closely related to the degree of lactose crystallization from the unfrozen phase of the frozen concentrate. Crystallization of lactose from the unfrozen solution temporarily raises its freezing point, causing additional water to freeze, thus increasing the concentration and promoting destabilization of casein micelles. 

Jouppila, K., Kansikas, J., and Roos, Y.H. (1997). Glass transition, water plasticization, and lactose crystallization in skim milk powder.J. Dairy Sci. 80, 3152-3160. 

Lactose exists in two isomeric forms a and p. It is possible to obtain the a-monhydrate, anhydrous crystalline a and p forms, as well as an amorphous form. Thepharmaceutical properties of these various types are different. The hardness of tablets obtained using amorphous lactose produced by lyophilization is 10 times that obtained using crystalline forms. During milling, it has been observed that the monohydrate loses part of its water of crystallization and of its crystallinity. Heat treatment of different lactoses has permitted the discovery of an anhydrous, unstable a form and a crystal containing a and p in the ratio 1 1. Under the influence of high degrees of humidity, amorphous lactose crystallizes and anhydrous forms tend to reconvert to the monohydrate. 

Vuataz, G. Preservation of skim milk powders role of water activity and temperature in lactose crystallization and lysine loss. Food Preservation by Moisture Control, C.C. Seow, ed., Elsevier Applied Science publishers, London, pp. 73, 1988. 

Figure 49.2 shows the rate of NEB as a function of RH for lactose and lactose-gelatinized starch (3 1) systems incubated at 70°C. Since lactose is a reducing sugar, NEB rate values were higher in the lactose systems than in the lactose-starch ones as a result of a dilution effect of the reactants. As previously indicated the purpose was to compare the shape of the curves and the location of the maximum in the RH scale. The maximum NEB rate for the lactose system was observed at 43% RH, in which the sample was completely crystalline. The kinetics of lactose crystallization was delayed by the incorporation of starch. However, the RH value at which crystallization started was the same as the lactose system, and the rate of NEB decreased only slighly above 52% RH (Figure 49.2). Several other studies have also indicated that the addition of a polymer to an amorphous matrix delayed the crystallization of the sugar (Roos and Karel, 1991 O Brien, 1996 Gabarra and Hartel, 1998 Mazzobre et al., 2001 Biliaderis et al., 2002). At higher RH, the NEB rate dramatically decreased for lactose, which was completely crystalline and where water had an inhibitory effect on the reaction. In the...

In a previous interlaboratory study it could already be shown that only a part of the crystallized water of a-lactose is detected by this method... 

Sweetened condensed milk is supersaturated with lactose, and the lactose crystals formed tend to settle and also give the product a sandy mouth feel. To prevent this, crystal size should be at most 8 pm, and one generally tries to achieve this by adding crystalline seed lactose, about 0.3 g per kg product. Calculate what the maximum size of the seed particles may be. One kg of sweetened condensed milk contains about 110 g lactose, 440 g sucrose, and 260 g water. Consult also Table 2.2. 

Hydrate. a-Lactose crystallizes as a monohydrate containing 5% water of crystallization and can be prepared by concentrating aqueous lactose solutions to supersaturation and allowing crystallization to occur below... 

Further liberation of free fat may occur under adverse storage conditions. If powder absorbs water it becomes clammy and lactose crystallizes, resulting in the expulsion of other milk components from the lactose crystals into the spaces between the crystals. De-emulsification of the fat may occur due to the mechanical action of sharp edges of lactose crystals on the fat globule membrane. If the fat is liquid at the time of membrane rupture, or if it becomes liquid during storage, it will adsorb on to the powder particles, forming a water-repellant film around the particles. 

Crystallization can be accounted for by the moisture dependence of Tg according to Roos and Karel (1991b). Crystallization releases water, which in closed containers is absorbed by the amorphous portion of the food. As a result when Tg drops, T-Tg increases and rapid crystallization follows. Products, which have high moisture transfer rates in the environment, will lose water, but the moisture content in the amorphous part remains fairly constant. Crystallization proceeds at a rate defined by a constant T-Tg. Crystallization leads to a complete change in physical structure. It may considerably decrease stability. Lactose crystallization in milk powders... 

The solution of support able to crystallize (i.e., sucrose/lactose in water) is concentrated till it reaches supersaturation. Then the aroma is added into the supersaturated sugar syrup under vigorous agitation. After nucleation, the crystallization is operated with emission of a substantial amount of heat. Then solid crystals (3-30 pm) with entrapped flavor and agglomerates are formed and separated, dried, and sieved (Chen et al., 1988 Beristain et al., 1996 Kim et al., 2001). 

Ice cream stabilizers (such as guar, carboxymethyl cellulose, xanthan) are used to produce smoothness in body and texture, reduce ice and lactose crystal growth during storage, provide product uniformity and resist melting [15,47]. Freezing is usually done at a temperature of about -7 °C, with about 75% of the water being turned to ice [38]. 

The most stable form is a-lactose monohydrate, C12H22O11 H2O. Lactose crystallizes in this form from a supersaturated aqueous solution at T < 93.5 °C. The crystals may have a prism-or pyramid-like form, depending on conditions. Vacuum drying at T > 100 °C yields a hygroscopic a-anhydride. Crystallization from aqueous solutions above 93.5 °C provides water-free P-lactose (P-anhydride, cf. Formula 10.10). Rapid drying of a lactose solution, as in milk powder production, gives a hygroscopic and amorphous equilibrium mixture of a- and P-lactose. 

---