Crystallization of lactose

As discussed in section 2.2.5, the solubility of lactose is temperature dependent and solutions are capable of being highly supersaturated before spontaneous crystallization occurs and even then, crystallization may be slow. In general, supersolubility at any temperature equals the saturation (solubility) value at a temperature 30°C higher. The insolubility of lactose, coupled with its capacity to form supersaturated solutions, is of considerable practical importance in the manufacture of concentrated milk products. 

The rate of nucleation is slow at low levels of supersaturation and in highly supersaturated solutions owing to the high viscosity of the solution. The stability of a lactose glass is due to the low probability of nuclei forming at very high concentrations. 

Once a sufficient number of nuclei have formed, crystal growth occurs at a rate influenced by  

Lactose glass. When a lactose solution is dried rapidly, viscosity increases so quickly that crystallization is impossible. A noncrystalline form is produced containing a- and j8-forms in the ratio at which they exist in solution. Lactose in spray-dried milk exists as a concentrated syrup or amorphous glass which is stable if protected from air, but is very hygroscopic and absorbs water rapidly from the atmosphere, becoming sticky. 

---

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. 

Sweetened condensed milk. Crystallization of lactose occurs in sweetened condensed milk (SCM) and crystal size must be controlled if a product with a desirable texture is to be produced. As it comes from the evaporators, SCM is almost saturated with lactose. When cooled to 15-20°C, 40-60% of the lactose eventually crystallizes as a-lactose hydrate. There are 40-47 parts of lactose per 100 parts of water in SCM, consisting of about 40% a-and 60% /1-lactose (ex-evaporator). To obtain a smooth texture, crystals with dimensions of less than 10 /an are desirable. The optimum temperature... 

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. 

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). 

Production of lactose essentially involves concentrating whey or ultrafiltration permeate by vacuum concentration, crystallization of lactose from the concentrate, recovery of the crystals by centrifugation and drying of the crystals (Figure 2.15). The first-crop crystals are usually contaminated with riboflavin and are therefore yellowish a higher grade, and hence more... 

Demineralization by electrodialysis and/or ion exchange, thermal evaporation of water and crystallization of lactose. 

As discussed in Chapter 2 (p. 38), crystallization of lactose as x-monohyd-rate exacerbates the situation. The combination of increased concentrations of Ca2 + and reduced pH causes destabilization of the casein micelles. 

Role of water in stickiness and caking of powders and crystallization of lactose... 

Some carbohydrates actively inhibit the crystallization of lactose, whereas others do not. Carbohydrates that are active possess either the /3-galactosyl or the 4-substituted-glucose group in common with lactose, so that adsorption can occur specifically at certain crystal faces (Van Krevald 1969). (3-Lactose, which is present in all lactose solutions [see Equilibrium in Solution (Mutarotation )], has been postulated to be principally responsible for the much slower crystallization of lactose compared with that of sucrose, which does not have an isomeric form to interfere with the crystallization process (Van Krevald 1969). Lactose solubility can be decreased substantially by the pres-... 

Processes for crystallization of lactose are well established, with production generally limited to a few large plants. Although a variety of cheese wheys and whey ultrafiltrates can be used for lactose production, sweet whey or ultrafiltrates are preferred (Woychik 1982). The crystallization process has three basic steps ... 

Roetman, K. 1972. Crystallization of lactose. Voedingsmiddelen-technoligie 3,(43), 230-234. Cited in Food Sci Technol Abstr. 6 2L110 (1974). 

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. 

The crystallization of lactose in frozen concentrated milk has been associated with a denaluration of casein which ultimately appears as a gel structure in the thawed product. Gelation in frozen milk can be retarded by enzymic hydrolysis of pan ol the lactose before freezing or by addilion of a polyphosphate salt. 

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. 

Prevent crystallization of lactose, which results in grainy, sandy texture... 

A faulty batch of tablets can sometimes be recovered by grinding up the tablets and recompressing them, a process which is known as reworking and is analogous to the dry granulation method of tablet manufacture. This can sometimes cause problems with a direct compression formulation. Many direct compression diluent particles are in the form of aggregates, e.g., spray-dried lactose is composed of small crystals of lactose embedded in amorphous lactose. If these aggregates are compressed, their structure may be broken down to such an extent that subsequent recompression will result in impaired tablet quality. 

Polyethylene glycol (PEG) 4000, when spray-dried with lactose, has been shown to accelerate the rate and extent of crystallization of lactose. It has also been shown that spray-dried lactose composite particles containing an ion complex of chitosan are suitable for the dry-coating of tablets. Spray-dried lactose and crystallized spray-dried lactose have been evaluated for dry powder inhalation.Amorphous spray-dried lactose has also been studied in composites with PVP. ... 

Price R, Young PM. Visualization of the crystallization of lactose from the amorphous state. J Pharm Sci 2004 93(1) 155-164. 

The Ter (onset) of spray-dried and freeze-dried materials obtained from dynamic DSC analysis are shovm in Figure 44.3 with respective water contents. The decreased with increasing water activities and water contents showing a similar behavior as Tg. T of freeze-dried lactose was higher than T r of spray-dried lactose in the anhydrous state and at all corresponding water contents (Figure 44.3). T for freeze-dried lactose was nearly the same as reported by Roos and Karel (1990) for freeze-dried anhydrous and humidified lactose over various RVP. Crystallization of lactose also occurred differently from differently dehydrated materials. 

Haque, M.K. and Roos, Y.H. Water plasticization and crystallization of lactose in spray-dried lactose/protein mixtures, /. Food Sci., 69, 23, 2004. 

When the model samples were stored in a 65.6% RVP environment, OD of the three systems was observed at 11-, 24-, and then at 24-h intervals up to 240 h, as shown in Figure 53.3d. OD in the trehalose/reactant system increased linearly from 11 h, which was followed by a leveled-off plateau. In lactose/reactant systems, from 11 to 24 h, rate constant was low, then started to increase linearly and rapidly because of the crystallization of lactose, and finally ended at the plateau stage. In agreement with the water-sorption results for the model systems at 65.6% RVP (Figure 53.1), crystallization of component sugars was observed from the loss of sorbed water at 5, 21, and 56 h for trehalose/reactant, lactose/reactant, and lactose/trehalose/reactant systems, respectively. The rapid linearly increasing region of OD in the trehalose/reactant and lactose/reactant systems seemed to be directly... 

The fact that the measured crystallization response is often split into two regions causes further confusion. For example, Sebhatu et al. (23) show a schematic version of the calorimetric response for the crystallization of lactose (Figure 8.4) and state that Part II is crystallization and Part III may be mutarotation from (1 to a-lactose. Therefore, they assess crystallinity by using the area under the curve for Part II,... 

We will illustrate the phenomenon for the crystallization of lactose. Lactose is a reducing sugar, and in solution the a and p anomers are in equilibrium with each other the ratio p over a is about 1.6. The crystallization of a-lactose monohydrate, the most common crystalline form, has been studied in detail. Figure 15.10 depicts a crystal as formed at... 

It was mentioned above that the crystallization of lactose can occur at a critical water content, just above the glass transition. It was further (implicitly) assumed that this would happen at the same mass fraction of water i/fw in skim milk powder. Experiments show that this is not precisely correct but that the critical conditions for crystallization are at the same water activity. Does this imply that the glass transition is determined by aw rather than i//w ... 

Lactase can also exert an indirect effect on the casein system in frozen, concentrated milk. In this case, the crystallization of lactose at —18° to — 12°C in frozen, concentrated milk has been correlated with destabilization and aggregation of the casein system resulting in undesirable flocculation of the casein (209), As shown in Figure 13 enzymic hydrolysis of the lactose by lactase prevents the aggregation of the casein system. This alteration in functionality of the casein system has been postulated to result from the hydrolysis of lactose and prevention of its crystallization which in some way destabilizes the casein system. 

---