CHEMISTRY OF MILK

Module 5. Carbohydrates of milk

Lesson 19
PHYSICAL PROPERTIES OF LACTOSE-PART II

19.1 Introduction

The body and structure of some of the milk products is influenced by the lactose crystallization. The crystallization of lactose will influence the physical properties of the milk product. The knowledge of this physical property helps in obtaining quality products and helps to avoid several defects in the body and texture of these products.

19.2 Crystallization of Lactose

The principal factor governing the crystalline habit of lactose is the precipitation pressure, the ratio of actual concentration to solubility. When the pressure is high and crystallization is forced rapidly, only prisms are formed. As precipitation pressure lessens, the dominant crystal form changes to diamond-shape plates, then to pyramids and tomahawks, and finally, in slow crystallization, to the fully developed crystal. Different relative growth rates on the crystal faces account for the various shapes observed. The rate of crystal growth increases rapidly as supersaturation (precipitation pressure) is increased. 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. The influence of a number of additives on growth rates has been studied; some additives resulted in marked retardation, whereas others accelerated growth on specific crystal faces. The axes and the faces of the tomahawk crystal is diagrammatically shown in figure 19.1.

The concentration of the additive can influence the relative importance of the acceleration of the crystallization reactions. The tendency toward spontaneous nucleation is also lowered upon repeated re-crystallization. Gelatin is an example of a crystallization inhibitor that reduces the growth rate to 1/3 to 3/4 of normal even at low gelatin concentrations. In highly supersaturated lactose solutions, however,gelatin cannot suppress nucleation, which explains its ineffectiveness in preventing sandiness in ice cream. Consequently various marine and vegetable gums are currently in wide use in ice cream formulations. Both methanol and ethanol accelerate crystallization by as much as 30 to 60% even at low (1%) concentrations, depending on which crystal face is being observed. The rate of lactose crystallization is also markedly increased at low pH (<1). Some carbohydrates actively inhibit the crystallization of lactose, where as others do not. Calcium chloride had the greatest growth-promoting effect; at the 10% impurity level.

Fig. 19.1 Tomahawk crystal α-lactose monohydrate
(Source: Fundamentals of Dairy Chemistry, Wong et. al., 1988)

19.3 Lactose Glass

It is also called as amorphous noncrystalline glass. When a lactose solution is dried rapidly, its viscosity increases so quickly that crystallization cannot take place. The dry lactose is essentially in the same condition as it was in solution, except for removal of the water. This is spoken of as “concentrated syrup’’ or an “amorphous”(noncrystalline) glass. Lactose glass is stable if protected from moisture, but since it is very hygroscopic, it rapidly takes up moisture from the air and becomes sticky. When the moisture content reaches about 8% or a relative vapour pressure near 0.5, the lactose achieves a maximum weight; a discontinuity is observed in the sorption isotherm, and water is desorbed from the lactose

When lactose crystallization occurs above 93.5°C, the crystals formed are anhydrous and have a specific rotation of +35.°C and a melting point of 252.2°C. They are composed of anhydrous. β-lactose, usually, α-lactose crystallizes as a hydrate containing equimolar amounts of lactose and water. The crystals are fairly hard and not hygroscopic. Above 93.5°C , anhydrous β-lactose crystallizes. β-lactose dissolves much faster than α-lactose hydrate at room temperature, as its solubility is about 10 times higher and the crystals are usually smaller with a larger surface area.

Amorphous lactose is formed when a solution (e.g. milk) is dried rapidly, as in a spray drier, or frozen. It is a very concentrated solution and it quickly dissolves or, rather, is diluted, on addition of water; but then,α-lactose hydrate may start to crystallize. If the water content of the amorphous lactose is low, say 3%, crystallization may be postponed almost indefinitely; nucleation rate is negligible because of the extremely high viscosity of the ‘solution’. The product is, however, very hygroscopic, and when moisture content rises to about 8%, α-lactose hydrate starts to crystallize which helps to make very small crystals. But when crystallization of lactose caused by moisture uptake occurs in milk or whey powder, the result is caking; powder particles are cemented together by crystalline lactose, forming large and stony lumps.

It is almost impossible to obtain pure crystals. For instance, α-­hydrate usually contains a few per cent of β-lactose, and vice versa. The different forms mentioned are different crystal polymorphs (i.e., they have different crystal lattices).

19.4 Density

The densities of the various lactose crystals differ slightly from each other. α-hydrate form is 1.540, anhydrous β is 1.589,anhydrous α formed by dehydration under vacuum is 1.544 and anhydrous αcrystallized from alcohol is 1.575. Densities of lactose solutions are not linear functions of concentration.

19.5 Relative Sweetness

It has been amply demonstrated that the relatives weetness of sugars changes with the concentration. Therefore it is misleading to say that one sugar is so many times as sweet as another, because this will be true only at certain concentrations. The relative sweetness of some common sugars is presented in table form. It should be noted that lactose is relatively sweeter at higher concentrations than at lower concentrations and is sweeter than is usually reported in reviews of food applications. β- Lactose is sweeter than α-lactose but β form of lactose is not appreciably sweeter than the equilibrium mixture except when the concentration of lactose solution equals or is greater than 7%. Since there is approximately 62.7% of β form in the equilibrium mixture, a β-lactose solution differs less in sweetness from a solution in equilibrium than does α-lactose solution.

Table 19.2 Relative sweetness of sugars

(Source: Nickerson, T. A. Fundamentals of Dairy Chemisty, 1974 )

(Lactose In: Fundamentals of Dairy Chemistry, Webb and Jhonson)

Lactose has a clean and sweet taste without any after taste. The sweetness profile resembles that of sucrose. However, the relative sweetness of lactose is small (only 20%) when compared to sucrose(100%). β-lactose appears to be somewhat sweeter than α-lactose, probably due to the fact that β-lactose dissolves somewhat quicker in the saliva of the mouth than α-lactose, hence reaching a higher concentration in the same period of time and thus giving rise to a higher sweetness sensation.