Chemistry of Milk

Module 5. Carbohydrates in milk

Lesson 18
PHYSICAL PROPERTIES OF LACTOSE-PART I

18.1 Introduction

The major constituent present in the soluble portion of the milk is carbohydrates and more specifically the lactose. It influences several properties of milk a thorough study of the various aspects of this carbohydrate will help in proper understanding of various phenomena occurring in milk during the milk processing.

18.2 Physical Properties

Lactose normally occurs naturally in either of two crystalline form α- monohydrate and anhydrous β or as an amorphous “glass” mixture of α- and β-lactose. Several other forms may be produced under special conditions. These designations refer to the configuration on the number one carbon of the glucose moiety.

18.3 Physical Forms

The physical forms of lactose are two one is α-lactose and the other is β-lactose. The physical properties of these two forms differ to some extent. The difference is mainly in their solubility, optic rotation and melting point.

18.3.1 α-Lactose
Ordinary commercial lactose is α-lactose monohydrate (C₁₂H₂₂O₁₁. H₂O). It is prepared by concentrating an aqueouslactose solution to super saturation and allow for crystallization to take place at a modern rate below 93.5°C . That α-hydrate is the stable solid form at ordinary temperatures is indicated by the fact that the other solid forms change to the hydrate in the presence of a small amount of water below 93.5°C . It has a specific optional rotation of +89.4°C (anhydrous weight basis) and a melting point of 201.6°C .

Fig. 18.1 Structural formula for α-Lactose
(Source: Harrington, J. Dairy sci., 1934)

18.3.2 β-lactose

This is the other isomeric form lactose. It exhibits a specific rotation of +35.0°C on anhydrous weight basis. Above 93.5°C crystallization or drying of lactose solutions yields β anhydrate.

18.4 Optic Property of Lactose

The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation ‘α ’ when plane-polarized light is passed through a sample with a path length of one decimeter and a sample concentration of one gram per millilitre. Therefore the specific rotation may be represented by the formula

[α] = 100 a/lc

Where

[α] = specific rotation at 20^◦C using D-line of sodium

a = degrees of angular rotation

l = length of tube in decimeter

c = concentration of substance in grams per 100 ml of solution

The specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature. Values should always be accompanied by the temperature at which the measurement was performed and the solvent in which the material was dissolved. Often the temperature is not specified; in these cases it is assumed to be room temperature. The formal unit for specific rotation values is deg dm^-1cm³per g but scientific literature uses just degrees. A negative value means levorotatory rotation and a positive value means dextrorotatory rotation.

18.5 Solubility

Lactose is freely soluble in water. However, the solubility of lactose is much lower than that of other common sugars. Solubility increases with increasing temperature. β-lactose dissolves more readily than α-lactose, as is apparent from their very different initial rates of solubility. Final solubility is the same for α- and β-lactose because of the mutarotation equilibrium that is eventually reached in solution. The particle size of the lactose influences its dissolving velocity. Coarselactose crystals dissolve much slower than tiny lactose particles. Dissolving velocity, and hence particle size, does not alter final solubility. Final solubility of lactose depends on temperature. The initial solubility is the true solubility of the form. The increasing solubility with time is due to mutarotation. As some of the α form is converted to β form, the solution becomes unsaturated with respect to a, and more α -hydrate dissolves. This process continues until equilibrium is established between α and β in solution and no more α -hydrate can dissolve, thus establishing the final solubility. This solution is saturated with respect to a, but a great deal of β-lactose powder can be dissolved in it because of the greater initial solubility of the β form The solution becomes saturated with along before the saturation point of β is reached. However, additional βdissolving in such a solution upsets the equilibrium, and mutarotation takes place. Since the solution was already saturated with a, a formed by mutarotation will crystallize to reestablish equilibrium.

Since β -lactose is much more soluble and mutarotation is slow, it is possible to form more highly concentrated solutions by dissolving β rather that a -lactose hydrate. In either case, the final solubility of the lactose in solution will be the same. Lactose solubility values at different temperatures are shown in Figure 18.2. The solvent and the presence of salts or sucrose influence the solubility of lactose, as well as the rate of mutarotation. The solubility of lactose increases with increasing concentrations of several calcium salts-chlorides, bromide, or nitrate-and exceedingly stable, concentrated solutions are formed. Acetone also reduces the solubility of lactose based on which a procedure has been developed to recover the lactose from whey.

18.6 Equilibrium in Solution (Mutarotation)

Lactose exists in two forms viz., α and β. By definition, a is the form with greater optical rotation in the dextro direction. The specific rotation of a substance is characteristic of that substance. Also important,besides the variables of the equation, are temperature of the solution,wavelength of the light source, and concentration of the solution. Regardless of the form used in the preparation of solution, the specific rotation will continue to change until +55.4° is reached at equilibrium. This is equivalent to the 37.3% in α form and 62.7% in β form. Since equilibrium rotation is the sum of the individual mix of α andβ forms. The rate of lactose mutarotation is influenced greatly by both temperature and pH. The rate is slow at low temperature but increases as the temperature rises, becoming almost instantaneous at about 75°C.

The presence of sugars and salts can also affect the rate of mutarotation. Although the effect is small in dilute solutions, a combination of salts equal to that found in solution in milk nearly doubles the rate of mutarotation. This catalytic effect is attributed primarily to the citrates and phosphates of milk. The presence of high levels of sucrose, on the other hand, has the opposite effect.

Fig. 18.2 Lactose solubility curve
(Source: Schoen, Ind eng.Chem, 1961)

The effect of sucrose is only slight at concentration up to 40% but as concentration increases above this level, mutarotation is rapidly decreased to about half the normal rate of the specific rotation.