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    Lactose in solid form
 
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::		 Molecular structure
Lactose in solid form
Physical properties
Chemical and biochemical properties
Physiological properties
Lactose in solid form can be crystalline or amorphous.
Crystalline lactose can exist in one of two distinct forms: β-lactose and α-lactose monohydrate. Crystallinity is the result of an highly ordered arrangement of the lactose molecules.

Amorphous lactose lacks crystallinity. The arrangement of the lactose molecules is more or less random in amorphous lactose.

The most frequently encountered forms of solid lactose are discussed here. The figure below shows the interrelations between the various forms of lactose.



Modifications of lactose


α-lactose monohydrate
The most common way to obtain lactose in solid form is crystallising it from solution.
When crystallisation is performed at temperatures below 93.5°C, only α-lactose is obtained. α-lactose has the peculiarity that, in its crystallineform, each lactose molecule is associated with one molecule of water. In other words, α-lactose crystallises as a monohydrate. The water is incorporated in the crystal and forms an integral part of it. It is not removed by normal drying processes. Due to this water of crystallisation, the normal water content of α-lactose monohydrate is around 5%. Only at temperatures as high as 140°C will the crystal water be completely removed.

Crystals of α-lactose monohydrate have a characteristic tomahawk-like shape. These crystals are very hard and brittle. On an industrial scale, α-lactose monohydrate is obtained by crystallising highly concentrated lactose solutions at low temperatures, separating the crystals from the mother liquor by. centrifuges, for example, and subsequently drying the moist, crystal mass.


Microscopic view of lactose monohydrate crystals


This type of lactose is used mainly for wet granulation processes and capsule filling. In dry powder inhalers, α-lactose monohydrate is used as a carrier.

β-lactose
When a highly concentrated solution of lactose is crystallised at temperatures above 93.5°C, only crystals of β-lactose are formed. Crystals of pure β-lactose have a characteristic kite-like form. β-lactose crystals are much softer than α–lactose monohydrate crystals. They do not contain crystal water. β-lactose is often referred to as anhydrous lactose. Industrially, β-lactose is produced by roller drying highly concentrated lactose solutions. The isomeric purity is approximately 80% β-lactose, the remaining 20% being α-lactose monohydrate and amorphous lactose (see below). Crystals of β-lactose in this product are very small and the kite form is rarely encountered. This type of lactose is used mainly for direct compression processes.


Figure 04 Microscopic view of a typical beta-lactose crystal


Electron microscopic view of beta-lactose crystals from roller-dried lactose


Amorphous lactose
When a highly concentrated lactose solution is dried very quickly, a glassy lactose mass results. This lactose glass is one form of amorphous lactose. This amorphous lactose glass contains both α- and β-lactose, the ratio being approximately the same as the equilibrium ratio of the original lactose solution at the drying temperature. Lactose glass is highly hygroscopic.

Although lactose glass can appear very hard, it lacks the brittleness of crystalline α lactose monohydrate. The material is deformable under high pressure. Industrially, this type of amorphous lactose is produced by spray drying a highly concentrated lactose solution. Spray-dried lactose is used mainly for direct compression processes. To improve the tabletting properties, the lactose solution can be mixed with a quantity of α-lactose monohydrate crystals before drying. The resulting product consists of a matrix of lactose glass in which lactose monohydrate crystals are embedded.


Electron microscopic view of spray-dried lactose
 
 
 
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