LACTOSE – C12H22O11
Chemical and Biochemical Properties
Lactose is very stable and inert from a chemical point of view. It has no significant tendency to react with the drug or other components of a formulation. Some remarks on the chemical properties of lactose are useful however, the low hygroscopicity of crystalline lactose supports its virtual chemical inertness. Most chemical reactions of lactose occur noticeably only in aqueous environment. Because lactose has a very low tendency to attract moisture, water in dry lactose preparations is normally not present in amounts sufficient for chemical reactions to proceed at a noticeable speed. The water of crystallisation is bound so tightly in the crystal lattice of the lactose that it is chemically inert.
Dehydration and Pyrolysis
At temperatures above 100ºC, α-lactose monohydrate gradually loses its water of crystallisation. At 140ºC, the loss of crystal water is completed. The loss of water of crystallisation is accompanied by a change of the crystalline structure of the lactose. The lactose becomes anhydrous. Further heating of lactose to higher temperatures causes the lactose to decompose. This process is called pyrolysis. The primary reaction products of pyrolysis tend to polymerize, resulting in brown and black coloured macromolecules. Eventually the lactose becomes black on heating.
In aqueous solution, lactose, as all reducing sugars, tends to react with compounds possessing primary- and secondary amino-groups, such as proteins and peptides. This reaction is called the Maillard reaction. High temperature and alkaline pH promote the reaction. In an advanced stage, yellow and brown polymers are formed, which ultimately turn the reaction mixture black. Purified lactose, even of pharmaceutical grade, still contains traces of proteinaceous matter. These are the reason why the Maillard reaction cannot be completely excuded, even from some dry lactose preparations. However, the...
References: "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
"Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
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