Galactose monohydrate

Monosaccharides, disaccharides, and the lower oligosaccharides have distinct melting points. Some boast two melting points, the first as a hydrate and the second for the anhydrous form. A classical example of this phenomenon is D-galactose monohydrate (rap. 118-120 °C) and an anhydrous anomeric mixture of D-galactose (rap. 167 °C). 

D-galactose, C HiiOe. Crystallizes in the pyranose form m.p. 1I8-120 C (monohydrate), 165-5" C (anhydrous). An isomer of glucose which is fairly widely distributed in plants. It is a constituent of raffinose and slachyose, of hemicelluloses, of pectin, of gums and mucilages, and of some glycosides. In animals it forms half the lactose molecule and is the sugar found in the brain. Chemically it is very similar to glucose. It has the structure... 

The above, indirect, procedure for glycoside synthesis is the Koenigs-Knorr method. Glycosidation may be effected from monosaccharides directly by treatment with an alcohol in the presence of a mineral acid catalyst. For example, when a-D-galactose (13) is heated in methanolic solution containing 2 per cent of hydrogen chloride the thermodymanically more stable methyl a-D-galacto-pyranoside (14) (Expt 5.113) is formed preferentially, and may be isolated from the reaction product by crystallisation as the monohydrate. The less abundant /i-anomer may be recovered from the mother-liquors. 

Lactose is a naturally occurring disaccharide containing one galactose unit and one dextrose unit. It is a constituent of all forms of mammalian milk, but is produced commercially from cow s milk, usually as a by-product of the cheese industry. Lactose can exist in two isomeric forms, a-lactose and p-lactose, and can be either crystalline or amorphous. Crystalline a-lactose occurs in both monohydrate and anhydrous forms, but p-lactose only exists in the anhydrous form. The temperature of crystallization is the principal determinant of which form is obtained. ... 

Lactitol as the monohydrate is nonhygroscopic and is stable under humid conditions. It is stable to heat and does not take part in the Maillard reaction. In acidic solution, lactitol slowly hydrolyzes to sorbitol and galactose. Lactitol is very resistant to microbiological breakdown and fermentation. Store in a well-closed container. When the compound is stored in an unopened container at 25°C and 60% relative humidity, a shelf-life in excess of 3 years is appropriate. 

The USPNF 23 describes lactose monohydrate as a natural disaccharide, obtained from milk, which consists of one galactose and one glucose moiety. The PhEur 2005 describes lactose monohydrate as the monohydrate of O-P-D-galactopyr-anosyl-(1 4)-a-D-glucopyranose. It is stated in the USPNF 23 that lactose monohydrate may be modified as to its physical characteristics, and may contain varying proportions of amorphous lactose. 

Lactose (C12H22O11) is a disaccharide of dextrose and galactose and is present in milk (Figure 3.5). Lactose is substantially less sweet than sucrose. It also has a relatively low solubility, as a result of which it can crystallize out of ice cream as a monohydrate i.e. for each lactose molecule there is also a water molecule in the crystal). The crystals... 

Lactose, milk sugar a reducing disaccharide. M, 324.3, a-form m.p. 223°C, [a]g-t-89.4°55.5° (water), p-form m.p. 252°C, [a]p-H34.9°- 55.3° (water). L. crystallizes from water as the p-form above 93 °C, and as a-lactose monohydrate below 93 °C. It consists of galactose P-1,4 glycosidically linked to glucose, both monosaccharide residues in the pyranose form. L. is not fermented by ordinary yeasts, but it is fermented by yeasts such as kefir. The souring of milk consists of the eonversion of L. into lactic acid by lactic acid bacteria. 

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