Disaccharides are sugars containing two molecules of monosaccharides. Disaccharides are formed by the condensation of two molecules of monosaccharides with the elimination of one molecule of water.
In disaccharides, monosaccharides are linked by the glycosidic bonds. The properties of the disaccharides depend to a great extent on the type of linkage. If the two potential aldehyde or ketone of both monosaccharides are involved in the linkage, the sugar will not exhibit reducing properties and will not be able to form osazones. eg. Sucrose. But if one of them is not bound in this way, it will permit reduction and osazone formation by the sugars eg. lactose and maltose which are known as reducing disaccharides.
Maltose is composed of two glucose molecules combined by a- 1,4 glycosidic linkage. It is commonly called malt sugar. Malt from sprouting barley is the major source of maltose. It is a rather sweet sugar and is highly soluble in water.
The structure of maltose shows that the potential aldehyde group of glucose -2 is blocked in the glycosidic linkage, whereas the potential aldehyde group of glucose -1 is free and can reduce alkaline copper solution. It is because of this free aldehyde in the first glucose molecule, maltose has reducing property.
Maltose is the end product of digestion of starch by the action of salivary amylase, in the mouth and pancreatic amylase in the intestine. Maltose is formed as an intermediate product in the intestine. Maltose is split into two molecules of glucose by the enzyme maltase of the intestinal juice before absorption.
Lactose is commonly called as milk sugar. It is present in the milk of mammals. However lactose is found in the urine of pregnant and lactating women. It is less soluble in water and less sweeter than sucrose.
Just as in maltose, lactose has a free potential adehyde group in the glucose molecule, not involved in the glycosidic linkage between glucose and galactose molecules. Whereas, the potential aldehyde group of galactose molecule is blocked in the linkage. Because of the presence of free aldehyde group in the glucose molecule, lactose can reduce Fehling’s solution and is therefore a reducing sugar.
When lactose is hydrolysed by acids or by the enzyme lactase, one molecule of glucose and one molecule of galactose are formed. The intestine of milk sucking infants has the enzyme lactase, which converts lactose into glucose and galactose. Then only it is absorbed in the body. Excess of lactose ingested into the body causes diarrhoea, abnormal intestinal flow and colic pain. Lactose is not fermented by yeast.
Sucrose is ordinary “table sugar”. It is also called as “cane sugar” as it can be obtained from sugar cane. It is widely distributed in sugar cane, beet root, pine apple, honey, carrot and ripe fruits.
Sucrose consists of one molecule of glucose and one molecule of fructose. The linkage between these molecules are formed between the aldehyde group of glucose and the ketone group of fructose. Thus, both the potential aldehyde group of glucose and the ketone group of fructose are blocked in the linkage and sucrose has no free reducing group. On account of this structural peculiarity sucrose is a non-reducing sugar. It does not reduce Tollen’s and Fehling’s solutions and does not form osazone.
Sucrose on hydrolysis by dilute acids or the enzyme sucrase or invertase gives a mixture of glucose and fructose. It is called as invert sugar.
Sucrose is dextrorotatory (+62.5°) but it’s hydrolytic products are levorotatory because fructose has a greater specific levo-rotation than the dextrorotation of glucose. As the hydrolytic products inverts the rotation, sucrose is known as invert sugar and the process is called as invertion. Honey contains plenty of ‘invert sugar’ and the presence of fructose accounts for the greater sweetness of honey.