Question

Which test is used to distinguish between lactose and maltose?

Answer 1

1. Monosaccharides containing an aldehyde functional group at one end of its carbon chain are called Aldose sugars. Two common examples of aldose sugars are Glucose and Galactose. The structures of glucose and galactose are shown below.

GLUCOSE

2. Lactose: Lactose is a disaccharide found in the milk of almost all animals. It is composed of $\mathrm{\beta }-\mathrm{D}-\mathrm{Galactose}$ and $\mathrm{\beta }-\mathrm{D}-\mathrm{Glucose}$ with a glycosidic linkage between C1 of Galactose and C4 of Glucose. The structure of Lactose is shown below.

3. Maltose: This is a disaccharide formed by two $\mathrm{\alpha }-\mathrm{D}-\mathrm{Glucose}$ units with C1 of one glucose unit being linked to C4 of the other glucose unit. The structure of maltose is shown below.

4. Aldose sugars react with Phenylhydrazine in the presence of glacial acetic acid to form Phenylosazones which appear as solid crystals.

5. Lactose and maltose react with Phenylhydrazine to form Lactosazone and Maltosazone respectively. Although both appear as solid crystals, on examination under a microscope, it is found that Maltosazone crystals are shaped like a sunflower while Lactosazone crystals are shaped like cotton balls.

6. Maltose and Lactose can be distinguished by the shapes of their respective Phenylosazone crystals.