monosaccharides. The most common monosaccharides encountered in the kitchen are glucose,
fructose and galactose; but there are many others. Glucose, fructose and galactose all have the same
chemical formula (C6H12O6) but the arrangement of atoms differs in each case.
Glucose and fructose are found in many fruits and in honey, in a mixture with other sugars, whereas
galactose, in contrast, is found more commonly in non fermented dairy products.
Sweet fruits and vegetables such as carrots and beetroot contain large amounts of these sugars.
Fructose is the sweetest of all the sugars, but its sweetness is reduced by about half on heating to
60°C. Glucose is less sweet than fructose.
However, these sugars are not found in this simple unjoined form in the kitchen very often. More often
the single sugar units are joined together to form larger sugar molecules. If a carbohydrate contains
two of these sugar units chemically bonded together it is called a disaccharide. When two
monosaccharides combine to from a disaccharide, an OH group from one of the sugar units is used to
form a link with another. This link is called a glycosidic bond. The most common dissacharides found
in the kitchen include:
a) sucrose – this is composed of one glucose unit joined to one fructose unit – this sugar is what
we know as table sugar. It is second sweetest sugar (after fructose), and is usually used to
make candy because it has a pleasant taste even at high concentrations and interesting
texture properties. (Some of the other sugars have a slightly bitter taste at very high
b) lactose – this is composed of one glucose unit joined to one galactose unit. It is rarely found in
the kitchen in its pure form and is the sugar naturally found in milk. It is much less sweet than
sucrose, so is not really used as a sweetener.
c) maltose – this is composed of two glucose units attached together, and is the sugar naturally
present in barley.
The structure of each of these sugars is shown below:
Sucrose Lactose Maltose