Carbohydrates: Disaccharides

How do disaccharides form?

joining two monosaccharides with a glycosidic bond by a condensation reaction.

How do disaccharides break?

Using hydrolysis

Adding water breaks glycosidic bond

What disaccharides are reducing sugars?

Maltose

What disaccharides are non reducing sugars?

sucrose

How do we test for non reducing sugars? Non reducing sugars have no effect on Benedict's reagent.

Hydrolysis into monosaccharide

If sample not in liquid form, ground up into water

Add 2cm^3 of food sample tested to the same volume of Benedict's reagent in a test tube and filter.

Place test tube into gently boiling water bath for 5 minutes.

If Benedict's solution doesn't change colour, a reducing sugar is not present.

Add another 2cm^3 of food sample into same volume of dilute hydrochloric acid in a test tube.

Place the test tube in gently boiling water bath for 5 minutes.

Dilute hydrochloric will hydrolyse any disaccharide present into its monosaccharides.

Slowly add some sodium hydrogen-carbonate solution to the test tube to neutralise hydrochloric acid. -> Benedict's reaction doesn't work in acidic conditions

Use pH paper to test if solution is alkaline

Re test resulting solution by heating it with 2cm^3 of Benedict's reagent in a gently boiling water bath for 5 minutes.

If non reducing sugar was present in the original sample, the Benedict's reagent will now turn orange-brown.

This is due to the reducing sugars that were produced from hydrolysis of the non reducing sugars.