Carbohydrates test

There are 2 types of carbohydrates, starch and sugars. We use different methods to test for starch and sugar.

Test for Starch

Add 1 drop of iodine to each dimple in a drop tray using a pipette. Add 1 drop of food solution to the iodine using a pipette. If starch is present the iodine turns from brown to blue black, if starch isn't present the iodine stays brown, as shown below.

Safety

Do not get iodine on your clothes as it stains.

Sugars test

Test for sugars 2 cm³of Benedicts solution was added to 2 cm³of food solution in a boiling tube. The food containing sugar is added.

The solution is heated in a water bath for 5 minutes at 70°C . If reducing sugars are present there is a colour change from blue to green to yellow to orange to brick red. The more sugars present, the more colour change, as shown below.

Safety

Wear goggles. If you heat with a Bunson burner, heat a beaker of water, and in the water have your boiling tube of Benedicts. Do not heat the boiling tube directly with the flame. Also, if you use a boiling tube use anti - bumping granules to stop the Benedicts exploding out of the tube.

The test for sugar actually only detects simple reducing sugars such as monosaccharides (e.g. glucose). More complicated sugars such as disaccharides (e.g. sucrose) need to be broken down into monosaccharides first. This is done by adding 2 cm³ of the sugar to 2 cm³ of hydrochloric acid. After two minutes the bonds have broken, and the acid is neutralised with sodium hydroxide. The sugar is then tested as above.