Food Test for Reducing Sugars
Reducing sugars are simple sugars with the ability to reduce copper (II) ions to copper (I). All monosaccharides (fructose, glucose, galactose) are reducing sugars as are some disaccharides, such as lactose and maltose. Simple sugars are all carbohydrates, and are used by the body as a source of energy.
To carry out food tests for reducing sugar in a given food sample.
Benedict's solution*, cooking pot, kerosene stove or charcoal burner, plastic spoon, droppers*, empty plastic bottles, test tube*, test tube holders*, and food sample containing a reducing sugar like glucose or onions.
1. Make a solution of a food sample containing a reducing sugar. This can be done by adding a spoonful of glucose to a litre of water or cutting an onion into quarters, grinding them in a mortar and pestle, and collecting and diluting the juice. Let the juice settle and decant the solution for use.
1. Put 2 mL of the food sample solution into a test tube.
2. Add 1 mL of Benedict's solution to the test tube.
3. Hold the test tube upright in the water bath and heat the solution to boiling.
1. Unused Benedict's solution should be stored in a labeled plastic bottle for future use.
2. Dispose of chemical waste in a pit latrine.
Copper is harmful to swallow and in large quantities is harmful to the environment.
The colour of the food sample will change to green, yellow, orange, and nally form a brick red precipitate. This indicates the presence of a reducing sugar.
Benedict's solution contains aqueous copper (II) sulphate, sodium carbonate, and sodium citrate. The citrate ions in Benedict's solution complex the copper (II) ions to prevent the formation of insoluble copper (II) carbonate. In the presence of a reducing sugar, however, the copper (II) ions are reduced to copper (I) ions which form a brick red precipitate of copper (I) oxide. The oxygen in the copper (I) oxide come from hydroxide; the purpose of the sodium carbonate is to provide this hydroxide by creating an alkaline environment.
Normally, sugar molecules formfivor six member rings and have no reducing properties. In water, however, the rings of some sugar molecules can open to form a linear structure, often with an aldehyde group at one end. These aldehyde groups react with copper (II) to reduce it to copper (I). Sugars that do not have an aldehyde group in the linear structure or that are not able to open are not able to reduce copper (II) ions and are thus called non-reducing sugars. Students do not need to understand this chemistry for their exam, but they may ask about what is happening in the reaction.