A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction.
Reducing sugars include glucose, fructose, glyceraldehyde, lactose, arabinose and maltose. All monosaccharides which contain ketone groups are known as ketoses, and those which contain aldehyde groups are known as aldoses. Significantly, sucrose and trehalose are not reducing sugars.
Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. Fehling's solution can also be used for the same purpose, as both contain copper (II) ions, which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated.
A reducing sugar occurs when its anomeric carbon is in the free form. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or ketal hydroxyl group is free, i.e. it is not locked, not linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is available for reducing copper (II) ions. When a sugar is oxidized, its carbonyl group (i.e. aldehyde or ketone group) is converted to a carboxyl group.
A non-reducing sugar can be tested for in much the same way, but first the non-reducing sugar must be hydrolised by using dilute HCL Hydrochloric acid, then neutralised. Then heat gently with Benedicts Solution, a positive result will show brick red.
To Carry Out the Benedict's Test Follow the Steps Below:
Procedure for Solid: 1. Place the specimen, which has been chopped into small pieces, into a test tube. 2. Add 2ml of distilled water. 3. Shake the mixture by holding the top firmly and hitting the bottom of the test tube with your finger. 4. Add 2ml of Benedict's Solution. 5. Shake the mixture in the same way. 6. Heat the mixture in a water bath.
Procedure for Liquid or Solution: 1. Place 2ml of the liquid or solution in a test tube. 2. Repeat steps (4) to (6) of the Procedure for Solid.
Observation: If a yellow, brick-red or orange precipitate is observed, reducing sugar is present. If the solution doesn't change colour (remains blue), reducing sugar is absent.
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