Benedict's solution

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The American chemist Stanley Rossiter Benedict is best known for his discovery of what is called Benedict’s reagent or Benedict’s solution. As its name indicates it is a chemical reagent used to testfor the presence of reducing sugars. The sugars it is able to detect are monosaccharides and specifically the disaccharides lactose and maltose. In general terms, Benedict’s solution detects thepresence of aldehydes – with the exception of the aromatic ones – and alpha-hydroxy-ketones.

One litre of Benedict’s test can be prepared using 100 grams of anhydrous sodium carbonate, 173 grams ofsodium citrate and 17.3 grams of copper (II) sulfate pentahydrate. It also contains blue copper (II) ions (Cu2+) that are reduced to copper (I) (Cu+). They are precipitated as red copper (I) oxide,which is insoluble in water.

Among all reducing monosaccharides glucose can be found, as well as fructose, glyceraldehyde and galactose. Besides these reducing sugars (monosaccharides), also manydisaccharides such as the previously mentioned lactose and maltose have a reducing form, when one of the two units may have an open chain with an aldehyde group. Of course there are some exceptions thatare not reducing sugars within disaccharides, like sucrose and trehalose, in which the anomeric carbons of the two units are linked together.

In glucose polymers as starch and starch-derivates, suchas glucose syrup, maltodextrin and dextrin the macromolecule begins with a reducing sugar or free aldehyde. More hydrolysed starch contains more reducing sugars indeed. The percentage of the reducingsugars present in those starch derivatives is called dextrose equivalent (DE).

In order to be able to understand in a best possible way what is mentioned above, a reducing sugar is any sugar that –in a solution – has an aldehyde (monosaccharides that contain an aldehyde group are known as aldoses) or a ketone (monosaccharides that contain ketone group are called ketoses) group. This is why...
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