Test for Reducing Sugars

Written by melissa kelly
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Food products can be tested to determine whether they contain reducing sugars through tests known as Benedict's test or Fehling's test. These tests can also be used to determine if sugars are present in certain bodily fluids, such as urine, which is important for diagnostic testing.

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What is a reducing sugar?

Any sugar that forms an aldehyde or ketone in the presence of an alkaline solution is a reducing sugar. Types of reducing sugars include glucose, fructose, glyceraldehyde, lactose, arabinose and maltose. Sucroses and trehaloses are not reducing sugars. Ultimately, a reducing sugar is a type of sugar that reduces certain chemicals through an oxidation reaction.

Benedict's Test

To test for the presence of reducing sugars, a food sample is dissolved in boiling water. Next, a small amount of Benedict's reagent is added and the solution begins to cool. During the next four to 10 minutes, the solution should begin to change colours. If the colour changes to blue, then no glucose is present. If a high amount of glucose is present, then the colour change will progress to green, yellow, orange, red and then a dark red or brown.

How Benedit's test works

Benedict's reagent is made from anhydrous sodium carbonate, sodium citrate and copper(II) sulphate pentahydrate. Once added to the test solution, reducing sugars reduce the blue copper sulphate from the Benedict's solution to a red brown copper sulphide, which is seen as the precipitate and is responsible for the colour change. Non-reducing sugars cannot do this. This particular test only provides a qualitative understanding of the presence of reducing sugars.

Fehling's test

To carry out Fehling's test, the solution is diluted in water and warmed until fully dissolved. Next, Fehling's solution is added while stirring. If reducing sugars are present, the solution should begin to change colours as a rust or red coloured precipitate forms. If reducing sugars are not present, the solution will remain blue or green.

How Fehling's test works

Fehling's solution is made by first making two sub-solutions. Solution A is made from copper(II) sulphate pentahydrate dissolved in water and solution B contains potassium sodium tartrate tetrahydrate (Rochelle salt) and sodium hydroxide in water. The two solutions are added together in equal parts to make the final test solution. The test is a detection method for monosaccharides, specifically aldoses and ketoses. These are detected when aldehyde oxidises to acid and forms a cuprous oxide. Upon contact with an aldehyde group, it is reduced to cuprous ion, which forms the red precipitate and inedicates the presence of reducing sugars.

Practical applications

Reducing sugar tests such as Benedict's and Fehling's test can be used to determine whether sugars are present in urine, which can be indicative of diabetes mellitus. They can also be used in a qualitative manner, such as in a titration experiment, to determine the amount of reducing sugars in a solution.

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