Title: Laboratory Practicals : Qualitative analysis of carbohydrates
Course: Biochemistry > Section: Week 5 - Unit 2-2_Carbohydrates

Purpose

Carbohydrates as Reducing Sugars: A reducing sugar is any sugar that, in a solution, has an aldehyde or a ketone group. The enolization of sugars under alkaline conditions is an important consideration in reduction tests. The ability of a sugar to reduce alkaline test reagents depends on the availability of an aldehyde or keto group for reduction reactions. A number of sugars especially disaccharides or polysaccharides have glycosidic linkages which involve bonding a carbohydrate (sugar) molecule to another one, and hence there is no reducing group on the sugar; like in the case of sucrose, glycogen, starch and dextrin. In the case of reducing sugars, the presence of alkali causes extensive enolization especially at high pH and temperature. This leads to a higher susceptibility to oxidation reactions than at neutral or acidic pH. These sugars, therefore, become potential agents capable of reducing   C u + 2 to Cu, A g + to A g. Most commonly used tests for detection of reducing sugars are Fehling’s Test, Benedict’s Test and Barfoed’s Test.

Over to you

This experiment aims to introduce you with the identification of unknown carbohydrates. To gain maximum benefit, observations should be related, as far as possible, to the structure of the substances examined.

Activity

Perform the Fehling’s test:

Fehling’s Solution (deep blue colored) is used to determine the presence of reducing sugars and aldehydes. Perform this test with fructose, glucose, maltose and sucrose.