Define the Fehling's Soxhlet method (Lane-Eynon method)?
This is a titrimetric method that is commonly used in food laboratories to estimate percentage of reducing sugars and total sugars. The method is also applicable to biological fluids like milk. The Lane-Eynon method is a titration method of determining the concentration of reducing sugars in a sample. A burette is employed to add the carbohydrate solution being analyzed to a flask consisting of a fixed amount of boiling copper sulfate solution and a methylene blue indicator.
The decreasing sugars in the carbohydrate solution react along with the copper sulfate present in the flask. Once all the copper sulfate in solution has reacted, any further addition of reducing sugars causes the indicator to change from blue to colourless. The volume of sugar solution needed to reach the end point is recorded. Reducing sugars like glucose, fructose, galactose, lactose and maltose can be estimated by direct titration. Non-reducing carbohydrates like sucrose, dextrins and starches cannot be directly estimated by this method. They have to be hydrolyzed into reducing monosaccharides and then estimated titrimetrically. The Fehling Soxhlet method or lane Eynon method is based on oxidation-reduction reaction. You are already familiar with oxidation-reduction tests used for identification of reducing sugars. The basic principle is that sugars in alkaline medium form enediols which reduce cupric ions to cuprous ions. In turn, sugars are oxidized to a mixture of sugar acids.