#10.Tests for carbohydrates

Tests for Reducing sugars, Non-reducing sugar and Starch.









1. Reducing sugar (Benedict's test)

• All monosaccharides and most disaccharides (except sucrose) will reduce blue CuSO4(II), producing a precipitate of red Cu₂O(I).
• Benedict’s reagent is an aqueous solution of Cu SO4(II), Na2CO3 and sodium citrate. 
• 2 cm³ test solution + ≥ 2 cm³ Benedict’s reagent. 
• Shake, and heat for a few minutes at 95°C in a water bath.
• The mass of precipitate or intensity of the colour indicates the amount of reducing sugar present  ---> the test is semi-quantitative. 

2. Non-reducing sugar (Benedict's test) 

Principles:

• Sucrose is a non-reducing sugar (not reduce CuSO4) ---> Benedict's test (-) .
• If it is hydrolysed to form glucose and fructose ---> Benedict's test (+) . 
• So sucrose is the only sugar that will give a (-) Benedict's test before hydrolysis and a (+) test afterwards. 

Steps: 

• Test a sample for reducing sugars to be sure it does not contain reducing sugars.  
• Boil the test solution with dilute HCl for a few minutes to hydrolyse the glycosidic bond. 
• Neutralise the solution by gently adding small amounts of solid NaHCO3 until it stops fizzing.
• Test for reducing sugar.

3. Starch (Iodine test)

• 2 cm³ of test solution + 2 drops of iodine/KI solution. 
• A blue-black colour indicates the presence of starch as a starch-polyiodide complex is formed. 
• Starch is only slightly soluble in water, but the test works well in a suspension or as a solid.

Additional resource: mrothery.co.uk

Syllabus 2015 (a) [PA] carry out tests for reducing and non-reducing sugars (including using colour standards as a semi-quantitative use of the Benedict’s test), the iodine in potassium iodide solution test for starch, the emulsion test for lipids and the biuret test for proteins;

Syllabus 2016  - 2018 Testing for biological molecules Tests for biological molecules can be used in a variety of contexts, such  as identifying the contents of mixtures of molecules and following the activity of digestive enzymes. a)   carry out tests for reducing sugars and non-reducing sugars, the iodine in potassium iodide solution  test for starch, the emulsion test for lipids and the biuret test for proteins to identify the contents of solutions b)   carry out a semi-quantitative Benedict’s test on a reducing sugar  using dilution, standardising the test and using the results (colour standards or time to first colour change) to estimate the concentration