Estimate the Amount of Reducing Sugars

To estimate the amount of reducing sugars in a grape, the test method for reducing sugars must be known first. To test the presence of reducing sugars in a solution, Benedict’s test can be carried out. In Benedict’s test, equal volume of Benedict’s solution should be added into the unknown solution. The mixture is then boiled. If reducing sugars are present, there should be some brick-red precipitates. This method can be done quantitatively. The amount of precipitates can indicate the amount of reducing sugars. The more precipitates that form, the more reducing sugars are present in the solution. So by comparing the amount of precipitates formed in the grape juice with the amount of precipitates formed in a standard sugar solution, one can estimate the amount of reducing sugars in a grape.

In this experiment, a standard glucose solution of concentration 0.01 M was used. It was diluted into different concentrations. The amount of reducing sugars was determined by Benedict’s test and the results were plotted in a standard curve.

Procedure

1. Prepare 1 mL of glucose solutions of different concentrations from a stock 0.01 M glucose solution by adding suitable amounts of distilled water. The dilution table is shown:
   
   
   
   
2. Peel off the epidermis of the grape, put the grape into a beaker, grind the grape tissue with a glass rod to get as much juice as possible, pour the juice into a measuring cylinder and add distilled water into it up to 25 mL.
3. Pipette 1 mL of the prepared diluted juice into a test tube G. Pipette 1 mL of the sugar solutions into test tubes A to F.
4. Place 2 mL of Benedict’s solution into each of test tubes A to G. Boil all tubes in a water bath for 5 minutes. Allow the precipitates to settle for 10 minutes. Estimate the amount of precipitates in each tube

Discussion

1. The skin and seeds should be removed. They will affect the grinding procedure.
2. Excess Benedict’s solution must be added so that all reducing sugars can react.
   
   
   
   
   
   
3. Allow the precipitates to settle down before the estimation.
4. The amount of reducing sugar estimated must be in term of the amount of glucose.
5. If the amount of reducing sugar in a grape is greater than the amount of reducing sugar in 0.01 M glucose solution, we can dilute the grape juice by a dilution factor and do the comparison again.