#31 Food test 2 - Benedict's test for Reducing Sugars

All simple sugars (e.g.glucose) are reducing sugars. They will react with a blue liquid called Benedict's solution to give a brick red color. We can use this reaction to find out if a food or other substance contains a reducing sugar.

Process 

• add a few drops of Benedict's solution
• heat the mixture for 2-3 minutes in boiling water bath
• a BRICK RED/ORANGE COLOR is a positive result: glucose is present
• The closer the color is to brick red, the more reducing sugar is present.

Observation (Final colour change)	Interpretation
No colour change (Blue)	No reducing sugars present
Green	Trace amounts of reducing sugars present
Yellow	Low amounts of reducing sugars present
Orange	Moderate amounts of reducing sugars present
Brick-red	Large amounts of reducing sugars present

Explanation

• Reducing Sugars are sugars that contain aldehyde groups, that are oxidised to carboxylic acids (R-COOH).

• They are classified as reducing sugars since they reduce the blue Cu²⁺(copper II ions) to Cu⁺ (copper I ions). These are precipitated in form of red Cu₂O (copper oxide), insoluble in water.  

Illustrations (Photo credit: cccmkc.edu.hk)

When the concentration of reducing sugar is low, the color of the Benedict's test may be light green or pale orange.

Videos: Benedict's test for reducing sugars

Additional source: cccmkc.edu.hk