Biochemical Tests (Part of 3.1.2) (A)

Question 1

Which sugars are reducing?

Answer 1

• All monosaccharides and disaccharides, except for sucrose

Question 2

Describe the test and positive result for reducing sugars

Answer 2

• A small amount of the sample is placed in a test tube with the same volume of Benedict’s solution
• This is heated to 95°C in a water bath
• Positive result shows a brick red precipitate

Question 3

Describe the test and positive result for non reducing sugars

Answer 3

• Perform the Benedict’s test for reducing sugars to confirm a negative result
• Hydrolyse another sample by boiling with dilute acid in a water bath
• When cooled neutralise with an alkali
• Repeat Benedict’s test for reducing sugars
• Positive result shows a brick red precipitate

Question 4

Describe the test and positive result for starch

Answer 4

• Add a few drops of iodine to the sample
• A blue/black colour is produced