3.4 Testing for Carbohydrates

Question 1

How do you carry out the Benedicts test? (For reducing sugars)

Answer 1

1. Place the sample in boiling tube (grind up/blend in water if solid)
2. Add an equal volume of Benedictus solution
3. Heat the mixture gently in a boiling water bath for five minutes

Question 2

What do the colour changes of the Benedicts reducing sugar test show?

Answer 2

Red = positive (high reducing sugar)
Orange = positive (medium reducing sugar) 
Yellow = positive (low reducing sugar) 
Green = positive (very low reducing sugar)
Blue = negative (no reducing sugar)

Question 3

How do you carry out the Benedicts test? (For non-reducing sugars)

Answer 3

Carry out the Benedicts test for reducing sugar. If a negative result is obtained then carry out the test for non reducing sugars :

1. Add dilute hydrochloric acid (HCl)
2. Boil with the HCl
3. Then look at the colour change to see how much sugar is present

Question 4

How do you test for starch?

Answer 4

1. Add drops of potassium iodide to sample

2. If there is a colour change from yellow/brown to purple/black then starch is present

Question 5

What are reagent strips and how are they used?

Answer 5

They can be used to determine the presence of reducing sugars
+ an advantage of using them is that, with the use of a colour-coded chart, the concentration of the sugar can be determined