Chemical tests

Question 1

Test for reducing sugars

Answer 1

Place 2 cm3 of your food sample into a test tube.
Add an equal volume of Benedict’s reagent.
Heat the mixture in a gently boiling water bath for 5 minutes.
If a reducing sugar is present, the mixture will change from a blue solution to a brick red precipitate.

Question 2

How can you determine conc of reducing sugars in a solution after benedicts test is done

Answer 2

Blue - This indicates no reducing sugar is present.
Green - This indicates a low concentration.
Orange - This indicates a medium concentration.
Brick-red - This indicates a high concentration.

Question 2

How to make a more accurate comparison of solutions containing reducing sugars after benedicts test

Answer 2

Use a colorimeter to measure the absorbance of each solution.
Filter the solution and weigh the precipitate.

Question 2

Test for starch

Answer 2

Place 2 cm3 of your food sample into a test tube.
Add a couple of drops of iodine solution and shake.
If starch is present, the solution will turn from orange to blue-black.

Question 2

Test for proteins

Question 3

How to carry out test for non reducing sugars

Answer 3

Carry out the test for reducing sugars, and if the result is negative, continue with the next steps.
Add 2 cm3 of the food sample to 2 cm3 of dilute hydrochloric acid.
Heat the mixture in a gently boiling water bath for 5 minutes (the acid will hydrolyse disaccharides into monosaccharides).
Neutralise the mixture by adding sodium hydrogencarbonate solution.
Retest this mixture using the test for reducing sugars.
If non-reducing sugars were present, the mixture will change from a blue solution to a brick red precipitate.