food tests

Question 1

test for reducing sugars

Answer 1

• reducing sugars (glucose, fructose, and maltose) will produce a red precipitate when boiled with Benedict’s solution while sucrose will not give a red precipitate
• place each food sample of equal volume into a test tube
• add the same volume of Benedict’s solution to the food sample
• shake the mixture and place the test tuve in a boiling water bath for 2-3 minutes
• blue - red precipitate : increasing amount of reducing sugar present

Question 2

test for fats

Answer 2

ethanol emulsion test :
for liquid sample :
- add ethanol to a drop of liquid sample in a test tube and shake the mixture thoroughly
- add equal volume of water to the mixture and shake the mixture
for solid sample :
- crush the solid sample into small pieces using mortar and pestle and place the pieces in a test tube
- add ethanol into the test tube and shake thoroughly
- allow solid particles to settle before carefully decanting the ethanol (pour off top layer of ethanol) into another test tube containing equal volume of water and shake the mixture
- fats present : cloudy white emulsion
- fats not present : remains clear

Question 3

test for proteins

Answer 3

biuret test
- to a liquid food sample, add a equal volume of biuret solution
- shake well and allow mixture to stand for five minutes
- violet colour : proteins are present
- blue colour : proteins are not present