RP4: Food Tests

Question 1

What is the aim of this experiment?

Answer 1

• Conduct qualitative chemical tests for starch, reducing sugars, proteins, and lipids

Question 2

What are the potential risks?

Answer 2

• iodine solution is a harmful irritant + corrosive
• ethanol is highly flammable; should be kept away from naked flames
• water in the water bath is very hot
• Biuret solution contains copper sulphate which is poisonous and sodium hydroxide which is caustic

Question 3

How would you prepare the food sample? (method)

Answer 3

1) use pestle and mortar to grind up small sample of food
2) transfer ground up food into small beaker and add distilled water
3) stir using stirring rod to allow some of food content to dissolve in the water
4) use filter funnel and filter paper to obtain as clear a solution as possible

if substance is liquid you don’t need to do this

Question 4

What is the test for (reducing) sugars including positive test?

Answer 4

1) transfer 5cm³ of food sample to test tube
2) place in water bath and set to 75ºC
3) add Benedict’s solution using pipette and leave for 5 minutes
4) if sample contains a reducing sugar, solution will change from blue to green, yellow or brick-red (highest conc) depending on conc; if no reducing sugar is present, it will remain blue

Question 5

What is the test for starch including positive test?

Answer 5

1) transfer 5cm³ of food sample to test tube
2) add few drops of iodine solution and gently shake
3) if sample contains starch, colour of solution will change from brown-orange to blue-black; if no starch is present, it will remain brown-orange

Question 6

What is the test for proteins including positive test?

Answer 6

1) transfer 2cm³ of food sample to test tube
2) add 2cm³ of biuret solution to sample and gently shake
3) if sample contains protein, solution will change from blue to purple; if no protein is present, solution will remain blue

Question 7

What is the test for lipids including positive test?

Answer 7

1) transfer 5cm³ of food sample to test tube
2) add few drops of ethanol to test tube and gently shake
3) pour solution in test tube of water
4) if lipids present, cloudy white emulsion will form

Question 8

Name some sources of error

Answer 8

• Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the
  concentration of the tested molecule is low.