food tests and results

Question 1

test for starch

Answer 1

• add iodine solution and colour changes from brown to a blue-black, starch is present

Question 2

benedicts test for reducing sugars

Answer 2

• reducing sugars are those with the ability to donate electrons

1) benedicts solution is added
2) heat
3) if a reducing sugar is present the colour of the sample will change to red

Question 3

test for non reducing sugars

Answer 3

1) heat the sample with benedicts solution. if there is no colour change, a reducing sugar is not present
2) boil a fresh sample with dilute acid (this hydrolyse the non reducing sugar)
3) neutralise with an alkali
4) now re-heat with benedicts solution and if the solution now turns red the explanation is that a non reducing sugar was present

Question 4

non specific

Answer 4

positive result tells us that a reducing sugar was present but doesnt tell us which one

Question 5

qualitative

Answer 5

colour change is used to determine the result so we cannot obtain a value for the concentration of reducing sugar

Question 6

semi qualitative

Answer 6

colour change depends upon the amount of reducing sugar present
blue
green
yellow
orange
red

Question 7

quantify

Question 8

test for proteins biuret

Answer 8

• add potassium hydroxide and copper sulphate to a sample
• positive = lilac
• negative = stay blue

Question 9

test for lipids

Answer 9

• using a completely dry clean test tube
• add ethanol to the sample
• shake thoroughly to dissolve any lipids in the sample
• then add water to the sample and shake gently
• a milky white emulsion indicates the presence of a lipid