biochemical tests

Question 1

name of test for reducing sugars

Answer 1

benedicts ONLY

Question 2

name of test for non-reducing sugars

Answer 2

benedicts test + with other steps

Question 3

name of test for starch

Answer 3

iodine

Question 4

name of test for proteins

Answer 4

biurets

Question 5

name of test for lipids

Answer 5

emulsion test

Question 6

colour change with benedicts (reducing sugars)

Answer 6

blue -> brick red

Question 7

colour change with biurets

Answer 7

blue -> purple

Question 8

colour change benedicts AT THE END of non-reducing sugars

Answer 8

blue -> brown/orange

Question 9

colour change with emulsion test

Answer 9

cloudy - white layer

Question 10

method for iodine test

Answer 10

1. Add 2cm3 of sample
2. Add 2 drops of iodine

Question 11

method for reducing sugars

Answer 11

1. Ground sample into water
2. Add benedicts solution
3. Heat in water bath

Question 12

colour change with iodine

Answer 12

orange -> blue/black

Question 13

method for biurets

Answer 13

1. Ground sample into water and place in test tube
2. Add equal volume of NaOH at room temp
3. Add a few drops of CuSO4 (very very dilute!!!) and mix gently

Question 14

method for emulsion test

Answer 14

1. Add 2cms of sample and 5cm3 of ethanol
2. Shake thoroughly to dissolve lipid
3. Add 5cm3 of water and shake
4. Milky/white emulsion should be seen

Question 15

method for non-reducing sugars

Answer 15

1. Ground sample to water and add 2cm3 of sample to 2cms of benedicts
2. heat in water bath for 5 minutes
3. Add 2cms food sample to 2cm3 of dilute HCl, place in water bath for 5 mins
4. add sodium hydrogencarbonate to neutralise pH (test with pH paper)
5. Re-test with 2cms benedicts in boiling water bath

Question 16

why do you add dilute HCl if there is no colour change with biurets

Answer 16

dilute HCl contains water, therefore it will hydrolyse disaccharides back into monosaccarides

Question 17

why should you add sodium hydrogen carbonate to solution before benedicts again

Answer 17

sodium hydrogen carbonate is alkaline therefore will neutralise the solution as benedicts doesn’t work in acidic conditions

Question 18

exam q
A biochemical test for reducing sugar produces a negative result with
raffinose solution.
Describe a biochemical test to show that raffinose solution contains a
non-reducing sugar. (3)

Answer 18

Heat with acid and neutralise;
Heat with Benedict’s (solution);
Red precipitate/colour;
Accept other colours eg orange/ brown/green

Question 19

exam q
there is a solid precipitate produced after benedicts
Suggest a method, other than
using a colorimeter, that this student could use to measure the quantity of
reducing sugar in a solution. (2)

Answer 19

Filter and dry (the precipitate);
2. Find mass/weight;