8/20/2019 11 Testing for Protein (1)

1/2

Experiment 11. Testing food samples for protein

(a) In your notebook draw up a table like the one below.

(b) Label six test-tubes 1-6.

(c) If the food is a solid, crush a small piece (about 5 mm cube in a mortar with about

1! cm" water. (If the food is a li#uid, simply pour about $! mm of it into a test-tube.

(d) %our about $! mm of the crushed mixture into a test-tube.

(e) &dd about 5 mm of sodium hydroxide solution. ('&) *

(f) &dd about 5 mm dilute copper sulphate solution and shake the tube +ently sideways

to mix the contents. eturn the tube to the rack and wait for a few seconds.

(g) ecord the results in the table in your notebook.

Food Result on adding sodium hydroxide and copper 

sulphateInterpretation

Milk

Egg white

Onion

 Apple

Beans

White meat

* '&). odium hydroxide is caustic and dissoles clothin+, skin and bench tops. It is

destructie, so if any is spilt on the bench, neutralie it at once with an e#ual olume of dilute

hydrochloric acid and wipe dry. If spilt on clothin+, do the same but follow with a wash in as

much water as possible. If spilt on the skin, do not add acid but wash under the tap until the

/soapy/ feelin+ is remoed.

Experiment 11 Discussion

1 In which of your samples do you think protein is fairly abundant0

2 In which samples do you think protein is absent or in ery low concentration0

3 If one of your samples +ies no purple colour with the biuret test, does this result mean

that the sample contains no protein0

4 If a food sample +ies a purple colour with the biuret test does this mean that it contains

only protein0

Experiment 11 Testing food samples for protein

ood tests 11.!1

ood tests 11.!$

8/20/2019 11 Testing for Protein (1)

2/2

Outline ood samples are crushed in water and the biuret test applied.

Prior no!ledge 2he biuret reaction (not essential. 2he caustic nature of sodium hydroxide.

"d#ance preparation and materials ood samples as on p. 1!.!1

3eans, peas, etc. should be placed to soak the day before the experiment.

2he white meat can be chicken or fish4 presered do+fish meat will do.

3oil the e++ white with tap water to make a suspension4 1! cm " e++ white per 1!! cm" water

is sufficient. &llow 5 cm" per +roup.

$ or 1! sodium hydroxide solution

1 copper sulphate solution

"pparatus- per groupsmall mortar and pestle 3unsen burner 

test-tube rack and 6 test-tubes 6 labels or spirit marker 

test-tube holder 6 watch-+lasses (for food samples

Experiment 11. Discussion $ answers

1 eat and e++ white should +ie a stron+ colour.

2 ilk and beans should +ie an indication of protein. &pple and onion should show little or no

 protein reaction.

3 %ossibilities7

(a the sample contains no protein.

(b the sample does not contain enou+h protein to react.

(c it contains a protein which has not been extracted by the crushin+ in water.

8 2he sample could contain other classes of food in addition to protein.

$!cm" per +roup