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GCSE level School biology revision notes: Food Tests for sugars, proteins, lipids (oils/fats)

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Food Tests for sugars, starch, proteins & lipids - methods and observations

Doc Brown's biology revision notes  Some of these biology revision notes on food tests are suitable for UK KS3 Science Biology (~US grades 6-8) as well as GCSE level students

 This page will help you answer questions such as ...  How do you carry out the test for a reducing sugar in food?   How do you carry out the test for starch in food?   How do you carry out the test for protein in food?   How do you carry out the test for lipids in food?


Sub-index for this page on food tests

1. Introduction - preparing the starting material

2. Benedict's test for reducing sugars

3. Testing for the carbohydrate starch with iodine solution

4. The Biuret test for proteins

5. Two tests for lipids (animal fats/vegetable oils)

6. The DCPIP test for vitamin C

See also Respiration page for a simple investigation of food calorimetry

The food tests described below are qualitative only, they area  NOT quantitative analysis, you simple get a 'YES or NO' result for the presence of particular type of food molecule in your prepared sample.

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1. Introduction - preparing the starting material (need picture of pestle and mortar)

Examples of biological molecules mentioned in conjunction with food tests

Carbohydrates like sugars and starch are a combination of the elements carbon, hydrogen and oxygen (C, H and O). They are synthesised in plants from the sugar molecules made in photosynthesis.

The synthesis of complex carbohydrates like glycogen from smaller sugar molecules.

Lipids (vegetable oils/animal fats) are mainly a combination of the elements carbon, hydrogen and oxygen (C, H and O), sometimes phosphorus (P) too.

These are synthesised in plants and animals from fatty acids, glycerol and other molecules.

The synthesis of lipids from fatty acids and glycerol

Proteins are mainly a combination of the elements carbon, hydrogen, nitrogen and oxygen (C, H, N and O), sometimes sulfur (S) too, enzymes proteins might contain metal ions like Zinc Zn2+.

However, in these tests, you are testing directly for a type of molecular compounds, not their constituent chemical elements.

These are synthesised in plants and animals from amino acids and other molecules.

synthesis of proteins from amino amino acids

Take the food sample and mash it up using a pestle and mortar.

Mix it with some pure water (distilled/deionised) and mix it all up.

Try to have a good 25 cm3 of food mixture solution from various types of food, i.e. enough to carry out a variety of tests on each food sample.

AND, remember that negative results are just as important as positive results!

AND, make sure you were safety goggles throughout the experiments.

With a filter funnel and paper, filter the mixture to remove the 'waste' bits of solid food.

The filtrate can now be tested for specific types of food with various chemical reagents.

In chemistry, the word 'reagent' just means a chemical you use in carrying out a test.

e.g. like silver nitrate or sodium hydroxide in chemical tests for ions in your GCSE chemistry

These four tests described, are qualitative, you don't get numerical data, so no graph work to do.

All you get is a positive or negative result.


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2. Benedict's test for reducing sugars

 

(a) A positive direct test for a reducing sugar

Sugars are an important ingredient in biscuits, bread, breakfast cereals, cakes and sweets.

There are two types of sugar - reducing and non-reducing sugars.

Glucose and fructose are reducing sugars. Sucrose is a non-reducing sugar.

You can use Benedict's reagent to test for reducing sugars.

Put a few cm3 of your previously prepared filtered food sample mixture into a test tube.

Set up a water bath and set the thermostat to ~75oC and wait for it to warm up.

This is safer than directly heating the test tube, but still wear safety glasses.

With a teat pipette, add at least half a dozen drops of Benedict's solution (blue) to the food sample solution and carefully shake the mixture gently.

(Benedict's solution is based on a complex copper salt mixture, hence the blue colour)

Place the test tube in the water bath using a test tube holder and leave it for 5 minutes - make sure the test tube points away from you in case of any accident.

If the food sample contains a reducing sugar, the solution in the test tube will change from the initial blue colour through a series of colours from a green solution ==> yellow solution ==> orange ==> brick-red precipitate.

The more reducing sugar in the food, the more likely you are to see the full colour sequence changes to give the final brick-red precipitate - the best confirming positive result that a reducing sugar was in the food.

If no reducing sugars are present, you should not see any significant colour change.

However, this does not mean that no sugars are present - there might be non-reducing sugars in the food ... read on in section (b) below

(b) Using Benedict's solution to test for a non-reducing sugar

BUT, follow the logic carefully, its an indirect test!

Put a few cm3 of your previously prepared filtered food sample mixture into a test tube and add a few cm3 of dilute hydrochloric acid and gently and carefully shake the mixture.

Set up a water bath and set the thermostat to ~75oC and wait for it to warm up.

Place the test tube in the water bath using a test tube holder and leave it for at least 10 minutes - make sure the test tube points away from you in case of any accident.

At this point you need to appreciate a bit of chemistry!

The hydrochloric acid catalyses and hydrolyses non-reducing sugars like sucrose into reducing sugars like glucose or fructose.

e.g. sucrose ====> glucose  +  fructose

C12H22O11  +  H2O  ===> C6H12O6  +  C6H12O6

Therefore, you can now test the solution for reducing sugars derived from non-reducing sugars!

Remove the test tube from the water bath and place it in a test tube rack.

You then add sodium carbonate to neutralise the acid.

Then test the neutral solution with Benedict's reagent.

If reducing sugars have been formed, the solution in the test tube will change from the initial blue colour through a series of colours from a green solution ==> yellow solution ==> orange ==> brick-red precipitate.

 

If the test with Benedict's solution is negative for both tests (a) and (b), the food probably didn't contain any sugar at all.


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3. Testing for the carbohydrate starch with iodine solution

The carbohydrate starch is present in many foods such as beans, bread, cereal grains, corn, pasta, peas, potatoes and rice.

Put a few cm3 of your previously prepared filtered food sample mixture into a test tube.

Add a few drops of a dilute iodine solution (pale orange-brown) and carefully shake the mixture gently.

If the food sample contains starch the mixture will turn from pale orange to a dark blue-black colour.

(If the solutions are very dilute, you might see a nice blue-purple colour.)

If no starch is present, you should not see any significant colour change - the solution remains a pale orange-brown.

(In chemistry you might test for iodine using starch solution - its the same test and result!)


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4. The Biuret test for proteins

Foods such as meat, fish and cheese are rich in protein.

Put a few cm3 of your previously prepared filtered food sample mixture into a test tube.

Add a few cm3 of potassium hydroxide followed by a few cm3 of copper sulfate solution to the food sample and carefully gently shake the mixture.

If protein is present in the food sample the solution should turn from blue to pink or purple.

Biuret reagent solution can be made from copper sulfate, sodium/potassium hydroxide and sodium tartrate.

Using the 'proper' Biuret reagent is better in my opinion.

If there is no protein in the food sample the mixture just stays the characteristic blue of a copper salt solution.


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5. Two tests for lipids (vegetable oils/animal fats)

Foods such as margarine, milk, vegetable oils like olive oil or sunflower oil contain lipids (chemically they are often fatty acid esters).

(a) The emulsion test for lipids

For this test you do not use the mixture prepared for the other tests.

You mash up a sample of food, as before, with a pestle and mortar and mix with a few cm3 of ethanol (NOT water) and mix up in a test tube for a few minutes (there is no need to filter it).

Let the mixture settle out.

The ethanol dissolves any lipid (fatty/oily) molecules present - lipids are NOT soluble in water.

You then carefully pour the contents of the 1st test tube into a 2nd test tube containing a few cm3 of pure water and carefully shake the mixture (try to avoid food particles getting into the test tube).

Since lipids are not soluble in water, any lipids dissolved in the ethanol are precipitated out as fine globules of oil, so you see a milky emulsion.

The more lipids in the food sample, the more milky the mixture becomes, but this is not a quantitative test!

If no fatty/oily lipid molecules are present, you don't see a milky emulsion.

(I wonder if you could add the Sudan III stain at this point to get the red colour described below?)

(b) The Sudan III test for lipids

For this test you do not have to use the mixture prepared for the other tests.

You mash up a sample of food, as before, with a pestle and mortar and mix with a few cm3 of water and there is no need to filter it.

With a teat pipette add three drops of Sudan III stain solution and carefully and gently shake the mixture. Sudan III stain is red-brown in colour

If the food sample contains lipids, the mixture will separate into two layers.

The 'thinner' top layer contains the lipids (insoluble oil) which becomes stained a bright red by the Sudan III stain if lipids are present.

If there are no lipids in the food sample, you will not get an upper separate red layer forming.


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6. The DCPIP test for vitamin C (Ascorbic acid)

DCPIP reagent is a redox indicator dye and turns from blue to colourless when it is reduced and accepts electrons.

Vitamin C is an antioxidant and therefore a reducing agent.

The blue DCPIP reagent is decolourised in the presence of Vitamin C.

This can be used quantitatively to measure the Vitamin C content of fruit juice.

When you add a few drops of the blue DCPIP reagent to a fruit juice, the vitamin C reduces it to a colourless state.

You can measure the amount of Vitamin C in the fruit juice by adding DCPIP until a blue colour persists.

You can do this accurately by titration using a burette and conical flask.


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