Investigating molecules in food using food tests

In a nutshell

Foods contain many different molecules. Different chemical tests, like the Benedict's test and Biuret test, can be used to test for these different molecules. In this practical you will learn how to test for the presence of sugars, starch, proteins and lipids.

Equipment list

The following equipment can be used to investigate the different types of molecules that a food sample contains.

Equipment	Use
Food sample	To test the types of molecule that the food contains.
Pestle and mortar	To grind up the food sample.
Beaker	To put the food sample in when dissolving.
Distilled water	To combine with the food sample when dissolving.
Glass rod	To stir the food sample.
Filter funnel	To separate the undissolved food sample.
Filter paper	To separate the undissolved food sample.
Test tube	To hold the food sample during the individual tests.
Iodine solution ( $0.01\space mol/dm^3$ )	To test for the presence of starch.
Pipettes	To transfer the test solutions and food sample.
Benedict's solution	To test for the presence of sugars.
Hot water bath	To incubate the test tubes at $75\ \degree C$ during the Benedict's test.
Test tube holder	To hold the test tubes whilst they are in the hot water bath.
Thermometer	To measure the temperature of the hot water bath.
Stopwatch	To measure the time that the tubes are in the hot water bath.
$10\ cm^3$ measuring cylinder	To measure the volume of the solutions.
Biuret solution	To test for the presence of proteins.
Sudan III solution	To test for the presence of lipids.
Ethanol	To test for the presence of lipids.

Safety precautions

When carrying out experiments it is very important to consider the safety precautions so nobody gets hurt.

Hazard	Risk	Safety measure
Biuret solution	Biuret solution contains copper sulphate which is poisonous. It also contains sodium hydroxide which is corrosive.	Be very careful when handling the solution. Wash your skin immediately if you touch the solution.
Ethanol	Ethanol is highly flammable.	Keep the ethanol away from the bunsen burner.

Preparing the food sample

Before carrying out any of the experiments below you will need to prepare the food sample.

Method

1.	Place your food into the pestle and mortar and grind it up into small pieces.
2.	Pour the ground food into a beaker and add distilled water to the beaker.
3.	Using a glass rod, stir the mixture to dissolve some of the food.
4.	Put a filter funnel over a clean beaker and place filter paper into the funnel.
5.	Pour the solution from step 3 into the funnel to filter out the solid particles that remain.

Now, you can use this solution for the experiments detailed below.

Experiment 1: Using Benedict's solution to test for sugars

Foods can contain reducing and non-reducing sugars. The Benedict's test is used to test for reducing sugars.

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable which depends on the independent variable, therefore it is the one you measure. The control variable is the variable that is kept the same during the experiments.

Independent variable	$The\space food\space sample.$
Dependent variable	$The\space presence\space or\space absence\space of\space sugars.$
Control variable	$The\space temperature\space that\space the\space water\space bath\space is\space set\space to,\space the\space amount\space of\space food\space sample\space used,\space\\ the\space amount\space of\space Benedict's\space solution\space used\space and\space the\space amount\space of\space time\space that\space the\space test\space \\tube\space is\space left\space in\space the\space water\space bath.$

Method

1.	Using a pipette, transfer $5\ cm^3$ of the food sample from the beaker to a test tube.
2.	Set a water bath to $75\degree C$ .
3.	Using a pipette, add $10$ drops of Benedict's solution to the test tube.
4.	Place the test tube in a test tube holder and place this in the water bath for $5\ minutes$ .
5.	Remove the test tube from the water bath and record the colour of the solution.

Analysis

Now you have the colour changes recorded, you need to know what these mean.

Initial colour	Colour of a positive result
Blue	Brick-red precipitate

Biology; Biology practicals; KS4 Year 10; Investigating molecules in food using food tests

There may be a range of colour changes that can be observed as the concentration of sugar increases (1.). If there is not much sugar in the sample it may appear green, yellow or orange.

Note: The precipitate can take a while to settle in the tube so you may just see a brick-red solution.

Conclusion

After analysing the colour change in the test tube, you will now be able to conclude whether your food sample contains sugar or not. If the colour of the sample in the test tube changes to brick-red and a precipitate is formed, then sugar is present. If the colour remains blue, then sugar is not present in your food sample.

Evaluation

After completing the experiment, you must comment on the quality of your data. You should also think about how your method could be improved.

A common error is using reagents that are out of date. As a result, the colour change may be too small to detect. The amount of glucose will impact the colour that you observe. If there is not much glucose in the food sample, you may mistake this as a negative result. To fix this error, you could use a colour chart to compare your tube against.

Another source of error could be that the food sample was prepared incorrectly. If it was not ground down enough at the start, the food sample will be less concentrated. It is also very common for students to put too much dye in which gives a false positive result.

Experiment 2: Using iodine to test for starch

Many foods contain starch. Foods like potatoes and pasta contain a lot of starch. Iodine can be used to test for the presence of starch using the following method.

Method

1.	Pipette $5\ cm^3$ of your prepared food sample into a test tube.
2.	Add a few drops of iodine solution to the test tube.
3.	Note any changes in colour.

Analysis

Now you have the colour changes recorded, you need to know what these mean.

Initial colour	colour of positive result
Yellow-brown	Blue-black

Biology; Biology practicals; KS4 Year 10; Investigating molecules in food using food tests

Conclusion

After analysing the colour change in the test tube, you will now be able to conclude whether your food sample contains starch or not. If the colour of the sample in the test tube changes to blue-black then starch is present. If the colour remains yellow-brown, then starch is not present in your food sample.

Evaluation

You should repeat the same process as in experiment one to evaluate your data.

Experiment 3: The biuret test for proteins

Protein is essential for your growth. Many foods, like meat, are rich in protein. The biuret test is often used to test for the presence of proteins in a food sample. The test is carried out using the following method.

Method

1.	Using a pipette, transfer $2\ cm^3$ of food sample to a test tube.
2.	Pipette $2\ cm^3$ of the biuret solution into the test tube.
3.	Gently shake the test tube to mix the solutions.
4.	Note any changes in colour.

Analysis

Now you have the colour changes recorded, you need to know what these mean.

Initial colour	Colour of a positive result
Blue (1.)	Purple (2.)

Biology; Biology practicals; KS4 Year 10; Investigating molecules in food using food tests

Conclusion

After analysing the colour change in the test tube, you will now be able to conclude whether your food sample contains protein or not. If the colour of the sample in the test tube changes to purple then protein is present. If the colour remains blue, then protein is not present in your food sample.

Evaluation

You should repeat the same process as in experiment one to evaluate your data.

Experiment 4: The Sudan III test for lipids

Lipids are present in oils and fats. You can test for the presence of lipids in a food sample using the Sudan III test. The method is described below.

Method

Biology; Biology practicals; KS4 Year 10; Investigating molecules in food using food tests

1.	Using a pipette, transfer $5\ cm^3$ of the food sample into a test tube.
2.	Using a new pipette, pipette $3$ drops of Sudan III stain solution into the test tube.
3.	Gently shake the test tube to mix the solutions.
4.	Note any changes you observe.

Analysis

Now you have the changes recorded, you need to know what these mean.

Initial colour	positive result
Depends on the food sample being tested.	Sample separates into two layers. The top layer will be bright red.

Conclusion

After analysing the changes in the test tube, you will now be able to conclude whether your food sample contains lipids or not. If the sample in the test tube separates into two layers and the top layer becomes red, then lipids are present. If the sample does not separate, then lipid is not present in your food sample.

Evaluation

You should repeat the same process as in experiment one to evaluate your data.

Experiment 5: Ethanol test for lipids

An alternative to the Sudan III for lipids is the ethanol test. This also tests for the presence of lipids.

Method

Biology; Biology practicals; KS4 Year 10; Investigating molecules in food using food tests

1.	Using a pipette, transfer $2\ cm^3$ of food sample into a test tube.
2,	Add $2\ cm^3$ of ethanol to the test tube and gently shake the tube.
3.	Measure out $4\ cm^3$ of water into another test tube and then pour the contents of the food/ethanol test tube into the water test tube.
4.	Note any changes you observe.

Analysis

Now you have the changes recorded, you need to know what these mean.

Initial colour	Positive result
Depends on the food that was used.	A white emulsion is formed.

Conclusion

After analysing the changes in the test tube, you will now be able to conclude whether your food sample contains lipids or not. If the sample in the test tube forms a white emulsion on the surface, there is lipid in the food sample. If no emulsion is formed then there is no lipid in the sample.

Evaluation

You should repeat the same process as in experiment one to evaluate your data.