Biology · Digestion and Enzymes
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Food Tests
How do you find out what is in a food? This lesson covers the required practical on food tests: the reagents for starch, sugar, protein and lipids, what a positive and a negative result look like, and how to carry the tests out safely.
Learning Objectives
- 1State the reagent used to test for starch, sugars, proteins and lipids.
- 2Describe the colour change that gives a positive result for each test.
- 3Describe a method for each food test, including how to prepare a food sample.
- 4Interpret the results of food tests to say which food groups are present.
- 5Describe the safety precautions and the controls used in the tests.
Retrieval practice
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1
Name the three types of large molecule in food.
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Carbohydrates, proteins and lipids.
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2
What is starch broken down into?
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Simple sugars.
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3
What are proteins broken down into?
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Amino acids.
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4
What is the name of the enzyme that digests starch?
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Amylase.
Finding out what is in your food
The labels on food packets tell you what they contain, but how would you find out for yourself? A food test uses a chemical, called a reagent, that changes colour or appearance when a particular type of molecule is present. These are qualitative tests: they tell you whether a substance is present or not, and they only give a rough idea of how much. This lesson covers the four tests you have to know. Three of them are about the large molecules from lesson 3: starch, protein and lipids. The fourth is for sugars, which are the products of digesting carbohydrates.
Preparing the food sample
Most of the tests need the food to be in a solution, so the first job is to prepare a sample.
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Break up the food
Use a pestle and mortar to grind a small amount of the food with a little distilled water.
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Make a solution
Transfer the mixture to a beaker and stir it with more distilled water, so that some of the food dissolves.
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Filter
Filter the mixture, so that the solid pieces are left behind. The liquid that passes through is the sample to test.
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The lipid test is different
For lipids, the food is shaken with ethanol, not water, because lipids dissolve in ethanol.
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Keep it clean
Use clean apparatus for each test, so that one test does not contaminate the next.
The four food tests
Each test uses a different reagent, and each gives a positive result that you can recognise.
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Starch: iodine
Add a few drops of iodine solution to the sample. The colour changes from orange-brown to blue-black if starch is present.
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Sugars: Benedict's reagent
Add Benedict's reagent, which is blue, and heat the mixture in a water bath. If sugar is present the colour changes from blue to green, yellow, orange or brick red.
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Protein: Biuret reagent
Add Biuret reagent, which is blue, to the sample. The colour changes from blue to purple if protein is present. No heating is needed.
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Lipids: ethanol emulsion test
Shake the food with ethanol, then pour the liquid into water. A cloudy white emulsion forms if lipid is present.
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A negative result
If the food group is absent, the reagent stays its original colour, or the mixture stays clear.
The colour changes in the four food tests
A positive result is a change from the original colour. A negative result is no change.
Carrying out Benedict's test for sugar
Benedict's test is the only one of the four that needs heat, so it needs most care.
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1
Add the sample
Put about 2 cm³ of the food sample into a test tube.
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2
Add the reagent
Add about 10 drops of Benedict's reagent. The mixture is blue.
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3
Heat it safely
Put the test tube in a hot water bath, at about 75 °C, for about five minutes. Do not use a flame.
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4
Watch for a colour change
If there is sugar the mixture goes green, yellow, orange or brick red. If there is none it stays blue.
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5
Compare
The further the colour moves towards brick red, the more sugar there is in the sample.
The other three tests
These tests are simpler, and none of them needs heating.
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Iodine test for starch
Put about 2 cm³ of the sample in a test tube, or in a well on a spotting tile. Add a few drops of iodine solution and look at the colour straight away.
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Biuret test for protein
Put about 2 cm³ of the sample in a test tube. Add Biuret reagent, or sodium hydroxide followed by a few drops of copper sulfate solution. Shake gently and look for a colour change.
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Emulsion test for lipids
Shake a small amount of the food with about 2 cm³ of ethanol, so that any lipid dissolves. Pour the clear liquid into a test tube of water. If there is lipid, it forms tiny droplets, and the water turns cloudy white.
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Sudan III
Some courses test for lipids with Sudan III stain instead. It forms a red layer on top of the mixture if lipid is present.
Safety and good practice
Some of the reagents are harmful, and you may be using hot water. Think about the risks before you start.
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Eye protection
Wear goggles. Biuret reagent contains sodium hydroxide, which is corrosive, and Benedict's reagent contains copper sulfate, which is harmful.
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Hot water
Use a water bath, not a flame, so that the liquid cannot boil over. Handle hot tubes with a holder and point the open end away from people.
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Ethanol
Ethanol is flammable. Keep it away from flames.
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Iodine
Iodine solution stains skin and clothes.
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Controls
Test distilled water with each reagent, to show what a negative result looks like. Test a food that you know contains the substance, such as glucose, starch or egg white, to show what a positive result looks like.
Interpreting food test results
A student tests a sample of food. The iodine stays orange-brown. Benedict's reagent turns brick red after heating. Biuret reagent turns purple. In the emulsion test the water stays clear. Which food groups does the sample contain?
Show the solutionHide the solution
- 1 Iodine stayed orange-brown There is no starch.
- 2 Benedict's turned brick red Sugar is present, and the colour suggests a large amount.
- 3 Biuret turned purple Protein is present.
- 4 The water stayed clear There is no lipid.
AnswerThe sample contains sugar and protein, but no starch and no lipid.
Case study
Testing urine for sugar
In the past, Benedict's reagent was used to test urine for glucose, because a high level of glucose in the urine can be a sign of diabetes. A patient's urine was heated with the reagent, and the colour showed roughly how much sugar there was. Today people use test strips and blood glucose meters, which are quicker and more accurate, but they work on the same idea as the food tests: a reagent that changes in the presence of one particular substance.
The four tests to learn
Iodine turns blue-black with starch. Benedict's reagent, heated, turns from blue to brick red with sugar. Biuret reagent turns purple with protein. Ethanol then water gives a cloudy white emulsion with lipids.
Name the reagent, say whether you heat it, and give the colour before and after.
Food tests
Food tests
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Starch
- iodine
- orange-brown to blue-black
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Sugar
- Benedict's
- heat
- blue to brick red
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Protein
- Biuret
- blue to purple
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Lipid
- ethanol then water
- cloudy white emulsion
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Safe and fair
- goggles
- water bath
- and controls
Summary and exam focus
- Starch: iodine solution, orange-brown to blue-black.
- Sugar: Benedict's reagent, heated in a water bath, blue to green, yellow, orange or brick red.
- Protein: Biuret reagent, blue to purple.
- Lipids: ethanol then water, a cloudy white emulsion.
- Wear goggles, use a water bath instead of a flame, and test controls so you know what a result looks like.
Exam focus
Describe how you would test a food sample for sugar, and what you would see if the result is positive. (3 marks) (3 marks)
Give the reagent, the condition (heat it in a water bath) and the colour change, from blue to green, yellow, orange or brick red. Saying that it turns red is not enough: say brick red, and say that it is from blue.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Reagent
- A chemical used to detect another substance because it changes colour or appearance.
- Qualitative test
- A test that shows whether a substance is present, but does not measure how much.
- Iodine solution
- The reagent that turns blue-black when starch is present.
- Benedict's reagent
- The blue reagent that turns green, yellow, orange or brick red when heated with sugar.
- Biuret reagent
- The blue reagent that turns purple when protein is present.
- Emulsion test
- The test for lipids using ethanol and water, which makes a cloudy white emulsion.
- Starch
- A carbohydrate made of many sugar molecules joined together.
- Positive result
- A result showing that the substance being tested for is present.
- Negative result
- A result showing that the substance being tested for is not present.
- Control
- A test that shows what a result looks like when nothing is present, or when it definitely is.
- Water bath
- A container of warm or hot water used to heat things gently and evenly.
Questions and answers
20 questions set on this lesson, with the mark schemes and model answers open.
Name the reagent used to test for starch.
Mark scheme — 1 mark available
- Iodine (solution) — 1 mark
Model answer
Iodine solution.
State the colour change that shows that starch is present in a food sample.
Mark scheme — 2 marks available
- Starts orange-brown / yellow-brown — 1 mark
- Changes to blue-black — 1 mark
Model answer
Orange-brown to blue-black.
Figure 1 shows four tests on a sample of food. Tube A contains iodine solution, tube B contains Benedict's reagent after heating, tube C contains Biuret reagent, and in tube D the food was shaken with ethanol and poured into water. Use Figure 1 to say whether the sample contains starch, sugar, protein and lipid.
Mark scheme — 4 marks available
- Starch absent: iodine stayed orange-brown — 1 mark
- Sugar present: Benedict's brick red — 1 mark
- Protein present: Biuret purple — 1 mark
- Lipid absent: no cloudy emulsion — 1 mark
Model answer
Starch is not present, because the iodine stayed orange-brown. Sugar is present, because Benedict's turned brick red. Protein is present, because Biuret turned purple. Lipid is not present, because the liquid stayed clear.
Describe how you would test a food sample for sugar.
Mark scheme — 3 marks available
- Add Benedict's reagent — 1 mark
- Heat in a (hot) water bath — 1 mark
- Blue changes to green / yellow / orange / brick red if sugar is present — 1 mark
Model answer
Add Benedict's reagent to the sample and heat it in a water bath. If sugar is present, the blue colour changes to green, yellow, orange or brick red.
A student heats a test tube containing Benedict's reagent in a hot water bath, and not in a Bunsen burner flame. Suggest two reasons for this.
Mark scheme — 2 marks available
- Less risk of the liquid boiling over / spitting / burns — 1 mark
- More even / controlled temperature — 1 mark
Model answer
The liquid cannot boil over or spit out of the tube, so there is less risk of burns. The temperature is more even and easier to control.
Describe how you would test a food for lipids, and what you would see if the result is positive.
Mark scheme — 3 marks available
- Shake the food with ethanol — 1 mark
- Pour into water — 1 mark
- A cloudy white emulsion forms if lipid is present — 1 mark
Model answer
Shake the food with ethanol and pour the liquid into a test tube of water. If lipid is present, a cloudy white emulsion forms.
A student also tests some distilled water with iodine solution. Explain why.
Mark scheme — 2 marks available
- It is a control / shows a negative result — 1 mark
- To compare with the food sample / so a colour change can be recognised — 1 mark
Model answer
Distilled water contains no starch, so it acts as a control. It shows what a negative result looks like, so the student can tell that a colour change in the food sample is a real result.
Food X gave these results: the iodine solution turned blue-black, Benedict's reagent stayed blue after heating, Biuret reagent stayed blue, and the emulsion test stayed clear. Say what food X contains and does not contain.
Mark scheme — 3 marks available
- Contains starch — 1 mark
- No sugar (Benedict's stayed blue) — 1 mark
- No protein (Biuret stayed blue) and no lipid (stayed clear) — 1 mark
Model answer
Food X contains starch. It does not contain sugar, protein or lipid.
Describe two safety precautions that should be taken when doing the food tests.
Mark scheme — 2 marks available
- Wear goggles / eye protection — 1 mark
- Use a water bath / keep ethanol away from flames / hold hot tubes with a holder — 1 mark
Model answer
Wear eye protection (goggles). Use a hot water bath instead of a flame, and keep the ethanol away from flames.
A student has three foods, P, Q and R. Describe how the student could use Benedict's reagent to find out which food contains the most sugar.
What the examiner wants: Include what must stay the same in each test, and say how you would tell which has the most sugar.
Mark scheme — 6 marks available
- Prepare a solution of each food in the same way — 1 mark
- Same volume of each sample and of Benedict's reagent — 1 mark
- Heat in a water bath for the same time and temperature — 1 mark
- Compare the colours — 1 mark
- Closest to brick red = most sugar — 1 mark
- Control with distilled water / repeat the test — 1 mark
Model answer
Prepare a solution of each food in the same way. Put the same volume of each sample in a test tube and add the same volume of Benedict's reagent. Heat all the tubes in the same hot water bath for the same length of time. Compare the colours. The food that goes closest to brick red contains the most sugar. Test distilled water as a control and repeat to check the results.
Which reagent turns purple when protein is present?
Why: Biuret reagent changes from blue to purple when protein is present.
A food is tested with iodine solution and the solution turns blue-black. What does the food contain?
Why: Iodine turns blue-black when starch is present.
What must be done after adding Benedict's reagent to a sample?
Why: The mixture must be heated in a water bath. Without heating, the colour change will not happen.
In Benedict's test, a sample turns brick red. What does this show?
Why: Brick red is the last colour in the series from blue, and it shows a large amount of sugar.
What is the positive result for the emulsion test for lipids?
Why: If lipid is present, the ethanol mixture forms a cloudy white emulsion when it is poured into water.
Why is the lipid test done with ethanol?
Why: Lipids dissolve in ethanol, but not in water, so the ethanol carries them into solution.
Why should a student use a water bath instead of a flame to heat Benedict's reagent?
Why: The liquid is less likely to boil over, and the temperature is more even and safer.
A food gives no colour change with any of the four tests. What can you conclude?
Why: All four tests were negative, so the food contains none of starch, sugar, protein or lipid.
Why is it useful to test a food that you know contains starch with iodine solution?
Why: It is a positive control, which shows what a positive result looks like for comparison.
These tests are qualitative. What does this mean?
Why: Qualitative tests show whether a substance is present, but they do not measure exactly how much there is.