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Food Test
In this practical, you should test a solution of unknown food substances for starch, protein, reducing sugars, non- reducing sugars, and fats/oils.
Record your procedure, observation, and conclusions,
Then answer questions about nutrition and the digestive system.
How to Carry Out Food Tests
When performing an experiment with the aim of identifying certain food content contained in the sample solution different reagents are used specifically in certain food sample as follows
TABLE FOR THE FOOD TYPE, REAGENT USED AND REAGENT COLOUR
FOOD TESTED/FOOD TYPE | REAGENT USED | REAGENT COLOUR |
| Starch | Iodine solution | Brown |
| Reducing sugar | Benedict’s solution | Blue |
| Non reducing sugar | Benedict’s solution | Blue |
| Dilute Hydrochloric acid (HCl) | Colourless | |
| Dilute sodium hydroxide (NaOH) | Colourless. | |
| Protein | Dilute sodium hydroxide (NaOH) | Colourless. |
| 1% Copper sulphate (CuS | Blue. | |
| Lipids | Sudan (III) solution | Red. |
Starch
Starch is a carbohydrate which made up by condensation of many glucose molecules.
Food sources which starch is obtained include potatoes, maize, wheat, rice e.t.c
The chemical reagent used to test starch is Iodine solution
Properties of starch
- They are insoluble in water.
- They are non-crystalline.
- They are macromolecules.
- They are not sweet
Uses of starch
- Provides the body with energy
- It is the food reserve in plant.
- It is the stored form of carbohydrate in plant
Procedures
1. Measure 2ml of the prepared starch solution in a test tube
2. Add 2 drops of iodine solution in a test tube containing the solution of starch
3. Shake it thoroughly
4. Observe the color of the mixture
5. If starch is present, the mixture will turn into blue black (dark blue)
6. If starch is absent, the mixture will retain the brown color of iodine solution
Lipids
Lipids are compounds of carbon, hydrogen and oxygen. They are insoluble in water
1. There are two forms of lipids which are fats and oil. Fats are in solid form while oils are in liquid form at room temperature (250).
2. Natural sources of lipids include sunflower, coconuts, avocado, groundnuts and fat from animal meat.
3. The chemical regent used in testing Lipids is Sudan III solution
4. White paper can also be used
Properties of lipids
1. They are insoluble in water
2. They are soluble in organic solvents such as alcohol, chloroform and ether.
3. When oil is shaken in water, the oil breaks down into droplets which disperse in water. After few minutes water and oil separates into two layers.
When water is added to a lipid that has dissolved in an organic solvent a white milky suspension is formed. This is an emulsion.
When fats rubbed against paper, the paper became translucent. When the paper is dipped in ether, translucent disappears because the oil dissolves in ether.
In mixtures of lipid and water, the lipid separates and moves to the top. When shaken with Sudan III solution, this lipid absorbs the dye of Sudan III solution, turns red, and produces
a "red ring" at the top of the test tube. Stained lipids globules will float on the top of water because they are less dense then water. This is the basis for testing lipids by using Sudan (III) solution.
Uses of lipids
1. They are source of energy. They can be broken down during respiration process to release energy.
2. Fat adipose tissues below the skin insulate the body against heat loss.
3. Lipids act as a source of metabolic water when fats and oils are oxidized. This is an alternative source of water for the animal living in desert area like camel.
4. Fats deposited around the major organs such as kidney, liver and heart protect those organs and they act as shock absorbers.
5. They form major structural component of the cell membrane. For example phospholipid is a special type of lipid which is a major component of cell membrane.
6. They are medium for storing fat soluble vitamins like vitamins A, D, E and K
Procedures
1. Measure 2ml of a given solution in a test tube
2. Add 3 drops of Sudan (III) solution then shake the content vigorously and let it settle for five minutes
3. Observe the color changes
4. If lipid is present a red ring will be formed on the top of the solution
5. If lipid is absent the solution will form a homogenous red mixture with Sudan (III) solution
Protein (Biuret test)
Proteins are long chains of amino acid. They are made up of carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain Sulphur and phosphorus
Natural sources of protein are categorized into plants, animal products and fungi.
1. Meat, fish eggs and milk are animal products
2. Mushroom and yeast are fungi proteins
3. Beans, coconut and ground nuts are proteins from plants Chemicals used to test protein are
4. Sodium hydroxide solution (dil NaOH)
5. Copper (II) Sulphate solution (1% CuSO4)
Properties of protein
1. They are insoluble in water.
2. They coagulate on heating. This means that small particles of protein clump together to form bigger particles when a mixture of protein and water is heated.
Protein reacts with sodium hydroxide and copper sulphate to produce violet colour/ purple clour. This is called biuret test.
Note the following during testing of protein
Sodium hydroxide is used to provide alkaline medium for the copper sulphate to react with nitrogen atoms of amino acids.
The purple colour formed during the test of protein is the result of a complex between four nitrogen atoms in the peptide bonds and the copper (II) ion (Cu2+) of Copper sulphate solution.
Uses of protein
1. They used for growth and repair of worn out body tissues. The protein we eat are used to make new cells which will make tissues to replace those which are worn out.
2. They provide body protection and defence against disease. For example protein make antibodies which provide the body with immunity.
3. They can be used to provide energy when the body lacks carbohydrate and fats. This happen during starvation and when the person falls sick.
4. They form the structural components to various materials such as nails, hair and muscle fibres.
5. They are required for the formation of enzymes, hormones, and haemoglobin.
6. They play role in clotting of blood. This is because the fibrinogen which control blood clotting is protein in nature.
7. They transport oxygen in the blood. This is because the hemoglobin which transport oxygen is protein in nature.
Procedures
1. Measure 2ml of the given solution in a test tube
2. Add 2ml of sodium hydroxide followed by 2 drops of cooper (II) sulphate drop by drop. Shake after each drop
3. Observe the color changes
4. The appearance of purple color indicates the presence of protein
5. Retention of the blue color of cooper (II) sulphate indicate the absence of protein
Reducing sugar
Reducing sugars are based units of all carbohydrates which are easily utilized by the body tissues.
1. The natural sources of reducing sugar include germinating cereals like maize, millet, onion, tomato, all sweet ripening fruits, honey and milk
2. Glucose and Maltose are example of reducing sugar
Properties of reducing sugar
1. They are soluble in water
2. They have sweet taste. They form sweet tasting solutions.
3. They are crystalline. They can form crystals. - They are small molecule.
They are reducing sugar. This is because the Benedict’s solution which contain copper (II) sulphate when is mixed with reducing sugar, the copper (II) ions of copper (II) sulphate will be reduced to copper (I) ions which form a brick red precipitate of copper (I) oxide. This for form basis for testing reducing sugar.
Uses of reducing sugar
1. It provides the body with energy. For example glucose is the most common respiratory which is oxidized during respiration to release energy.
2. Play role in synthesis of disaccharides. For example when two molecule of glucose are combined through condensation reaction, maltose is formed.
3. They are used is synthesis of polysaccharides such as glycogen. Glucose is particularly important in this role.
Procedures
1. Measure 2ml a solution put it in the test tube
2. Add 2ml of Benedict’s solution to the solution
3. Heat the mixture to boil
4. Observe the color change
5. If reducing sugar is present in the food sample, the mixture will experience a series of color change from blue to green to yellow then orange and finally to brick red, this depends with the quantity of reducing sugar present in the solution, brick red color indicates the presence of large quantity of reducing sugar
6. If reducing sugar is absent the solution will be of either blue or green color
Non-reducing sugar
1. Non-reducing sugar is component of carbohydrate formed when two molecules of reducing sugar are condensed and release a molecule of water. Natural sources of non- reducing sugar include sugarcane, beetroot, malt and unripen fruits
2. Chemical reagents used when testing non-reducing sugar are
3. Dilute hydrochloric acid (dil HCl)
4. Sodium hydroxide solution (NaOH) and
5. Benedict’s solution.
Properties of non-reducing sugar
1. They have sweet taste. They form sweet tasting solutions.
2. They are crystalline. They can form crystals. - They are small molecule.
3. They are called non-reducing sugar because they cannot reduce copper sulphate of Benedict’s solution.
4. Note the following during testing of non-reducing sugar :-
Dilute hydrochloric acid is used to break down (hydrolyze) non-reducing sugars to reducing sugars/converting complex sugar to simple sugar
Sodium hydroxide is used to neutralize the dilute hydrochloric acid.
Uses of non-reducing sugar
They provide the body with energy
Procedures
1. Measure 2ml of a given solution and put in test tube
2. Add 1ml of dil hydrochloric acid, shake it then boil the mixture and cool it.
3. Add 1ml of sodium hydroxide solution to neutralize the acid
4. Add 2ml Benedict’s solution then boil the mixture
5. Observe the color change
6. If non-reducing sugar is present, the color of the mixture will pass through the series of color from blue to green to yellow and finally to brick red
7. If non-reducing sugar is absent in the solution it will retain the blue color of benedict’s solution or turn to green
Nutrition - Food Test: Biology Practical Preparation Towards NECTA Exams (CSEE)
HOW TO WRITE A FOOD TEST PRACTICAL REPORT
Principles of writing the food test report
1. The report should be presented in a tabular form
2. It must be written in a reported form (passive speech)
3. Solutions and reagents must be quantified, e.g 2ml or 2 drops and not few drops or little amonuut of solution
4. Commanding language like add, put should be avoided
Sample of the report for the provided solution V
Test for | Procedures | Observation | Inference |
| Starch | To 2ml of | The mixture | Starch was |
| solution V in a | turned into blue | present | |
| test tube, 2 | black | ||
| drops of | The solution | Starch was | |
| iodine solution | retained the | absent | |
| were added | color of iodine | ||
| then shaken | solution | ||
| Lipids | To 2ml of | A red ring was | Lipids was |
| solution V in a | formed on the | present | |
| test tube 3 | top of solution | ||
| drops of | V | ||
| Sudan (III) | Solution V | Lipids was | |
| solution were | retained the | absent |
added then shaken | color of Sudan (III) solution | ||
| Protein | In a test tube | The mixture | Protein was |
| containing 2ml | turned in | present | |
| of solution V | purple | ||
2ml of sodium hydroxide were added followed by 2 drops of 1% Cooper (II) | |||
The mixture retained the blue color of cooper (II) sulphate | Protein was absent | ||
| sulphate | |||
| solution | |||
| dropwise | |||
| while shaking | |||
| after each drop | |||
| Reducing | To 2ml of | A series of | Reducing |
| sugar | solution V in a | color change | sugar was |
| test tube, 2ml | was observed | present | |
| of Benedict’s | in the mixture | ||
| solution were | from blue to | ||
| added then | green to yellow | ||
| boiled | and finally to | ||
| brick red | |||
| The solution | Reducing | ||
| retained the | sugar was | ||
| blue color of | absent | ||
| Benedict’s | |||
| solution |
| Non- | To 2ml of | The series of | Non reducing |
| reducing | solution V in a | color change | sugar was |
| sugar | test tube 1ml | was observed | present |
| of dil | in the mixture | ||
| hydrochloric | form blue to | ||
| acid was | green to yellow | ||
| added shaken | and finally to | ||
| then boiled | brick red | ||
| and cooled. | The solution | Non reducing | |
| 1ml of Sodium | retained the | sugar was | |
| hydroxide was | blue color of | absent | |
| added | Benedict’s | ||
| followed by | solution | ||
| 2ml of | |||
| Benedict’s | |||
| solution and | |||
| the mixture | |||
| was re-boiled |
Preparation of solution
You can be provided with a specimen and then asked to prepare a solution by yourself then conduct the food test
1. The procedure of preparing the solution should be included in the reported and they should be written in the reported speech (passive voice)
2. Indicate each procedure that is taken while preparing a solution
THE HUMAN DIGESTIVE SYSTEM
The table summarizing the digestion process
Part of the e alimentary y canal (Site of action) | pH medium (optimum m pH of the substrate e) | Source of enzyme | Digestive juice (secretion) | Enzyme | Food digested | Produ cts of digest ion |
| Mouth | Alkalin e | Salivary gland | Saliva | Salivary amylase (ptyalin) | Starch | Malto se |
| Stomach | Acidic | Gastric gland | Gastric juice | Pepsin | Protein | Pepti des |
| Rennin | Soluble milk protein (caseino gens) | Insol uble milk protei n (casei n) | ||||
| Duodenu m | Alkalin e | Pancreati c gland | Pancreatic juice | Pancreatic amylase | Starch | Malto se |
| Pancreatic lipase | Lipids | Fatty acids and glyce rol | ||||
| Trypsin | Protein | Pepti des | ||||
| Small intestine( ileum ) | Alkalin e | Intestinal wall | Intestinal juice (Succus entericus) | Erepsin (peptidase) | Peptide s | Amin o acids |
| Maltase | Maltose | Gluco se | ||||
| Sucrase | Sucrose | Gluco se and |
| Fruct ose | ||||||
| Lactase | Lactose | Gluco se and Galac tose | ||||
| Lipase | Lipids | Fatty acids and glyce rol |
Addition notes
Adaptations of the ileum for digestion
1. It has secretory glands which produce digestive enzymes.
2. It is connected to the liver by the bile duct. The bile duct enables bile to reach the ileum.
3. The ileum has mucus-secreting gland to ensure a constant supply of mucus.
Adaptations of the ileum for absorption
1. The lining of the ileum has finger-like projections called villi. On the villi are even smaller projections called microvilli. These structures increase the surface area for the absorption of food.
2. The ileum is very long (about six metres). It therefore has a large surface area for the absorption of soluble products of digestion.
3. The villi have a large network of blood capillaries. These capillaries transport absorbed food to all parts of the body.
4. The inner lining of the ileum is folded. This increases the surface area that is in contact with food.
5. The lining of the ileum is only one cell thick. Thus, the digested food diffuses through a short distance into the capillaries.
PHOTOSYNTHESIS
What is photosynthesis
It is the process whereby green plants make their own food using water, carbon dioxide and energy from the sun
Balanced chemical equation for photosynthesis
Chlorophyll
6CO2 +12H2O C6H12O6 + 6H2O + 6O2
Sunlight
Testing for starch
Plants are living things that are able to make their own food (glucose) and store it in form of starch
Iodine solution and starch form blue black color when they mix. The test for starch therefore is to add iodine solution in a leaf to see if it goes to blue black, the leaf must be treated as follows
1. Heat some water to boiling point in a beaker
2. Use forceps to dip a leaf in hot water for 30sec. this kills the cytoplasm and denatures the enzymes and make the leaf more permeable to iodine solution.
3. Remove the leaf from the leaf from the beaker
4. Push the leaf to the bottom of the test tube and cover it with alcohol (ethanol). Place the tube in hot water for alcohol to boil and dissolve out most of the chlorophyll. This makes colour change with iodine easier to see
5. Remove the leaf from the test tube and dip it once more into the hot water to soften it
6. Spread the decolorized leaf on a white tile and drop iodine solution onto it. The parts containing starch will turn into blue black and those parts with starch will stain brown colour of iodine
Sample question
You have been provided with a variegated leaf and iodine solution. Carefully follow the instruction given below
1. Heat some water in a beaker to boiling and then turn off the source of heat
2. Use forceps to dip the leaf in hot water for 1 minute
3. Then push the leaf into bottom of the test tube and cover it with ethanol.
4. Place the tube in hot water until the alcohol boils together with the leaf.
5. Remove the leaf from the test tube containing ethanol and deep it into hot water
6. Spread the decolorized leaf on a white tile and pour few drops of iodine solution on it
Questions
1. What as the aim of the experiment?
2. Why was the leaf dipped in hot water for 1 minute?
3. Give the reason why the leaf was boiled in ethanol.
4. Why was the leaf dipped in hot water again?
5. Give the interpretation of the result observed when few drops of iodine solution were poured onto the decolorized leaf
6. Mention the physiological process investigated in the experiment.
7. Draw a well label the diagram of a leaf as it appeared before the experiment and after the iodine solution was added
Answers
1. The aim of the experiment was to test if the leaf contain starch.
2. The leaf was dipped for 1minute in hot water so as to stop the reaction taking place in the leaf.
3. The leaf was boiled again in methanol to dissolve the chlorophyll present in it.
4. The leaf was then dipped in hot water so as to remove alcohol and soften it.
5. When few drops of iodine solution were poured on the decolorized leaf, blue black colour occurred in some parts of the leaf. This shows the presence of starch.
6. The physiological process investigated in the experiment was photosynthesis
Diagram of a leaf before and after the experiment.
Before experiment After experiment