Biology and Health Sciences

Key unit competency

To be able to carry out chemical tests on a variety of foods to identify the nature of food substances.

Learning objectives

After studying this topic, I should be able to:

Introduction

Have you ever visited a tea or coffee processing factory? What happens during grading of coffee or tea? Look at the picture below. What is the person using to tell whether the coffee of great quality?

Coffee or tea is tasted to determine its quality during processing. The fragrance of the brewed tea or coffee, the deep flavours when drinking and the pleasant after-taste are used to tell the quality.

Based on this, do you think it is possible to know the components of the various foods that we eat by tasting? Why? What can we use to do this?

5.1 Components of food substances

The foods that we eat contain different types of nutrients. These nutrients serve different functions in the body. It is therefore essential that we know the components of the food that we eat in order to live healthy lives.

Drinks and foods that are manufactured in factories need to be tested as well. This is done to confirm the type and percentage of nutrients they contain. This enables the consumer to be informed on the product contents.

Food contains mainly two classes of nutrients; organic and inorganic nutrients. The inorganic nutrients include mineral salts like calcium, phosphorous and others like water. The organic nutrients include proteins, carbohydrates, lipids and vitamins.

Chemical test for food substances

The process by which foods and drinks are taken to the laboratory to confirm the type of food components in them is called food testing. Food testing is done to confirm the presence of starch, reducing sugars, proteins, fats and vitamin C in a food sample.

5.2 Testing for starch and reducing sugars

a) Test for starch Starch

is a polysaccharide comprising of glucose monomers linked together. Starch is insoluble in water. It does not have a sweet taste and on hydrolysis, it readily converts back to monosaccharides. Some foods containing starch include sweet potatoes and cassava.

Activity 5.1 Testing for starch

Requirements

• Iodine solution (brown)

• Test tubes and test tube rack

• Droppers • Water

• Starch solution (sweet potato,  or plantain extracts)

• Glucose solution

• Measuring cylinder

Procedure

1.  Using the measuring cylinder, measure about 2 cm³ of the following solutions into separate test tubes: starch solution, glucose solution and water. 

Precaution: Each time, rinse the measuring cylinder thoroughly before use.

2.  To each test tube, add a few drops of iodine solution and note any colour change.

3. Record your results in the table below

Starch is the storage form of glucose in plants. Each starch molecule has about 300 - 1000 glucose units. Most starch in plants is found in seeds and storage organs like potato tubers. Starch gives a blue-black colour with Iodine in potassium iodide solution. This is the laboratory test for starch. The colour of Iodine turns from brown to blue-black if starch is present. If there is no starch in the food sample, the brown colour of iodine persists.

Money matters!

You should not waste laboratory reagents because they are usually very expensive.

b) Testing for sugars

Based on their reaction with Benedict’s solution sugars are grouped into two.

             • Reducing sugars

             • Non-reducing sugars

Reducing sugars

Some simple sugars, for example glucose can be made to reduce copper in blue copper sulphate to red copper oxide. This reaction can be used as a test for some sugars. Copper sulphate is mixed with other chemicals to be used as Benedict’s solution which is used in the test. Other examples of reducing sugars

include: maltose, lactose, fructose and galactose.

All monosaccharides are reducing sugars, they taste sweet and are soluble in water.

Activity 5.2: Test for reducing sugars

Requirements

• Benedict’s solution

• Test tubes and test tube rack

• Glass rod and a spatula

• Dropper

• Labels

• Conical flask (100 cm³) or small beaker

• Motor and a pestle

• Source of heat

• Food extracts, for example, banana juice

• Glucose, starch and sucrose solutions

• Distilled water

Procedure

1. If the food to be tested is in liquid form, go straight to procedure (2). If it is solid do the following to form an extract:

2. Using a dropper put 2 cm³ of each of the solutions to be tested into separate test tubes.

3.  Label the test tubes with the solutions in them.

4. To each test tube, add 2 cm³ of Benedict’s solution.

5. Heat for about two minutes and observe the colour change.

6. Record your observations in the table below.

Benedict’s solution is the chemical reagent used to test for the presence of reducing sugars. When boiled with glucose, which is a reducing sugar, an orange precipitate forms.

A change in colour from blue to green, yellow and finally red indicates the presence of reducing sugar. The colour changes depend on the amount of reducing sugars present.

• Green colour indicates small amounts of reducing sugars.

• Yellow/orange shows high levels of reducing sugars.

When Benedict’s solution, which is blue in colour, is boiled with substances with non-reducing sugar, its blue colour persists.

Note: Water acts as the control.

Health Check!

•  Wear eye protection for eye safety.

•  Take care while heating and handling test tubes. This minimises risk of laboratory accidents.

•  Do not consume any food provided in the laboratory, it may be poisoned or contaminated.

Non-reducing sugars

Some sugars such as sucrose are unable to reduce the copper ions in Benedict’s solution. This makes the colour of Benedict’s solution to persist when these sugars are boiled with it. Such sugars are called non-reducing sugars. Most nonreducing sugars are disaccharides. This means they contain two single sugar units.

Activity 5.3: Testing for nonreducing sugars

Requirements

Procedure

1. Arrange the four test-tubes in a test tube rack and label them 1 - 4.

2. Put 2 cm³ of sucrose solution into the test tube labelled 1 and add 2cm3 of Benedict’s solution then boil.

          •  Note down any observations.

3. Measure 2 cm³ of dilute hydrochloric acid and pour it into the test tube labelled 2. Add to it 2 cm³ of Benedict’s solution and boil.

          •  Note down any observations.

4. Follow the following procedure for test tube labelled 3.

5. Compare the colour of the contents of test tube 3 with that in test tube 1 and test tube 2.

6. Record your result in a table similar to table below. 

There are no special reagents used to test for non-reducing sugars. The same reagents used for reducing sugars are instead applied. This time hydrochloric acid is also added to hydrolyze these non-reducing sugars to reducing form. Later when Benedict’s solution is added, the test becomes positive.

There is no change in the colour of Benedict’s solution when it is heated with sucrose solution. This is because sucrose is not a reducing sugar. Benedict’s solution changes colour only in the presence of a reducing sugar. Boiling the sucrose solution with dilute hydrochloric acid hydrolyses sucrose to glucose and fructose. Both of these are reducing sugars and as such, the colour of the Benedict’s solution changes from blue to orange/brown precipitate.

Any excess acid is neutralised by the sodium hydrogen carbonate. Boiling is a requirement for the hydrolysis to occur. Heating the Benedict’s solution with the acid is to rule out the possibility of the acid being the one changing the colour of Benedict’s solution from blue to orange.

Note: Any excess hydrogen carbonate does not affect Benedict’s solution.

Self-evaluation Test 5.1

1. With examples, distinguish between reducing sugars and non-reducing sugars.

2. What is the purpose of sodium hydrogen carbonate in the test for non -reducing sugars?

3. Starch is stored in seeds, stems and roots among many other places in plants. Where is starch stored in animals?

5.3 Testing for proteins

Proteins are essential nutrients for the human body. They are one of the building blocks of the human tissue and can also serve as a fuel source. They contain as much energy as carbohydrates but less than lipids. Amino acids are the building blocks of proteins. Most of the foods we eat contain these amino acids. Milk, eggs, meat, beans and peas are examples of foods that contain proteins.

There are two major ways in which scientists test for the presence of proteins in food. They are:

(a)  Millon’s test

In this test a reagent known as Millon’s reagent is used to test for proteins.

Activity 5.4: To test for the presence of proteins using Millon’s reagent

Requirements

• Test tubes and test tube rack

• Source of heat

• Dropper

• Measuring cylinder

• Syringes

• Cooking oil

• Egg albumen

• Distilled water

• Milk

• Millon’s reagent

Procedure

Carry out the following procedures and record your observations and deductions in the table below.

The table below shows the results that may be obtained during a Millon’s test

Health Check!

Care should be taken while boiling to avoid laboratory accidents. Millon’s reagent is very poisonous! Be careful when handling it

Quality Check!

Buying quality apparatus reduces the risk of breaking and therefore it is cost effective.

b) Biuret test

The Biuret’s test uses several reagents that are mixed in equal proportions. Copper (II) sulphate and sodium hydroxide make up the reagent. The normal colour of the reagent is blue. The reagent turns violet in the presence of peptide bonds.

These bonds hold amino acids together to form a protein.

Activity 5.5 Test for proteins using Biuret reagent

Requirements

 Note: No heating is involved during the Biuret test.

Procedure

1.  Place 2 cm³ of soya bean, distilled and water and orange juice into three separate test tubes.

2.  To each of these test tubes, add 2 cm³ of sodium or potassium hydroxide solution.

3.  Add 1cm3 of 1% copper sulphate solution to each test tube then shake.

4.  Record your observations in a table similar to the one given below.

Study questions

(a) What colour change was observed in each of the test tubes?

(b) What is the role of the test tube with distilled water?

Copper sulphate is pale blue in colour. In the presence of a protein, its colour changes to purple. The appearance of purple colour, therefore, is a confirmation for the presence of protein in a food sample.

Self-evaluation Test 5.2

1.  The _________ test uses several reagents. The reagents are mixed in equal proportions. _________  and __________  make up this reagent.

2. The bonds that hold protein structure are known as _________

3. To test for proteins, two reagents are used. Millon’s reagent and Biuret reagent. Which of these two tests does not involve heating?

5.4 Testing for lipids

Lipids are composed of fats and oils. Fats are solid at room temperature while oils are liquid at room temperature. Cooking oil, animal fats, groundnuts, meat among others, all contain lipids.

Two types of tests can be carried out on a food sample to identify if it contains lipids. These are:

(a) Ethanol emulsion test

In this procedure ethanol is used as the reagent. Water is also added. A white emulsion confirms the presence of lipids in the food sample.

Activity 5.6: Testing for presence of lipids using the ethanol emulsion test

Requirements

Procedure

Carry out the following tests and record your observations and deductions in the table below.

Study Questions

(a) Which food components are found in cooking oil?

(b) Why is it important to eat foods containing lipids?

(c) Which health problems are related to too much lipids in the diet?

An emulsion forms when the solution of oil and alcohol is shaken with water. This is because the oil does not dissolve in water. A cloudy or white liquid is formed. If a food sample is used, the alcohol extracts the oil from the food by dissolving it.

(b) The translucent spot test

Activity 5.7: Testing for presence of lipids using the translucent spot test

Requirements

Procedure

• Put a drop of oil or fat on some absorbent paper.

• Hold it up against light from the window and note what you observe.

A translucent spot indicates presence of oil or fat.

5.5 Testing for vitamin C

A reagent known as Dichlorophenol indophenol (DCPIP) is used to test for vitamin C. DCPIP is a deep blue reagent in colour. When vitamin C is present in a food sample, the blue colour disappears (decolourised). Fresh fruits and green vegetables contain vitamin C. Table 5.8 shows the test for vitamin C using DCPIP.

Activity 5.8

Further Activity 5.9

You are provided with the following:

    • Specimen S which is an orange,                      • Millon’s reagent             • DCPIP

    • Specimen J which is milk • Source of heat        •  Scalped                        •  Test tubes and test tube rack

    • Distilled water                                                   • Droppers                       • Syringe

    • Iodine solution                                                   •  A stirrer

Procedure

1.  Cut a transverse section of specimen S and obtain about 10 cm3 of juice from the specimen by squeezing into a test tube.

2. Mix the orange juice in 1 above with the milk to form a solution. Label the solution Extract A.

3.  Carry out the following tests on this extract A using the reagents provided and record your observations in table like the one shown below:

Study questions

(a) From your observations name the food substance(s) present in the extract.

(b) What is the biological significance of the food substance named in (a) above?

Self-evaluation Test 5.3

1.  Name diseases that result due to lack of vitamin C in the diet.

2.  Why is it not advisable to take too much food containing lipids?

3. Which of the following substances is the most abundant compound in a living cell?

Unit summary

• Starch is a polysaccharide that functions as a carbohydrate store and is an important constituent of the human diet.

• Food testing is the process that is carried out to determine the food components in a food sample.

• Starch is tested in a food sample by using Iodine solution. If it turns blue-black, it indicates the presence of starch.

• Benedict’s solution is used to test for the presence of reducing sugars. Reducing sugars change the colour of Benedict’s solution from blue to orange.

• Non- reducing sugars do not change the colour of Benedict’s solution. Unless they are first boiled with dilute hydrochloric acid to hydrolyse them.

• Presence of proteins in a food sample can be tested by Biuret test or Millon’s reagent.

• The alcohol –emulsion test is used to test for presence of lipids in a food sample.

• Presence of vitamin C is tested by a chemical called DCPIP.