Lesson Explainer: The Chemistry of Food Biology

In this explainer, we will learn how to carry out chemical tests on food to determine the presence of sugars, starch, protein, and fats.

Our bodies, and those of every living thing, are entirely composed of biological molecules. The main groups of biological molecules that living organisms require are carbohydrates as a source of energy, proteins for growth and repair, and lipids for insulation, storing energy, and making membranes. As humans, we need to gain our nutrition from the food we eat. Other organisms, such as plants and some bacteria, make their own nutrition.

Definition: Biological Molecules

Biological molecules are substances that are produced by cells and living organisms.

Biological molecules are generally polymers, which is a broad term referring to a long-chained molecule. These molecules are made up of a chain of smaller molecules called monomers joined together by chemical bonds. We will start by seeing which smaller units carbohydrates, proteins, and lipids consist of.

Definition: Monomer

A monomer is a small molecular subunit that bonds together with other monomers to form a polymer.

Definition: Polymer

A polymer is a large molecule made of many repeating subunits called monomers.

As visible in Figure 1, carbohydrate polymers are long chains of simple sugar monomers. Starch is an example of a carbohydrate polymer made up of lots of glucose molecules joined together. Bread is full of starch, and the reason it tastes more sugary as you chew it is because the starch molecules are being broken down into sweeter glucose monomers by enzymes in your saliva.

Proteins are polymers made up of a string of different amino acid monomers bonded together. There are about 20 standard amino acids that make up proteins in the human body, and they can be arranged in different combinations to form a massive range of different protein polymers.

Lipids, sometimes simply called fats, are biological macromolecules made up of glycerol and fatty acid subunits. A typical example of a lipid is the triglyceride molecule shown in Figure 1, which consists of one glycerol molecule and three fatty acid “tails.” Just like carbohydrates and proteins, there are many different lipids that have different compositions of glycerol and fatty acids. Fats are a subgroup of lipids that are solids at room temperature, and oils are another subgroup that are liquids at room temperature.

As our bodies require these biological molecules in appropriate proportions to remain healthy, it is important to know which specific molecules the food we eat contains and to know how much energy they provide us with.

Example 1: Describing the Composition of Biological Molecules

Complete the statements to correctly describe the composition of the biological molecules.

  1. A protein is made of many monomers called .
  2. Many sugar molecules, like glucose, join together to form .
  3. A basic lipid molecule is formed of one and three fatty acids.

Answer

Biological molecules are long-chained molecules made up of smaller subunits joined together. When the subunits in a macromolecule are repeated versions of the same small molecule, these subunits are called monomers, and the macromolecule is called a polymer.

Part 1

Proteins are polymers made up of a string of different amino acid monomers bonded together. There are about 20 standard amino acids that make up proteins in the human body, which can be joined together in various combinations to produce different proteins. You can see a small section of a protein molecule consisting of different amino acid subunits in the diagram below.

Therefore, a protein is made of many monomers called amino acids.

Part 2

Carbohydrate polymers are long chains of repeated simple sugar monomers joined together. For example, as you can see in the diagram below, when many molecules of glucose are joined together, they form a macromolecule called starch.

Therefore, many sugar molecules, like glucose, join together to form carbohydrates.

Part 3

Lipids, sometimes called fats, are biological macromolecules made up of glycerol and fatty acid subunits. As lipids are not made of the same repeating subunits, they cannot technically be called a polymer. An example of a lipid is a triglyceride molecule, which consists of one glycerol molecule and three fatty acid “tails.”

Therefore, a basic lipid molecule is formed of one glycerol and three fatty acids.

Scientists have developed food tests that identify which molecules are present in different foods. They work by adding a specific chemical to a food sample and observing a color change that confirms the presence of each particular biological molecule. This color change, or another similar observation indicating a molecule in the food, is called a positive result. If no change is observed, we call this a negative result, and it means the biological molecule is not present in the sample.

We will look into the different food tests for several biological molecules and the positive results they provide, and then we will examine the process of calorimetry, which determines the energy content of a food sample.

Glucose is an example of a simple reducing sugar. This means that it has an OH group attached to one of its carbons that can change or reduce a different molecule. Carbohydrates with a single carbon ring such as glucose are called monosaccharides, and those with two rings are called disaccharides. Polysaccharides are those made of more than two carbon rings. All monosaccharides and some disaccharides are reducing sugars as they have this same property.

The chemical test for identifying the presence of reducing sugars such as glucose in food is called the Benedict’s test, and the procedure for using it is explained below.

How To: Testing for Reducing Sugars (e.g., Glucose) in a Food Sample

  1. If it is solid, crush the sample of food using a pestle and mortar.
  2. Add the crushed sample to a test tube with some water.
  3. Add several drops of Benedict’s solution.
  4. Place the test tube into a water bath at 90C (Note: Take care as the water bath will be very hot. A water bath is used instead of a direct flame from a Bunsen burner, as it is easier to maintain a consistent temperature by using a water bath.)
  5. Wait for at least 8 minutes.
  6. Observe and record any color change.

Positive result: If glucose (or another reducing sugar) is present, it will turn from blue to green, yellow, orange, or red depending on the concentration of glucose present as you can see in the figure below.

Starch is also a carbohydrate made up of lots of small glucose molecules joined together. The chemical test for identifying starch in food is called iodine, and the procedure is explained below.

How To: Testing for Starch in a Food Sample

  1. If it is solid, crush the sample of food using a pestle and mortar.
  2. Add the crushed sample to a test tube.
  3. Add a few drops of iodine solution.
  4. Observe and record any color change.

Positive result: If starch is present, it will turn from orange brown to blue black.

The chemical test for identifying protein in food is called the biuret test, and the procedure is explained below.

How To: Testing for Protein in a Food Sample

  1. If it is solid, crush the sample of food using a pestle and mortar.
  2. Add the crushed sample to a test tube.
  3. Add a few drops of biuret solution.
  4. Carefully shake the test tube to ensure that the sample and reagents mix well.
  5. Observe and record any color change.

Positive result: If protein is present, it will turn from blue to purple.

Lipids are insoluble in water, so the two substances do not mix together. Instead, lipids tend to form a layer on top of water, as you may have seen occur with cooking oil that contains a lot of lipids. Lipids are however soluble in an alcohol called ethanol, so these substances do mix together. These properties are used in the emulsion test and the Sudan IV test to identify the presence of lipids in a food substance.

In the emulsion test, ethanol is added to a food substance containing lipids and the lipids dissolve in the ethanol. When water is then added to this solution, white and cloudy droplets appear and disperse throughout the water, which is called an emulsion. The emulsion tends to eventually separate into distinct layers, with a cloudy ethanol and lipid layer above a clear layer of water.

Sudan IV solution is also insoluble in water, but it is soluble in lipids, staining them red orange if they are present in a sample.

Definition: Emulsion

An emulsion is a mixture of two liquids that would not normally mix together, where one liquid contains a suspension of tiny particles of the other liquid.

How To: Testing for Lipids in a Food Sample

Emulsion Test:

  1. If it is solid, crush the sample of food using a pestle and mortar.
  2. Add the crushed sample to a test tube.
  3. Add 2 cm3 of ethanol to the food sample.
  4. Carefully shake the test tube to ensure that the sample and reagents mix well.
  5. Allow the solution to settle for two minutes.
  6. Add 2 cm3 of distilled water to the solution.
  7. Observe and record any change.

Positive result: If lipids are present, a white emulsion will form.

Sudan IV Test:

  1. If it is solid, crush the sample of food using a pestle and mortar.
  2. Add the crushed sample to a test tube.
  3. Add Sudan IV solution to the sample.
  4. Carefully shake the test tube to ensure that the sample and reagents mix well.
  5. Observe and record any color change.

Positive result: If lipids are present, the Sudan IV solution will stain them reddish orange.

Example 2: Interpreting Composition of Biological Molecules from Food Tests

An egg yolk was tested for different food groups and the results are provided.

Reagent Added/Test PerformedIodineBenedict’sBiuretEmulsion
ResultOrangeBluePurpleWhite emulsion layer

According to these results, which of the following is the correct conclusion about the biological molecules found in the egg yolk?

  1. The egg yolk contains starch, sugars, protein, and fats.
  2. The egg yolk contains fats and proteins, but neither starch nor sugars.
  3. The egg yolk contains sugars and proteins, but neither starch nor fats.
  4. The egg yolk contains sugars and starch, but not protein.

Answer

Different biological molecules can be identified in food samples using food tests.

To test for starch, iodine solution is added. If starch is present, the sample changes color from orange to blue black.

To test for simple reducing sugars such as glucose, Benedict’s solution is added. If glucose is present, a color change is observed from blue to green, yellow, orange, or red depending on the concentration of sugars in the sample.

To test for proteins, biuret solution is added. If proteins are present, a color change is observed from blue to light purple.

To test for lipids, sometimes called fats, ethanol is added to the food sample to dissolve the lipids and then distilled water is added to the solution; this is called an emulsion test. As if lipids are present in the food sample, a white emulsion forms that eventually separates into a distinct cloudy white layer above a clear layer of water.

This food sample remained orange when iodine was added, showing that no starch was present. It remained blue when Benedict’s solution was added, so no simple reducing sugars such as glucose were present. It turned purple when biuret solution was added, so it does contain protein. It formed a white emulsion layer when an emulsion test was carried out, so it does contain fats.

The correct answer is therefore B: The egg yolk contains fats and proteins, but neither starch nor sugars.

The main food tests are summarized in Table 1 below, including the chemical solutions that must be added, whether a water bath is required, and the positive result we would observe if the chemical was present. Remember that if the biological molecule, which each specific chemical tests for, is not present in the food sample, no color change (or emulsion in the case of lipids) will be observed.

Example 3: Interpreting Composition of Biological Molecules and Food Tests

Complete the table to show the correct biological molecule being tested for, or the name of the test.

Molecule Being Tested forStarch2Protein4
Name of Reagent/Test1Ethanol/Emulsion test 3Benedict’s solution

Answer

Different biological molecules can be identified in food samples using different food tests. To test for starch, iodine solution is added. If starch is present, the sample changes color from orange to blue black. To test for glucose (simple sugars), Benedict’s solution is added. If glucose is present, a color change is observed from blue to green, yellow, orange, or red depending on the concentration of sugars in the sample. To test for proteins, biuret solution is added. If proteins are present, a color change is observed from blue to purple. To test for fats, ethanol and water are added, termed an emulsion test. If lipids are present, a white emulsion layer forms above a layer of water.

Our correct answers are therefore the following:

  1. Iodine
  2. Lipids
  3. Biuret
  4. Sugars

Food calorimetry is a process used to measure the energy contained in food.

A calorimeter is the instrument used to carry this out. It works by burning a sample of food below a set volume of water held in a container. As the food burns, the chemical energy contained within the food is released and transferred to thermal (heat) energy, which is absorbed by the water, heating it up.

By measuring the temperature of the water at the start and the end of the procedure and calculating the difference between the two, the amount of energy that has been transferred to the water can be estimated. The larger the change in the temperature of the water, the more energy the food sample initially contained. This energy is worked out using the temperature change of the water and the mass of water and the food sample, using the equation below.

Equation: Calculating Energy Released in Food Calorimetry Experiments

EnergyreleasedfromfoodpergramJmassofwatergtemperatureriseCmassoffoodsampleg()=()×()×4.2().

The energy released from the food per gram is estimated by using several different measurements.

One of the values that must be measured is the mass of the water in the calorimeter, as we are calculating the energy required to change the temperature of this specific mass of water.

The temperature rise of the water represents the amount of energy transferred from chemical energy in the food sample to this mass of water in the calorimeter. The more the temperature of the water increases, the more energy the food sample has released into the water.

The value given as 4.2 is the specific heat capacity of water, which is the energy required to increase 1 kg of water by 1C.

These three values are multiplied together to calculate the energy released from the food in total. We then divide this value by the mass of the food sample used, as the equation is asking us for the energy released per gram of food.

A simple explanation of how to carry out food calorimetry is described below.

How To: Using Food Calorimetry to Measure the Energy Content of Food

  1. Measure a set volume of water into the calorimeter (e.g., 20 mL).
  2. Clamp the calorimeter to a clamp stand.
  3. Measure and record the temperature of the water in the calorimeter with a thermometer.
  4. Choose a sample of food, and measure and record its mass in a results table using a balance.
  5. Impale the food carefully onto a mounted needle and hold using tongs.
  6. Light the food sample using a spirit burner and hold in the flame until the food has fully burnt (Note: Try to keep the food sample close to the calorimeter so that as much of the heat from the burning food is transferred to the water as possible.)
  7. Measure and record the temperature of the water again.
  8. Repeat steps 1–7 for different samples of food, ensuring you use fresh water each time.
  9. Calculate the energy released from each sample of food using the formula energyreleasedfromfoodpergramJmassofwatergtemperatureriseCmassoffoodsampleg()=()×()×4.2().

Example 4: Describing the Observations and Trends in Food Calorimetry

The diagram provided shows a simple setup of a calorimeter, which can be used to estimate the amount of energy in different foods.

  1. What will happen as the food burns?
    1. The temperature of the water will rise as the food releases heat energy.
    2. The temperature of the water will fall as the food releases heat energy.
    3. The temperature of the food will fall as the food absorbs heat energy.
    4. There will be no change in the temperature of the water or the food.
  2. Which of the following correctly describes the trend between the energy contained in a food source and the expected result?
    1. The less energy a food sample contains, the more the temperature of the water increases.
    2. The more energy a food sample contains, the more the temperature of the water increases.
    3. There is no correlation between the energy a food sample contains and the temperature change of the water as heat energy cannot be transferred through the air to the container.

Answer

Part 1

Food calorimetry is a process used to measure an approximate value for the energy contained in food. A calorimeter is the instrument used to carry this process out. It works by burning a sample of food below a set volume of water held in a container.

As food burns under a calorimeter, the chemical energy contained within the food is transferred to thermal (heat) energy released by the food, which heats up the water.

Therefore, the temperature of the water will rise as the food releases heat energy.

Part 2

By measuring the temperature of the water at the start and the end of the procedure, the amount of energy that has been transferred to the water can be estimated. The higher the change in the temperature of the water, the more energy was transferred from the food sample to the water and the more energy the food sample initially contained.

Therefore, the more energy a food sample contains, the more the temperature of the water increases.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Biological molecules such as carbohydrates, lipids, and proteins are polymers made up of long chains of monomers or subunits.
  • Benedict’s test can be used to test for the presence of reducing sugars (e.g., glucose) in a food sample.
  • Iodine can be added to food samples to test for the presence of starch.
  • Biuret reagent can be added to a food sample to test for the presence of proteins.
  • The emulsion test, or Sudan IV test, can be used to test for the presence of lipids in a sample of food.
  • Food calorimetry can be used to measure the approximate energy contained within a food sample.

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