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Chemistry of food

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Chemistry of food

We often dig into our favourite foods without thinking about what they contain. Despite this, the macromolecules that make up our foods profoundly affect our bodies. We require food for energy and nutrients, with different parts of our daily diets having both different sources but also functions. A lack of any specific macromolecules is not good, but neither is too much.

Molecules in Food

There are many different molecules found in food that we need for our body to function properly. Have a look at some of the most important molecules in food below.

Carbohydrates

The first important molecule that we are going to discuss is carbohydrates. Carbohydrates are an energy source in living organisms. They are the main energy source in our bodies and make up a lot of the food plate of a balanced diet!

We get carbohydrates from many different types of food. Have a look below at some of the foods which are a source of carbohydrates:

  • Potatoes

  • Rice

  • Bread

  • Cereals

  • Pasta

  • Fruit

  • Sweets

As we mentioned above, carbohydrates are an energy source. All carbohydrates are made up of simple units. These are called monosaccharides. They join to form disaccharides and then polysaccharides. The main carbohydrate that we use is glucose, which also happens to be a monosaccharide! In order to get glucose into the blood, it has to be absorbed into the bloodstream from the small intestine.

If you want to learn more about the digestive process, have a look at our article on Digestion!

Monosaccharides are single units of carbohydrates. They include fructose and glucose.

Disaccharides are two monosaccharides joined together. Sucrose is an example, and consists of glucose and fructose joined together.

Polymers are large molecules made up of small repeating units (monomers).

Starch and cellulose are both polymers and examples of storage molecules.

  • Starch is a polymer of glucose. It must be broken down into glucose molecules to pass into the bloodstream.

  • Cellulose makes up plant cell walls. It is also a polymer of glucose. It cannot be broken down by the body, as we don't have the enzymes to do this.

Here are some of the forms carbohydrates take in the human body and plants:

  • Glucose is converted into starch for storage in plants.

  • Glucose is converted into cellulose for cell wall synthesis in plants.

  • Glucose is stored as glycogen in humans and animals.

Make sure you know the difference between the roles of carbohydrates and how they are stored in plant and animal cells!

Proteins

Another extremely important class of molecule that makes up foods is proteins. Proteins are essential in the growth and repair of muscles and cells in the body, which is why many athletes are so concerned about how much protein they consume! We get protein from a variety of different foods! Here are some:

  • Meat

  • Eggs

  • Cheese

  • Beans

  • Nuts

  • Seeds

As with carbohydrates, proteins need to be absorbed into the bloodstream before the body can use them. This absorption happens in the small intestine. However, proteins are large molecules, unlike glucose, so they need to be broken down before absorption.

We break proteins down into amino acids, which are the building blocks of proteins. There are 20 amino acids occurring naturally and are joined and folded to make proteins and give them their unique characteristics. They are small molecules made up of carbon, hydrogen, oxygen and nitrogen. Once these amino acids get into the bloodstream, we rebuild them into proteins. Any amino acids not being built back into proteins are broken down in the liver.

Lipids

Lipids are the final important molecule that we will discuss. Lipids have many different roles in the body. For example, they are great energy stores. Lipids are also involved in buoyancy, protection of vital organs and insulation. Importantly, lipids also make up a significant part of our cell membranes!

Buoyancy is the ability of an object to float in a fluid.

As with carbohydrates and proteins, there are many sources of lipids in foods. Here are some:

  • Meat and fish

  • Nuts and seeds

  • Plant oils

  • Butter

Similarly to proteins, lipids are too big to be absorbed directly into the bloodstream from the small intestine, so they have to be broken down first. Lipids are broken down into fatty acids and glycerol. The fatty acids that makeup lipids can be the same or different to each other! Have a look at Figure 1 to see what these fatty acids are!

As with proteins and amino acids, lipids are then reassembled from fatty acids and glycerol once they get into the bloodstream and reach the cells that need them. They can then be used for any of the functions that we discussed above!

Qualitative Biochemical Food Chemistry Tests

It is useful for scientists to know what foods contain each of the molecules we discussed above; proteins, carbohydrates and lipids. So, each of these molecules has qualitative tests we can use to identify them.

Qualitative refers to something that does not output a measurable value but rather a yes or no answer. This is in comparison to a quantitative test which measures the amount of a substance present, rather than simply if it is present or not.

Detecting Sugars

The first biochemical test that we look at is the test for sugars, also known as carbohydrates. The test that we use for sugars is called Benedict's Test. Learn the steps below for Benedict's test:

  • Add Benedict's solution to the food.

  • Heat in a water bath.

  • The solution will change from blue to brick red if the test is positive.

  • If the test is negative, the solution will remain blue.

  • In a positive test, the colour change can go from green to orange to red. The stronger the colour, the more carbohydrate/sugar/glucose present.

There are two different types of sugars, reducing and non-reducing sugars:

  • Non-reducing sugars = sucrose.

  • Reducing sugars = glucose, fructose, lactose and maltose

Only reducing sugars will give a positive test in Benedict's solution. When carrying out this test, we must wear goggles and avoid contact with the solution. Have a look at Figure 2 to see the colour changes involved in a positive Benedict's test.

It is important to remember that Benedict's solution needs to be heated in a water bath!

Detecting Starch

We do not use Benedict's test specifically for the carbohydrate starch. Instead, we use the Iodine test. To test for starch, follow these steps:

  • Add iodine solution to the food.

  • The food will turn from yellow/brown to blue/black if positive.

  • If negative, the food will remain yellow/brown.

Again, we have to wear safety goggles and avoid contact with our skin and eyes, as, like Benedict's solution, iodine solution is an irritant.

Detecting Proteins

Next, we will look at the qualitative test for proteins. To test for proteins, we use the Biuret test. Biuret solution either needs to be mixed from two other solutions or sometimes it can come already prepared. Here is how we test for the presence of proteins:

  • Add 1ml of biuret solution A to the food.

  • Mix the liquids.

  • Add 1ml of biuret solution B and shake.

  • If positive, the solution will turn from blue to purple

  • If negative, the solution will remain blue

  • Any detection of the colour purple equals a positive result!

Detecting Lipids

The final test to talk about is the test for lipids. For lipids, we can use two different tests. To start with, we will discuss the Sudan III test. This test follows these steps:

  • Add equal amounts of food and water to a test tube.

  • Add droplets of Sudan III into the solution.

  • Shake the test tube.

  • If positive, a red layer will form in the test tube.

  • If negative, no red layer will form.

We need to wear goggles for the Sudan III test and avoid contact with skin and eyes. We also have to be careful as this solution is flammable!

The next test for lipids is the emulsion test. Emulsion refers to a mixture of substances, where one substance is in fine droplet form, and the other is a liquid. Have a look at the steps needed for the emulsion test:

  • Add ethanol to a test tube with food that has been broken into small pieces.

  • Pour liquid from this test tube into another test tube containing water.

  • If positive, a cloudy liquid will form in the test tube.

  • If negative, a clear liquid will form in the test tube.

Have a look at Figure 3 to see what a positive and negative emulsion test looks like!

Chemistry of Food - Key takeaways

  • Carbohydrates, proteins and lipids all play important roles in the body.

  • All carbohydrates are made up of simple units called monosaccharides.

  • Proteins are made up of amino acids.

  • Lipids are made of fatty acids and glycerol.

  • Proteins, lipids and larger carbohydrates have to be broken down before they can be absorbed into the blood.

  • We use qualitative tests to test for the presence of proteins, lipids and carbohydrates in different foods.

Frequently Asked Questions about Chemistry of food

Food chemistry analyses the chemical processes that impact the biological and non-biological components of food. This largely consists of looking at the interactions of lipids, carbohydrates and proteins, but also includes other substances such as water, enzymes, vitamins and minerals, amongst other substances. 


It also looks at the interactions of food with various processing methods, aiming to either enhance or limit these interactions, depending on how desirable they are. 

The main function of protein in food is to be broken down into its components, to then be reassembled into proteins the body needs. 

The main functions of lipids within the body are to act as a store of energy and to provide protection and insulation to areas of the body. They are also used in signalling, and to help absorb certain vitamins such as A, D, E and K. Finally, they form the main structural component of cell membranes. 

Carbohydrates act as an important energy store within the body. They also form several structural compounds, such as cellulose and chitin. Finally, they play an important structural role in many molecules within the body, including ribose, the backbone of DNA, RNA and many co-enzymes. 

Digestive enzymes are enzymes, biological catalysts, which are used to break down large macromolecules, polymers of smaller sub-units, into their smaller parts. This allows for easier absorption into the body.


They are generally found within the bodies digestive tract, along with inside of cells. Digestive enzymes are generally classified according to the substances they break down, such as lipases breaking down lipids and amylases breaking down carbohydrates. 

Final Chemistry of food Quiz

Question

What are carbohydrates broken down into?

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Answer

Carbohydrates are broken down into glucose and other small sugars

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Question

What are proteins broken down into?

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Answer

Proteins are broken down into amino acids

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Question

What are lipids broken down into?


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Answer

Lipids are broken down into glycerol and fatty acids

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Question

What does qualitative mean? (In reference to a biomolecular test)


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Answer

It refers to tests in which results are decided on based on whether something is present or absent. Results are not given by counting or calculations.

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Question

What is the test for proteins?


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Answer

The test for proteins is the Biuret solution. A positive test goes from blue to purple

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Question

What is the test for carbohydrates?


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Answer

The test for carbohydrates is Benedict’s solution. A positive test goes from blue to brick red, with colours like green and orange also being observed.

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Question

What two tests can we use for lipids?


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Answer

We can use the Sudan III test or the emulsion test to check for the presence of lipids in foods

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Question

What test do we use for starch?


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Answer

To test for the presence of starch in food, we use the Iodine test. A positive test changes from yellow/brown to brown/black.

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Question

What would a positive biuret test show?


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Answer

Blue

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Question

What would a positive Benedict’s test show?


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Answer

White

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Question

 What would a positive emulsion test show?


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Answer

Red

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Question

What are proteins broken down into?


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Answer

Amino acids

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Question

What are lipids broken down into?


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Answer

Amino acids


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Question

Which of the following refers to the quality of something?

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Answer

Qualitative

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Question

What would a negative iodine test show?

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Answer

Yellow/Brown

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Question

Define qualitative test

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Answer

A qualitative test tells us whether a substance is present or not.

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Question

Define quantitative test

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Answer

A qualitative test tells us how much of a substance is present.

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Question

What is a reducing sugar?


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Answer

A reducing sugar is a sugar that causes another substance to be reduced. 

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Question

Give an example of a reducing sugar.

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Answer

Glucose

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Question

​Give an example of a non-reducing sugar.


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Answer

Sucrose

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Question

What is the test for starch?

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Answer

Iodine test

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Question

What is the test for reducing sugars?


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Answer

Heat with Benedict's solution in a water bath.

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Question

Why do we use a water bath for Benedict's test?

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Answer

We use a water bath as this allows more accurate temperature monitoring.

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Question

What colour change is observed during a positive iodine test?

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Answer

A colour change from yellow/brown to blue/black.

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Question

What colour change is observed during a negative biuret test?


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Answer

A negative biuret test stays blue.

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Question

Which biomolecule is the emulsion test used for?

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Answer

Test for lipids.

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Question

Which biomolecule is the biuret test used for?


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Answer

Test for proteins

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Question

What colour change is observed during a negative Sudan III test?

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Answer

A negative sudan III test shows no red ring or red solution being formed.

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Question

A colour change from a colourless solution to a white emulsion during an emulsion test shows what type of result?

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Answer

A positive result

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Question

A colour change from blue to brick-red precipitate during a Benedict's test shows what type of result?

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Answer

A positive result

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