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.

• 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.

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!

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.

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.

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!