Lesson Video: The Chemistry of Food Biology

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


Video Transcript

In this video, we’ll learn about the structure of three types of biological macromolecules in our food and our bodies, how to identify the presence of these molecules using simple laboratory analyses, as well as a method to ascertain the amount of chemical energy released from food as the larger molecules are broken down into smaller ones. And we’ll work some example problems along the way.

We need food to build, maintain, and keep our life processes like breathing and many more running. And we’re gonna look at how food does this for us. Here are common food items that are also recognizable organisms, although even a Cheeto or a hi-tech protein bar are also made of combinations of formerly living organisms. When food is consumed, large molecules within it are broken down, and the resulting smaller molecules can be used in our bodies. The types of large molecules that we’re gonna focus on include complex carbohydrates, proteins, and fats, which are a type of lipid. These types of molecules are three of the biological macromolecules that all organisms use in their bodies, and they’re also polymers.

The word root “poly” means many, and the word root “mer” means part. And a polymer is a large molecule made of many similar parts. You can see this easily in the carbohydrate and protein diagrams, since they look a little bit like long trains made out of smaller train cars. But fats are a little bit different, and not all references consider them to be real polymers. But we will be considering them as such.

During digestion, these large molecules are broken down into the smaller parts that they’re made out of which are sugars for carbohydrates, amino acids for proteins, and glycerol and fatty acids for fats. The smaller parts that polymers are broken into can be called monomers, which means one part. But how will a bunch of monomers help this dinosaur here? Well, she can use the sugars for energy or to build up larger molecules. She can use the amino acids to make her own proteins, which have many functions. And the glycerol and fatty acids can be used to make cell membranes or as energy-rich fuel and more. So when we eat food, we’re really reorganizing other organisms into ourselves. And if you have a run in with a tiger, you may be part of that tiger for a very long time.

Next, let’s look at a few laboratory tests that we can use to identify the presence of these molecules in our food. Let’s begin with Benedict’s test for sugars. Sugars are carbohydrates themselves, but they’re also the monomer unit of larger complex carbohydrates. First, we put a small sample of the food to be tested into a test tube. If the food is a solid, mix it with a little water and then add about the same amount of blue colored Benedict solution. Then place the tube in a water bath that’s a little below boiling and let it heat for a few minutes to see if the color changes. If the color doesn’t change, that means it’s a negative result, in other words, no sugar. But if the color changes from blue to green, to orange, or even brick red, that indicates a positive result for sugar. In fact, these colors indicate the concentration of sugar as well, with green being lower, orange is higher, and brick red is the highest.

Here’s another carbohydrate test, but this time it’s for a complex or large carbohydrate called starch, which was made out of many, many glucose units bonded together. Place a few drops of an iodine solution on the food sample and see if a color change occurs to a black or bluish black. If no such color change occurs, that’s a negative result, meaning there’s no starch present. But if you see the color change to a black or a bluish black, that indicates that starch is there.

The next test we’ll look at is called Biuret’s test, and it’s used to determine the presence of proteins. Add a few drops of blue Biuret’s solution, which contains potassium hydroxide and copper sulfate to the liquid or liquefied food sample, and then see if there’s a color change to purple. If it does, that’s a positive result for protein.

And last but not least, the emulsion test for lipids. Again, we place a small sample of food in a test tube and add about twice as much ethanol then mix. After letting the mixture settle for a bit, add about an equal amount of water to the test tube. Or for clearer results, pour the upper portion of the sample into a test tube with about an equal amount of water. If the resulting mixture becomes cloudy, that’s a positive test for lipids. And a clear solution indicates that fats are not present. Cloudiness is evidence of an emulsion. And an emulsion is a mixture of two liquids that don’t normally mix unless one disperses and the other is tiny droplets.

While these tests determine if certain macromolecules or sugars are present, there is another test to run that determines the amount of energy released by food in a process that is surprisingly similar to digestion. And that’s the combustion or burning of food. The name of this method is calorimetry, since the units that we measure the energy in food are called calories, and the suffix -metry is all about the process of measuring things. One of the simplest calorimeters is an insulated container with a thermometer and water. Record your starting or initial temperature. And then the fun part, set the food sample under the calorimeter and light it on fire. Once the food is completely burned, record the final temperature.

The amount of energy transferred from the chemical energy in the food to the water in the calorimeter is represented by the change in temperature, and this is used to calculate the number of calories or amount of energy released from the food sample. So, the key idea here is that the more energy or calories a food sample contains, the more the temperature of the water will increase. Up next, let’s work through some practice problems.

Complete the statements to correctly describe the composition of biological molecules. (a) A protein is made of many monomers called blank. (b) Many sugar molecules, like glucose, join together to form blank. (c) A basic lipid molecule is formed of one blank and three fatty acids.

Key knowledge required to complete these statements correctly is the structure of biological molecules, also known as biological macromolecules. So, let’s review what these molecules are made of and the patterns in their structure as we go through this question. The question asks us to describe the composition of biological molecules. So, we need to describe how the parts of a biological molecule are put together. The biological molecules of concern here are three types of large molecules that we require from our food and in our bodies, and these include carbohydrates, proteins, and lipids. And specifically, we’ll be talking about fats. These types of molecules are called polymers.

The word polymer literally means many parts. And you can see in the diagram here that carbohydrates can be made of many parts. And typically, they’re many, many times larger than what we have shown here. The parts can be bonded into a single chain, as shown here, or they’re branched chain carbohydrates as well. A couple common examples of large or complex carbohydrates include starch, which is used for energy storage, and cellulose, a structural component of plant cell walls.

Polymers are made of many parts, and we call those parts monomers, which literally means one part. The monomer of carbohydrates are sugars and individual sugars are considered carbohydrates as well. In fact, some other carbohydrates are made of only two sugars bonded together, such as table sugar or sucrose. So, the number of sugars in a carbohydrate can range from one to thousands. Protein polymers also have a large range of sizes, and they bond together to form a single chain. A couple common examples of proteins that you may have heard of before are the hormone insulin and the structural protein collagen. The monomer unit of a protein is called an amino acid, and there’s 20 different kinds of amino acids that we build our proteins from.

Lipids are a little different, and while some references don’t consider lipids to be polymers, we will recognize them as polymers. And while lipids also include more than just fats, we’ll limit our scope to just fats here. Fats are made of a molecule of glycerol bonded to three fatty acid chains, which can vary in length and bond type. The monomer units of fats are glycerol and the fatty acids that attach to the glycerol. The fatty acids vary in length, and some of them can even have bends. The combination of the different fatty acids that attach to a glycerol give the fat its identity.

And it looks like we’re ready to complete the statements in the question. Statement (a) says, a protein is made of many monomers called blank. If we look at our table down here, proteins have a monomer unit called amino acids. Statement (b) says, many sugar molecules like glucose join together to form blank. And we now have the names of a couple example sugars in our table, glucose and fructose. So, if we put together many glucose sugars, we’ll get a larger carbohydrate. Statement (c) says that a basic lipid molecule is formed of one blank and three fatty acids. Looking at our diagram, we can see one, two, three fatty acids are connected to one glycerol.

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

Key knowledge required to complete this table is knowing the appropriate food test for each type of biological molecule, sometimes also called biological macromolecules. The table contains the names of two biological molecules, starch and protein, and also the names of two food tests, the ethanol/emulsion test and also a test that uses Benedict’s solution as a reagent. Let’s review three biological molecules common in both our food and our bodies, carbohydrates, lipids, and proteins, as we determine how we should fill out this table.

These biological molecules are polymers, which means they’re made out of many repeating similar units. And those units are called monomers. The monomers that bond together to form larger carbohydrates are called sugars, but sugars can be standalone carbohydrates by themselves as well. The monomers of lipids that we’ll consider here are the glycerol and fatty acids found in fats, which are a type of lipid. And the monomer units that bond together to form proteins are called amino acids. Some examples of carbohydrates include glucose and lactose, which are both simple sugars, and also some larger carbohydrates, such as starch or glycogen. And fats can also be divided into whether they’re saturated fats or unsaturated fats. A couple examples of proteins include myosin and actin, which are in our muscle cells and allow us to move.

Next, we’ll list the tests used to identify these biological molecules in food samples. In carbohydrates, there’re two different tests for two different kinds of carbohydrates, both the reducing sugars, which include all the simple sugars, and starches. The test for sugars is a test that requires a solution called Benedict’s solution. And once the Benedict solution is added to the food sample, it has to sit in a warm water bath for a few minutes. The test that’s used for starches, which is again many glucose molecules bonded together, is the iodine test. And an iodine solution is simply applied to the food sample. The food test for lipids uses ethanol, and it looks to see if the food sample will form an emulsion, which is cloudy. And the test used to identify protein in a food sample is called Biuret test.

And we’re ready to start filling in our table. To identify starch in a food sample, we need to use the iodine test. And the reason for adding ethanol to a food sample to see if an emulsion will form is if you’re checking for lipids. And when we wanna look for protein in a sample, we wanna use a Biuret test. If you leave a food sample in a warm water bath with Benedict’s solution, that would be a test for sugars.

Let’s review some key points from the video. The biological molecules that we discussed are polymers, since they’re made out of many repeating similar units, and those units are called monomers. We discussed three kinds of biological molecules, carbohydrates, proteins, and lipids. The monomer unit of carbohydrates is sugars, and the test for sugars is Benedict’s test. A positive result for Benedict’s test is green if there’s a low concentration of sugar, orange for a higher concentration of sugar, and even brick red for the highest concentration of sugar. If you’re testing for starch, you use the iodine test, and the positive result would look black or bluish black. The monomers of proteins are amino acids, and the test for proteins is called Biuret’s. The positive result for this test will give you a purple or mauve color.

The lipids that we talked about in this video are called fats, and they have two monomer units, a glycerol attached to three fatty acids. The test for fats is an emulsion test, and the positive result looks cloudy or whitish. Of these tests, only Benedict’s test for reducing sugars requires a hot water bath. We also discussed calorimetry, which is a technique to determine the amount of energy that’s released by food when it’s combusted or burned.

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