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​B2 F) Food Tests
We can undertake a few different tests to see whether a food sample contains certain substance. All of the tests start with the same general process, which is:
  1. Get a sample of the food and break it up using a pestle and mortar.
  2. Put the ground up food in a beaker and add some distilled water (pure water).
  3. Mix the distilled water and ground up food to create a solution.
  4. Filter the solution to get rid of any solid bits of food. We filter the solution by pouring the solution through a funnel that is lined with filter paper. The water and dissolved food substances will pass through the filter paper, and the solid bits of food will be stuck on top of the filter paper.
 
We now have our sample with the dissolved food substances to test. The tests for carbohydrates (sugars and starch), proteins and lipids are explained below
Carbohydrates
Carbohydrates are the body’s main source of energy. They make up about 5% of the body’s mass.
 
Carbohydrates come in the form of sugars or starch.
 
Sugars
Sugars are soluble in water and they taste sweet. Glucose is an example of a sugar. Many naturally sweet tasting foods contain sugars. For example, most fruits contain fructose and milk contains lactose. Sugar that we add to hot drinks or sprinkle on top of cereal is known as sucrose (e.g. caster or granulated sugar).
 
There are two different types of sugars – reducing and non-reducing sugars. We can test for reducing sugars by using Benedict’s test. To test for reducing sugars we:
  • Add 5 cubic centimetres of our food sample to a test tube.
  • Use a water bath to heat the test tube up to 75°C.
  • Add about 10 drops of Benedict’s solution so that the solution becomes blue.
  • Leave the test tube in the water bath for around 5 minutes. Make sure that the test tube is facing away from you when it is left in the water bath (for safety reasons).
 
After 5 minutes, we look at the colour of the mixture in the test tube. If no reducing sugars are present, the mixture in the test tube will remain blue. If reducing sugars are present, the colour of the mixture will change. The extent of the change depends on the quantity of reducing sugars that are present. Here are what the different colour outcomes mean:
  • Blue – no reducing sugars
  • Green – a trace of reducing sugars
  • Yellow – low levels of reducing sugars
  • Orange – moderate levels of reducing sugars
  • Red – high levels of reducing sugars
Starch
The second type of carbohydrates are starch. Foods that are high in starch are pastas, potatoes and rice.
 

We can test for the presence of starch by using iodine solution. We carry out the test by transferring around 5 cm3 of our food sample into a test tube. We then add a few drops of iodine solution and gently shake the test tube. Iodine is a brown-orange colour. If starch is present, the starch will react with the iodine, which results in the solution changing from brown-orange to black-blue. If no starch is present, the mixture will remain brown-orange.

Proteins

Proteins make up around 18% of the body’s mass. Proteins are polymers that are made from amino acids. Amino acids are used to build thousands of different proteins, such as enzymes, haemoglobin (in red blood cells) and many more. Meats, fish and cheeses are foods that are high in protein.

 

We test for proteins by using the biuret test. We transfer a small sample of the food (around 2 cm3) to a test tube. We then add 2 cm3 of Biuret solution and shake the test tube. Biuret solution is a blue solution. If proteins are present in our sample, the solution will change from blue to pink or purple. If no proteins are present in our sample, the solution will remain blue.

Lipids

Lipids make up around 10% of our body’s mass. They are used as long-term energy stores. They are stored under skin, and they act as an insulator preventing heat loss. They are also stored around the kidneys and the heart for protection. Lipids are made from fatty acids and glycerol. A lipid is a molecule of glycerol joined to three fatty acid molecules (see diagram below).

We can test whether a food contains lipids by using the Sudan III test. We prepare the sample in pretty much the same way that was outlined at the start of this section, except we do not filter it. We then transfer about 5 cm3 of our sample into a test tube. The next step is to add a few drops of Sudan III into the test tube and gentle shake. If lipids are present in the mixture, a red layer will form on the top of the sample. If no lipids are present, no red layer will form on the top of the sample. 

End Note
The names for the different food tests can be quite tricky to remember. Therefore, it is worth getting them down on a revision card. Here is a summary:
  • Reducing Sugars – Benedict’s test: the mixture goes from blue to green, yellow, orange or red if reducing sugars are present. The mixture will stay blue if no reducing sugars are present.
  • Starch – iodine solution: the mixture goes from brown-orange to blue-black if starch is present. If starch is not present, the mixture will stay brown-orange.
  • Proteins – biuret test: the mixture goes from blue to pink or purple if proteins are present. The mixture will stay blue if no proteins are present.
  • Lipids – Sudan III: a red layer will form at the top of the mixture if lipids are present. If no lipids are present, no red layer will form.