Views: 14 Author: Anne Marie Helmenstine, Ph.D. Publish Time: 2020-06-19 Origin: ThoughtCo.
4 Simple Chemical Tests for Food
By Anne Marie Helmenstine, Ph.D.
Updated August 07, 2019
Simple chemical tests can identify a number of important compounds in food. Some tests measure the presence of a substance in food, while others can determine the amount of a compound. Examples of important tests are those for the major types of organic compounds: carbohydrates, proteins, and fats.
Here are step-by-step instructions to see if foods contain these key nutrients.
01 Benedict's Solution
Carbohydrates in food can take the form of sugars, starches, and fiber. Use Benedict's solution to test for simple sugars, such as fructose or glucose. Benedict's solution doesn't identify the specific sugar in a sample, but the color produced by the test can indicate whether a small or large quantity of sugar is present. Benedict's solution is a translucent blue liquid that contains copper sulfate, sodium citrate, and sodium carbonate.
How to Test for Sugar
1.Prepare a test sample by mixing a small amount of food with distilled water.
2. In a test tube, add 40 drops of the sample liquid and ten drops of Benedict's solution.
3.Warm the test tube by placing it in a hot water bath or container of hot tap water for five minutes.
4.If sugar is present, the blue color will change to green, yellow, or red, depending on how much sugar is present. Green indicates a lower concentration than yellow, which is a lower concentration than red. The different colors may be used to compare the relative amounts of sugar in different foods.
You can also test for the amount of sugar rather than its presence or absence using density. This is a popular test for measuring how much sugar is in soft drinks.
02 Biuret Solution
Protein is an important organic molecule used to build structures, aid in the immune response, and catalyze biochemical reactions. Biuret reagent may be used to test for protein in foods. Biuret reagent is a blue solution of allophanamide (biuret), cupric sulfate, and sodium hydroxide.
Use a liquid food sample. If you are testing a solid food, break it up in a blender.
How to Test for Protein
1.Place 40 drops of liquid sample in a test tube.
2.Add 3 drops of Biuret reagent to the tube. Swirl the tube to mix the chemicals.
3.If the color of the solution remains unchanged (blue) then little to no protein is present in the sample. If the color changes to purple or pink, the food contains protein. The color change can be a bit hard to see. It may help to place a white index card or sheet of paper behind the test tube to aid viewing.
Another simple test for protein uses calcium oxide and litmus paper.
03 Sudan III Stain
Fats and fatty acids belong to the group of organic molecules collectively called lipids. The lipids differ from the other major classes of biomolecules in that they are nonpolar. One simple test for lipids is to use Sudan III stain, which binds to fat, but not to proteins, carbohydrates, or nucleic acids.
You'll need a liquid sample for this test. If the food you are testing is not already a liquid, puree it in a blender to break up the cells. This will expose fat so it can react with the dye.
How to Test for Fat
1.Add equal volumes of water (can be tap or distilled) and your liquid sample to a test tube.
2.Add 3 drops of Sudan III stain. Gently swirl the test tube to mix the stain with the sample.
3.Set the test tube in its rack. If fat is present, an oily red layer will float to the surface of the liquid. If fat is not present, the red color will remain mixed. You're looking for the appearance of red oil floating on water. There may only be a few red globules for a positive result.
Another simple test for fats is to press the sample onto a piece of paper. Let the paper dry. Water and volatile organic compounds will evaporate. If an oily stain remains, the sample contains fat. This test is somewhat subjective, because the paper may be stained by substances other than lipids. You can touch the spot and rub the residue between your fingers. Fat should feel slippery or greasy.
Chemical tests may also be used to test for specific molecules, such as vitamins and minerals. One simple test for vitamin C uses the indicator dichlorophenolindophenol, which is often just called "vitamin C reagent" because it's much easier to spell and pronounce. Vitamin C reagent is most often sold as a tablet, which must be crushed and dissolved in water just prior to performing the test.
This test requires a liquid sample, like juice. If you're testing a fruit or solid food, squeeze it to make juice or liquefy the food in a blender.
How to Test for Vitamin C
1.Crush the vitamin C reagent tablet. Follow the instructions that came with the product or dissolve the powder in 30 milliliters (1 fluid ounce) of distilled water. Don't use tap water because it can contain other compounds that could affect the test results. The solution should be dark blue.
2.Add 50 drops of vitamin C reagent solution to a test tube.
3.Add a liquid food sample one drop at a time until the blue liquid turns clear. Count the number of drops required so you can compare the amount of vitamin C in different samples. If the solution never turns clear, there is very little or no vitamin C present. The fewer drops required to change the color of the indicator, the higher the vitamin C content.
If you don't have access to vitamin C reagent, another way to find vitamin C concentration is using iodine titration.