What’s in the Food You Eat???

The goal of this lab is to show the students what kinds of nutrients are in the foods they eat so they can understand nutrient labels and to give them experience with using different chemical indicators and understanding their properties (indophenol, Biuret solution, Benedict’s, and Lugol’s iodine).

The students are provided with a couple different foods by the teacher, and their job is to test them for glucose, sucrose, lipids, protein, starch, or vitamin C. There is a different indicator test to test each one of these nutrients, and the teacher should go through the experiment before and do all of these, to have a set of positive controls ready so the kids will have something to compare their results to. The accompanying table also provided on this site should be passed out beforehand, on which students will write down the foods provided, their prediction (+ or -), and then once they finish the test, the result.

The different nutrient tests are as follows:

Fats and Oils (Lipids): Rub a bit of ground up food on brown paper; hold the paper up to light after the water in the sample has evaporated…if fats and oils are present, a greasy semi-translucent spot should appear on the paper.

Protein: Place about 5 mL of ground-up food into a test tube, and add 10 drops of Biuret solution. There should be a pink to purple reaction if there is protein…if no change, then protein is not present. Be sure to let the kids know that Biuret is an irritant and can damage clothing…take all safety precautions.

Starch: Add 5 drops Lugol’s iodine solution (which can stain things easily) into a 5 mL sample of crushed food. The Lugol’s will turn blueish black if there is starch; no color change means no starch.

Sugar: Add 3 mL Benedict’s to a 5 mL sample of ground up food; place the test tube in a beaker of boiling water, and heat for 5 minutes. The Benedicts will show an orange or brick red color is glucose is present and green if sucrose is present. No color change occurs if there is no sugar. Be careful with the heating and boiling water.

Vitamin C: Add 8 drops of indophenol solution to a 5 mL sample of ground up food. Blue indophenol becomes colorless in the presence of Vit C. There may be an intermediate pink stage. No color change means no Vitamin C.

If there is a lack of time, have each group do one food with the four different tests, and then report back to the class afterwards. Groups of 4 or so are ideal. Ask the kids questions afterwards as a class, such as how might the natural colors of the foods affect the results. Before the lab it may be a good idea to do the tests on foods known to have the nutrients and do the tests on them as controls beforehand, and then provide these foods, such as oatmeal, liver, cereal, candy, etc. to the kids. Test tubes, test tube holders, and a hot plate will be necessary materials for this lab, in addition to the foods and chemical indicators.