Experiment 25 The Detection of Fats, Proteins, and Carbohydrates in Foods

Name: Larisa Elias

Experiment 25: The Detection of Fats, Proteins, and Carbohydrates in Foods

Experiment 25, The Detection of Fats, Carbohydrates, and Proteins in Foods, is a lab that applies the tests we did previously on carbohydrates, along with additional tests for lipids and proteins. This lab is not well suited for presentation through video. Instead of doing an experiment, you will view a couple of YouTube videos that illustrate the tests of proteins and lipids. Additionally, we will review some of the carbohydrate tests you did previously. Finally, you will analyze the results of test as described for a specific food sample.

First, review the tests that you previously performed on carbohydrates (p. 355-362 in the lab manual). Also read the procedures given in Experiment 25, p. 344. Concentrate on the procedures, as well as the observed results for positive and negative tests, for the Molisch test, the Benedict test, and the Iodine test for starch.

Second, read the instructions for the grease spot test on p. 345 (just 3 sentences). Click on this link to watch the YouTube video showing the performance of the grease spot test. It is very simple.

Third, read the instructions on p. 345-6 on how to perform the Xanthoprotic Test, the Biuret Test, and the Ninhydrin Test. Click on this link to watch a short video demonstrating the Xanthoprotic Test. Click on this link and this link to watch a pair of short videos demonstrating the Biuret Test. Click on this link and this link to watch a pair of short videos demonstrating the Ninhydrin test.

Please fill in the following table which summarizes the chemical tests used in this lab.

Test Name Tests for What? Positive Result (describe) Negative Result (describe)

Carbohydrates:

Molisch Test Carbohydrate Purple color: presence of carbohydrate Colorless:

Benedict Test Reducing sugars Changes from blue, to green to yellow to orange Blue color. Retain color of benedict solution

Iodine Test Starch Blue-black solution Yellow color. Retain color of iodine solution

Lipids:

Grease Spot Test Fats and triglycerides Translucent patch No translucent patch

Proteins:

Xanthoprotic Test Test for amino acid contain tyrosine, tryptophan and phenylamine After adding HNO3 a yellow precipitate appears. Adding NaOH makes the yellow solution darker No color change, solution remains light yellow.

Biuret Test Test for peptide bonds Color change to deep purple Blue color

Ninhydrin Test Test for free amino groups Appearance of deep blue or purple color No color change

An applied problem:

A sample of about 5 grams of a glazed donut was tested as follows. The sample was first ground up in a mortar and pestle with about 10 mL of hexane. The sample was poured through filter paper to separate the liquid (hexane) from the ground-up solid.

Because hexane is a nonpolar solvent, any lipids present in the donut were dissolved into the hexane. A few drops of the hexane were placed on a piece of paper, and the hexane was allowed to evaporate. When the hexane had evaporated, the paper had a translucent spot where the hexane was placed.

The ground-up solid portion of the donut was allowed to dry, then divided up into six equal portions. Each portion was placed into one of six test tube. Each test tube was used to perform one of the following tests. The results of the tests are described below.

Carbohydrate tests:

The Benedict test gave a blue-colored solution that was clear except for the undissolved crumbs of the donut.

The Molisch test gave a violet color at the interface between the water (upper) layer and the sulfuric acid (lower) layer.

The iodine test gave a dark blue-black color as soon as iodine-potassium iodide solution was added to a water suspension of the ground-up donut. The color was darkest on the undissolved crumbs of donut.

Protein tests:

Biuret Test: About 2 mL of deionized water was added to the ground donut in one test tube. The contents were agitated for about a minute. 5 drops of 6 M NaOH was added to the mixture, mixed, and allowed to stand for about a minute. 3 drops of 2% copper sulfate solution was added to the test tube, and the test tube was mixed again. The resulting mixture had a purple color.

Xanthoprotic Test: About 1 mL (20 drops) of deionized water was added to the ground donut in one test tube. 5 drops of concentrated nitric acid was added to the test tube. The contents of the test tube were mixed, and the test tube was placed in a hot-water bath for 3 minutes. The contents of the test tube turned bright yellow. The color was most pronounced on the undissolved solid particles of ground donut. The test tube was cooled with tap water. 15 drops of 6 M sodium hydroxide solution were added to the test tube, and the contents was mixed. Testing a drop with pH paper showed that the solution was strongly basic. When the sodium hydroxide was added, the color of the mixture changed from bright yellow to dark orange.

Ninhydrin Test: About 1 mL (20 drops) of deionized water was added to the ground donut in one test tube. 2 drops of 0.1% ninhydrin solution was added to the test tube, and the contents was mixed. The test tube was placed in a hot water bath for 2 minutes (instead of heating with a Bunsen burner). At the end of two minutes, no color change was seen.

Please answer the following questions about the donut.

Does the donut contain lipids? Why or why not; what is the supporting observation?

The donut contains lipids because there was formation of a translucent spot on the paper because the lipid in the hexane, and when hexane evaporated the lipid formed a patch that was translucent.

Does the donut contain carbohydrates? What is the supporting observation or observations?

Yes, the donut contains carbohydrate. In the Molisch test there was formation of a violet color.

Does the donut contain reducing sugars? What is the supporting observation or observations?

No, the donut did not contain reducing sugar because in the benedict test, the solution remained blue. However, there was an indication of reducing sugars on the undissolved crumbs meaning the donut had been glazed with a reducing sugar.

Does the donut contain starch (polysaccharides)? What is the supporting observation or observations?

Yes, the donut contains starch because there was color change to dark blue-black a positive indication of starch

Does the donut contain protein? What is the supporting observation or observations?

Yes, the donut contains proteins because there was formation of purple color from the mixture

Based on the result of the Xanthoprotic test, what can you conclude about the donut?

The donut contained either tryptophan or tyrosine thus the color change from yellow to dark orange after addition of NaOH.

Based on the result of the ninhydrin test, what can you conclude about the donut?

The donut did not contain any free amino group that is why there was no color change.

Bonus question: Based on the results of the tests performed, can you tell what sugar was used to glaze the donut? What is your supporting evidence?

I would say fructose was used in glazing because of the positive benedict test on the crumbs.