Experiment 8

Analysis of Foods for Starch and Vitamin C

Background

The three main classes of foods used by people and animals are carbohydrates, proteins and fats. For good nutrition our diet should contain the proper proportions of all three types of foods as well as Vitamins, minerals and dietary fiber.

This experiment deals with the detection of starch, a carbohydrate, in foods and with the determination of Vitamin C in juices. Because of this, the following discussion will be limited to the description of carbohydrates and Vitamins. A more detailed discussion of food and nutrition can be found in Chapter 18 of the reference at the end of this page.

Carbohydrates
These are compounds of the elements carbon, hydrogen and oxygen, and include digestible simple sugars such as glucose and fructose, disaccharides such as sucrose and maltose, and polysaccharides such as starch and cellulose. The structures of these carbohydrates are shown below. In these structures, the carbon atoms in the 5- and 6-membered rings are not shown so that the rest of the molecules are easier to see. Note the difference between the two forms of glucose. This will be important when we look at the structures of starch and cellulose.

alpha-glucose	beta-glucose	fructose

sucrose

Note that the sucrose molecule is made up of a fructose molecule and a glucose molecule joined together. In the body sucrose is broken down into glucose and fructose which are then used by the cells of our bodies for energy.

Starch and cellulose molecules are much more complex than those of sucrose. Both are polymers of glucose. Starch is a polymer of alpha-glucose and cellulose is a polymer of beta-glucose. The two polymers have different properties because of their different structures. Starch can be broken down in the human body to give glucose, whereas cellulose cannot be digested by the human body, and has no food value. One purpose of this experiment will be to detect the presence of starch in various foods.

starch
cellulose

Vitamins
These are organic compounds which are required in relatively small amounts to maintain health. Since Vitamins are not synthesized by the human body they must be included in the diet.

The chief sources of one of these Vitamins, Vitamin C, are fresh fruits and vegetables. A deficiency of Vitamin C in the diet results in a disease called scurvy. Another purpose of this experiment is to determine the concentration of Vitamin C in various fruit juice drinks.

The Experimental Method

Starch reacts with iodine, I₂ to give a blue-black compound:

This reaction forms the basis for the experiment.

Part 1

In this part of the experiment you will test several foods for the presence of starch by placing a few drops of an iodine solution on the food. The formation of a blue-black color where the iodine contacts the food indicates the presence of starch.

Part 2

This part involves the use of the starch-iodine reaction to analyze fruit juices for Vitamin C. Vitamin C reacts with iodine (I₂) according to reaction (2) below, which consumes Vitamin C.

(2)

A solution of iodine is added dropwise to a juice sample containing starch. The reaction between Vitamin C and I₂ will occur as long as Vitamin C is available. Once all of the Vitamin C has been used up, the reaction can no longer occur. Any additional I₂ which is added to the juice sample will then react with the starch, according to reaction (1), to give the blue-black starch iodine compound. Therefore, the appearance of the blue-black color is a signal that the amount of I₂ added is just sufficient to react with all of the Vitamin C in the sample. The more Vitamin C a sample contains, the more I₂ solution will be needed to cause the blue-black color to appear. Thus, the volume of I₂ solution added is a measure of Vitamin C in the sample. The procedure described above is called a titration. In this case, the juice is titrated with the iodine solution.

The goal of this part of the experiment is to measure the Vitamin C content as milligrams (mg) of Vitamin C in 20 mL of the juice. In order to do this it will be necessary to determine the mg of Vitamin C which will react with 1.0 mL of your iodine solution and then determine how many milliliters of the iodine solution react with 20 mL of your juice sample. A standard solution of known Vitamin C content will be prepared by dissolving a 250 mg Vitamin C tablet in water.

Examples of the two types of calculation that you will need to do are shown below:

Example 1:

A 250 mg Vitamin C tablet is dissolved in 100 mL of water and mixed thoroughly. A 20 mL sample of this solution is then titrated with an iodine solution. The volume of iodine solution required to cause the appearance of the blue-black color is 7.50 mL. How much Vitamin C reacts with 1.0 mL of the iodine solution?

Solution: The original Vitamin C solution contains 250 mg in 100 mL. Therefore 20 mL contains

250 mg x 20 mL/100 mL = 50 mg

These 50 mg of Vitamin C require 7.50 mL of iodine solution to react completely. The amount of Vitamin C which reacts with 1.0 mL of iodine solution is obtained by dividing 50 mg by 7.50 mL

50 mg/7.50 mL = 6.7 mg Vit. C per mL of I₂ soln.

Example 2:

The iodine solution in Example 1 is then used to titrate 20 mL of a fruit juice. The volume of iodine solution required to cause the appearance of the blue-black color is 2.0 mL. How many mg of Vitamin C are contained in 20 mL of juice?

Solution: From Example 1, 1.0 mL of I₂ solution reacts with 6.7 mg of Vit. C. Therefore, 2.0 mL will react with

2.0 mL of iodine solution x 6.7 mg Vit. C per mL of iodine solution = 13.4 mg of Vitamin C

Therefore, 20 mL of the juice contains 13.4 mg of Vitamin C.

Reference

1. Chemistry, a Contemporary Approach, G.T. Miller, Jr., D.G. Lygre, and W.D. Smith, Wadsworth Publishing Co., Belmont.

 

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Purpose and Conclusions

Procedure and Results