Chemistry of Food Additives

Introduction

Are food additives beneficial or unhealthy? Often people associate food additives as being unhealthy and as strictly used for convenience food. Demonstrate the following activities to help students gain perspective on food additives and nutrition.

1. Nails for Breakfast—Many breakfast cereals, among other foods, are fortified with iron and other essential vitamins and minerals. Find out what form of iron is in your breakfast cereal.
2. Iodized Salt—Is sprinkling your food with salt only harmful? Iodine, in the form of potassium iodide, was the first food additive to be approved for use by the federal government. Potassium iodide is added to salt to prevent thyroid diseases caused by lack of iodine in the diet. Demonstrate the presence of iodide in two forms of iodized salt.
3. Does Salt Sense^® Make Sense?—Several salt products are on the market that claim to contain less sodium. How is this possible? If it is possible, is it worth using?
4. Fruit Facts—When a person cuts up an apple purchased at the grocery store, within a short amount of time the fruit has already started to turn brown. How can companies sell pre-cut fruit without having the same effect?

Concepts

• Food additives
• Nutritional benefits
• Consumer chemistry

Materials Included In Kit

Additional Materials Required

Materials

Safety Precautions

Acetone is a flammable liquid and dangerous fire risk. Keep away from flames and other sources of ignition. 1,10-Phenanthroline is highly toxic by ingestion. Hydrochloric acid is a skin and eye irritant. Avoid contact with eyes and skin. Chlorine water is toxic by ingestion and inhalation. Do not breathe chlorine vapors. Conduct the Iodized Salt demonstration in a well-ventilated laboratory and dispense the chlorine water in a hood. The materials used in Does Salt Sense^® Make Sense? and Fruit Facts are considered nonhazardous. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any food items in the chemical laboratory and do not remove any remaining food items after they have been used in the lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. The waste solutions and excess hydrochloric acid in Nails for Breakfast may be neutralized with base and rinsed down the drain with an excess of water according to Flinn Suggested Disposal method #24b. Acetone may be allowed to evaporate in a hood according to Flinn Suggested Disposal Method #18a. The waste solutions in Iodized Salt may be rinsed down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. Allow excess chlorine water to stand in an open container under a hood in a well-ventilated, protected area. The water remaining after the chlorine gas has evaporated may be rinsed down the drain. All chemicals used in Does Salt Sense® Make Sense? may be disposed of in the regular trash according to Flinn Suggested Disposal Method #26a. Remaining apple pieces may be disposed of according to Flinn Suggested Disposal Method #26a. The solutions from Fruit Facts may be flushed down the drain with an excess of water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

Part II. Iodized Salt

1. Prepare the chlorine water no longer than 48 hours before this demonstration. Chlorine water has a short shelf life. Freshly prepared solutions yield optimal results. Prepare the solution in a fume hood.
2. To prepare 30 mL of chlorine water: Mix 5 mL of 5% sodium hypochlorite (bleach) solution with 10 mL of 1 M hydrochloric acid in a hood. Dilute to 30 mL with distilled or deionized water.

Procedure

Part I. Nails for Breakfast

1. Obtain 30 g of cereal (approximately one serving size) and grind the cereal to a powder using a mortar and pestle.
2. Transfer the cereal powder to a 400-mL beaker and add 250 mL of warm tap water.
3. Measure and record the mass of a magnetic stirring bar and add the stir bar to the cereal mixture.
4. Place the beaker on a magnetic stirrer and stir the cereal-water mixture for about 30 minutes. To obtain quantitative results, 30 minutes is optimum. Metallic iron will be visible on the stir bar after 15–20 minutes.
5. While the cereal mixture is stirring, pour 50 mL of acetone into a 250-mL beaker.
6. Wearing gloves, remove the stir bar from the beaker. Try and grasp the stir bar in the middle region to avoid dislodging the iron from the stir bar.
7. Rinse the stir bar containing metallic iron with a gentle stream of water from a wash bottle.
8. Wearing gloves, quickly dip the stir bar containing metallic iron into the 250-mL beaker containing acetone.
9. Remove the stir bar from the acetone and rinse. Allow remaining acetone to evaporate in a fume hood.
10. Measure and instruct students to record the mass of the stir bar with the iron attached. This information will be used by students to calculate the mass of iron extracted from the cereal. Compare the amount of iron obtained with the information provided on the Nutrition Facts label of the cereal. Note: The typical yield of iron from one serving of Total cereal is 13 mg. The iron content reported on the nutrition label is 18 mg per serving.
11. Remove the iron filings from the magnetic stir bar: hold a mega-magnet in place under a weighing dish. Bring the stir bar from above over the top of the weighing dish. Note: The iron will fall into the weighing dish as it is attracted to the stronger mega-magnet.
12. Remove the mega-magnet vertically straight down from under the weighing dish. Caution: If the mega-magnet is removed to the side of the weighing dish, the iron will literally “fly through the air” to the magnet!
13. Scrape the iron from the weighing dish into a 150-mL beaker.
14. Add 50 mL of 0.1 M hydrochloric acid to the beaker with the iron and allow the mixture to sit undisturbed overnight. Note: Some of the iron will dissolve to give a pale gray or green solution.
15. Dilute 10 drops of HCl solution from step 14 with 2 mL of distilled water in a small test tube. Using a pipet, add 1 drop of 1,10-Phenanthroline.
16. Instruct students to record the color of the solution in the test tube on the Chemistry of Food Additives Worksheet.
17. Compare the color change in step 16 with a known or reference solution containing Fe²⁺ ions by diluting 10 drops of acidified 0.01 M iron(II) sulfate solution with 2 mL of water in a small test tube. Add 1 drop of phenanthroline solution and observe the color change.
18. Instruct students to record the color change in the second test tube on the worksheet.

Part II. Iodized Salt

1. Label three large test tubes A, B and C and add the following reagents to each test tube.

2. Add 5 mL of distilled or deionized water to each test tube. Note: The solids in test tubes B and C will not fully dissolve. The excess solid is used because KI is present in very small amounts in iodized salt.
3. Add 5 drops of 0.5% starch solution to each test tube, followed by 2–3 drops chlorine water.
4. Compare the color changes observed in each test tube and discuss the qualitative evidence for the presence of iodide in table salt.

Part III. Does Salt Sense^® Make Sense?

1. Label graduated cylinder 1 and 2. Place graduated cylinder on the balance. Read the mass and instruct students to record the mass on their Chemistry of Food Additives Worksheet.
2. Pour table salt into the graduated cylinder to the 5-mL mark and place on balance. Instruct students to record the mass of the salt plus the graduated cylinder.
3. Repeat steps 1 and 2 using the second graduated cylinder and Salt Sense instead of salt.
4. Pour the 5 mL of table salt from the graduated cylinder into a mortar and carefully grind using the pestle.
5. Carefully return ground salt to its original graduated cylinder. Instruct students to note the measured volume on their worksheet.
6. Repeat steps 4 and 5 using Salt Sense.

Part IV. Fruit Facts

1. Obtain two 200-mL beakers and label them 1 and 2.
2. Fill a 100-mL graduated cylinder with 100 mL of distilled or deionized water.
3. Transfer the 100 mL of distilled or deionized water to beaker 1 and set aside.
4. Repeat steps 2 and 3 for the second beaker.
5. Using a centigram balance measure 2.0 g of ascorbic acid.
6. Add the 2.0 g of ascorbic acid to the second beaker and stir until all the ascorbic acid has dissolved. Set beaker aside.
7. Using a knife or scalpel cut an apple approximately in half. Discard one half of the apple.
8. Remove the core from the other half of the apple and cut into smaller pieces, resulting in approximately 9–12 pieces.
9. Divide the apple pieces in half and place 4–6 pieces in beaker 1 and the other 4–6 pieces in beaker 2.
10. Using a separate glass stirring rod stir the contents of each beaker.
11. Allow two beakers to sit overnight. Compare differences in the appearance of the apple pieces after approximately 24 hours.

Student Worksheet PDF

12763_Student1.pdf

Lab Hints

• This demonstration is best broken up over two 50-minute class periods. Parts I and IV may both be started on day one and the results may be observed the following day.
• Parts I and IV require the experiment to sit for 24 hours.

Teacher Tips

• Reaction of metallic iron with 0.1 M hydrochloric acid in Nails for Breakfast simulates the process of iron dissolving in the acidic contents of the stomach (pH 2). The process is very slow at room temperature. The rate of the reaction can be accelerated by increasing the temperature (try 37 °C, the internal temperature of the human body).

• Phenanthroline forms colored (dark red), stable complex ions with iron(II) ions in acidic solution. Ferroin, a complex of phenanthroline with iron(II) sulfate, is widely used as an indicator in oxidation–reduction titrations.
• Be sure to inform students of the iron content of the cereal studied in Part I so they are able to complete their post-laboratory questions.
• In Iodized Salt, potassium iodide is added to salt in amounts ranging up to 0.01%. Calculate the mass and number of moles of potassium iodide that would be present in one tablespoon of iodized salt. Express the mass of potassium iodide in micrograms and compare to the amount with the RDA for iodine in young adults (150 micrograms per day).
• Salt Sense^® is not a salt substitute; it is table salt that has been processed to yield 33% less sodium by volume.

Sample Data

Part I. Nails for Breakfast

Part II. Iodized Salt

Part III. Does Salt Sense^® Make Sense?

Answers to Questions

Part I. Nails for Breakfast

1. Compare the amount of iron filings obtained in Part I to the amount which should be present according to the nutrition facts label on the box. Calculate the percent recovery between the measured mass and the theoretical mass.

The iron content reported on the nutrition facts is 18 mg per serving. The yield recovered in class was 13 mg. Therefore the percent recovery is
13 mg/18 mg • 100% = 72% recovery

2. What color did the 1,10 phenanthroline indicator turn upon adding to the iron obtained from the breakfast cereal? What does the color of the phenanthroline indicate about the type of iron found in breakfast cereal?

The phenanthroline turned from its original blue to red upon interacting with the iron. Phenanthroline forms colored (dark red), stable complex ions with iron(II) ions. Since the phenanthroline also turned red in the presence of iron(II) sulfate it indicates that the iron in breakfast cereal is present in the Fe²⁺ form.

Part II. Iodized Salt

3. Based on the results of obtained in the Iodized Salt demonstration, is iodide present in NaCl (tube 2) and Iodized Salt (tube 3). Give evidence to support your answer.

Yes, there is iodide present in both NaCl and iodized salt. The potassium iodide turned a blue-black complex upon addition of chlorine water. Since both NaCl and iodized salt also turned blue-black in certain regions of the test tube there is iodide present in those areas.

Part III. Does Salt Sense^® Make Sense?

4. Based on the masses of salt and salt sense obtained in Table 2, what is the percent mass difference between the two?

Both are present in the same volume of 5.0 mL. The mass of the salt is 6.40 g the mass of the salt sense has a mass of 3.52 g.

5. Calculate the percent volume remaining of both regular salt and salt sense after each has been ground with a mortar and pestle.

Salt—originally 5.0 mL, after grinding 4.9 mL

4.9/5.0 • 100 = 98% volume remaining

Salt Sense—originally 5.0 mL, after grinding 3.7 mL

3.7/5.0 • 100 = 74%

6. Examine the change in volume of regular salt after being ground compared to Salt Sense calculated in Question 5. What does this relate to the fact that Salt Sense claims to contain 33% less sodium than regular salt?

Student answers may vary from sample answer based on experimental results. While the regular table salt stayed virtually the same, the Salt Sense decreased to 74% of its original volume. This indicates that around 25% of the Salt Sense product is strictly air but that it does contain other ingredients as the volume was not reduced to 67% percent of the original.

Part IV. Fruit Facts

7. How did the apples kept in the beaker containing distilled water differ from those in the beaker containing ascorbic acid after 24 hours?

The apples in the distilled water were more brown colored after 24 hours while the apples in the ascorbic acid solution looked as if they were freshly cut.

8. In this demonstration L-ascorbic acid was found to reduce the browning effects of cut apples. What other foods might benefit from treatment with L-ascorbic acid?

L-ascorbic acid would be useful in preserving most foods in which oxygen exposure causes phenol oxidase activity to result in brown discoloration. L-ascorbic acid would be expected to be useful on potato slices as well.

Discussion

Iron is an essential element in good nutrition. It must be supplied by the diet—meat and poultry are the most important natural sources of iron in the typical American diet. Because of its nutritional value, iron is added to many foods, especially breads and cereals. Iron supplied by natural food sources is in the form of iron(II), which is readily absorbed by the body. In contrast, most iron-fortified foods supply iron in its fully reduced or metallic form, Fe(0). Elemental iron must be oxidized by the stomach before it can be absorbed by the body. The reaction of iron with hydrochloric acid (Equation 1) simulates the process that occurs when elemental iron dissolves in the acidic contents of the stomach.

Iodine is an essential nutrient. It is required by the body for the synthesis of the thyroid hormone thyroxine. The primary symptom of iodine deficiency is a goiter, an enlargement of the thyroid gland. The widespread use of iodized table salt has largely eliminated this health problem in society. The presence of potassium iodide in iodized salt is identified by the oxidation of iodide ion to iodine (Equation 2). Iodine is detected by means of its familiar blue-black complex with starch.

Upon completion of the Salt Sense demonstration students will clearly see why Salt Sense claims to contain less sodium than regular table salt. The predominant reason Salt Sense contains less sodium than regular table salt is evident when the volumes are measured upon grinding. Initially, both salt and Salt Sense have the same volume of 5.0 mL. Upon grinding the salt stays nearly the same while the Salt Sense typically decreases in volume by 20–25%.

Why do apples and potatoes turn brown shortly after being sliced? The cells which make up apples and potatoes contain enzymes that do work required for a cell to live. Once fruit is cut certain enzymes will turn the fruit brown when exposed to air. The brown color is a result of oxidation of phenolic compounds due to the action of the polyphenol oxidase enzyme. Ascorbic acid is often used to prevent discoloring of several foods. Ascorbic acid is added to deionized water, typically in concentration of 1–2%. This decreases the pH of the solution therefore inhibiting phenol oxidase activity.

References

We would like to thank Kathleen Dombrink of McCluer North High School in Florissant, MO, for providing us with the idea for Part III of this activity.

Parts I and II of this activity were adapted from Chemistry of Food, Flinn ChemTopic™ Labs, Volume 23; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2003).

Shimi, N. M. 1993 Control of enzymatic browning in apple slices by using ascorbic acid under different conditions. Plant Foods Hum Nutr.. University of Alexandria. http://www.ncbi.nlm.nih.gov/pubmed/8464847 (Accessed June 2008)