Students will learn the concept of pH, and the impact of acids and bases on plant pigments, explore the impact of acids and bases on plant cell structure, and discover the health benefits of consuming vegetables. Grades 6-8

acid: a substance that ionizes in water to produce hydrogen ions (H )

base: any substance that in water solution is slippery to the touch, tastes bitter, changes the color of indicators (e.g., turns red litmus paper blue), reacts with acids to form salts, and promotes certain chemical reactions (base catalysis)

chemical change: a change that results in the formation of a new chemical substance through the making or breaking of bonds between atoms

pH scale: a scale that ranges from 0 to 14 indicating the level of acidity (concentration of H ions) with substances closer to 0 being more acidic, substances closer to 14 more basic, and substance close to 7 being neutral

physical change: a change in a substance that does not alter its chemical identity, including changes in shape, physical state, size, or temperature; this type of change is usually reversible

phytochemicals: the broad term for a chemical compound that is naturally produced by plants

pigment: the chemical compound that produces color

FoodMASTER (Food, Math and Science Teaching Enhancement Resource) is a compilation of programs aimed at using food as a tool to teach mathematics and science. For more information see the Background & Introduction to FoodMASTER. This lesson is one in a series of lessons designed for middle school. 

Most Americans do not eat the variety or amount of vegetables they need for healthy living. For this reason, it is important to learn about the science and nutrition of cooking vegetables. In this chapter, students will explore vegetables by learning about pigments, the physical and chemical changes that occur when acids and bases are cooked with vegetables, and the many health benefits associated with eating vegetables.

This lesson is separated into two parts. In Part A, students will put cabbage juice indicator in two unknown samples. Using a pH chart, students will then determine if the samples are acids or bases. In Part B, students will view the cabbage indicator results, determine pH using a chart, and classify additional household substances as acids, bases, or neutral. They will also identify the two unknowns.

Acids and bases are determined by the gain or loss of hydrogen ions on the pH scale. The pH scale ranges from 0-14. A pH of 7 is considered neutral, a pH below 7 is an acid, and a pH above 7 is a base. Acids lose hydrogen ions while bases gain hydrogen ions. Cooking in acidic or alkaline solutions can affect the color and structure of a vegetable. Depending on the pigment the vegetable contains, its color may or may not change when cooked in an acidic or basic solution. For example, when a red cabbage is cooked or heated in a liquid, the color of the liquid can indicate pH. If an acid is added the the liquid, the color will turn red; however, if a base is added to the liquid, the color will turn blue.

The most common base used in cooking is baking soda. Its primary function in cooking is to leaven and raise baked products. When green vegetables are cooked in baking soda, the color will become bright green. This occurs because alkaline substances break down chlorophyll into chlorophyllin. Chlorophyll is the pigment found in green vegetables. The most common acid used in cooking is cream of tartar. Its primary function in cooking is to activate baking soda or to stabilize egg whites. Cream of tartar and baking soda are the components that make baking powder. Other acids used in cooking include vinegar and lemon juice, but these foods can greatly impact taste. Cooking green vegetables in an acid can cause the color to become dull green and the texture to become mushy. This occurs because acidic substances break down chlorophyll into pheophytin. Pheophytin is the gray-green color of chlorophyll after it has been broken down. Typically acids are not recommended when cooking green vegetables due to its negative effect on texture and color. Acids and bases also affect purple, red, and white vegetables containing the flavonoid pigment. The effect of acids and bases on this pigment, however, is opposite of that on chlorophyll. Orange and yellow vegetables containing the carotenoid pigment are not affected by cooking with acids and bases.

The color of the vegetable is also a sign of the phytochemical it contains. Phytochemicals may function as antioxidants or anticancer agents. Vegetable pigments provide many benefits for the body because of their phytochemical content. Some red vegetables, such as tomatoes, contain lycopene. Lycopene may reduce the risk of cancer and protect the heart and lungs against diseases. Flavonoids are found in red/purple vegetables and research has found that they may reduce the risk of heart disease, cancer, blood clots, and stroke. Some examples of red/purple vegetables are red cabbage and red bell peppers. Orange vegetables, like carrots and sweet potatoes, contain beta-carotene, which can help your immune system, protect your eyes, skin, and bones, and prevent heart disease. Lutein is found in yellow/green vegetables, such as corn, green beans, spinach, and green bell peppers. Lutein helps keep your eyes and heart healthy.

Green vegetables include broccoli, Brussels sprouts, cabbage, kale, and cauliflower. These green vegetables contain indoles, which may help to protect against cancer. Onions and other white vegetables contain allicin, which also may help to prevent cancer. Some red or yellow vegetables, such as beets, contain betalains. Researchers have found that betalains provide anti-inflammatory and detoxification properties, which helps to prevent disease. Orange vegetables, such as carrots, contain carotenoid pigments. Carotenoids may play a role in improving vision and aiding in the reduction of eye disease. White vegetables, such as onions, contain anthoxanthin pigments. Anthoxanthins may help lower cholesterol and blood pressure. Vegetables, such as asparagus, contain chlorophyll, which are green pigments. Research has shown that chlorophyll can provide cancer-fighting properties. It has been found to treat and prevent certain types of cancer when used as chlorophyllin. Chlorophyll is converted to chlorophyllin when cooked in an alkaline solution resulting in a bright green appearance. Vegetables, such as red cabbage, contain anthocyanins, which include red-purple and blue pigments. Anthocyanins are most notably known for their circulatory benefits. Research has found a relationship between anthocyanins and a reduction in blood pressure and prevention of cardiovascular diseases.

1. Ask students to brainstorm and list common vegetables that we eat. After several vegetables have been listed, ask the following questions:
   • What classifies a vegetable?
   • What are some different ways vegetables are prepared to eat? (raw, boiled, steamed, roasted, etc.)
   • In terms of chemistry, what is an acid and a base?
   • Are there any common cooking ingredients that are either an acid or a base?
   • Are there any cooking methods (with or without the addition of other ingredients) that change the availability of the nutrients found in vegetables?
2. Give each student one copy of the Fabulous Phytochemicals student handout. This assignment can be completed in class or as a homework assignment. See the attached Fabulous Phytochemicals Teacher Key for answers to the Investigating Your Health lab questions If completed in-class, allow students to work in small groups on the to explore the topic and respond to questions.
3. Follow-up with a class discussion about student findings related to the health benefits of vegetables and student generated ideas for increasing vegetable consumption.

Lab 1: Exploring Acids & Bases

Teacher Preparation:

1. Review information found in the Background Agricultural Connections section of the lesson, lesson Procedures, and the attached Essential Files.
2. You will need to produce at least 200 mL per group of students (Part A), plus an additional 1000-2500 mL for Part B. Boil shredded red cabbage in large pots for 5-10 minutes with 2.5-4 liters of water and pour liquid into pitcher (this will be the pH indicator). If you do not have a pot large enough to hold 2.5-4 liters of water, the indicator can be made in smaller batches. The water should be a deep purple color. Remember to save some cabbage for tasting!
3. Part A - Prepare the following for each group:
   • Plastic cup with Unknown Sample A- put in 1/2 Tbsp. baking soda (label cup “Unknown Sample A” with permanent marker)
   • Plastic cup with Unknown Sample B- put in 50 mL vinegar (label cup “Unknown Sample B” with sharpie)
     • Timesaver: To save time, pre-measure 200 mL of cabbage juice for student groups ahead of time in a beaker or jar.
4. Part B - Prepare the following for the class:
   • 2 or 3 labeled plastic cups, beakers, or jars containing 1 tsp. of cream of tartar and 100 mL of cabbage juice
   • 2 or 3 labeled plastic cups, beakers, or jars containing 1 tsp. baking soda and 100 mL of cabbage juice
   • 2 or 3 labeled plastic cups, beakers, or jars containing 1 tsp. of salt and 100 mL of cabbage juice
   • 2 or 3 labeled plastic cups, beakers, or jars containing 50 mL of vinegar and 100 mL of cabbage juice.
     • TIP: The number of observational containers needed depends on class size. A minimum of two is recommended. Preparing the containers may be a class demonstration. If time and materials allow, Part B may be done as a lab investigation by providing each student group with 4 labeled clear plastic cups containing the baking soda, cream of tartar, salt and vinegar, 500 mL beaker or jar containing 400 mL of cabbage juice, 1 – 100 mL graduated cylinder or measuring cup, and 6 plastic spoons.
5. See slideshow for pictures illustrating the lab and teacher preparation steps.

Lab Procedures:

1. Distribute lab materials. It is recommended that materials are organized into stations for easier distribution. Students should be arranged in small groups of 4-5. Each group should receive the "Part A" lab supplies outlined in the Materials section as well as 1 copy of the Exploring Acids and Bases lab sheet and the Cooking With Chemistry handout.
2. Ask students to read Cooking with Chemistry and complete the focus questions in the "Think About It" section.
3. Before beginning the lab investigation:
   1. Require students to wash their hands.
   2. Allow students to taste a sample of the red cabbage prior to beginning any investigation procedures. This process is important for increasing student exposure to healthy foods and decreasing the likelihood that students will be tempted to taste foods included as investigation materials.
   3. Emphasize the importance of practicing good food safety behaviors by not consuming substances used as part of the lab investigation.
4. Launch Part A of the lab by asking students to observe and respond to the lab question. Follow specific investigation procedures outlined in the Exploring Acids and Bases lab sheet. Students should see the following results:
   • Sample B: When vinegar is mixed with the indicator (cabbage juice), the solution should turn ruby red.
   • Sample A: When baking soda solution is mixed with indicator, the solution should foam and turn a bluish color. The color change may be faint and similar to the purplish color of the pH indicator. Encourage students to hold the jar up to a light source to determine a true color change.
5. Allow students to work in small groups on the lab sheet to further explore the topic and respond to lab questions. After completing initial acid/base observations and conclusions, students should be prepared to begin Part B.
6. Part B should begin by showing students the provided video lab demonstration (Part I: Exploring Acids & Bases Demonstration Video) or allowing students to observe the pre-prepared containers that contain pH indicator mixed with cream of tartar, baking soda, salt, or vinegar. See lab sheet for specific investigation procedures for students. 
7. Distribute additional materials needed for Part B and allow students to work in small groups to finish the Exploring Acids and Bases lab sheet.
8. Follow-up with a class discussion about pH and the impact of various acids and bases on pigments.

Lab 2: Cooking with Acids and Bases

Teacher Preparation:

1. Review information found in the Background Agricultural Connections section of the lesson, lesson Procedures, and the attached Essential Files.
2. Cut broccoli, carrots, and white onion into small pieces for each group.
3. Make a plastic sandwich bag of raw vegetables containing 1 sprig (small shoot) of broccoli, 2 small carrots, and 1 piece of white onion for each group.
   • (Optional) Make a plastic sandwich bag of very thinly sliced onion (to be used for lab extension). To cut, lay a piece of onion flat on a cutting board. Run the blade of a knife across the top of the onion to slice a thin piece (with the grain).
4. Begin boiling vegetables before or at the start of the lesson. Boil vegetables for 5-10 minutes. Vinegar (4 tbsp. - acid) and baking soda (1/4 cup – base) should be added to one pot of vegetables each before boiling.
   • Tip: Vegetables may be cooked the day before. Cover and refrigerate overnight.
5. Prepare samples of each vegetable type for students to observe. Place cooked vegetable samples into each of the bags labeled “acid” and “base”. If possible, refrigerate samples until you are ready to use them for observational purposes.
   • Tip: One set of vegetables cooked in vinegar and baking soda is recommended; however, if your class size is smaller or larger, a different number may be needed.
   • Timesaver: Paper plates with cooked vegetables for each group can be prepared before class ahead of time. Cover with plastic wrap and refrigerate. Another option is to have the first class prepare the paper plates as indicated by the directions and save the plates for use in other classes. Both of these methods will save on the amount of vegetables needed.

Lab Procedures:

1. Distribute lab materials. It is recommended that materials are organized into stations for easier distribution. Students should be arranged in small groups of 4-5. Each group should receive the lab supplies outlined in the Materials section as well as 1 copy of the Cooking with Acids and Bases lab sheet.
   • Note: Be sure students have completed the Cooking with Chemistry handout as outlined in Lab 2.
2. Before beginning the lab investigation:
   1. Require students to wash their hands.
   2. Allow students to taste a sample of raw broccoli, carrots, and onion prior to beginning any investigation procedures. This process is important for increasing student exposure to healthy foods and decreasing the likelihood that students will be tempted to taste foods included as investigation materials.
   3. Emphasize the importance of practicing good food safety behaviors by not consuming substances used during the lab investigation.
3. Launch the lab by showing students the video lab demonstration, Part II: Cooking with Acids & Bases Demonstration Video. 
4. Allow students to make predictions about what will happen to each vegetable when cooked in acidic and basic solutions. Follow exact procedures as outlined in the student lab sheet.
5. Pass out plates containing cooked vegetables and allow students time to observe the cooked vegetable samples and work in small groups on the lab sheet.
   • Tip: Students may be directed to cover the cooked vegetable plates with plastic wrap so that the plates can be used with other classes.
6. For conclusion question #4, pass out a copy of the Red Cabbage and Apples Recipe. Instruct students review it prior to answering the question.
   • (Optional) Launch the lab extension by allowing students time to observe a plant cell (onion) under the microscope. Students should examine the plant cell for the nucleus, cytoplasm, and cell wall.
7. Follow-up with a class discussion about the impact of various acids and bases on cell structure.

After conducting these activities, review and summarize the following key concepts:

• Vegetables provide valuable nutrients for a balanced diet.
• Methods of cooking vegetables (and other foods) utilize principles of science and chemistry.
• Acids and bases are determined by the loss or gain of hydrogen ions on the pH scale.
• Vegetables contain phytochemicals which serve as antioxidants or anticancer agents.

1. http://www.lifehack.org/articles/lifestyle/20-incredible-facts-about-eating-fruits-and-vegetables-that-you-probably-didnt-know.html
2. https://www.quora.com/What-are-some-interesting-facts-about-vegetables

This lesson was partnered with East Carolina University. The FoodMASTER program was supported by the Science Education Partnership Award (SEPA) which is funded from the National Center for Research Resources, a component of the National Institutes of Health.

• Primary Authors:
  • Virginia Stage, PhD, RDN, LDN
  • Mary White
  • Ashley Roseno, MAEd, MS, RDN, LDN
  • Melani W. Duffrin, PhD, RDN, LDN
• Graphic Design: Cara Cairns Design, LLC

• Eat Happy Project video series
• Interactive Map Project
• Nutrition Research Articles
• Producepedia
• Vegetables Flowchart
• Yum! Fruit and Vegetable Wraps