What factors affect the oxidation of apples?
Grade level(s):Elementary School (K-5), Middle School (6-8), High School (9-12), Grade 5, Grade 6, Grade 7, Grade 8, Grade 9
Subjects(s):Biology/Life Science, Chemistry
Apples and other fruit turn brown once the cut surface is exposed to air due to a reaction with oxygen in the surrounding air, a chemical reaction called oxidation.
(Advanced: Some fruits contain enzymes that facilitate oxidation and coloration of the fruit when exposed to air.)
The browning process can be slowed down or prevented in several ways, including reducing the contact with the surrounding air and using anti-oxidative chemicals such as Vitamin C.
chemical reaction, oxidation, enzyme, inactivation, acid, base, anti-oxidant, hypothesis, variable, control
What you need:
- Apples (or other browning produce)
- Instruments: Plastic knives, petri dishes or small plastic plates, pipettes, Ziploc bags, cellophane wrap, wax, labeling pens, safety goggles, stop-watches, cups, stirring rods, safety goggles,
- "Chemicals": Lemon juice, ascorbic acid solution, citric acid solution, sodium bicarbonate (baking soda) solution (see "getting ready" section of lesson plan)
Students will work in pairs or small group to develop and conduct their experiments.
The whole class will be together for the introduction of the lesson and for the groups' presentation and discussion of the results.
A regular classroom works fine. However flat table surfaces and sinks are very helpful for this lesson.
Introduction and planning of experiments: 30-60 minutes (Depending on students' grade level and previous experience designing experiments)
Conduction experiments: ~60 minutes
Presentation of results & conclusions: ~5-10 minutes per presenting group/pair
Wrap-up: 10-20 minutes
Students observe the browning of apples after cut and being exposed to air and brainstorm ideas about why this might be happening.
Students think about ways to slow down or prevent the browning effect and in teams create and conduct a simple experiment to test their ideas.
Students will be able to
- design and conduct a simple experiment to answer a scientific question
- analyze their results and use logic to draw conclusions
- list some ways in which natural oxidation processes can be slowed down or prevented
Why do some fruits and vegetables turn brown after you cut them?
The cells of apples and other produce (e.g., pears, bananas, peaches, potatoes) contain an enzyme (called polyphenol oxidase or tyrosinase) that, when in contact with oxygen, catalyzes one step of the biochemical conversion of plant phenolic compounds to brown pigments known as melanins. You see the browning when the fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals. This reaction is known as enzymatic browning and occurs at warm temperatures when the pH of the plant material is between 5.0 and 7.0. This reaction is also sped up by the presence of iron (such as an iron or rusted knife) or copper (such as a copper bowl). The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or another acids), reducing the reaction rate by storing the fruit in the refrigerator, reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide).
- Prepare a solution of Ascorbic Acid:
One crushed Vitamin C pill dissolved per cup of water. Depending on the grade level, you can also just provided the ingredients and equipment have the students make their own solution.
- Prepare a 0.1% solution of Citric Acid:
Dissolve 1g of Citric Acid in 1000ml of destilled water.
- Prepare a Sodium Bicarbonate solution
Dissolve one teaspoon (~6g) per 100ml of water for a 6% solution of NaHCO3. Depending on the grade level, you can also just provided the ingredients and equipment have the students make their own solution.
- Make some fruit salad, use lemon juice on half of it. Prepare a small dish containing fruit salad with lemon juice and one dish without for each table/group of students.
Lesson Implementation / Outline
Give each table or pair of student two bowls with fruit salad (one looking nice and fresh treated with lemon juice, one with brown fruit pieces (no lemon juice).
(If you want to simplify, just show pictures of a bowl of fresh fruit salad and browned fruit salad).
Ask students to pick which of the fruit salad servings they would prefer to eat. Call on some students and have them explain why they preferred one over the other (The one containing lemon juice looks more fresh and appealing).
Ask students why they think the salads look so different. Collect responses from students and write them on the board or a large chart paper. Suggestions might be: one is older then the other; one was covered up with foil and one wasn't; one was in the fridge and the other one was left outside; one was sprinkled with lemon juice and the other one wasn't.... If they suggest that one is older than the other, tell them that you prepared them at the same time but that you did something differently to one than to the other. Tell students that it is their challenge now to find a way to prevent fruit, more specifically apples, from turning brown.
After the intro activity have students complete the following sentence starter: "I think that I can slow down or prevent apples from turning brown by ___________________________. It will slow down or prevent the apple from turning brown because ________________________________" (see attached student hand-out). For students in lower grades, testing one possible way to prevent browning might be sufficient. Older grades could test several ways and compare their effectiveness. Show students all the available materials and ask them to design a simple experiment to test their ideas. Students should work in small groups, preferably pairs to work on this. Depending on how experienced the students are in designing their own experiments, more or less scaffolding is needed. Classroom discussions about the importance of control experiments as comparison and about what constitutes a "fair test" as well as supporting the planning process with a template (see attached for sample) might be needed.
Depending on the grade level and experience of the students, the data collection can be more or less "sophisticated". Students can simple record time at which the apples are cut into pieces and at which time they notice each of the apples beginning to brown. At certain time intervals, students can record qualitative data (darkness of color, percentage of coverage on apple, etc.) and/or quantitative data (a ranking of the six different conditions from lightest brown (1) to darkest brown (6)). Students will record their results in data tables they created or on a provided template, again depending on the situation. Another idea is to have the students take photos of the apples at certain time intervals to capture the browing progression and to compare different treatments more accurately. Students can use photos in their final presentations and/or in their lab report (if done).
- Before the experiments: Check in with student teams to discuss their experimental plans.
- Have teams share out their findings to the whole class. This can be done with posters the students created or just verbally. Encourage "audience" to ask clarifying questions and point out flaws in the experimental set-up if applicable.
- Have students respond to a reflection prompt: "Based on the class experiments, what is the most effective way to prevent browning of fruit? Explain why."
After all teams presented their findings, have students summarize what the most effective way to prevent browning in fruit is and why.
Talk about other oxidation process, for example the rusting of a nail and explain that all of these processes include the reaction with oxygen.