Homework answers / question archive / Fruit turns brown when exposed to air because a reaction is happening when a cut piece of fruit is exposed to oxygen

Fruit turns brown when exposed to air because a reaction is happening when a cut piece of fruit is exposed to oxygen

Biology

Share With

---

Fruit turns brown when exposed to air because a reaction is happening when a cut piece of fruit is exposed to oxygen. This is called enzymatic browning. The name enzymatic browning comes from the fact that an enzyme located in the fruit reacts with oxygen from the air to turn the fruit brown.

The general reaction can be simplified to:

Polyphenol oxidase + O2 → melanin (brown color)

Oxygen activates the compound polyphenol oxidase in the fruit to turn the fruit brown. Polyphenol oxidase is the enzyme.

In this experiment you will perform a controlled experiment where you treat apple slices in various conditions and gather data on how much the apple turned brown. If you do not have the ingredients to conduct this experiment at home, see if you can reach out to one of your classmates and borrow their data in order to analyze it. I will set up a discussion board to help with this. Be sure to properly reference that individual.

 

For this experiment, use your information about bonds found in protein structure and be able to hypothesize/predict what you may expect to see for the following treatments of apple slices compared to the untreated control:

Adding lemon juice or vinegar

Adding salt

Covering with plastic wrap

Keeping at room temperature vs. in a refrigerator

 

If you're a parent, you might consider having your child(ren) help as a fun project!

 

Materials:

Apple

Knife

Lemon juice or vinegar

Salt water solution (approximately 1 teaspoon in ¼ cup water)

Plastic wrap

Tongs or other utensil

Plates

Bowls

Paper towels

Timer

 

Procedures:

1. Prepare the solutions. Note that you only need a small amount of each, it is sufficient to just wet the exposed surfaces of the apple, i.e., you may wet the one side of the apple slice, flip it over, and then wet the other side.
2. Cut off both ends of the apple and then cut the apple into five slices (control, lemon juice, salt, plastic wrap, refrigerator).
3. Set one untreated apple slice at room temperature (this is your control).
4. Set one untreated apple slice into your refrigerator.
5. Wrap one apple slice in plastic wrap; try and press the wrap closely against the apple surface to exclude air.
6. Place one apple slice in the lemon juice or vinegar for 30 seconds.
7. Place one apple slice in the salt water solution for 30 seconds.
8. Be sure to label and keep track of each slice (e.g., place each on a paper plate/napkin, and write the treatment on the paper plate next to each slice).
9. Observe, photograph, and record your qualitative data after 10 minutes, 30 minutes, 1 hour, and 1 day in the table below. Use a scale of 0-5, where 0 is unchanged, and 5 is very brown. Please hold on to this information as you will be asked to submit it.

Table 1. Browning of apple slices over time for different treatments.

 

Apple slice 10 minutes 30 minute 1 hour 24 hours

 

Untreated control

Lemon juice or vinegar

Salt water

Plastic wrap

Refrigerator

Reflection Questions:

?    Is the data that you are collecting in the above table quantitative or qualitative? Explain why.

?     Which treatment had the least amount of browning? Which had the most? Why do you think you obtained these results?

?     Remember that the enzyme polyphenol oxidase is a protein! For each treatment, apply your knowledge of how temperature, pH, and salt concentration affect enzyme activity and explain why you got the results that you did. Include bonds and the levels of protein structure that you explored in Activity A in your answer.

?     How does temperature impact the rate of enzyme activity? If you were to leave the apple in the refrigerator longer, why would it eventually brown? Explain based on what you know about enzyme activity.

?     How does pH and its impact on specific types of bonds explain the results you obtained in your lemon juice treatment? Include bonds and levels of protein structure in your answer.

?     How does salt and its impact on specific types of bonds explain the results you obtained in your salt treatment? Include bonds and levels of protein structure in your answer.

?     How did the plastic wrap affect the browning reaction? Explain.

 

1. For the control and each treatment, what do you predict will happen, and why? Briefly explain and reference each relative to the control.

2. Submit your completed "Table 1. Browning of apple slices over time for different treatments" showing the results of your experiment.

3. Which treatment had the least amount of browning? Which had the most? Why do you think you obtained these results?

4. How do you think temperature, pH, salt, and plastic wrap affected the browning reaction in terms of protein structure (primary, secondary, tertiary, quaternary) and other factors? Please explain briefly