Enzyme Lab: What Stops an Apple from Browning?

A cut apple turns brown in minutes. Find out which treatments slow the change, and use enzyme structure to explain why.

30–45 minutes plus observation
Groups of 3–4
Very low cost
Biology BIO.1B.2
Foundations FB.2.6

Question

Which treatments slow enzyme browning in cut apple, and what do they do to the enzyme?

Background

When you cut an apple, you break open cells. An enzyme called polyphenol oxidase meets chemicals in the apple and oxygen from the air, and the reaction makes brown pigments. Anything that slows the enzyme, changes its shape, or keeps oxygen away can slow browning. Temperature and pH both affect an enzyme's shape, so they are good variables to test.

enzyme
substrate
denature
pH
independent variable
dependent variable
control

Plan your investigation

Choose four treatments plus an untreated control. Good choices: water, lemon juice (acid), diluted vinegar (acid), baking soda solution (base), a slice dipped in hot water, a slice kept in the refrigerator. Write a hypothesis in the form If …, then …, because ….

Independent variable: the treatment. Dependent variable: browning score. Keep constant: kind of apple, size of slice, how long each slice is treated, room and light.

Browning score
Score	What you see
0	No color change from the fresh cut
1	A few light tan spots
2	Light tan over about a quarter of the surface
3	Tan to light brown over about half the surface
4	Brown over most of the surface
5	Dark brown over the whole surface

Safety

Do not taste any of the apple slices or solutions. Check for food allergies before the lab.
Use plastic knives or slices your teacher cut ahead of time. Keep hot water away from the edge of the table.

Materials

Apple slices of the same size (about 8 per group)
Cups or small bowls, labels, and a marker
Water, lemon juice, vinegar, baking soda solution (as chosen)
Hot water (not boiling) and a refrigerator or ice
Timer, paper towels, and a camera or phone for photos (optional)

Procedure

Label a cup for each treatment and for the control.
Place one slice in each cup. Cover it with its treatment liquid for 1 minute. For the hot-water slice, dip it for 30 seconds. For the refrigerator slice, put it on a labeled plate in the refrigerator.
Remove the slices, blot them, and lay them cut-side up on labeled plates.
At 0, 15, 30, and 60 minutes, score the browning from 0 to 5 using the color scale. Take a photo of each plate at each time if you can.
Record your scores in the data table.

Data

Rename the rows on your own paper with the treatments you chose.

Analysis

Which treatment slowed browning the most? Use your scores as evidence.
What did your untreated control show, and why do you need it?
Acids lower the pH. Explain how a change in pH could slow an enzyme, using the words shape and denature.
A slice dipped in hot water may brown less. What happened to the enzyme?
The browning reaction needs oxygen. How could you slow browning without changing the enzyme?
Write a claim-evidence-reasoning answer for the best way to keep apple slices fresh.

Teacher notes

Preparation and tips

Use one kind of apple, such as Gala or Red Delicious, and cut all slices at the same time.
Browning scores are subjective. Have each group agree on one scorer, or use phone photos and the color scale.
Some treatments slow browning by changing pH, and some also react with oxygen first. Vitamin C (crushed tablet in water) is a good extension, but it works by a different chemical route than pH, so discuss that difference.
Hot water should be hot, not boiling, so students do not get burned.

What to expect

The untreated slice browns steadily. Lemon juice and diluted vinegar slow browning noticeably. Baking soda solution often does not.
The hot-water slice and the refrigerated slice both brown more slowly, for different reasons: heat can denature the enzyme, and cold slows the reaction.

Alignment

Biology BIO.1B.2 and Foundations FB.2.6 (enzymes). Science and engineering practices: plan and conduct investigations; analyze and interpret data.