Understanding Enzymes in Apples and How to Prevent Browning

What Causes Apples to Brown?

Have you ever cut an apple, only to see it turn brown within minutes? This process is called enzymic browning, a chemical reaction that occurs when oxygen interacts with specific enzymes in the fruit. When an apple is cut, oxygen enters the damaged plant tissue. Inside the apple cells, polyphenol oxidase (PPO) enzymes, located in the chloroplasts, react with naturally occurring phenolic compounds. This reaction produces o-quinones, which subsequently react with amino acids, forming brown-colored melanins.

Different apple varieties contain varying levels of PPO enzymes, meaning some brown faster than others. While this natural process does not make the apple unsafe to eat, it can affect the fruit’s appearance, texture, and taste, leading to significant food waste.

How Can We Prevent or Slow Down Apple Browning?

To slow down enzymic browning, we must either reduce PPO oxidation activity or lower the availability of the substrates (oxygen and phenolic compounds) that the enzyme acts upon. It is very common for growers to put harvested apples into cold storage where they limit oxygen exposure. Our titratable acidity test is actually utilized to monitor titratable acidity in apples and inform harvest and storage decisions to help ensure when it gets to the consumer it is crisp and fresh!

One effective approach is using acidic substances, which lower the pH and make PPO enzymes less active. Many fruit juices, such as lemon and pineapple juice, contain antioxidants and acids that help delay browning. Another method involves using substances that create a barrier between the apple surface and oxygen, such as sugar or salt solutions.

Experiment: Testing Different Methods to Prevent Apple Browning

To determine the most effective method for slowing enzymic browning, we can conduct a simple experiment using different solutions.

Materials

  • Apple slices
  • pH test strips (we recommend using a strip specifically for testing acidic solutions)
  • Several fruit juices (e.g., lemon, lime, pineapple, orange, white grape, apple juice)
  • Vinegar
  • Tap water
  • Plastic cups
  • Spoons
  • Paper plates
  • Cutting board
  • Knife (adult supervision recommended)

Procedure

  1. Prepare Solutions: Pour samples of each fruit juice and vinegar into labeled cups. Prepare salt and sugar solutions by dissolving 1 teaspoon of salt or sugar in ¼ cup (60 mL) of water.
  2. Cut Apple Slices: Slice an apple into uniform pieces.
  3. Treat Apple Slices: Place each slice into a different cup for about 5 minutes.
  4. Observe Changes: Remove the slices and place them on corresponding labeled plates. Set one slice aside as a control (no liquid treatment).
  5. Monitor Browning: Observe and record changes at intervals (10 minutes, 1 hour, 3 hours, and beyond) to determine the effectiveness of each treatment.
  6. Check pH Levels: Use pH test strips to measure the acidity of each solution and record the results.

Results and Discussion

  • Acidic Solutions: Lemon juice and vinegar, with their low pH levels, are likely to be the most effective in preventing browning. The acid alters the enzyme structure, reducing its activity.
  • Salt and Sugar Solutions: Salt can interfere with enzyme function by breaking up protein structures, while sugar forms a protective coating that limits oxygen exposure.
  • Water: Submerging apple slices in water can reduce oxygen exposure, slowing browning to some extent.

Conclusion

Enzymic browning is a natural oxidation reaction that can be slowed down by modifying pH levels, blocking oxygen exposure, or interfering with enzyme function. Based on this experiment, acidic solutions such as lemon juice are among the most effective ways to maintain the freshness of apple slices. Understanding these simple preservation techniques can help reduce food waste and keep fruit looking and tasting fresh longer.

Next time you prepare apple slices for a snack or a recipe, try using an acidic fruit juice to keep them looking crisp and delicious!