Why Do Apple Slices Brown If Left Out In The Air?

When fruits or vegetables are peeled or cut, enzymes contained in the plant tissue are released. In the presence of oxygen from the air, the enzyme polyphenol oxidase (phenolase) catalyzes one step in the biochemical conversion of plant phenolic compounds to brown pigments known as melanins. This reaction, called enzymatic browning, occurs readily at warm temperatures when the pH is between 5.0 and 7.0.

The presence of iron or copper can increase the rate of reaction. This can be easily observed when fruit is cut with a rusty knife or mixed in a copper bowl.

Bruising or other injury to the plant tissue disrupts the structural arrangement of constituents within the cells and allows the contents to make contact with oxygen. This may lead to browning of uncooked fruit tissue.

Enzymatic browning can be a significant problem, limiting the shelf life of many fruits and vegetables which have had little heat applied during processing. However, enzymatic browning is not always a defect. The browning reaction contributes to the desirable color and flavor of raisins, prunes, coffee, tea, and cocoa.

Several substances have been used in the food industry to prevent browning of fruits and vegetables. They are: sulfites, ascorbic acid (vitamin C), citric acid and acetic acid (vinegar), and fresh water.

Let's monitor the level of enzymatic browning of apple slices with the following experiment.

Which Substance Works Best To Slow Apple Browning?

Materials You Will Need:

Fresh apple slices

Test solutions for dipping: 0.1% ascorbic acid, 0.1% citric acid, 0.1% acetic acid, and 1.0% acetic

acid

Cup of water

Tongs

Paper towels

Your Procedure:

1. Place an untreated apple slice on a paper towel. Label the towel "Control."

2. Using tongs, dip another apple slice into one of the test solutions for 30 seconds, place it on the towel, and label the towel with the name of the solution. Rinse the tongs and repeat the same procedure for the other three solutions.

3. Soak one slice in water for 30 seconds. Place it on a towel and label the towel "Water Soak."

4. Note the time and temperature in your data table. Observe the slices every 10 minutes for one hour and record your observations.

The Result:

Apple slices treated with citric acid and acetic acid brown the least.

Your Conclusion:

Citric acid and acetic acid retard the browning of the apple slices because they lower the pH which decreases the phenolase activity. Citric acid also ties up copper ions and prevents them from participating in the reaction. Ascorbic acid prevents oxygen from reacting with the phenolase. Soaking in water alone temporarily reduces the level of browning by restricting the amount of oxygen in contact with the apple tissue.

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