Kids’ activity: Can you stop a banana from turning brown?

Let’s Talk Science and the Royal Society of Canada have partnered to provide Globe and Mail readers with relevant coverage about issues that affect us all – from education to the impact of leading-edge scientific discoveries. Let’s Talk Science offers a number of fun activities to get youth engaged in STEM. These hands-on activities encourage active learning and discovery using materials commonly found at home.

Explore the chemistry behind why bananas turn brown. Can chemistry also help prevent this?

What You Need

● Lemon Juice (1/2 cup)

● Chewable Vitamin C tablet

● Water (1 cup)

● Spoon

● 3 Plastic Cups

● Variety of fruit: apple, banana, peach or pear

● Knife (use with adult supervision only)

● 3 Plates

● Pastry Brush

What to Do

*Safety first! Use caution with sharp knives while cutting fruit. Check with your parent/guardian first.

• Crush the Vitamin C tablet with the back of a spoon and dissolve it in half a cup of water.
• In two separate cups, pour half a cup of water and half a cup of lemon juice.
• Set cups aside along with the cup from Step 1 and let all three liquids come to room temperature.
• With a parent’s permission or help, cut up one or more types of fruit into pieces of nearly equal size.
• Quickly put some of each fruit on three separate plates and label the plates with the words “water”, “lemon” and “Vitamin C.”
• Using a pastry brush, paint the cut surfaces of the fruit with water, lemon juice or the vitamin C solution - according to the label on the plate.
• Arrange the fruit so that the cut surfaces are exposed to the greatest amount of air.
• Let stand at room temperature for an hour or more and observe what happens to the fruit on each plate.

What do you think the fruit on each plate will look like? Which liquid did the best job at keeping the fruit from turning brown?

What’s happening?

The cells making up the fruit contain many chemicals, some of which are called aldehydes. When you cut the fruit, many of the cells are damaged and they release aldehydes. When the aldehydes mix with the oxygen in the air they turn brown, and thus change the colour of the cut surface of the fruit.

Vitamin C, also known as ascorbic acid, is found naturally in citrus fruits, such as lemons and oranges. It can also be made artificially and put into vitamin pills. Either way, the chemical that slows down the oxidation of cut fruit is the mighty vitamin C! It is often added to foods as a preservative to interfere with the natural process of browning.

Why does it matter?

Compounds like vitamin C that slow down the oxidation reaction that we can observe on cut fruit can also slow down oxidation in the human body! Oxidation reactions can damage cells, proteins and molecules in plants, animals, and people. Getting enough vitamin C (ascorbic acid) can slow or prevent these oxidation reactions from happening. This is one of the reasons why vitamin C is so important. Ascorbic acid is often used in food manufacturing as a food preservative.

Investigate further

● Explore the effect of putting fresh fruit into other liquids, such as orange juice, apple juice, milk, water, soda water and other types of pop. What liquids do a better job at preventing the fruit from going brown? What liquids would you rather use to keep your fruit from browning? Why?

● Look for different acids on labels of packaged foods that come from the grocery store. What types of acids do you find listed on labels? Find out why these are included.

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