Question Answered by Alana Jensen, Environmental Educator, INL ESER Program
Apples on the tree all start out green. The green color of apples is due to a pigment called chlorophyll. Chlorophyll is also present in leaves and helps the plant convert sunlight into energy so it can grow.
At the height of photosynthesis, during the summer when the days are long, plants continually make chlorophyll. The green from the chlorophyll covers up any other color that may be in the leaf. However, when the days become shorter and the temperatures become cooler, trees stop producing chlorophyll.
Without chlorophyll present, the apples (and leaves) change color as other pigments are exposed. Anthocyanins are red pigments and are produced only in the fall when the temperatures are cool and the days are short. Anthocyanins can also be found in cranberries, cherries and strawberries.
Granny Smith and other green apples, do not produce red pigments at all and they stay green because the chlorophyll is much slower to break down. In Golden Delicious apples the chlorophyll does break down and they turn yellow, because of another group of pigments called carotenoids. This also happens in the leaves of many trees in autumn.
Anthocyanins are water soluble pigments that change color with pH. The lower the pH level, the redder the pigments get. Neutral pH levels, turn the anthocyanin green. More alkaline pH levels turn anthocyanin purple. Anthocyanins are found in apple skins, plums, poppies, red cabbage, cornflowers, blueberries, and grapes. Can you think of other red, blue or purple flowers, fruits and vegetables that contain anthocyanins?
Let’s make a pH indicator with anthocyanins.
Materials: several red apples, knife or peeler (and an adult helper), water, pan, stove top, coffee filter, jar, spoon, eye dropper, vinegar, laundry detergent, saucer
- Peel the apples. The anthocyanin is primarily in the fruit’s skin, so separating the white pulp to retain the skin will produce better results.
- Add a small amount of water. High concentration is key, so try to use as little as possible.
- Boil the mixture of apple peels and water about five to 10 minutes. If it starts to dry up, add a little more water. Remove the mixture from the heat and allow it to cool.
- Pour the mash into the coffee filter and collect the liquid in the small jar. You may need to compress the filter gently with the spoon to get more liquid out of it.
- Extract some of the liquid from the jar, using the eyedropper. Deposit a pool of the liquid about the size of a quarter onto the saucer.
- Flush the eyedropper with water before using it to apply a few drops of vinegar to the mixture. The acid in the vinegar should turn the red juice to a lighter color, which would be slightly pink.
- Rinse the saucer and repeat steps 5 and 6, but this time use laundry detergent instead of vinegar. Laundry detergent is a base, so it should turn the reddish solution purple or dark blue.
- Try this experiment with another anthocyanin-rich plant. Would this experiment work with red cabbage, tomatoes, or red rose petals?