Ask Dr. Universe – Why are ripe fruits sweet?

Dear Dr. Universe: Why are ripe fruits sweet and why is it so important?
-Alexa, Schenzhen, China

Dear Alexa,

My friend Kate Evans said the answer really depends on whether you want the perspective of a person, a plant, or even a cat. Evans is a plant scientist at Washington State University in Wenatchee, where she investigates fruit in the Apple Capital of the World.

She explained how long ago, wild apples actually grew in forests. Without farmers around to plant them in orchards, trees had to scatter their own seeds to survive.

For some trees, the key to survival is growing sweet, ripe fruit. This way they can get forest critters to help spread their seeds.

Bears, for example, were drawn to the apple’s bright colors and sweet taste. Fruit on the outer edge of the tree ripens quickest, so that made it even easier for bears to get to the good stuff. Evans said they would eat the apples and carry the seeds across the forest floor in their bellies.

“The seed came out the other end of the bear, and hence, a new tree could grow,” she adds.

Moving the seed was really important for a tree’s healthy start, so it wouldn’t compete with the mother tree for water or sun.

“It’s a very smart move by the plant,” said Amit Dhingra, my friend and a WSU plant scientist who researches cherries, apples, pears, and all kinds of fruits.

Dhingra said plants are the reason there is life on the planet. Plants have the machinery to convert energy from the sun into their own food and give us the oxygen we need to breathe. Plants and their fruit can also give the human body energy. Dhingra encourages people, and especially growing kids, to eat more of it.

As you eat fruit and other food, a chemical in your saliva makes it easier to digest. The chemical is called amylase.

Fruit has amylase, too. It breaks down starches in the flesh into sugar. That is how fruit can ripen.

To tell how ripe a piece of fruit is, scientists can measure its sugar content. They can cut an apple in half and test it with purple iodine. When an apple is sweet, the purple iodine won’t show up. That’s when they know they have a ripe one.

Humans, and most mammals, can experience sweetness because of their tongues. Most mammals, including humans, have a special combination of two taste receptors in their taste buds. Scientists actually discovered we cats don’t have them. We can taste sour, though.

Not all fruits are sweet. If you’ve ever bitten into a lemon you know it’s a whole other experience. But some insects and animals have their own unique tastes and enjoy lemons, which helped the trees survive.

So, sweet fruit is sweet because plants are actually pretty smart. Using their ripe fruit, trees can move their species forward without ever picking up their roots.


Dr. Universe

Got a question? Ask Dr. Universe. You can send her an e-mail or visit her website at