When you bite into and enjoy the sweet taste of an apple, you are eating what was once a starch, or carbohydrate. The carbohydrate is converted into sweeter-tasting sugar as your apple ripens and matures. The effects of starch on apples determine when they're ripe and ready to eat.
Starch is categorized as a carbohydrate, which means it can be broken down into smaller sugar components. If you write out the chemical composition of a starch molecule, it has repeating glucose -- starch -- units. Apples aren’t the only food that contains starch. Potatoes, corn and wheat all have starch as a component. Starch is similar to another chemical, cellulose, found in fruit and vegetable peels, like the peel of an apple. Like starch, cellulose gives foods a crunchy, fibrous texture.
You can tell how ripe an apple is by how much starch it contains. A very ripe apple will contain little starch, while a young apple will be mostly starch, particularly when compared to sugar. As an apple ripens, the starch is converted into sugar, so the fruit grows more palatable as it ripens. The conversion process starts at the apple’s core, then progresses outward to the apple’s skin.
You can tell the level of starch in a particular batch of apples by conducting a simple iodine test. Cut an apple in half, then brush brown iodine, which is available at most health food and medical supply stores, over the apple. You should notice the brown color starting to turn purple in some areas. The purple portions indicate the presence of starch in the apple. If your apple has few regions of purple starch, this indicates ripeness, according to Nova Scotia Apples.
The presence or lack of starch can affect an apple’s flavor. An apple that has not had sufficient time to ripen may lack sweetness because more starch is present than sugar. As an apple matures, the increasing amount of sugar will cause a sweeter flavor. This does not mean apples are a significant source of sugar, however. Apples rate rather low on the glycemic index that measures sugars in food, particularly when compared to high-sugar fruits. Apples have a glycemic load of 6 per serving, which is very low compared to raisins, which have a glycemic load of 28, or a ripe banana, which has a glycemic load of 13, according to Harvard Health Publications.