Why do bananas seem to get brown spots faster in the summer? Also, why do greenish bananas taste different than yellow bananas with brown spots? asks a reader.
Bananas are part of a color-changing legion. Most fruits change from green to another color as they ripen. Take strawberries. Until the seeds speckling their surface are fully developed, strawberries are white-green, fading shyly into the leaves of the strawberry plant. Unripe berries are also hard and bitter. But a ripe, juicy strawberry is a red beacon against its green background, the better to attract hungry animals who will spread the plant's seeds.
Fruits like strawberries do most of their ripening before they're picked. But bananas are one fruit that actually ripen better detached from their plants (which aren't trees, but 25-foot tall herbs). Bananas left on the stem ripen and rot quickly, and taste more mealy than sweet. Picked green, their starches slowly turn to sugar.
An enzyme called amylase speeds the makeover. By the time its green chlorophyll has disintegrated and a banana turns yellow, more than 80 percent of its starch has transformed into sugars. In a brown-spotted banana, the starch-to-sugar process is nearly 100 percent complete. Which is why a fully ripe banana tastes so sweet.
Banana plants grow best near the equator, in rainy tropical and subtropical climates. Summer heat and humidity speed the ripening process. Cold slows it down. So even though the peel of a banana may turn brown in the fridge, the ripening process will slow to a crawl, keeping a banana from going bad as quickly.
While we see yellow and think ripe, other animals may have different color criteria. In 2008, scientists in the U.S. and Austria reported the results of a study of bananas under black lights. It turns out that in ultraviolet light, ripe bananas are not sunny yellow, but bright blue.
According to the researchers, the color shift occurs as the green pigment chlorophyll disintegrates. The (colorless) fragments left behind in the peel actually fluoresce, or glow, in UV light. And dying cells around the peel's brown spots show up as brightly glowing rings.
So a banana under UV light is, like a black-light poster, a spookily decorative object. But there may be more to blue bananas than meets (or doesn't, in our case) the eye. One researcher noted that many of the animals that find bananas tasty can see UV light. So just as a strawberry turns from green to red to signal its ripeness, bananas may attract diners by taking on a blue tint in the UV light of the Sun.