What Makes Fruit Ripen?

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Sometimes the old wives get it right. When your mother (perhaps even when she was a “youngish wife”) told you that putting an apple in a bag with a banana would make it ripen, you might have scoffed a little. But then you found yourself with a rock-hard green McIntosh, you tried it, and soon you were enjoying red juicy goodness! But why did it work?

Journey with me now to the land of plant communications, specifically those with “fleshy fruits.” We’re talking tomatoes, bananas, citrus, pineapples, dates, persimmons, pears, apples, mangos, avocados, papayas and jujubes (if you’re like me and have never seen a jujube, check out the July/August 2007 Mail Call). This kind of plant is so interested in seed dispersal that it creates fruit intended to tempt some animal to come along and carry it off.

Many things happen as fruit ripens. Unripe fruit is often green, sour, odorless, hard and mealy. The ripening process makes the fruit more appealing – the color of the skin changes as chlorophyll (the green stuff in plants) is broken down and in some cases new pigments are made, the acids that make the fruit sour are broken down, the mealy starches are converted into sugar, hard pectin is softened, and larger molecules are made into smaller ones that then evaporate as aroma. Suddenly, we have a soft, juicy, sweet, fragrant, colorful animal-attractor.

This produce makeover is accomplished by a group of enzymes that are made on cue. They take their cue from a ripening signal – a burst of a gas called ethylene. Ethylene is a simple hydrocarbon gas produced when a fruit ripens. Ethylene flips the switch to trigger the genes that in turn make the enzymes that cause ripening.

Plants send signals all the time using hormones. This ripening signal is unique, though, because it involves an airborne hormone (the ethylene). Ethylene is produced by rapidly growing tissue (the tips of roots, flowers, ripening fruit, damaged tissue). Thus, a wound can activate ethylene production; just the act of picking green fruit can cause the ripening process to begin.

By controlling and manipulating the ethylene signal, fruit producers can put perfectly ripe, beautifully colored fruit in the produce section. Bananas can be shipped green (to save on bruising) then put in a “ripening room” where a low concentration of ethylene is applied to trigger the ripening process. (Refrigeration can slow the process, so if you want to keep your bananas at a certain ripeness longer, put them in the fridge.) Oranges also get “oranged-up” by adding a little ethylene.

Put unripe apples in a refrigerated warehouse where you scrub ethylene from the air, and the ripening process can be slowed. Take one of these apples home, put it in a paper bag, which keeps the ethylene close but allows oxygen in, add a ripening banana (one of the highest producers of ethylene), and you can speed up the whole process. Put some in a barrel, add a damaged, ethylene-producing one (that is, a really bad apple), and the rest will start driving fast cars, staying out past curfew, and sneaking into the theater, too.   

Associate Editor Jenn Nemec actually prefers green Granny Smith apples to their softer, sweeter counterparts. It’s probably due to sheer stubbornness on her part.