Ask A Scientist: Banana Gas
By Anonymous on May 17, 2012
Question: Why do some fruits and vegetables ripen after picking and others don’t?
Answer: the reason that some produce ripens on your kitchen counter whilst other types just rot is simply a matter of what type of produce it is. Some produce is climacteric, which means that it has the ability to ripen after harvesting (and thus gives it a longer shelf life) and other types are nonclaimacteric, meaning that they have a short shelf life as they cease development after harvesting. Some examples of climacteric produce include bananas, apples, pears, tomatoes, avocados and cantaloupes; nonclimacteric produce examples are citrus fruit, grapes, strawberries, blueberries and cherries.
I could just stop right there as the answer is technically complete. However, as true to my nature, I won’t. Especially when the question at hand involves ethylene gas!
Ethylene gas serves as a plant hormone. Naturally speaking, plants will produce ethylene gas when they perceive that they’re in danger as a means fo self preservation, mostly in the form of ripening, blooming or shedding. (Triggers for ethylene production include flooding, wounds, drought, chilling and pathogen attack… All the fun ones.) The gas regulates the plant’s ability to ripen as well as leaf shedding, seed germination, flower blooming, root hair stimulation and a host of other protective measures. Ethylene gas basically puts a plant onto full on counteraction mode.
For centuries, people may have had an inkling of harnessing the power of ethylene but it wasn’t really defined as we would use it today until the early part of the twentieth century. Farmers as far back as those in ancient Egypt would score figs to stimulate their ripening. Various produce ripening hacks have been employed since then with a variety of different results, but it wasn’t until 1901 when Dimitry Neljubow, a Russian scientist, noted that the reason why plants located near gas street lamps had less leaves and thicker plant stalks was due to the ethylene gas byproduct circulating around. However, the correlation between plants producing ethylene and ripening wasn’t really proven until 1935. Since that day when it was proven that ethylene could hasten the ripening of produce, its been used to extend the shelf life of harvests for minimal waste.
The climacteric produce that I was mentioning really just means that it has a high cell respiration rate, has reached the end of its maturation period and is now beginning to age. If your produce is climacteric and ethylene gas is present in the right conditions the produce will ripen quickly. Lets take the example of a banana, which is oftentimes gassed to hasten its ripening. The perfect condition to ripen that banana is in a sealed chamber that’s held at about 68 degrees with controlled oxygen levels. Green bananas are held in the chamber for 24-48 hours when they’ll be subjected to a constant environment of ethylene gas and oxygen. Carbon dioxide, a byproduct of ethylene gas ripening, is also closely monitored and filtered out as these levels may increase up to 10 percent during chamber ripening and slow down the process slightly. After the gassing is complete the bananas will be the standard yellowish color that we’re used to seeing in the stores. From there they may be prepared for packing and shipping.
Alternately, to slow down ripening of climacteric fruit farmers will oftentimes store their fruit in climate and atmospheric controlled conditions. Apples are oftentimes stored for long periods of time where the temperature is about 32 degrees and carbon dioxide gas levels are higher. Oxygen is lower in these conditions as well. The cooler temperatures reduce ethylene gas production as well as less oxygen and hydrogen molecules for the ethylene to bond to. Apples can be held for quite a bit of time in these conditions with minimal ripening.
Also, as in every relationship, there are those who give and those who take. The same idea applies to nonclimacteric produce. Although not every plant produces good results from being in the presence of ethylene gas, most plants are receptive to it. So in theory, if you had a couple of bananas and a underripe box of strawberries and put them in a sealed container on the counter for a day or two, you may theoretically be able to ripen those berries further. However, to really get the most out of nonclimacteric fruit in regards to palatability, the produce really has to be ripened by allowing the growth cycle to complete before harvesting.
So there you have it. A very long and semidetailed description of why your produce will either hold beautifully on your kitchen counter or die a slow death. Go forth and impress your friends, dear reader, with your newfound knowledge of plant hormones.
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By Karen Ballum