WAIT FOR IT: The plant hormone ethylene triggers the ripening process.
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Bananas hanging on a tree or sitting in the produce section of the grocery store start out green, plenty hard and none too tasty. Over time, of course, they become softer and sweeter. The cause of fruit ripening is a natural form of a chemical synthesized to make PVC (polyvinyl chloride) piping and plastic bags—namely, a gaseous plant hormone called ethylene.
For thousands of years, people have used various techniques to boost ethylene production even if they did not quite know it. Ancient Egyptian harvesters slashed open the figs they collected to stimulate ripening, and Chinese farmers would leave pears in closed rooms with incense burning. Later research showed that wounding and high temperatures trigger plants to produce ethylene.
In 1901 Russian scientist Dimitry Neljubow showed that ethylene could affect plant growth after he identified it as the active ingredient in vapors leaking from a gas main. The vapors were causing surrounding plants to grow abnormally. Three decades later, researchers found that plants not only responded to ethylene, but they could produce their own, and production of the gas increased when the scientists cut (injured) the fruit with a knife.
Researchers later discovered that plants produce ethylene in many tissues in response to cues beyond the stress from heat and injury. It is made during certain developmental conditions to signal seeds to germinate, prompt leaves to change colors, and trigger flower petals to die. Because the gas diffuses easily it can travel within the plant from cell to cell as well as to neighboring plants, serving as a warning signal that danger is near and that it is time to activate the appropriate defense responses.
Special receptors in plant cells bind to the ethylene. The first known plant genes involved in this process, ETR1 and CTR1, were identified in 1993; they keep the fruit ripening genes from activating until ethylene is made. Once that happens, ETR1 and CTR1 turn off, which allows a cascade that ultimately turns on other genes that make various enzymes: pectinases to break down cell walls and soften the fruit; amylases to convert carbohydrates into simple sugars; and hydrolases to degrade the chlorophyll content of the fruit resulting in color change. Such changes invite animals to consume the fruit and disperse the mature undigested seeds via their defecation.
The evolution of the ethylene pathway, from the production of the gas to end responses like cell death, still puzzle scientists. Land plants are the only organisms known to contain the entire response system. Cyanobacteria can sense ethylene, but whether they can produce the compound is unknown. These microorganisms have an ETR1-like gene, but no CTR1 gene, so their ethylene response system would have to be different from that of land plants. Green algae, generally thought to lie between cyanobacteria and land plants in the evolutionary tree, do not perceive ethylene, so how ethylene responses jumped from cyanobacteria directly into land plants also interests researchers.
For economic reasons, scientists continue to explore the biomolecular details of the ethylene production–response cycle, in hopes of developing better methods of preventing fresh-picked fruit from ripening during transport over long distances. The trick is to ensure that the fruit does not become ethylene-insensitive so that it never ripens. After all, who wants to eat green bananas that taste like fiberboard?
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7 Comments
Add CommentI'm curious about an idea that's so simple it's probably already been tried (or so simplistic it's been missed). What about storing the picked, not-yet-ripe fruit in an inert gas (argon, etc) such that the ethylene cannot reach a sufficiently high density to cause the ripening process to begin?
Reply | Report Abuse | Link to thisi was thinking the same thing, but maybe i am missing something in the article because it seems like it would be a logical first choice to keep the activating substance away from the reactor until you want the reaction to happen.
Reply | Report Abuse | Link to thisI think what you're missing is that the fruit produces it's own ethylene.
Reply | Report Abuse | Link to thisThe article did not explain how banana's are handled from the time of harvest. They are boxed and placed under refrigeration as soon as possible, usually at 56 F. and kept at that temperature until they are "ripened" at destination by raising the temperature and injecting ethylene into sealed banana ripening rooms. Ripening may take 3 to 6 days, or longer depending on market conditions, by temperature regulation. Banana's may also be packed in sealed plastic bags under vacuum and held for 30 or more days under refrigeration without natural ripening. Temperature is critical in holding the fruit in its green condition.
Reply | Report Abuse | Link to thisBanana's also don't grow on trees. Check Wikipedia.
@DiscomBob: I understand that. If the inert gas atmosphere is maintained under an elevated pressure, then the ethylene density will remain low, perhaps postponing ripening.
Reply | Report Abuse | Link to this@bonzalph: Thank you for that. I never knew! Are other fruits treated similarly, or does each type have pretty much its own style of postponing ripening? I'm actually surprised that a vacuum environment works, considering, as mentioned in the article and by DiscomBob, that the fruits generate their own ethylene. It would seem then that temperature is the larger piece of the control system.
Or we could all just eat locally grown fruit that has never been exposed to artificial gas environments that have unknown effects on those consuming it. We could save billions of barrels of oil transporting foods in sealed gased containers. We could employ scientists outside of the private sector to do things that actually advance science instead of finding ways to make Dole and Chiquita more money. Oh, that's just silly isn't it. We have to have bananas! I think it's even in the constitution.
Reply | Report Abuse | Link to this@Economist -- What you say sounds like it makes sense -- and is therefore very populist -- but "locally grown" fails under deeper analysis:
Reply | Report Abuse | Link to this1) Locally grown produce is generally grown on a smaller scale. Therefore, it tends to be grown with smaller, less efficient farm machinery. I love locally grown corn, for instance, but it it clearly not harvested with the efficiency of a large combine in the Midwest!
2) Locally grown produce is generally transported by smaller, less efficient vehicles. The small trucks pale in pounds transported per gallon relative to semis or trains. Depending on your assumptions and where you live, locally grown produce can easily use more fuel before it gets to your table than nonlocal produce!
3) Locally grown produce is grown at much lower efficiency. In fact, large portions of the world population would starve if everyone lived in your locally grown utopia. Further, much larger amounts of land would have to be cultivated... so say goodbye to any nearby forests or open spaces.
4) Not all produce is available locally or for large portions of the year. This is not just an argument about standard of living. Consider that preserving local produce so it's available in the Winter requires energy for refrigeration, canning, etc. (Or are you suggesting we all dig root cellars?) Further, the nutritional aspects of, say, not being able to get green vegetables need to be addressed: do we manufacture vitamins? Or use energy to keep frozen vegetables?
So, while I thank you for the populist soundbite, I hope you will address the issues above before prescribing such a naive idea for all of us.