This NBC Learn video, one in a 6-part "Cheeseburger Chemistry" series, examines the role of the plant hormone ethylene, or C2H2, in ripening tomatoes (and other fruits); lycopene's connection to tomatoes' red color; and how ethylene gas diffuses.
The Chemistry of Tomatoes
AL ROKER, reporting:
It’s hard to find images of cheeseburgers without them: slices of ripe, red tomatoes. Here are some things that help explain the chemistry of tomatoes: racing flags, the Oracle of Delphi, werewolves, carrots, hairspray, and the U.S. Supreme Court, which ruled in an 1893 customs and tariffs case, that tomatoes were to be classified as vegetables even though, botanically, they’re not.
DR. KENT KIRSHENBAUM (New York University): Tomatoes really are fruits in that they have seeds and they ripen in many of the same ways that fruits do. Tomatoes start off being small and green. They don’t contain too much sugar. As they begin to grow and ripen, they get larger and finally, in the ripening process, they go from being green to red.
ROKER: How does that happen?
KIRSHENBAUM: The ripening process actually relies upon the action of a small molecule, a really very simple molecule called ethylene.
ROKER: Ethylene, its chemical formula tells you what it’s made of: four atoms of hydrogen, bound to a pair of carbon atoms connected by a double bond. Organic compounds made of only hydrogen and carbon are hydrocarbons; hydrocarbons with at least one double bond between carbon atoms are unsaturated hydrocarbons, also known as alkenes. Ethylene is the smallest, simplest alkene.
Here’s where the Oracle of Delphi comes in. This ancient Greek priestess would predict the future after going into a trance, which scholars believe was caused by ethylene gas coming out of ground faults.
Yes, ethylene is a gas, one of the three common states of matter. And it can seep out of the ground as natural gas, but it is also produced by plants, especially those that produce fruits, including bananas, apples, peaches and plums, avocadoes, potatoes and tomatoes.
Ethylene is a plant hormone, and as every teen knows, hormones trigger major changes in living things. In plants, this is ethylene’s main job.
KIRSHENBAUM: The tomato plant needs a signal that it’s time to fully ripen and ethylene gas is that trigger, that flag at the beginning of the race that drops and tells that tomato plant, now it’s time to fully ripen.
ROKER: When receptors in the tomato plant sense the release of ethylene gas, the chemical process of ripening revs up. Starches in the sour green fruit are converted into fructose, the natural sugar in most fruits.
KIRSHENBAUM: The presence of the ethylene gas also triggers the generation of lycopenes, giving that tomato its beautiful red color.
ROKER: Lycopene, one of a class of natural pigment and antioxidant compounds called carotenoids, that create colors in fruits and vegetables, yes, including carrots, that’s beta-carotene. Lycopene is what makes ripe apples and tomatoes red. All these changes in flavor, texture, color are a sign to tell insects, animals and people: “I’m ripe and tasty! Eat me! Throw my seeds around and grow more of me!” But in nature, bright red colors can also signal, “Stay away! Don’t eat me! I’m poisonous!”
In fact, and we’re getting to the werewolf part here, the tomato plant has been associated with danger for centuries. Why? One hint: the term lycopene. Lycos is a Greek word meaning wolf. In legend and myth, a being who transforms into a wolf is a lycanthrope, or werewolf, a shape-shifter who can be summoned with nightshade, that’s a family of plants ranging from Deadly Nightshade to tobacco to, yes, tomatoes. Scientific name: lycopersicum.
Back to the bright red color, which these days, isn’t always a sign that the tomatoes are juicy and tasty. Too many are tasteless and tough, taken off the vine before they get ethylene’s cue to ripen.
KIRSHENBAUM: We can actually control the ripening process of tomatoes. We can pick them when they’re mature but still green, transport them, and then we can give the tomato the appearance of being ripe. We can develop the lycopenes, we can develop the color profile of a ripe tomato by controlling its exposure to ethylene gas.
ROKER: Witholding it until it’s time to sell the tomatoes, then piping ethylene gas into warehouses or greenhouses where the gas diffuses, spreads widely, the way the liquid and scent molecules in hairspray disperse into the air. You can’t see the ethylene molecules moving between and through fruits, but you can see where they’ve been. It only takes a little, one part ethylene per million parts of air, to trigger the ripening process.
KIRSHENBAUM: If you have a tomato that’s not quite ripe and you want to ripen it up, you put it right next to another plant that’s generating ethylene gas and those tomatoes will ripen really quickly.
ROKER: And keep ripening to the point of rotting as long as there’s exposure to a concentration of ethylene, which, since you now know that apples generate ethylene, explains that old saying about “one bad apple” spoiling the whole barrel. So, the chemistry of tomatoes: not the whole story, but a pretty good slice.
Looking for a sign of the apocalypse? Consider this. Our global obsession with guacamole and avocado toast has helped spawn record avocado prices, financial woes for millennials and even an uptick in avocado-related crime.
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