There’s a reason people crowd the tomato stands at the farmers market. Its diverse and vibrant tomatoes seem to taste better than the perfectly red ones at the supermarket, even if parts of the fruit aren’t as ripe as others.
A study in the current issue of Science lends credence to what your taste buds have been telling you.
Researchers from the University of California, Davis; Cornell University; and other institutions have pinpointed the genetic change that makes many commercial tomatoes ripen uniformly. But they also discovered that the same genetic change makes a tomato produce less sugar -- so it is less sweet and flavorful.
“What this paper shows is that a pretty tomato comes at a cost in flavor,” said Harry Klee, a University of Florida horticulturalist not involved in the study.
Some tomato varieties, when unripe, have dark green “shoulders” topping an otherwise light-green tomato. This makes the top of the tomato redden more slowly than the rest as it ripens.
For some consumers, a partially red tomato is less appealing. Salsa companies, for example, don’t want green chunks of tomato in their glass jars. And uneven ripening makes commercial and mechanical harvesting of tomatoes more difficult.
So for 70 years, since breeders discovered a naturally occurring variety of tomato that ripened uniformly, lots of tomatoes have been bred that way. These tomatoes are some of the most flawlessly red supermarket ones, and are in nearly all pizza sauces, tomato soups and ketchups.
Breeders knew they were selecting tomatoes to have a particular version of some gene. But they didn’t know which one, or what it did. James Giovannoni, an author of the study at the Boyce Thompson Institute for Plant Research at Cornell, likened it to having a Google map zoomed in only as far as California. Now, he said, “We’re at your house in Sacramento.”
The researchers discovered the gene, GLK2, and the version of it with a DNA difference that makes a tomato ripen uniformly.
By turning other genes on and off, it controls a tomato’s chloroplasts, the factories that convert the sun’s energy into sugar and make plants green.
Tomatoes with the uniform-ripening version of the gene have fewer, smaller and more evenly distributed chloroplasts. This gets rid of an unripe tomato’s dark green shoulders, so it ripens evenly.
But fewer chloroplasts also mean less sugar -- about 10 percent to 15 percent less in the uniform-ripening tomato, said Ann Powell of UC Davis, lead author of the study.
Sugars can accentuate flavors in a tomato, just as you might “sprinkle some sugar on top of your berries to bring out the flavor,” Powell added.
Sweetness is also important for processing tomatoes, which nearly all have the uniform-ripening gene version. “The sweeter it is to start off with, the less they have to cook it” to concentrate the sugar, Powell said.
It’s a “double whammy,” Klee said.
Researchers have found that the same gene version also makes a tomato have less lycopene, beta carotene and other compounds that give it a nuanced taste and smell -- not to mention nutrition.
Giovannoni described a tomato that ripens uniformly as “dimmer in all senses.”
“Breeders do a good job because they can get tomatoes harvested at the right time and that consumers will buy,” he said. But “you push a little bit one way, and it gives the other way. It’s hard to get everything you want.”
“No one would have predicted that the color would lead to fruit quality,” Powell said.
David Francis, an Ohio State University tomato breeder and geneticist not involved in the study, is cautious about its claim of reduced field-grown tomato quality as a direct consequence of breeding.
“They’ve shown what gene affects uniform ripening. What I’m challenging is whether uniform ripening affects fruit quality in any way other than visual,” he said, citing the authors’ experimental methods.
“That wasn’t something they tested,” agreed Dina St. Clair, a UC Davis tomato breeder not involved with the study. “I guess what one can say is that it’s speculation.”
Breeders also never breed for just one thing, she said.
While it lends some insight, the discovery of the gene does not fully explain a fresh supermarket tomato’s dullness. “Think of fruit quality as an orchestra. It’s a violin ... but it’s not the only violin,” said St. Clair.
While only some supermarket tomatoes are uniform-ripening, most have been picked early and unripe. Before they make it to the shelves from the truck, they are gassed with hormones to make them fully ripen. But they don’t get the extra sugar they would have otherwise gotten on the vine.
“That’s why they have gotten a bad rap,” said St. Clair. “What I say is, ‘Hey, make sure they’re ripe.’ ”
The finding in Science may affect tomorrow’s tomatoes by providing an easy way for breeders to test the DNA of tomatoes and accelerate breeding, said Powell.
Eventually, a tomato may meet the demands of commercial production and also be mouthwateringly delicious.
“Every time you understand a gene that underlies a trait, even without genetic engineering, you have the possibility of better exploiting natural diversity,” Giovannoni said.
For now, we may just have to change our preferences.
“I personally would eat a tomato that had that uneven color if I knew I was going to get a tomato that tasted better,” said Klee.
“It was like discovering a new universe or something,” said Milt Whaley, who grows more than 90 heirloom varieties north of Sacramento. “If more people shopped at farmers markets, they would realize the (taste) differences that are within their reach to experience.”
And Whaley doesn’t mind that tomatoes don’t all ripen uniformly. “I’m all for the green shoulders on a tomato.”