Science explains why refrigerators sap the flavor from ripe tomatoes

If you’re one of those people who puts tomatoes in the fridge, you are going to want to stop. Now.

Sure, chilling a tomato will keep it looking fresh for a longer period of time than if you left it on the counter, but it will also drain all that earthy, slightly grassy, distinctive tomato taste right out of the fruit. (And, yes, tomato is a fruit.)

Scientists and foodies have known for some time that cooling tomatoes is detrimental to their flavor, but they were not exactly sure why — until now.

According to new a study in the Proceedings of the National Academy of Sciences, when a tomato’s environment drops below 68 degrees, the genes responsible for making it taste like a tomato get turned off.

“Basically, the tomato gets cold and tells itself to stop making these aroma compounds,” said Denise Tieman, a research associate professor at the Plant Innovation Center at the University of Florida in Gainesville.

“The change is irreversible,” she added.

Tieman has been studying the science of tomato flavor for at least a decade. Previously she discovered that tomatoes taste the way they do because of a combination of sugars, acids and a collection of chemicals that scientists call volatile compounds or aroma compounds.

“Aroma compounds are what you smell, and they make up the wonderful part of the flavor,” Tieman said. “The sugars and acids are what you taste on the tongue, but there would be no excitement to the flavor without the aroma compounds.”

In previous work, the authors had shown that a refrigerated tomato has the same amount of sugars and acids as a freshly picked tomato, but significantly less aroma compounds.

The first experiments in the new study were designed to determine how long it takes for a tomato to start losing its aromatic chemicals. The group compared tomatoes that had been chilled for one, three and seven days to those that had just been picked. They found that tomatoes that had been chilled for seven days showed the least number of volatiles, but that even after three days of chilling there was still a significant decrease in the number of aroma compounds compared with a freshly picked tomato.

In a taste test, volunteers rated the chilled tomatoes as less tasty than fresh tomatoes.

Further experiments revealed that the proteins responsible for assembling the aroma compounds were less abundant in chilled tomatoes than in freshly picked tomatoes. Further analysis revealed that the genes that code for these aroma compounds are turned off in a process called methylation when a tomato stays in a chilly environment for too long.

Tieman said the research team doesn’t know why a tomato turns off its flavor genes when it gets cold. She suggested tomatoes in nature might have evolved to conserve energy when it’s cold, but added that is only a guess.

In the meantime, the group’s next step is to look for tomato varieties that are tolerant to chilling. Even if consumers can be convinced to keep their tomatoes out of the fridge, most tomatoes at the supermarket are chilled at a distribution center, and then again at the store to keep them looking pretty enough for purchase.

“I don’t think we can change the whole tomato production network, but we could suggest varieties that can handle the cold without losing so much flavor,” Tieman said. 

deborah.netburn@latimes.com

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