The typical supermarket tomato is ripe-red, firm to the touch and free of blemishes—as well as of flavor. Since at least the 1970s, U.S. consumers have lamented the beautiful but bland fruits that farmers breed not for taste but rather for high yield and durability during shipping. Recently organic farmers and foodies have championed the superior flavors of heirloom tomatoes—older varieties that come in an assortment of shapes, sizes and colors. In a study published in June in Current Biology, researchers took a close look at the chemical composition of both standard tomatoes and more than 100 different heirloom varieties, which they also fed to 170 volunteers in a taste test. Their new findings confirm what scientists have learned in recent years: a tomato's flavor depends not only on the balance of sugars and acids within the fruit but also on subtle fragrant compounds—many of which are lacking in the modern supermarket tomato.

Harry Klee of the University of Florida has been studying tomato flavor for the past 10 years. Some of the shortcomings of supermarket tomatoes, he explains, arise because farmers have bred the plants to produce as much fruit as possible. The more fruit an individual tomato plant produces, the less sugar it can invest in each tomato, Klee says. Knowing that tomato flavor depends on so much more than sugar, however, Klee and his colleagues began a research project three years ago to analyze the chemical potpourri that determines a tomato's taste. Klee thinks what he has found suggests a new way to enhance the flavor of tomatoes without sacrificing the economy of high-yielding plants.

Klee's team grew 152 varieties of heirloom tomatoes in fields and greenhouses at the University of Florida and bought standard tomatoes from a local supermarket. The scientists sliced up the fruit and offered the wedges to volunteers who carefully chewed, tasted and swallowed each piece of tomato, rating the texture and the intensity of sweetness, sourness and bitterness, as well as the overall flavor and how much they enjoyed eating that particular sample. As expected, the volunteers in Klee's new study preferred the flavor of tomatoes with a lot of sugar to less sweet fruit—but sugar content did not entirely explain their preferences. Chemicals known as volatile compounds, which drift into our nostrils once a fruit has been sliced or bitten, also contributed to flavor.

In Klee's analysis, the most abundant volatile compounds in a tomato—the C6 volatiles—barely influenced what people thought of the fruit's flavor. Instead a less prevalent volatile compound called geranial made a huge difference. Geranial, Klee concluded, somehow improves a tomato's overall flavor, perhaps by enhancing innate sweetness. Compared with heirloom varieties, stan-dard tomatoes have less geranial and other volatile compounds. “They're kind of like light beer,” he says. “Even if all the chemicals are there, they are at lower levels.” By breeding or genetically modifying tomatoes to contain lots of the volatile compounds taste testers prefer, scientists could produce supersweet and flavorful varieties without increasing the sugar content.