Sing praises to the vine-ripe tomato, that high-summer symphony of taste and texture, brimming with the goodness of nature, not to mention polygalacturonase, pectin mythylesterase and the master molecule of tomato flavor: (Z)-9-hexenal.
People say that nothing tastes quite like a vine-ripe tomato. And tomato scientists are learning they're right.
Tomatoes are being studied as never before by a small army of industrial and academic researchers who want to understand in new detail the cascade of events that occur within a tomato during the still mysterious process of ripening. In the process, large suites of genes are switched on and off like lights in a pinball machine, activating enzyme systems that soften the fruit, change its color and texture and manufacture the sugars and acids and an elusive complex of volatile and short-lived perfumes that give tomatoes their distinctive flavor.
"The tomato is a very interesting system," said Alan Bennett, professor at the University of California at Davis who studies the molecular biology of tomato ripening. "It is also understood that the creation of a better tasting tomato would have great economic value."
Ripening Is Not Rotting
Indeed, the tomato industry's own scientists admit there is little comparision between a healthy tomato that ripens on the plant under optimal conditions and one that is picked green and hard, and then refrigerated, trucked and gassed with ethylene. If the ripening process can be understood, however, and if the genes that control the activity can be harnessed, scientists could help commercial growers produce a store-bought tomato that tastes more like the home-grown variety. But much remains to be learned.
Botantists have long appreciated a fact often overlooked by consumers: A tomato is alive. Even plucked from the plant, cut off from its support system, the tomato remains a living, breathing, metabolizing entity. Ripening is not rotting. Even sitting on the kitchen counter, green and hard as an apple, the tomato's cellular machinary whirls away with purpose. Genes click on and off. Enzymes are manufactured in precise order to do their work. Gases are exchanged with the outside world. In fact, the tomato's breathing actually becomes heavier during ripening.
Proof Is in the Picking
The journey to ripeness begins weeks before the fruit turns red. A young, green tomato is fed by the plant's leaves, where the engines of photosynthesis grind away. The leaves make the sugar sucrose, which the immature fruit converts to starch and stores. As the fruit matures, it breaks down the starch molecules, turning them into the sugars fructose and glucose.
It is the sugar, in concert with citric acid and malic acid, that gives the best tomatoes their balance of sweet and tart. An underripe tomato is very acidic. As it ripens, the acids decrease.
Most commercial growers try to harvest tomatoes when they reach a state known as "mature green," when the seeds are viable and the fruit is capable of ripening without substances supplied through the vine. Some growers wait until the tomatoes become "breakers," just beginning to turn orangey-pink. The fruit is picked early because hard tomatoes stand up better to mass picking, packing and shipping.
Alas, growers often misjudge. They pick tomatoes before all the fruit has reached "mature green." These tomatoes, though they can be forced to change color and even to soften, are the truly wretched examples loathed by vine-ripe enthusiasts.
A tomato plucked green and hard from the vine at the right stage will ripen. Some tomato scientists insist that such a tomato allowed to ripen slowly on its own, without refrigeration, will rival the taste of its vine-ripened counterpart.
"It would be hard to tell the difference," said Allen Stevens, pioneer tomato researcher and director of research for California's Petoseed Company, a supplier of commercial tomato seed. "The tastes would be similar." But as Bennett counters: "The operative word is similiar. They're not the same."
The two researchers explain that ripening begins in earnest with "the master signal," ethylene gas, whose production is initiated when a certain gene is turned on. This gene, in fact, was isolated and cloned only in the past few months. Many fruits and vegetables produce ethylene. If you put bananas or tomatoes in brown paper bag, for example, and allow them to sit a day and then take a whiff, you can smell the ethylene. Commercial growers speed and synchronize ripening by putting their tomatoes in ethylene gas chambers.
The chemical triggers a cascade of events. Scientists suspect the ethylene somehow turns on other genes that direct the manufacture of dozens of different enzymes. The enzymes perform at least three crucial jobs.
The first job is to soften the tomato by producing enzymes that weaken the rigid walls that enclose cells and the pectin that glues them together.
The second job happens at the same time, turning a green tomato red. Other enzymes replace green chlorophyll with other pigments that give the tomato its orange and then red color.
But the third and perhaps greatest act in ripening occurs at the very last moment. Ronald G. Buttery of the Agriculture Department's Food Quality Research Laboratory in California explains that the master molecule of flavor, the chief ingredient that gives the tomato its most distinctive aroma, is not produced in quantity until the tomato is sliced.
"When you smell a whole red tomato is has very little odor. But cut it open and that's when you get the aroma. That's when the enzyme system in the tomato breaks down the fatty acids and releases the volatile compounds," Buttery said.
Aroma Peaks After Slicing
Apparently, cutting or chewing a tomato disrupts the cells and releases enzymes that break down a certain fatty acid in the tomato, converting it to the aromatic compound known as (Z)-9-hexenal. Within seconds, this chemical is wafting into the air. Three minutes after slicing the tomato, the aroma has peaked. The hexenal mingles with other fruit perfumes -- probably dozens -- to produce the bouquet of scents that the nose recognizes as "tomato."
Buttery and his colleagues say that the worst thing to do to a tomato is refrigerate it. Even worse, is to cut a cold tomato. "If you cut the tomato when it's cold, it's like any chemical reaction," Buttery said. "The cold turns off the enzymes and slows the whole reaction down."
As to why the store-bought varieties taste so lame, botantists contend the reasons vary. Some suspect that the prematurely picked tomato may be cut off from additional sugars and from plant hormones that help initiate ripening. Another problem is that commercial tomatoes have been refrigerated, sometimes for a month. Also, they are often gassed, which accelerates the ripening process but may rob the fruit of time to develop the subtle aromatics.
"A real scientific understanding of the tomato continues to elude us," Bennett said. "It is a lot more complex than you'd think."