by Bob Holmes
There is one crop, however, where flavor matters above anything else, even yield: wine grapes. The whole point of growing wine grapes, of course, is to make a wine with a distinctive, appealing flavor. If anyone knows how soils and farming methods affect the flavor of a crop, it’s going to be viticulturists. For an example that’s been worked out in great detail—an example, moreover, that you can taste for yourself tonight—let’s journey halfway around the world from Harry Klee’s tomato lab to New Zealand.
Mike Trought loves to talk wine—and there’s probably no one in the world who knows more about the acclaimed white wines made from the sauvignon blanc grape, particularly in the Marlborough region of New Zealand’s South Island. A cheerful, balding fellow who’s spent more than three decades in Marlborough as a researcher, university lecturer, winery consultant, and viticulturist, Trought recalls the time, in the midnineties, when he made a pilgrimage to the world-renowned oenology department of the University of California, Davis. At the time, New Zealand wines were just beginning to emerge onto the world stage, and Trought poured two bottles of Marlborough sauvignon blanc for a group of Davis researchers. Everyone dismissed them, calling them too acidic, too herbaceous, unsubtle—in short, unripe and inferior. Two decades later, the joke’s on the folks from Davis. “New Zealand sauvignon blanc has now become a benchmark for sauvignon blanc around the world,” says Trought. “We can’t produce enough.” One of the wines Trought poured at Davis that year, Cloudy Bay, quickly became so popular that wine shops couldn’t keep it in stock, especially in Britain. (Trought wonders, a bit impishly, whether wine experts might actually hinder progress in the wine world.)
If you’re at all fond of wine, you have probably encountered the distinctive flavor of New Zealand sauvignon blanc. Sip one of those wines, especially from Marlborough, and you’ll be struck by the pronounced aromas of passion fruit, green pepper, and what’s often described as boxwood, or “cat’s pee on a gooseberry bush”—the latter being an actual, if unconventionally named, commercial wine. The intense, distinctive flavors make New Zealand sauvignon blanc an ideal test case for understanding where a wine’s flavor comes from and how growers and winemakers can influence the outcome. As an added bonus, New Zealand’s wine industry is relatively new, so tradition doesn’t get in the way of science.
What gives Marlborough wines their distinctive flavor? It’s certainly not the grape variety alone. Virtually all of New Zealand’s sauvignon blanc vines are descended from a single clone originating in the vineyard of France’s fabled Chateau d’Yquem, where they yield a vastly different wine. Instead, a lot depends on the vineyard soils. Not, perhaps, in the way you’d think. The notion that you can somehow “taste the soil” in a wine is completely false. Grape vines take up only water and simple nutrients like nitrogen, potassium, and calcium from the soil. They make all their more complex biomolecules—including the flavor volatiles—in-house. To put it more bluntly, none of the volatile molecules that determine a wine’s flavor come directly from the soil. (This calls into question one of wine writers’ favorite buzzwords these days: “minerality.” You won’t find the term much in wine writing before the 1980s, but it’s become a term of high praise today. Whatever the term means—and the experts don’t exactly agree—it’s not the flavor of the vineyard. One study suggests that “minerality” is a description that emerges only when a wine lacks any other distinctive flavor.)
Instead, a vineyard’s soil affects flavor indirectly, by altering how the vine grows and, especially, how quickly the grapes ripen. In Marlborough’s Wairau Valley, the vineyards sit on an old river flood plain, where the soil is a jumble of sand, gravel, and cobble deposited by the river channel as it meandered across the plain. Soil quality can change rapidly as you walk through the vineyard, so that grape vines only a few yards apart experience very different soils. Where the soil is shallower, vines tend to be less vigorous, and their fruit ripens earlier. (Trought isn’t sure why the smaller vines on stony soils ripen earlier, but he suspects it might be because the vines put more of their energy into the grapes when growing conditions are poorer.) When harvesters go through the vineyard, the patchwork of soils means that some bunches of grapes will be harvested at a riper stage than others. The volatiles responsible for the green pepper flavor, known as methoxypyrazines, form early in grape development, so they are more prominent in less ripe grapes. Meanwhile, the thiols that account for the passion fruit notes dominate in riper grapes. This mix of ripeness, and the different flavors it delivers, helps give the Marlborough wines their complexity. “To some extent, that’s the characteristic of Marlborough sauvignon blanc,” says Trought.
There’s more to the story, though, as Trought and his colleagues discovered. “When we started our sauvignon blanc program, we did it because we thought it was going to be easy,” he says ruefully. “As we got into it more and more, we realized it was much more complex. It’s not just what’s in the vineyard that matters.” If you pluck a grape off the vine and chew it, you won’t notice much passion fruit flavor, because the thiol molecules haven’t formed yet—only their odorless precursors are present. The thiols themselves form during fermentation, as the yeast attack the precursors and split off thiol molecules. Rough handling of the grapes causes them to accumulate more of the precursors, so machine-harvested grapes yield wines with about ten times as much thiol as handpicked ones. This, incidentally, may be part of the reason that New Zealand sauvignon blanc, which is generally mechanically harvested, tends to have a much more pronounced passion fruit flavor than French sauvignon blanc, which is usually hand harvested. Even trucking the grapes from vineyard to winery leads to more thiols in the finished wine.
The biggest effect on the final flavor of a wine comes from fermentation, as wine yeasts and other microbes attack the sugars, proteins, and other molecules in the grape juice and convert them to alcohol and flavor volatiles. Each strain of yeast approaches this task with its own unique tool kit of genes and enzymes, and as a result different yeasts can yield very different wines from the same juice. Winemakers are very aware of this, and put a great deal of thought into their choice of yeast. Here, too, regional differences matter, because every winegrowing region—and, quite possibly, every vineyard—harbors its own unique microbial ecosystem. Winemakers rarely sterilize their grapes before fermentation, so these microbes end up in the fermentation tank. In fact, many winemakers rely exclusively on natural microbes for fermentation. So it makes sense that part of the regional character of a wine—its “terroir,” to use the term beloved of wine critics—might be the result of different microbial actors taking the stage during fermentation.
Plausible, but until recently, untested. A few years ago, geneticist Sarah Knight and her colleagues at the University of Auckland, New Zealand, set out to see whether it really was true. To rule out any differences in the grapes themselves, Knight started with a single batch of Marlborough sauvignon blanc grapes and sterilized them to kill any resident microbes. Then she divided the juice into a series of tiny fermentation tanks, and sowed each one with a different wine-yeast variant gathered from one of New Zealand’s six main winegrowing regions. Same juice, same fermentation conditions—the only difference was the yeast itself. In the end, the yeast variants from each region produced a wine with a detectably different aroma profile. Theory confirmed! Not only that, but Knight’s study probably underestimated the effect of microbial differences, because she used only wine yeasts, not the whole microbial flora.
For other crops, too, any effect of the soil on flavor is likely to be indirect. The soil a plant grows in determines how much water and nutrients it has access to, and therefore its energy and materials budget for the sugars and volatiles that determine flavor. You’d think that more would always be better, but it’s more complicated than that.
To explain, I turned to Carol Wagstaff, a crop scientist at the University of Reading in England—just a few minutes’ drive, actually, from chef Heston Blumenthal’s Fat Duck res
taurant in Bray. Reading’s research group is one of the few to actually study how growing conditions, shipping, and storage affect the nutritional value and flavor of crop plants. Wagstaff has unruly, long brown hair and a large, strong face that lights up when she talks about her work. If conditions are too easy, she says, plants have little need for secondary compounds and put all their energy into growing as fast as they can. Only when they begin to feel a budget crunch do they invest in defending what they’ve already got. “A bit of controlled stress doesn’t go amiss. When a plant is stressed, you’ll get more secondary compounds, and that means more flavor and more nutrients,” she says. That’s likely why Whitaker’s strawberries also profit from a little water stress. Exactly what that stress response means in flavor terms is likely to depend on what Wagstaff calls the “metabolic bureaucracy” of the plant—that is, its particular genetic endowment of enzymes and the particular balance of secondary chemicals they favor. Much of Wagstaff’s research centers on arugula, which the British call rocket, and that’s exactly what she sees there. “You can see quite clearly that some genotypes of rocket will preferentially shunt production in one direction, and other genotypes head down another route when they’re stressed,” she says.
Soil microbes, too, could play some role in determining the flavor of the crops they grow with. For example, baby corn—a popular vegetable in some Asian cuisines—contains a volatile flavor molecule called geosmin, the same molecule that gives red beets their earthy flavor. Researchers think the young corn plants don’t make the geosmin themselves, because corn grown in an English greenhouse lacks the compound. Instead, they think microscopic fungi living with the roots of the corn plants make the geosmin. The plants take up the fungi through their roots, and the geosmin comes along for the ride. It’s possible that soil microbes affect flavor in other ways, too, but so far there’s little actual evidence.
So far, we’ve been talking as though more flavor was always better—but for many vegetables, particularly members of the sharp-tasting mustard family, like arugula and brussels sprouts, that’s not necessarily true. Many people—especially those who carry the bitter-sensitive version of the T2R38 taste receptor—find the bitterness of their secondary compounds off-putting and would prefer that their brussels sprouts have less flavor, not more. “Horticulture is essentially messy,” says Wagstaff. “You’ve got the variable genotypes of your plants, you’ve got the environment you’re growing them in, and you’ve got the varying genotypes of the consumer.”
Once a fruit or vegetable has been picked, its flavor continues to change during storage and en route to your grocery store. Partly, that’s because volatile flavor molecules leak out into the air, as we’ve seen happens with tomatoes. At the same time, though, enzyme activity in the tissues can produce new flavor molecules or alter old ones. Occasionally, this can mean that a fruit or vegetable actually improves with storage. Arugula, for example, continues to produce glucosinolate molecules during cold storage after harvest. When you chew a leaf, they turn into flavorful isothiocyanates. That’s good news for your salad: The arugula you buy in the grocery store—if it’s relatively fresh—may actually be more flavorful than if you’d picked it in your own garden this afternoon. After a few more days in the refrigerator case, though, that advantage goes away, as the “fresh” set of flavor compounds gives way to nastier products that result from fat breakdown. This happens at different rates depending on the variety of arugula—some store better than others, Wagstaff has found.
Some other vegetables last a long time with little or no change in quality. An onion or a potato, for example, is meant to just sit there like an inert lump—that’s its job, as a storage organ for next year’s growth. So it makes sense that we don’t notice much of a decline in flavor. Others, such as corn and carrots, are sweetest just after picking, because enzymes convert their sugar into starch, and no new sugars arrive after picking. They’ll last, but their flavor will be disappointing. But a head of broccoli or an asparagus spear hasn’t evolved to be long lasting. Far from it—both are rapidly growing shoots, and as soon as you pick them their flavor starts to degrade. One Spanish study, for example, found that more than 70 percent of the glucosinolates in a freshly cut head of broccoli have vanished after a week in cold storage, and another 10 percent disappear after another three days on a grocer’s display. That’s a lot of lost flavor.
Many people think that another way to ensure tastier fruits and vegetables is to buy organic when possible. It makes sense, in theory: If a little stress is good for flavor, then you’d expect that organic crops ought to benefit, flavor-wise, from the extra insect damage and weed competition they experience. Hundreds of scientific studies have compared the flavor—or, more often, the nutritional content—of organic and conventional crops. The results, unfortunately, are a mess. Some studies show that organic crops are indeed better, while others find no difference. Even the so-called meta-analyses—in which researchers scour the library for every comparison they can find, then add up the results to get a majority opinion—haven’t reached consensus on whether organic is better.
A big part of the problem is that the answer you get depends on how you ask the question. You could go to the grocery store, buy a head of conventional and a head of organic broccoli, and measure—or taste—the difference in secondary compounds. But if the conventional broccoli was harvested two weeks ago in Mexico, and the organic was picked yesterday just down the road, that difference in freshness might have much more impact on the flavor than any organic versus conventional effect. It’s hard to generalize, though. The Mexican broccoli could have gone straight into a refrigerated warehouse and stayed under refrigeration right until the time you put it in your shopping cart, while the local one could have spent a hot summer’s afternoon in the back of a pickup truck, followed by hours in the sun at the farmer’s market. In that case, local might not mean fresher.
Ideally, you’d like to compare the flavor of identical crops grown side by side with organic or conventional methods, because that cuts out a lot of the potential sources of confusion. Researchers at Kansas State University did exactly that a few years ago, planting onions, tomatoes, cucumbers, and several leafy greens in greenhouses in identical soils. When the crops were harvested, about one hundred volunteers tasted organic and conventional samples of the same vegetable—without knowing which was which—and rated how much they liked them and how intense the flavors were. The results? It didn’t matter one bit whether the vegetables grew organically or conventionally. The taste testers liked them all equally (or, in the case of mustard greens and arugula, disliked them equally—evidently Manhattan, Kansas, is not the place to get rich from an arugula greenhouse). The only difference was that people thought the conventional tomatoes had a little more flavor, probably because they were also a little riper.
That’s not to say that you won’t find organic produce more flavorful. As we have seen, our expectations play a big role in flavor perception—for example, wine tastes better when we think it’s expensive. That bias probably comes into play here as well: If you think organic produce will taste better, then it probably will, to you. Consider what happened when Swedish researchers gave unwitting university students two identical cups of coffee, telling them that one was “eco-friendly” and the other conventionally grown. Sure enough, most volunteers thought the eco-friendly coffee tasted better—and the effect was strongest for people with the strongest environmental consciousness.
Even if organic farming or other differences in growing conditions do make a difference to crops’ flavor, it’s likely to be less important than flavor differences among varieties. If so, then breeders, not farmers, may be the critical link in producing tastier fruits and vegetables. In upstate New York, for example, Cornell University plant breeder Michael Mazourek has been working on breeding a more flavorful squash. Squash and other vegetables are the poor cousins of the agricultural world, Mazourek says. You can go into any grocery store and find perhaps
a dozen different apple varieties, each offering its own recognizable flavor profile. And we know them all by name: a Granny Smith will be tart and firm, a Spartan sweet and softer, Golden Delicious rich in estery fruit flavors. No doubt you have your favorites. But can you name your favorite variety of broccoli, or your favorite butternut squash? I’ll bet not.
“Vegetables are still part of a commodity system, where sameness is one of the overarching goals,” says Mazourek. “There’s not value in people being able to tell that the bell pepper in the grocery store is different from the one last month. It’s an antivalue.” With all the commercial pressure working in the direction of sameness, there’s little incentive for anyone to develop a tastier version.
Mazourek is trying to change that. He starts by seeking out heirloom squashes noted for their good or unusual flavor and crossing them with commercial varieties, then planting the progeny out in his field. When the fruits ripen, he gathers them and selects the most promising ones for taste testing. “I can’t possibly eat some of every squash and stay sane, so we have some proxies that narrow the pool that we do the taste tests on,” he explains. First, he picks fruits with the highest level of dissolved solids—that is, sugar and other molecules that might contribute to flavor. Then from those he selects the ones with the deepest yellow flesh. Those have the highest levels of carotenoid pigments, key precursors of many flavor compounds. Unlike Harry Klee’s work with tomatoes, Mazourek doesn’t yet know which of the many flavor molecules are most important in a good-tasting squash, so he can’t measure them directly with a gas chromatograph. Mazourek has to do his flavor analysis the old-fashioned way: he roasts several varieties of squash and sees which ones he likes best. (By the way, here’s a squash specialist’s advice for the most delicious way to roast a butternut squash: halve it and scoop out the seeds, cover and roast in a four-hundred-degree oven for forty-five minutes. Then uncover the squash, baste it with butter or oil, and continue roasting until tender. “It’s not what Betty Crocker says,” Mazourek admits. “Roasting them for longer and hotter really is a way to bring out a lot of the savory flavors layered on top of the sweetness.”)