by Bob Holmes
Even so, we know that breeders of many crops have focused for decades on traits like disease resistance; yield; appearance; uniform size; and ease of packing, shipping, and processing—all the traits that make the crops easier to grow and deliver to distant markets. Their focus hasn’t been on flavor. As one horticulturist told me, kiwi fruit are considered “good quality” if they’re the right size and free of blemishes. Flavor doesn’t even enter into the equation.
Despite the lack of good, scientific studies that measure crop flavor directly, there might be a backdoor way to document its decline. Fruits and vegetables that are more nutritious are also likely to be more flavorful, because at least some of the molecules that make them more nutritious—the antioxidants in leafy greens, for example—are volatile, or break down into flavor volatiles. Comparisons of the nutrient content of foods over time are easier to find, and they do indeed show that nutrient levels in modern crops, by and large, are as much as 40 percent lower than they used to be. Not every nutrient shows the same decline, and not every nutrient has a direct effect on flavor. However, the overall trend is hard to ignore.
The industrialization of agriculture must also share some of the blame for the tasteless stuff in grocery stores. The peach or cantaloupe in my grocery store in wintry Canadian February has traveled thousands of miles to get there, and to survive the trip, it was almost certainly picked before it was perfectly ripe. This premature harvest cost it the chance to get the full load of sugars and volatiles that a full-term fruit could have acquired. For most fruits, which don’t continue to make sugars after harvest, there’s no way of making up for the loss. Even in August, when supply chains are shortest, many large-scale producers can’t afford to handle fruits carefully enough to let them ripen fully on the tree or vine.
But scientists like Klee are finding ways to put the flavor back into our fruits and vegetables. You’ll recall that some aromas—vanilla, for example, or strawberry—can make a sugar solution taste sweeter. If some of the volatiles in tomatoes can pull off the same trick, Klee thought, then maybe growers don’t have to sacrifice flavor for yield. He gathered up a wide variety of tomatoes—152 in all, mostly heirloom varieties but including commercial ones as well—and measured the amount of sugars and flavor volatiles that were present in each. The varieties differed enormously, with some volatiles varying by as much as three thousandfold from one variety to another.
Klee chose sixty-six varieties with very different sugar and volatile profiles and, working with Linda Bartoshuk, fed them to a taste-testing panel made up of ordinary people from the Gainesville area. For each tomato, the tasters rated its sweetness, its aroma, its tomato intensity (which Klee defines as “that concentrated ‘Wow, that’s a tomato!’”), and a few other attributes. They also rated how well they liked that tomato on a scale from negative one hundred to positive one hundred, with the endpoints being the worst and best thing they’d ever experienced. “Effectively, the tomatoes go between 0 and 35,” says Klee. “Thirty-five would be a fabulous tomato. Zero is absolutely neutral.” (I think I’ve eaten a few zeros on fast-food burgers or in February salads.)
The panelists generally liked the sweetest tomatoes best. But when Klee looked closer, he saw something more interesting: The level of sweetness that tasters perceived sometimes didn’t have much to do with the amount of sugar actually present. Tasters thought a variety called Matina, for example, was about twice as sweet as the Yellow Jelly Bean variety, but the analysis showed that Yellow Jelly Bean actually contained more sugar. Matina tastes so sweet, despite its low sugar content, because it’s rich in volatile odor compounds such as geranial that make our brains think “sweet.” (Geranial, by the way, is derived from lycopene, the molecule responsible for a tomato’s red color. Orange- or yellow-colored tomato varieties make less lycopene and hence less geranial—so they taste about 25 percent less sweet than redder varieties. Something to keep in mind when you’re buying tomatoes.)
It’s worth taking a moment here to examine why plants have these volatile aroma molecules in the first place. The volatiles that account for the flavors of the plants we eat are what botanists call “secondary metabolites.” The term refers to the fact that, for the most part, they’re not absolutely essential to the life of the plant, as molecules like chlorophyll, sugars, proteins, or DNA are. Instead, these secondary metabolites serve more subtle functions, often in defense or signaling, or are merely by-products, molecular garbage left behind as the plant performs some other biochemical task.
“Usually, I can best explain what a secondary metabolite is by comparing with humans,” says Kirsten Brandt, a plant scientist at Newcastle University in the United Kingdom. “In humans, the main secondary metabolite we have is melanin, a brown pigment. Most people have it in their hair—if you don’t, you’re blonde—and most of us can make it in the skin. It protects the skin from UV light. Plants make lots of chemicals that they could actually survive without, but they need them to interact with the world around them.”
Often, those secondary compounds are there to defend the plant against predators. The bitter taste of broccoli and mustard greens comes from molecules called glucosinolates that are poisonous to many animals, particularly insects, that might otherwise munch on the plants. They’re not especially toxic to humans—we dodged that one—but even cattle are more sensitive to the chemicals, which is why canola breeders have made low-glucosinolate varieties for growers who want to feed them to cattle. Similarly, most of the pungent flavors we know from culinary herbs are actually pretty effective deterrents to feeding. (When’s the last time you sat down and ate a plateful of rosemary or sage?)
Fruits, on the other hand, want to be eaten. The whole point of a tasty, sugar-filled fruit is to tempt some animal into eating it, carrying the seeds away to be dropped somewhere they won’t compete with the parent plant. To help achieve that end, plants endow their fruits with a suite of volatile chemicals that shout out, “Good stuff here! Come and get it!” As Klee notes, many of the flavor compounds in a fruit like the tomato are closely related to essential human nutrients such as particular fatty acids and amino acids that our bodies can’t make on their own. That makes these flavor compounds a cheat-proof signal of the fruit’s nutritional quality: the plant can’t make the flavor compounds without having the nutrients, as well.
The fact that fruits want to be eaten, but only once their seeds are ripe, also explains why “ripeness” only applies to fruits, not to vegetables. Immature fruits contain sour acids and astringent polyphenols—think of an unripe apple, or an immature persimmon—that discourage their consumption; as the seeds mature, the chemical content of the fruits changes from discouraging to encouraging. Vegetables, on the other hand, are always trying to dissuade you from eating them, so ripeness isn’t an issue.
But both fruits and vegetables have a common interest in having a recognizable flavor. Remember how we use flavor to learn which foods we want to eat and which to avoid? This is the other side of the exact same coin. Vegetables want us to remember them like Dana Small remembers Malibu and 7UP: that was awful, and I never want to ingest that again. Fruits want us to remember them for their good consequences—except, maybe, for the seeds themselves. The seeds of the coffee plant, for example, pack a potent neurotoxin, caffeine, that most of us are familiar with. In nature, far from espresso machines, this poison teaches an important lesson. “We can learn that we should not eat that plant, because it makes us giddy,” says Brandt. “But we need to be able to recognize that plant. It’s important to us, and to the plant, that we should recognize the taste.” So the coffee plant has evolved distinctive-tasting seeds, and we (that is, mammals) have evolved the taste and odor receptors to recognize those distinctive flavors. After millions of years of this coevolution, says Brandt, “you’re not in doubt, when you’re eating something, whether it’s pea or potato or broccoli. All those past defences are now sitting around being useful for us—and the plants—as labels, to point us in the righ
t direction.” Even flavor compounds that today have no toxic effect at all may well have arisen as toxins sometime in the distant past, she notes. And the pressure to evolve new compounds—new flavors—is ongoing. “Anything the plant has been using for a while, their enemies will have evolved countermeasures. So you have an arms race.”
Thanks in part to that arms race, tomatoes have at least four hundred volatiles in their fruits. However, only about two dozen are important to the flavor of the fruit, Klee finds—and the important ones aren’t necessarily those that are easiest to smell. Until recently, tomato scientists had always winnowed the hundreds of volatiles by comparing their concentration to people’s measured detection thresholds. Those compounds whose concentration soared well above threshold, they assumed, must be the most important, while those that fell below the detection threshold could be discarded as unimportant. But when Klee actually tested what made for a tasty tomato, he found that that obvious assumption didn’t hold. Some of the most prominent volatile odorants, such as the classic “tomato stem” smell you get when you brush against a growing tomato bush—and which always brings me back to happy memories of the backyard garden—make no difference to whether people like the tomato. On the other hand, some volatiles that turn out to be really important contributors to flavor are present at below-threshold concentrations. Several below-threshold volatiles, it turns out, can work together to alert the brain to their presence—just like Paul Breslin’s rose-sweet chewing gum.
Those volatiles are the secret to sweeter-tasting tomatoes, says Klee. It takes a lot of sugar to sweeten a tomato, so growers can’t do it without crippling the yield. That’s why today you can buy excellent tomatoes in the store only if you’re willing to pay a lot more for them. But volatiles don’t cost a tomato plant much—they’re present in such small quantities anyway that tomato breeders could crank up volatile levels manyfold while barely denting the yield at all. “All of a sudden, you’re doubling the perception of sweetness,” says Klee. That should make sweeter, richer-tasting tomatoes possible at a price everyone can afford.
Volatiles, incidentally, are the reason why you should never, ever put a tomato in the refrigerator. A tomato is constantly leaking volatiles into the air (which you can easily verify by sniffing a good, ripe one) and replenishing the loss by making new ones. Chilling turns off the enzymes that make the volatiles—and one of the peculiarities of the tomato, a tropical plant, is that the enzymes stay off, even after you take the fruit out of the fridge. Volatile content goes down as molecules leak out into the air and aren’t replaced, so a tomato that’s spent time in the fridge tastes less sweet and has less tomato flavor. (And, by the way, most of the volatiles leak out the stem scar at the top of the tomato—so, all else being equal, a tomato sold “on the vine” with a bit of stem attached ought to be a little more flavorful than one without.)
Klee has already taken the first steps down the road to the flavorful tomato of the future. In 2014, his team released its first two new varieties, Garden Gem and Garden Treasure, that were created by crossing high-volatile heirlooms with modern, high-yielding varieties. The hybrids yield nearly as much as the commercial varieties but keep almost all the flavor of the heirlooms, he says. As I talked tomatoes with Klee, five golf ball–sized Garden Gem tomatoes sat between us on his desk. After a couple of hours, he offered to cut them up so I could taste what we were talking about. It wasn’t an ideal test—these were April tomatoes, after all, ripened when days were short and temperatures relatively low, so there was little chance they’d reach the glorious heights of a midsummer heirloom. They didn’t—but they certainly tasted sweeter and tomatoier than anything I was likely to get in the grocery store at the time. Grown under better conditions, Klee’s two new varieties have definitely gotten people excited. As I write this, the varieties are not yet commercially available, but Klee has sent seeds to more than thirty-two hundred people who donated money to the tomato research program, and the feedback is enthusiastic. “We’ve had several people write to tell us they’re the best tomatoes they’ve had in their lives, which makes us feel good,” he says. “People really want these things. It just shows how big the pent-up demand is for good tomatoes.”
Just an hour down the road from Klee’s lab, in the sandy soils of central Florida, a plant breeder named Vance Whitaker is trying to solve the flavor problem for another fruit that’s often disappointing in the grocery store: strawberries. The big problem with strawberries is that they’re what’s called a “nonclimacteric fruit,” meaning that they don’t ripen any further after harvest. You can’t treat a strawberry like a banana or an apple or a pear—or a tomato, for that matter—and prod it into ripeness in the warehouse with ethylene gas. All you can do is let it ripen on the bush as long as you dare: once you pick the berry, it’s all downhill. It’ll never get any better. And because strawberries are so fragile, growers can’t risk letting them ripen fully before picking, because they’d never survive the rigors of shipping and handling. The result is that a grocery-store strawberry will rarely be as ripe as one you’d pick yourself—and the clearest sign of that is the white “shoulders” you’ll see at the stem end of most grocery-store berries.
What to do? Scientists could try to find a way to help the strawberries last longer, so that growers can afford to pick them when they’re riper. Or they could look for a way to boost the flavor directly. Whitaker chose the second route, and started digging deep into the chemistry of strawberry flavor, using the same techniques that Klee used for tomatoes. (In fact, the two research teams share several scientists in common, including Klee himself and Linda Bartoshuk.) Strawberries are a lot like tomatoes, he found: People like sweeter ones better, and they also prefer a more intense strawberry flavor, which depends on the volatile chemicals. And, just like tomatoes, if the plants make too many berries, they can’t afford to stock them with enough sugar. That puts breeders like Whitaker in a bind. “We could increase yield by a pretty sizeable percentage in just a couple of generations,” he explains, “but we would drastically reduce sugar content.”
One solution might be to breed a more vigorous strawberry plant that photosynthesizes more energy, so that it can afford a bigger sugar budget. A more likely option, though, might be to take a page from Klee’s playbook and tinker with the volatiles. Sure enough, when Whitaker and his colleagues looked at the volatiles in strawberries, they found several that made the berries taste sweeter, independent of the amount of sugar that was actually present. (Curiously, even though many of the same volatiles turn up in strawberries as in tomatoes, different ones affect sweetness in the two fruits. It’s all in the context.)
Strawberry flavor intensity, too, depends strongly on the mix of volatiles that are present in the berry. Whitaker is looking closely at a molecule called gamma-decalactone—the very same peachy-smelling molecule that bridged top notes and bottom notes in the artificial strawberry flavor Brian Mullin designed for me at Givaudan. Some strawberry varieties have it, some don’t. Whitaker’s team sorted through the genotypes of the haves and the have-nots—a harder task than it sounds, because strawberries have not two but eight copies of each gene—to find a single gene variant that accounted for the difference. With a clear target identified, breeders will have an easier time ensuring that any new varieties have the good gene for this important flavor compound. They can use the same technique—and all of Whitaker’s genotyping work—to find other flavor genes more quickly.
There are a few other, nongenetic secrets to growing tasty strawberries, says Whitaker. Cool temperatures, especially at night, help the plant store more sugar in the berries. As a result, Florida strawberries always taste best early in their growing season, in December and early January; quality declines as the weather heats up into February and March. (It seems counterintuitive to put strawberries at the top of your shopping list during the darkest days of winter, but that’s exactly what you should do, at least if your grocer gets berries from Florida. Berries that come from Ca
lifornia or Mexico have different seasons for peak flavor.) And just a little bit of water stress, or just a little bit of fertilizer limitation, also tends to improve flavor by slowing growth and giving the plants plenty of time to stock the berries with sugar and volatiles. In contrast, good soils don’t seem to make any difference. Whitaker’s soils in Florida are nothing but coarse sand, and many growers in Asia and the Netherlands produce delicious berries hydroponically, with no soil at all.
Tomatoes and strawberries are unusual in having crop scientists pay much attention to flavor. Most other fruits, and almost all vegetables, exist in a vast, undifferentiated sea of produce where one head of broccoli is interchangeable with the next. “The guy who’s buying the stuff for the supermarket, he wants it to taste the same as last time,” says Brandt. “What most of the supply chain wants is predictability and low price. There are no consumers wanting special broccolis. They’re just not there.” In such a milieu, it’s not surprising that the science of flavor on the farm is almost nonexistent.
We don’t know much, for example, about how a farm’s soil affects the flavor of the crops. (Not much at all, in the case of hydroponic strawberries!) Here’s Alyson Mitchell on one of the crops she studies, spinach: “I don’t think there has been a single sensory study done on spinach looking at the effect of growing environment on flavor. I would be shocked to find it.” And most other crops are in pretty much the same boat. Once again, we can get a little more information by asking about nutritional quality instead of flavor, since that attracts a little more research money, but even there, big lessons aren’t easily found.