by Bob Holmes
Lab chemists starting with pure amino acids and sugars have documented at least 621 different Maillard products; real foods, with their vastly greater diversity of chemical starting points, almost certainly produce even more products. We’ll leave their detailed identification to the flavor chemists. For now, suffice it to say that Maillard products are responsible for all the toasty, roasty flavors we get in baked goods, roasted and grilled meats, and anything else with a browned crust. Most of these Maillard products are present in only tiny quantities, but our senses are exquisitely sensitive at detecting them. On the downside, the Maillard reaction can also produce molecules like acrylamide and other carcinogens. Chemists are hard at work trying to find ways to guide the reaction into stream courses that enhance the beneficial flavors and avoid these unhealthy molecules.
For the cook, the most important thing to know about the Maillard reaction is that it requires high temperatures, typically well above the boiling point of water. That’s why fried and grilled foods brown, but stewed, steamed, and simmered foods don’t. It’s also why conscientious cooks dry the surface of their meat before searing—with less moisture to evaporate, the meat reaches Maillard temperatures more quickly so that more flavor develops. (Actually, the Maillard reaction does happen at lower temperatures, too—but so slowly that it rarely figures in cooking. Low-temperature Maillard reactions explain why powdered eggs sometimes turn brown after long storage, the impetus for some of the early research on the Maillard reaction. And black garlic, a cutting-edge ingredient these days, owes its complex, caramel-like flavor in part to Maillard reactions that take place over the span of a month at temperatures well below the boiling point.)
Because the Maillard reaction requires amino acids, or the proteins formed from them, it works most dramatically for protein-rich foods like meats, though most grains and vegetables also contain enough protein to generate some Maillard products. For some vegetables, especially sugar-rich ones like onions, a second browning reaction—caramelization—is also important. In caramelization, sugar molecules react with one another, rather than with amino acids, to form a similar cascade of flavorful products. Since sugars lack the nitrogen and sulfur atoms found in amino acids, however, caramelization produces a narrower range of flavor compounds, and less of the meaty, roasty flavors of Maillard products. From the cook’s point of view, though, both can be treated as a single, high-temperature browning process.
As complex as browning is, cooks can still steer the process to some extent. Meat that contains more fat will feed more fatty acid breakdown products into the reaction, producing more of the roasty furans that make a rib roast of beef or a leg of lamb so delicious—a big reason we like to roast these cuts without trimming off all the surface fat. Cooking temperature makes a big difference, too, by pushing the flow down one branch or another of the Maillard stream.
For any carnivore, of course, all this raises a practical question: What’s the best way to grill a steak? As it turns out, this has been the subject of sober scientific study by a meat scientist in (where else?) Texas named Chris Kerth. I phoned Kerth in his office at Texas A&M University, a hotbed of agricultural research, to get the scoop.
The hotter you cook a steak, the more you shift the balance from the beefy, brothy fatty acid breakdown products toward the roasty, nutty Maillard products. “You have a whole continuum that you can play with to customize the flavor,” says Kerth. “There’s a lot of restaurants where that’s their claim to fame, is cooking their steaks at 1,800 degrees—which is obviously going to be for a very, very short period of time. That’s what creates their signature flavor.” For thicker steaks, the outside would burn black before the middle cooks adequately, so these restaurants often sear their steaks to develop the right Maillard crust, then finish cooking in a gentler oven.
Most of us don’t have access to temperatures like that. To find out what works best at more typical cooking temperatures, Kerth embarked on the lab version of a cook-off. He bought whole beef strip loins and cut them into steaks either one-half, one, or one and one-half inches thick, then cooked the steaks to well done at one of three different temperatures: 350, 400, or 450 degrees Fahrenheit. It’s hard to get the temperature exact on a grill, so Kerth cooked his steaks in preheated cast-iron skillets instead. (Science demands some sacrifices. The bigger sacrifice here, actually, is that Kerth fed his cooked steaks not to hungry Aggie volunteers, but to a gas chromatograph, again in the interest of greater precision.) As you’d expect, the thinner steaks (and those in the hotter pans) cooked through more quickly, which left less time for roasty Maillard flavors to develop. In thin steaks, as a result, tallowy, fatty, green flavors tended to dominate, while thicker steaks were more roasty, nutty, and buttery—but also had more acrid flavors. Since then, Kerth has also cooked experimental steaks for actual people, and he reports that most of them preferred the flavor of thick steaks cooked at relatively low temperatures. “That’s been my recommendation, is to find a little bit lower temperature,” he told me. “There’s also an impact on tenderness—the lower, slower grilling results in more tender meat.”
The third main way that cooks can create flavor in the kitchen is through fermentation, the process that creates such diverse flavor wonders as cheese, bread, soy sauce, kimchi, and beer and wine. Actually, fermentation is probably better described as a form of herding than cooking, because what we’re really doing is managing the microbes that are doing the hard work of breaking down sugars and other molecules in the food, releasing volatile flavor molecules as they go. Often, a whole ecosystem of microbes—bacteria, yeasts, and other fungi—is involved in a fermentation. As we saw for wine in the previous chapter, the outcome of fermentation depends on exactly which microbes are involved.
That’s easiest to see in the case of cheese. Several species of lactic acid bacteria attack the lactose in milk, generating sour-tasting lactic acid as a waste product. As the milk acidifies, its proteins curdle into a semisolid mass that the cheesemaker strains and presses to form the basic starting point for the cheese. Now things get far more diverse, as cheese makers can encourage different sets of microbes to take over the job. If a fungus called Penicillium camemberti settles in, its microscopic filaments form a whitish rind on the outside of the cheese and secrete enzymes that break down the casein protein, gradually liquifying the center of the cheese and generating the sharp, ammonia aromas of degraded proteins that mark a ripe Camembert. On the other hand, the related Penicillium roqueforti favors a different set of enzymes that break down the milk fats in the cheese, yielding sharp-flavored fatty acids and 2-heptanone, the signature flavor compound of blue cheeses such as Roquefort. Bacteria in Swiss cheese produce propionic acid, which contributes the nutty flavor of that cheese. The reddish rind of Limburger cheese is rich in the bacterium Brevibacterium linens, which produces sulfury by-products that give the cheese its stinky, body-odor quality (an apt analogy, since a related species lives in human armpits). Many other microbes contribute minor notes to the flavor of cheeses—indeed, the complexity these minor microbes add is the main reason for the deeper, more complex flavors of raw-milk cheeses. (These complex microbial ecosystems are also what makes sourdough bread more flavorful than bread risen from plain old, cultured baker’s yeast.)
One of the big questions both professional chefs and amateur home cooks want to know about flavor is which ingredients go well together. Until recently, however, every cook—from the most primitive tribeswoman to world-famous chefs with three Michelin stars—has worked entirely by trial and error. We learn what goes well together by combining ingredients and seeing if the result tastes good. (Actually, most people cook what their culture has always cooked: Vietnamese savor fermented fish sauce, hot chilis, and lime; those from southern India favor mustard seed, coconut, and tamarind; southern Italians mix tomato, garlic, and basil. But this merely pushes the trial and error into the distant past.) The approach has obviously worked well, as any stroll through the restaurant districts of N
ew York or San Francisco would reveal. But it’s hard to get very far off the beaten path using trial and error. To really explore the far reaches of possibility, it would help a lot if we could find some general principles that underlie and guide our choice of flavor combinations that work.
You’ll sometimes hear chefs cite the principle, “What grows together, goes together,” as a basis for their flavor pairings. It’s not hard to come up with some outstanding examples: morels with asparagus, lamb with thyme and rosemary from the Mediterranean hillsides where it once grazed, venison with cranberries and wild forest mushrooms. Zearfoss particularly likes the combination of apricots with the chanterelle mushrooms that grow in apricot orchards. But is there any scientific reason this principle should hold true?
At one level, yes—because it directs your attention to ingredients that are local and in season, and therefore most likely to be at their peak of flavor. Why would you not pair morels and asparagus in the springtime, when they’re both at their best? At a slightly deeper level, the grows-together/goes-together principle can be seen as an endorsement of traditional flavor pairings. After all, for most of the history of civilization, cooks had no choice but to combine foods that grew together—local, seasonal food was all that was available (especially if you think of winter storage foods as another type of seasonal ingredient). Over the course of generations, cooks learned which combinations were most pleasant, and these became fixed by tradition. Meanwhile, no one much notices the pairings that didn’t make the cut. (Spinach grows well in the springtime, too, but no one makes a big deal about pairing spinach with morels.) The upshot is that the grows-together/goes-together pairs that come to mind are largely the ones that have passed our ancestors’ taste tests. Following one of those pairings is likely to yield a better result than you’d get with a random pairing of two ingredients—chili peppers and turnips, say—that aren’t naturally found together and therefore haven’t been vetted by tradition.
On the other hand, there’s probably no basis in the science of flavor chemistry for expecting that ingredients from the same place would combine particularly well. As we’ve seen, the molecules that give fruits and vegetables their flavor do not come from the soil directly but are made by the plants themselves. That means there’s no reason why two plants that grow together should be more likely to make similar flavor molecules, or molecules that are compatible in some other way.
We can push this a little further, though. Because our expectations and our previous experience play a big role in our perception of flavor, and especially in our flavor preferences, we might predict that familiar, traditional combinations of ingredients—based, of necessity, on foods that grow together—would strike us as more pleasant, on the whole, than novel combinations. What grows together, goes together not necessarily because it’s intrinsically better, but because we’ve tried it before and we expect to like it.
There’s another problem inherent in puzzling out pairs of ingredients that go well together: it can quickly become overwhelming, because the number of possible combinations explodes far too fast to evaluate all of them. Consider the pizza problem: If I have twenty-five different toppings, I can make twenty-five different one-topping pizzas, so it’s reasonable to ask you which one you like best. But if I’m making a two-topping pizza, you’ve got six hundred different combinations to evaluate (that’s 25 × 24, for the math geeks—we won’t consider pepperoni and pepperoni as a two-topping option), and only an obsessive would work through the entire list to pick the best pair. And if you’d like a three-topping pie, you’ve got nearly fourteen thousand combinations to choose from. It’s little wonder most pizzas use the same standard set of toppings, over and over and over again.
A few years ago, Michael Nestrud, a sensory scientist then at Cornell University who is also a CIA-trained chef, realized that an arcane branch of mathematics called graph theory might provide fresh solutions to the pizza problem by helping identify appealing food combinations more quickly. Despite its name, graph theory has nothing to do with the bar charts and zigzag lines most of us call graphs. Instead, it’s all about groups of connected objects—in this case, foods that go together. Nestrud’s insight was that you should be able to recognize a good three-topping pizza by seeing that each topping pairs well with the other two. Mathematically, this is identical to picking out “cliques” of Facebook friends where every member of the clique is friends with every other member.
So Nestrud made a list of pairs of possible pizza toppings and asked several hundred university students to give a thumbs-up or thumbs-down to each suggested pairing. From their answers, he compiled a list of “good” topping pairs, such as pepperoni and mushroom. Then he used the mathematics of graph theory to pull out sets of three or more toppings for which all the pairs were on the “good” list. These three-topping pizzas also turned out to be more popular than you’d expect by chance.
Of course, you don’t need advanced mathematics to top a pizza. However, Nestrud’s approach attracted serious interest from the U.S. Army, which desperately wanted to make tastier field rations. Soldiers in combat situations need food that is light, nutritious, and—above all—durable, and for decades, that has meant the dreaded MRE (the acronym stands for “meal, ready-to-eat”). MREs are precooked meals sealed into foil pouches. From the Army’s point of view, MREs are great. They’ll last for years, and soldiers can just grab them and go. The problem is, soldiers quickly get bored with the meals. It’s already hard to get soldiers to eat enough when they could be shot or blown up at any moment, and boring food doesn’t help matters. So the Army puts a lot of effort into making MREs as appealing as possible under the circumstances.
Each MRE contains an entree, side dishes, fruit, dessert, snacks, condiments, candy, and beverages, each chosen from up to thirty-two different options. These components could conceivably be mixed and matched in any which way—more than twenty-two billion different combinations in total. Which ones would the soldiers like? The Army hired Nestrud—fresh out of graduate school with his PhD in pizza toppings—to figure it out.
Using the same approach he took with the pizza, Nestrud designed a questionnaire that listed possible pairs of items and asked soldiers whether they’d want to eat them at the same meal: beef roast with vegetable couscous, meatballs and gravy with barbecue sauce, beef taco filling with jalapeño cheese spread, chicken fajita with bacon cheese spread, and so on. Using the pairs most commonly approved by the soldiers, Nestrud could then assemble entire MRE menus that he predicted would prove popular (top of the list: chili with beans, Mexican mac and cheese, herb-citrus seasoning, crackers with chunky peanut butter, fruit, cookies, and cheese pretzels), and others that he predicted the soldiers would hate. When he showed those menus to actual soldiers and asked them to rate their compatibility, the soldiers’ ratings matched his predicted ones almost perfectly—real-world validation that you really can use Nestrud’s approach to predict flavor pairings.
Nestrud’s next move was to a consulting company, where he used his graph-theory technique to help identify which snack foods people like to buy at the same time. Grocery stores and fast-food restaurants could then display these “go togethers” next to each other in the store so that consumers who bought one might happen to buy the other, as well—seemingly on a whim, but really the result of careful thought and planning by the seller. (Nestrud has no idea whether his clients actually put his recommendations into practice.)
In his current job as sensory scientist with Ocean Spray, America’s largest cranberry company, Nestrud is trying a different twist to this flavor-association game. Every day during the winter of 2015–2016, he searched through Twitter’s daily archives and gathered every tweet that mentioned certain flavorrelated key words. (The details are secret, of course, and Nestrud was careful not to mention the word “cranberry” when I spoke to him, but it’s a pretty safe bet that it was one of his key words.) Cleaning up the data took a lot of work. He had to toss out key word hits that did
n’t really refer to flavor, such as references to cranberry-colored paint for the bathroom or mentions of orange that referred to the color of the uniforms of the Denver Broncos football team. And conversely, he had to ensure that “cranberry,” “cran-apple,” and “cran-raspberry” grouped together as similar flavors, and likewise “orange,” “mandarin,” and “tangerine.”
In the first four months, Nestrud accumulated nearly twelve thousand relevant tweets—enough to get a good sense of what other flavors the Twitterverse thought of when it thought of cranberry. Better yet, his sample included both Thanksgiving and Christmas, as well as the postholiday lull, so he could see how flavor pairings changed through the seasons. The results might not lead directly to new products, but they’re the first step in a long creative process. “The ultimate goal is not to make any final decisions,” says Nestrud. “It’s to generate hypotheses about products we wouldn’t have thought of on our own that we can then go out and validate with real consumer testing.”
Professional chefs like to push the limits of tradition, and so do adventurous eaters. One of the great joys of eating is to discover a novel mix of ingredients that works unexpectedly well together, breaking us out of the comfortable confines of tradition and into a new world of possibility. We can find these new combinations through trial and error, of course, or we can rely on the intuition of gifted cooks, which is essentially trial and error inside the cook’s imagination. But perhaps our search for delicious novelty can also get some guidance from what we know about the science of flavor.