According to those who tried it last week, lab-grown beef doesn't really taste like meat.So what exactly gives meat its flavor and makes us beg for more?We've long known why ice cream and chocolate appeal so much to our taste buds: It's that blissful mixture of sugar and fat. But what's so special about bacon and steak that, for most people, it trumps the growing pile of scientific data on meat's detrimental health effects?The answer, according to scientists, lies in meat's unique mixture of fat and umami (more about this taste later), spiced up in a process called the Maillard reaction - the browning that happens when we cook a piece of meat. "These are powerful stimuli to humans," says Paul Breslin, a nutritional sciences professor at Rutgers University.As much as 95 percent of what we think of as meat's taste is actually its aroma, according to a Barb Stuckey, author of "Taste: Surprising Stories and Science About Why Food Tastes Good." And the lack of strong scents is one reason why raw meat is not very appealing.Even animals seem to agree: If mice could cook, many would turn up their noses at raw meat. The first time Rachel Carmody of Harvard University offered her lab mice mini-steaks, both roasted and raw, the animals eagerly went for the cooked meat. In similar experiments, chimps, gorillas and orangutans were clear about their preferences, too: Roasting, grilling and stewing appealed to them.One of the main reasons for that is the Maillard reaction, the marriage between carbohydrates and amino acids that occurs in a slightly moist, hot environment and that produces aromas so delightful that they "make us go weak at the knees," says Adam Drewnowski, director of the Nutritional Sciences Program at the University of Washington. More than 1,000 chemical compounds are responsible for the scents of meats, and many are created by this reaction. Some smell fruity, other musty, while still others have the scent of nuts, mildew, marshmallow and even crushed bugs. When taken together, they create the enticing aroma of a backyard barbecue or a stove-top saute.Drewnowski points out that the Maillard reaction is also behind our love of baked cookies, crusty bread and roasted coffee, but the magic that occurs during the grilling of meat is probably the most potent. The Maillard reaction is also the reason why the lab-created beef burger, which tasters said was unappetizingly bland, nonetheless smelled quite good as it was being cooked.There may be a strong evolutionary basis for our idea of what smells good. As Drewnowski explains, "the Maillard reaction is a way for us to notice that the product has been cooked. The aroma of the Maillard reaction meant the meat was safe to eat."•••-Beyond the Maillard reaction, our desire for meat is often a desire for fat, scientists say. "When you think you're craving meat, most likely what you're really craving is fat," Breslin says.Fat is more energy-dense than sugars, and as such it was highly desirable for our ancestors' survival. Fat in meat has a particular texture, an appealing creaminess and juiciness. When you cook a piece of pork or beef, it's not just the Maillard reaction that occurs; fats also start to oxidize, creating delicious scents that rush toward your nose. The lab burger, created from stem cells extracted from a cow's shoulder, contained muscle fibers only and had no fat. It had been spiced with a dash of salt and colored with saffron and beet juice, and the meaty scents it emitted were there when it was being cooked in a pan - but they were "subtle," said Richard McGeown, the chef who cooked it. And without fat, the mouthfeel was not particularly good.•••Another reason why cooked meat is so hard to resist is that it's loaded with the taste known as umami (Japanese for "delicious"). This fifth taste - in addition to salty, sweet, sour and bitter - was identified a century ago, but only in recent years has it become widely accepted by scientists.In nature, three substances are responsible for umami taste: glutamate (as in Parmesan cheese, soy sauce, sun-dried tomatoes and many meats), inosinate (which is found in meat and fish) and guanylate (think dried mushrooms).When two or three of these substances are combined, the umami taste of a dish is vastly magnified, in an effect that chefs call a "u-bomb." And meat, says Toshihide Nishimura, a professor of food science at Nippon Veterinary and Life Science University in Tokyo, is a particularly good example of the mouthwatering synergy of umami substances.Humans like umami. Gary Beauchamp, director of the Monell Chemical Senses Center in Philadelphia, found that if given a choice, we go for dishes with added glutamate. We learn its taste even before we're born: Amniotic fluid is loaded with glutamate. Some scientists believe that the role of umami is to indicate the presence of proteins in food, which helps us choose good sources of that nutrient. Cooking releases glutamate, making foods more delicious. That is why, Breslin says, the umami taste - like the Maillard reaction - signals that meat has been cooked and thus is safer to eat.Does all this mean humans will always have a craving for meat? Not necessarily. If you want to cut down on meat, for health or other reasons, try a few simple tricks.To enjoy the Maillard reaction, Drewnowski suggests, choose freshly baked breads, crunchy toasts, roasted veggies. To make up for the meat fats that you're forgoing, try avocados, cheese and nuts that contain healthful fats. As for umami, Breslin suggests that tofu cooked with soy sauce, a little peanut butter and mushrooms should satisfy your craving.For lab beef to taste more like conventional meat, we will have to wait until scientists figure out how to add fat tissue to the muscle fibers. This, according to the Dutch professor who created the lab meat and provided a taste of it last week, will take "another couple of months."