Have you ever noticed that a whole onion smells different from one that’s been cut? Have you ever wondered why? Here’s the answer: Physically, an onion is 90 percent water, trapped in a fairly flimsy network of cellulose. Within that network is a subnetwork of smaller cells, called vacuoles. These vacuoles separate a variety of chemical components suspended in the water. It’s only when the vacuoles are ruptured, either by cutting or by smashing, that these chemical components combine and then recombine again and again in a cascade of chemical reactions, creating the smell and taste we associate with raw onions. Most simply put, what happens is that the contents of these separate vacuoles combine to form a variety of sulfur-rich compounds called sulfonic acids. These acids in turn combine to form still more compounds that provide most of the fresh-cut onion character. What’s more, this chain of reactions happens in a flash. It’s a little miracle. In fact, not until the 1970s had science advanced to the point that it could begin to decipher what happens in that fleeting instant between the time your knife touches the onion and the fumes reach your nose.
Think about it: the chopping of an onion is one of the most common acts in all of cooking. Any good cook has done it thousands, probably millions of times. Yet how many have ever stopped to think about what is really going on? All of this is neither trivial nor purely technical. For example, it’s important to realize that these sulfonic acids are extremely unstable, meaning they go away quickly. One of the places they go, of course, is right up your nose, which triggers the crying response we associate with chopping onions (for this reason, these chemicals are called lachrymators, from the Latin word for tears). More critically, they are both water-soluble and heat-sensitive, which means that the chemicals will dissolve in water and will vaporize when heated. In short: soak an onion or cook it and those acrid flavor characteristics go away. By the same token, and perhaps just as useful, chill an onion or rinse it under cold water and you won’t cry as much when chopping it. Also, a sharper knife will damage far fewer cells than a dull one. And what about those so-called sweet onions, the Vidalias or Mauis or whatever you want to call them? Though sweet onions cost significantly more, they usually contain no more sugar than plain five-pounds-for-a-dollar yellow storage onions. They taste sweeter because they are much lower in the acrid sulfuric compounds (as well as in the enzyme that produces much of the onion flavor). The practical application of this is that while raw sweet onions are delicious on hamburgers or in salads, it is spendthrift to cook one. Take away those sulfuric acids by cooking, and a yellow storage onion will actually taste much sweeter than the so-called sweet. You can even make raw storage onions taste sweeter by soaking them in several changes of cold water (hot water is more effective at dispersing the acids, but even that small amount of heat will begin to cook the onion, breaking up the delicate physical framework and robbing it of its crispness). Each time you rinse the cut onions, you will note that the water becomes milky. That is the trail of the sulfurous compounds. Use vinegar to rinse them, as they do in Mexico, and your onions will seem even sweeter, because the remaining sulfuric acids are overshadowed by more palatable acetic acids.
There are other lessons for the cook in this little bit of onion chemistry. For example, now it should be clear why the size of the dice you cut is important. The smaller the pieces of onion, the faster the cellulose framework breaks down and the faster the sulfuric compounds go away. Chop an onion small and it will melt into the background, its residual sweetness forming an almost imperceptible harmonizing flavor. Leave it large if you want both texture and flavor to retain some bite. You can control the effect by how you cook the onion as well. In a hot pan, it will cook so quickly that some of the sharp flavor will remain, as will some of the crisp texture. Cook it slowly and, again, it will melt into the background, flavoring everything else without retaining much of its original identity. What’s more, all of these things are equally true for the other members of the onion family: garlic, shallots, chives, green onions and leeks. They are all built the same way; the differences in flavor are due to subtle differences in the chemicals involved. Garlic, for example, follows the same process but breaks down into a slightly different set of chemicals.
The kitchen is full of such little miracles, froom the browning of meat to the emulsion of a sauce. How are various meats different from one another? Why do you cook pork differently from bbbbbeef? How do various cuts within the same type of meat differ? Why do you cook a leg of lamb longer than a rack? And what about chicken and fish? How is frying different from roasting, and how is steaming different from either of these? Why are some potatoes better for boiling and others for baking? Why can you stick your hand in a 450- degree oven but not in 212-degree boiling water? Cooking is full of questions that science can help answer -- questions you might not have even thought about asking but that can make you a better cook. In the good old days, you learned to cook in the kitchen. You worked at the elbow of a master -- your mother, a great chef or the fry cook down the street -- and you absorbed the basics. You learned by watching and repeating. You saw what they did and then you tried to do it yourself, mimicking as exactly as possible every act they performed. When you had absorbed a sufficient amount of knowledge, you then became the teacher, passing along exactly the same lessons in exactly the same way. There is much to be said for tradition, but as a method of instruction, it has its drawbacks. In the first place, it puts an enormous burden on the talents of that one teacher. If your mom/chef/fry cook was, let us say, something less than supremely skilled, bad habits may have been passed along every bit as easily as good ones. It’s a fairly limited way of cooking too. If you ever want to move beyond your teacher’s range of dishes, you’ve got to find another mentor, or you’re out of luck. There’s the story about the daughter who is learning to cook. Her mother teaches her that when cooking a ham, you always cut off the shank end. She asks why, and her mother explains that that’s the way her mother taught her and that it is done to tenderize the meat. She asks her grandmother why, and the grandmother tells her that that is the way her mother did it and it’s because the meat tastes better that way. Puzzled, she visits her great-grandmother out on the farm and asks her for her story. “Well,” she says, pointing at the little old wood-fired stove, “That’s the only way I could get it to fit.” But perhaps the biggest drawback to tradition as a way of instruction is that it assumes there’s someone around who is actually cooking. More and more, that seems to be a dangerous assumption. While microwave meals and TV dinners may be a boon to busy working families, they have been deadly to the tradition of home cooking. We are now three and sometimes four generations removed from the age of real cooks -- those who made do using the raw ingredients at hand, without the aid of food industry shortcuts. But by now this is an old song. What’s rarely asked is whether we would be willing to listen even if those imaginary teachers of the past were still around. We’ve changed, and the unquestionin...