Try this the next time you have an afternoon with absolutely nothing better to do: Take two potatoes, boil one and roast the other. Do the same thing with rutabagas, then turnips, parsnips, carrots, beets. . . . Just how much time do you have, anyway?
The results are startling. The flavors and textures of the roasted vegetables are so different from the boiled that you might think you've cooked two different plants. From the oven, the vegetables are intensely flavored with a crusty, chewy exterior and a puffy, light inside. From the pot, the flavors are softer and less definite; the textures more uniformly waxy or buttery.
While lightly cooked, bright, crisp vegetables might be just the thing for spring and summer, on a cold, rainy winter day, there's nothing like the warm, soft, intensely "cooked" flavor of roasted vegetables.
It seems almost magical how cooking roots this way--toss the vegetables with a bit of seasoned olive oil or melted butter, put in roasting pan, roast--is so simple and can conjure up such complex and intense flavors.
Really, it all comes down to chemistry.
When any food is heated, a chain of chemical changes begins. One is called the Maillard reaction, after the turn-of-the-century French scientist Louis-Camille Maillard. Actually a series of reactions, the final stage occurs when amino acids and sugars--which are found in almost all foods--are heated together to around 335 degrees and the complex "roasted" smells and tastes we cooks call "browning" begin to be created. This is different from (and frequently confused with) caramelizing, which occurs when sugar alone is heated.
Harold McGee, in a wonderful chapter on browning in his "The Curious Cook" (Collier Books: $13), even tries an experiment in which he heats corn syrup and adds the contents of an amino acid capsule of the type sold at health food stores. Depending on the specific amino acid (or combination of acids) used, the smells ranged from frying onions to roasting meat.
Of course, browning is impossible in a boiled vegetable. Water boils at 212 degrees--well below the threshold temperature. But it's equally obvious that when vegetables are boiled, something has happened. When heated to an intermediate temperature (at approximately 200 degrees to 220 degrees), vegetables develop flavors that are buttery rather than roasted.
The textures of the cooked vegetables are also different. When starchy vegetables are heated, the closely bound, waxy starch molecules begin to absorb water and swell, resulting in a puffy, soft texture. Because root vegetables are so high in water (ranging from 79% for parsnips to 91% for turnips), this happens whether the vegetables are boiled or roasted.
But when starchy vegetables are roasted, something else happens. While the insides get puffy from absorbing the interior water, the outside gets crusty. This is caused not by heat alone, but by the circulation of air. When dry hot air circulates over the vegetable, its surface dries out, creating a crusty shell. (Remember how you feel when the Santa Anas blow?)
For another experiment, try roasting potatoes on a flat baking sheet and in a deep roasting pan of the type frequently recommended for turkey. What happens? While the potatoes on the baking sheet develop a fine crust, those in the roasting pan will be closer to boiled or steamed than roasted. The high sides of the pan prevent the air from circulating over the vegetables.
How does all this work in the real world? In general, roasted vegetables are sweeter, with a more concentrated flavor and a more varied texture. That is not always for the best. For example, in the brassica vegetables (such as rutabagas and turnips) roasting not only develops a browned sweetness, it intensifies the vegetables' naturally bitter "horseradish-y" edge. Paired with a fatty, faintly sweet meat such as duck or pork, they work well, but by themselves they might be too assertive.
On the other hand, it would be difficult to find something bad to say about roasted potatoes. Little smooth-skinned potatoes--either red or white--cut in half and filmed with olive oil, become well-browned and crusty on the outside and fluffy on the inside, with a sweet, intense potato flavor. Carrots are also delicious, with a dark, sweet, concentrated taste.
"I roast root vegetables," says Michael Roberts, former chef at Trumps restaurant and a cookbook author. "Why? Because they taste better that way. Think of the difference between a baked potato and a boiled potato, that's the difference between any root vegetable cooked by wet heat or dry heat."