Although they're variations of the same process, these methods are essentially distinct. Here's why it's good to know the difference.
While neither simmering nor boiling is difficult, both are essential techniques used to prepare everything from pasta to green vegetables to stewed meats. They're really degrees of the same thing, but the effect each has on food is profoundly different. These two basic cooking methods are used in most kitchens every day and require little more than a heavy-bottomed pot or saucepan to evenly distribute the heat.
Unlike the French, who are gifted with a vocabulary that describes the stages of a liquid about to boil (such as fremir, which means to tremble or shake), we have no equivalent words to describe variations in simmering. But for most purposes, a simmer is the stage when the water is in motion but almost no bubbles break the surface; they're trying to, but the water's surface tension holds them in place. Boiling, though, refers to liquid that's in full motion, with bubbles rapidly rising to the surface. The recipes and tips in this package will distinguish between the two and illustrate when each works best.
This technique cooks food at a relatively high temperature―212 degrees is the boiling point for water at sea level. When liquids boil, bubbles break through and pop on the surface while the whole batch of liquid churns vigorously. Bubbles are caused by water vapor, a gas, rushing to the surface.
What boiling does. In the case of pasta, churning, boiling water keeps the food in motion, prevents sticking, and cooks quickly so the pasta doesn't get soggy. Green vegetables are tossed into boiling water to cook as quickly as possible so they retain their flavor and bright color in a process called blanching; if they were to simmer gently in a covered pot, their color would dull, and they would lose much of their texture. Boiling causes speedy evaporation, a useful effect for reducing sauces, where the volume of the liquid decreases and flavors are concentrated.
Boiling liquid. When ingredients are boiled, they are done so in water, sometimes containing salt and oil or butter for flavor and texture. The food is usually added to the liquid once it reaches a boil.
Best bets for boiling. This intense cooking method is well suited for pasta, some grains, and green vegetables. Boiling is also useful for reducing sauces.
A cooking method gentler than boiling, simmering refers to cooking food in liquid (or cooking just the liquid itself) at a temperature slightly below the boiling point―around 180 to 190 degrees. It's trickier than boiling because it requires careful regulation of the temperature so that the surface of the liquid shimmers with a bubble coming up every few seconds.
What simmering does. Simmering cooks food gently and slowly. Delicate foods such as fish are poached at or below a simmer to prevent them from breaking apart. Meats that are simmered remain moist and fork-tender, while boiled meats are often dry and tough because the heat of boiling liquid can cause their proteins to toughen. Stocks are simmered so the fat and proteins released by any cooking meat or bones float to the top, where they can be skimmed off instead of being churned back in, which can make the stock cloudy and greasy.
Best bets for simmering. This technique is more versatile than boiling and lends itself to a variety of foods. Simmering is used to cook proteins (fish, poultry, and meats), often in the form of poaching (cooking in enough liquid to cover the food) and braising (cooking in a small amount of liquid). It's also essential when making broth or stock. Whereas boiling works well for tender green vegetables, tough, fibrous root vegetables (such as potatoes, turnips, and beets) are best simmered so they cook evenly throughout.
Maintaining a simmer. A constant simmer isn't always easy to regulate, especially on a gas stovetop. Even at the lowest setting, the heat may be too intense and cause the liquid to boil. Turning the flame too low may cause it to extinguish, or the self-lighting mechanism may click incessantly. To avoid this, put the pot to one side of the flame, or use a device called a flame tamer or heat diffuser (or sometimes called a simmer ring) to absorb some of the stove's heat.
Simmering liquid. Food is usually simmered in flavored liquid, such as broth/stock or wine, but sometimes water is used. As a general rule, add meat to cold liquid, and bring it up to a simmer. If you add uncooked meat to already-simmering broth, the meat immediately releases proteins that cloud the broth. When you start the meat in cold liquid, these proteins are released more gradually and become entangled with one another in a frothy mass that's easy to skim off the surface. Fish are an exception. If you start poaching small pieces of fish in cold liquid, by the time it comes to a simmer, the fish will be overcooked.
The bottom line.
• When simmering, a small bubble or two should break through the surface of the liquid every second or two. If more bubbles rise to the surface, lower the heat, or move the pot to one side of the burner.
• If simmering meat or large pieces of fish, place the food in cold water, and then bring it up to a simmer.
• When boiling vegetables or pasta, add the uncooked food to water that's fully churning.