Simply stated, the difference between boiling, simmering, and poaching is just a matter of degrees. At 212°F, boiling is the hottest of the three methods. Next is simmering, in the 185° to 205°F range. Finally, there’s poaching, the most gentle method, from 160° to 180°F. But you don’t have to consult a thermometer to tell the difference—your eyes are all you need. The real issue for the cook is matching each method to the right food.
Poaching requires the most finesse of the three methods.
Keeping the temperature constant takes practice as well as a burner that can hold a very low heat. We poach the most delicate of foods, like eggs, fish, fruit, and some organ meats. Part of the nuance of poaching is knowing when the liquid (water, stock, or wine, for example) has reached the right temperature. The surface of the liquid should just shimmer with the possibility of a bubble. The food must be completely submerged, which is why some recipes suggest covering the food with parchment. (Covering the pot with its lid would quickly raise the temperature to a simmer.)
Simmering is ideal for braises, stews, soups, stocks, or any food that needs a high but gentle heat.
High-protein foods like meat and poultry fare much better when simmered, since the proteins are less apt to toughen and dry out at low temperatures. Meat cooks more slowly, giving you more control and a longer window to gauge doneness. Simmering is also essential for naturally tough cuts, like pot roast, where the slow, gentle heat melts the connective tissue into succulent gravy. A simmer, sometimes called a gentle boil, is characterized by the slow, small bubbles that periodically rise to the surface. The gentler and slower the bubbles, the lower the temperature. You can simmer with a lid, but remember that the temperature inside the pot will rise, and the simmer can easily turn into a boil.
Most people can recognize a liquid at a full boil by the vigorous bubbles that rise to the surface and break.
The high temperature and turbulence of a boil disperse the starch in pasta or cook sturdy vegetables like green beans and beets. But the high heat can easily overcook food, making vegetables limp and meat tough—ever wonder what gave boiled dinners such a bad name?