Simmering is a wet-heat cooking method whereby food is submerged in liquid at a temperature near, but below the boiling point. Simmering is but one train station on the track from poaching to boiling. To fully appreciate simmering, the entire journey should be reviewed.
Poaching, simmering, and boiling are all means of cooking foods in hot liquid. In all three methods the food is cooked via conduction, (from contact with the water molecules) and convection (from the movement of the water molecules). The rate of energy transmission is significant. Because water is denser than air, foods will cook faster in hot water than in an oven. Poaching, simmering, and boiling are distinguished by the temperature of the fluid medium. Poaching is performed at 160 to 185 degrees. Simmering lies between 185 and that elusive concept known as a full boil. Boiling is of course, the temperature at which a full boil is achieved.
Boiling is the process by which water is heated to the point that the energy it has absorbed causes the molecules to break free and dissipate into the surrounding air. In a word, it vaporizes. The boiling point of water is 212 degrees Fahrenheit at sea level and decreases with altitude. Water molecules must overcome atmospheric pressure to escape their liquid state. The lower your altitude, the more amount of atmosphere bearing down on you, and hence the higher temperature required to boil the water. So for the sake of our discussion, assuming you’re not boiling lobsters in Denver, simmering lies between a temperature of 185 and 212 degrees. (In Denver by the way, the boiling point of water is 202 degrees). As for visual indicators, when poaching only a few bubbles are lazily making their way to the surface while at a full boil there is a roiling barrage of bubbles. Simmering is obviously in-between.
Returning to numerical indicators, the temperature differences between poaching, simmering and boiling are not arbitrary and have significant ramifications for the food being cooked. The hotter the liquid, the more destructive it’s force, not only from the higher temperature but from the increased turbulence as well. You would never put a fish fillet into boiling water. The heat and agitation would disintegrate it. Boiling is ideal for structurally stout foods that require more intense heat to soften them such as pasta or dried beans. At the other extreme, poaching is ideally suited to more fragile foods that require gentle heat such as that fish fillet we mentioned or eggs. Simmering, being in the middle temperature zone of wet heat cooking encompasses a greater variety of foods since fewer examples are found at the extremes.
Many times foods are brought to a full boil first and then reduced to a simmer for extended cooking. The initial and brief boil may be used to kick start the softening of the food, dissolve certain types of ingredients, bring starch based thickeners to full thickening power, or facilitate the reduction of the fluid medium. Soups, stews, and braises are the quintessential “bring to a boil and reduce to a simmer and cook” type foods. (Braises and stews however should be performed at a gentle simmer, almost knocking on the door of poaching).
The nature of the simmering fluid and the cooking duration is based on the type of food being cooked and the recipe. I apologize for being so vague but the examples truly run the gamut here. Foods can be simmered in plain salted water, water that has been flavored by the dish’s preceding ingredients, a previously crafted fluid such as stock or court bouillon, a natural fluid such as cream, or some combination thereof. Likewise, cooking times vary depending on the goals: reduction of the fluid to intensify the final concoction, cooking/softening of the ingredients, flavor development, gelatization of starch molecules, or again, some combination thereof.
An issue with simmering that many recipes annoyingly don’t mention is whether the food should be covered or not. If the recipe fails to address the cover issue, here’s what must be kept in kind. The liquid will reduce minimally if the food is simmered with the cover on. Obviously then, if your goal is to reduce the fluid, you will need to simmer your preparation uncovered. Sometimes however you may start it covered, to ensure there is enough fluid to cook the food, and then uncover it at some point to reduce it. Recipes requiring prolonged cooking may even necessitate adding extra liquid along the way. Also, keep in mind that covering the food traps more heat so maintaining the same temperature uncovered will require a higher dial setting on your stove.
Finally, be mindful of the nature of the ingredients to be simmered. Specifically, one must be cognizant of ingredients that require protracted simmering and those that do not. Hard root vegetables, and tough meats with plenty of connective tissue demand longer cooking. Of course the size they are cut into influences cooking duration as well. Greens generally take less time but even within this category there is variability. Some, like collard greens or kale are a little tougher and take more time. Fresh spinach on the other hand quickly becomes mushy if cooked too long. Delicate herbs such as parsley, basil, cilantro, or dill should be simmered briefly or not at all. It doesn’t take much cooking to completely dissipate their gentle essences. Hardier herbs such as rosemary, thyme and bay leaves can be added earlier.