Boiling is one of the most basic cooking methods. It's used to cook everything from pasta to fresh vegetables to meats. However, there can be different stages of boiling, all of which are defined by different temperatures of the liquid during the cooking process. A rolling boil is the highest, hottest cooking level possible, while a gentle boil, also known as a simmer, is a more moderate cooking level.
Both rolling boils and gentle boils are moist-heat cooking methods, where the food is submerged in the cooking liquid. The primary difference between the two is the temperature of the cooking liquid, although there are other visible characteristics.
Moist-heat cooking methods mean that foods are cooked with water, stock or steam. Moist-heat cooking methods keep foods moister, as the natural moisture in the foods is retained rather than dissipated, as would be the case in dry-heat cooking methods, such as broiling or grilling.
In addition to rolling and gentle boils, other moist-heat cooking methods include poaching and steaming.
All moist-heat cooking methods can be done with water, with or without seasonings. Common additions for seasoning liquids for a rolling or gentle boil are: fresh or dried garlic, fresh or dried onion, whole peppercorns, bay leaves, dried chili flakes or fresh herbs, such as basil, oregano or rosemary.
Foods can also be cooked at a rolling or gentle boil in liquids other than water, or water and a mix of other liquids. Common choices include: wine, broth or stock.
A rolling boil disturbs food more, although foods will be cooked more quickly. Rolling boils jostle and disturb food, and cause the outside of foods to be cooked well before the insides are. Gentle boils, or simmers, lead to less disturbance during cooking, which leads to less disintegration. However, foods cooked at a gentle boil will take longer to be fully cooked.