Boiling occurs when vapor pressure exceeds air pressure. In cooking, this technique refers to boiling water and other water-based liquids, such as stock and milk, for instance. Simmering is gentle boiling, while poaching is when the cooking liquid of the food moves but scarcely bubbles. Food can be boiled in two ways — it can either be placed in already rapidly boiling water and left to cook, or the food can be placed in a cooking vessel with cold water and the two may be heated together till the food is cooked.
How Does Boiling Work?
Boiling food is a safe way of ensuring that all disease-causing bacteria in the water or the food are destroyed. Typically, in places where water is unfit for consumption, water is boiled, cooled and then consumed. Boiling water for a few minutes kills most bacteria, amoeba and other microbial pathogens. It can help prevent illnesses such as cholera, dysentery and other diseases caused by micro-organisms. As for food, boiling makes it is easy to digest.
Boiling has a variety of effects on different kinds of foods. In meat, it toughens fibres and dissolves connective tissues; in egg, it toughens the albumin; and in cereals and vegetables, it softens the cellulose. 
When water boils, evaporation takes place all over the surface of the water. Therefore, the greater the surface exposed, the more quickly water turns to steam and its quantity decreases. This is why curries, sauces and syrups become thicker when cooked open, without a lid. Covering a dish while cooking helps in the retention of heat and water, enabling the food to be cooked faster. Boiling food on a high flame changes the shape of the food being cooked, while boiling on a low flame helps retain the shape.
Types of Boiling
As a cooking technique, there are several ways to boil food. The most direct is, of course, putting vegetables such as potatoes, beetroot, peas, carrots; meats such as chicken and lamb; cereals such as pasta, rice and noodles; and eggs directly into water.
- Simmering: This involves boiling at low heat. This is to ensure that food is cooked through and through. Simmering enhances the flavour of food and, typically, one brings foods such as soups and curries to a boil and then lets them simmer for a longer time.
- Poaching: Under this method, an item is cooked by submerging it in a liquid that is just barely simmering. It’s a far gentler technique compared to boiling. The surface of the liquid should be just "shivering", and temperature must be carefully controlled between 170°C and 180°C. This is critical as boiling can toughen meat and fish, and can make eggs and delicate fish disintegrate.  Though it’s not a very common method of cooking, it is popular with chefs and just about anything can be poached.
- Steaming: This method is a more common way to cook food. It involves placing food in a perforated container, which is then itself placed in a container of boiling. The water used for both the steaming and in the food is very nutritious.
Healthy Cooking Technique
Boiling food involves no fat, so in a sense it is healthy, but food tends to lose soluble vitamins in water. Therefore, the water that the food is boiled in is actually healthier and it can be used for other purposes, such as using it as stock or for making soup. This water is extremely nutrient rich.
Steaming, rather than boiling, particularly vegetables, is a healthier way to cook food. Green leafy vegetables such as mustard greens, turnip tops, collards and spinach are best cooked this way. Poaching too is a healthy way to cook as it retains both nutrients and flavour.
It does not matter whether food is boiled on high heat or low flame, except for the fact that cooking on high heat uses up more energy and fuel. Boiling any liquid with a lid on saves energy and hastens cooking. Of course, different recipes require different boiling techniques, be it boiling, simmering, steaming or poaching. Boiling is a slow method of cooking and does at times take longer to cook. Boiling food in high altitude areas also takes longer. Variations in composition and pressure, the boiling point of water is almost never exactly 212°F/100°C, but is close enough for cooking.
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