Food contains proteins, fats, carbohydrates, and water, along with salt, vitamins, flavor elements and color pigments.
Carbohydrates are starches and sugars found in vegetables, fruits, grains, beans, nuts, meats and fish. Two types of carbohydrates caused by heat are carmelization, the browning of sugar and gelatinization, when starches absorb water and swell. Gelatinization is a major factor when trying to thicken sauces and the production of breads and pastries. If you ever wish to thin out a sauce thickened with flour or starch use an acid, because acid stops gelatinization.
Fiber gives structure and firmness to plants and fruits. When you cook fruits and vegetables, you are breaking down the fiber, which softens the fruit or vegetable structure. Sugar makes fiber firmer so fruit cooked with sugar keeps its shape better. Baking soda should never be cooked with vegetables, because it will make them mushy.
Proteins are major components of meats, poultry, fish, eggs, milk, and milk products. As the temperature increases, proteins shrink, become firmer and lose moisture. Too much heat toughens protein and makes them dry. Most proteins are finished cooking at 160degreesF to 185degreesF. Acids, like lemon juice, vinegar and tomato products, speed coagulation, and help dissolve connective tissues. Meat with a lot of connective tissue is tough, but when cooked slowly with moisture they become tender.
Fats are present in most foods. They are important for frying and can be either solid or liquid at room temperature, liquid fats are oils. Fats begin to break down when heated and when hot enough they begin to smoke, this is called smoke point. A stable fat, one with a high smoke point is important when deep fat frying. Fats are important carriers of flavor. So when fats melt and are lost from food, some flavors, and vitamins are lost with them.
Minerals and Vitamins are important to the nutritional value of food. You should becareful not to destroy the vitamins, minerals and other important elements in food by long cooking. This is why it is very important to use proper cooking methods that preserve as much nutrients, taste and appearance in foods as possible.
Water is in almost every food. Dried foods contain at least 1% of water, but fresh meats, fish, vegetables, and fruits consist mostly of water. Water freezes at 32degreesF and turns to steam at 212degreesF. When water molecules turn to steam and escape into the atmosphere, this is called boiling. Evaporation is when water turns to gas at any temperature. Evaporation occurs more slowly when the temperature is lower, and is responsible for the drying of foods. Water is also a carrier of flavor and nutritional value. When water carries salt or sugars its freezing point is lowered and its boiling point is raised.
Heat is transferred in three ways, conduction, convection, and radiation. During a cooking process more than one of these methods of transfer may be happening at the same time. Example: food on a grill may be heated by conduction from the hot metal grill, by convection from hot air rising from the burner or charcoal, and by radiation from the glowing burner or coals.
Conduction occurs either when heat moves from one item to something touching it or when heat moves from one part of something to an adjacent part of the same item.
Convection happens when the movement of air, steam, or liquid spreads heat. Two kinds of convection are natural or mechanical. Stirring is a form of mechanical convection. Thick liquids cannot circulate as quickly as thin ones, so the rate of natural convection is slower. This is why it is important to constantly keep stirring sauces and thick soups, because the heat is not carried away from the bottom of the pan quickly enough and it scorches or burns the food. Stirring redistributes the heat and prevents this. Try using heavy pots made of material that conducts heat well, because the pot conducts the heat more quickly and evenly across the bottom and up the sides. Convection is the process that carries the heat from the heat source to the food. When the carrier of heat comes in contact with the food, the heat is transferred from the carrier to the food by conduction. A convection oven cooks faster than a conventional oven.
Radiation happens when energy is transferred by waves from a source to the food. Two kinds of radiation used in cooking are Infrared and Microwave. Broiling is the most popular example of infrared cooking; high-intensity infrared ovens are designed to heat food rapidly. With microwave the radiation generated by the oven penetrates partway into the food, where it agitates the water molecules. This agitation creates heat, which cooks the food. So if you put food in a microwave that does not contain any water the food will not get hot, because microwaves penetrate no more than about 2 inches (50mm) into foods, heat is transported to the center of large pieces of food by conduction, the same as in roasting.