Starch absorbs water, swells and becomes partially soluble in water. This begins at about 150 degrees F. Dry starch begins to change to dextrine at about 320 degrees F.

Sugar is not changed at low temperatures unless acid is present. It melts at about 365 degrees and begins to caramelize at about 420 degrees F. Sugar, boiled with acid, changes slowly to glucose or non-crystallizing sugar.

Cellulose itself is not affected by cooking, but the connecting substances are softened and it may be separated.

Protein foods are hardened somewhat by heat. Albumen coagulates completely at 160 degrees F. and will no longer dissolve in water. Other proteins, as gluten of flour, casein of milk, legumen of peas and beans, myosin of meat, are hardened somewhat.

Gelatin is formed from gristle and connecting tissue of meat, and from bones, by long continued heating in the presence of water.

Fat is not changed except at a high temperature, when it is broken apart - "split" - into fatty acid and glycerine. Some of the glycerine is changed into "acrolein" at very high temperatures, 5000 and over, which is very irritating to the mucous membrane, as is recognized by the smarting sensation given to the eyes and nose when fats are overheated. Butter begins to "split" at about 256°F., lard at 360° F., beef suet at 440° F., cottolene and snow drift at 450° F., especially prepared cottonseed oil and olive oil at 600° F. Baking Powder, a mixture of cooking soda and an acid substance, as cream of tartar, or phosphates, or alum, undergoes chemical change, whereby carbon dioxide is set free and salts - as Rochelle salts, or phosphate, or alumina compounds - are formed. The heat of the oven expands the air or gas in the food, evaporates part of the water and drives out volatile substances like alcohol. All these changes are, for the most part, physical rather than chemical in their nature. For example, in a cake after baking, the sugar is still sugar, the starch is still starch, the fat is still fat, and the albumen is still albumen. All the materials have been blended, flavors having been developed through minor but complex chemical changes and a small proportion of the starch and sugar in the crust have been changed to dextrin and caramel.