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

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

Albumen is hardened, "coagulated," and will no longer dissolve in water. Temperature about 160° F. Other proteins, as the 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.

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

Fat is not changed, except at a very high temperature, 500° F. and over, when it is broken apart - "split" - into fatty acid and glycerine. Some of the glycerine is changed to "acrolein," which is very irritating to the mucus membrane, as is recognized by the smarting sensation given to the eyes and nose when fats are heated too hot. Butter begins to "split" at 374 ° F, lard at 446 ° F, olive oil at 6300 F.

Baking Powder, a mixture of cooking soda and an acid substance, as cream of tartar, or phosphates, or alum, undergoes chemical change; the acid part of the mixture drives out the carbon dioxid gas of the soda and salts - as Rochelle salts, or phosphates, or alumina compounds - are formed.

The heat of the oven expands any 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.