The term baking is usually applied to flour-based foods, for example wheat, oats, maize and sorghum. These grains and flours have a relatively long shelf life. The main purpose of baking is to change the eating quality of the staple and to add variety to the diet. It is also a means of preservation, extending the shelf life by removing moisture and inhibiting enzymes and bacteria. After baking, goods such as breads and pastries have a shelf life of 2 to 5 days and other goods such as biscuits and some cakes have a shelf life of several months so long as they are correctly packaged. During baking food is heated by the hot air in the oven. Moisture at the surface is evaporated by the heat and a dry crust forms. Biscuit production involves slower heating so that moisture is also lost from the inside of the product. Baked goods are produced from either doughs or batters which are a mixture of flour and water made by mixing, beating, kneading or folding. The processing method depend on the ingredients being used and the product being made.