Making flour-mixtures light has been brought about in different ways through the ages that cooking has been practiced. Present-day methods probably include something from each of those of the past. But they are now applied with more accurate knowledge of what will happen. They can therefore now be used to do what is desired, while avoiding what would be unfavorable for human food. Better results are thus possible.

Air that fills the spaces between the cells of food, when heated, expands. So does air that is beaten into food. When beaten egg-white is added to a mixture, air-leavening is the method of raising or making that food light. This is not equally applicable to all types of flour-mixtures.

Through experience with such mixtures and foods in general it was observed that foods allowed to stand changed, but not always in the same way. Sometimes the change improved the food, sometimes it left it unfit for use. By studying these changes it was discovered that the atmosphere seemed to contain something invisible that caused this, as it did not occur when air was excluded.

Among the changes noted were rising and molding of bread, souring of milk, ripening of cheese and game, decomposing of meat. It was further noted that some of these changes in food-substances were accompanied by gases being given off.

From early times it has been known that a mixture of flour and water when it stood in a warm place would rise. The cause of this was finally found to be the growth in the mixture of yeast plants that entered it from the air. In growing and taking their food for growth from the mixture it was discovered that they so broke up some of its constituents as to form the gas that expanded in the warm mixture and raised the mixture.