Prior to World War I, ice was the most widely used means of refrigeration for perishable items. After the war, electrical refrigerators became commonly available. The availability of these systems allowed for more constant and efficient cooling, in turn allowing perishable meats, dairy products, and other items to be stored, or even frozen, for long periods of time. As a result, food producers could increase their output, since excess food could be stored for later sale or consumption. The concept of the wholesale selling of bulk foods (known as the supermarket) was, in part, made possible by improved refrigeration. The same method used in food refrigeration was also developed for use in air conditioners.