Foods to be dried must be washed, and some peeled and cut. Others may be precooked. Cut fruits are subject to darkening through enzyme action, and must be either blanched, treated with salts or exposed to the fumes of burning sulfur (to provide sulfur dioxide). Certain vegetables may be pretreated in the same manner. Sulfuring may also be required to limit nonenzymatic browning (the Maillard reaction). Browning refers to the development of brown color. Various dried egg products (egg white, dried egg yolk and dried whole egg products) are also subject to browning and are susceptible to the development of off-flavors. In this case, the reaction involves a combination of a small amount of glucose, which is naturally present, with the proteins. Because of this, dried egg products, especially egg white, are either allowed to undergo a natural fermentation (this involves the growth of bacteria) or are treated with enzymes (glucose oxidase and peroxidase). Enzyme treatment converts the glucose to a compound which does not react with proteins. The natural fermentation method of removing sugars is not desirable from the standpoint of sanitation. However, by holding the product at 130 F (54.4 C) for some hours after drying, any disease-causing bacteria which might have survived or even increased in the product during the natural fermentation process should be eliminated.