One reason why breakfast foods, such as rolled oats, are partially cooked by the manufacturer, is because they keep longer. As has been stated, the nutrients of the grain are found inside the starch-bearing and other cells, and the walls of these cells are made of crude fiber, on which the digestive juices have little effect. Unless the cell walls are broken down, the nutrients can not come under the influence of the digestive juices until the digestive organs have expended material and energy in getting at them. Crushing the grain in mills and making it still finer by thorough mastication breaks many of the cell walls, and the action of the saliva and other digestive juices also disintegrates them more or less, but the heat of cooking accomplishes the object much more thoroughly. The invisible moisture in the cells expands under the action of heat, and the cell walls burst. The water added in cooking also plays an important part in softening and rupturing them. The cellulose or cell wall is also changed by heat to a more soluble form. Heat makes the starch in the cells at least partially soluble, especially when water is present. The solubility of the protein is probably, as a rule, somewhat lessened by cooking, especially at higher temperatures. Long, slow cooking is therefore better, as it breaks down the crude fiber and changes the starch to a soluble form without materially decreasing the solubility of the protein. The fireless cooker is particularly desirable in the cooking of cereals. In experiments made with rolled oats at the Minnesota Experiment Station, it appeared that cooking (four hours) did not make the starch much more soluble. However, it so changed the physical structure of the grains that a given amount of digestive ferment could render much more of it soluble in a given time than when it was cooked for only half an hour. On the basis of the results obtained, the difficulty commonly experienced in digesting imperfectly cooked oatmeal was attributed to the large amounts of glutinous material which surrounds the starch grains and prevent their disintegration. When thoroughly cooked the protecting action of the mucilaginous protein is overcome, and the compound starch granules are sufficiently disintegrated to allow the digestive juices to act. In other words the increased digestibility of the thoroughly cooked cereal is supposed to be largely due to a physical change in the carbohydrates, which renders them more susceptible to the action of digestive juices.