For digestibility, therefore, boiled, broiled, and roasted foods are preferable to foods cooked in fats.
Such food as fried potatoes, mush, eggs, French toast, and griddle cakes, cooked by putting a little grease into a frying pan, are more difficult of digestion than foods cooked by any other means, particularly when the fat is heated so that it smokes.
Cooking of Cereals
One safe rule is to cook most foods too much rather than too little; overcooking is uncommon and harmless, while undercooked foods are common and difficult of digestion.
All partially cooked cereal foods should be cooked at least as long as specified in the directions.
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.