Science in the Kitchen. E. E. Kellogg
The time required for the digestion of food also depends upon the condition under which the food is eaten. Healthy stomach digestion requires at least five hours for its completion, and the stomach should have an hour for rest before another meal. If fresh food is taken before that which preceded it is digested, the portion of food remaining in the stomach is likely to undergo fermentation, thus rendering the whole mass of food unfit for the nutrition of the body, besides fostering various disturbances of digestion. It has been shown by recent observations that the length of time required for food to pass through the entire digestive process to which it is subjected in the mouth, stomach, and small intestines, is from twelve to fourteen hours.
Hygiene of Digestion.—With the stomach and other digestive organs in a state of perfect health, one is entirely unconscious of their existence, save when of feeling of hunger calls attention to the fact that food is required, or satiety warns us that a sufficient amount or too much has been eaten. Perfect digestion can only be maintained by careful observance of the rules of health in regard to habits of eating.