Digestion is primarily synonymous with solution. All solid food materials must become practically soluble before they can pass through the walls of the digestive system. Starch and like materials must be transformed into soluble substances before absorption can take place. Cane-sugar, though soluble, has to undergo chemical change before it can be absorbed. By these changes it is converted into grape and fruit sugars. These and milk sugar are taken directly or with little change into the circulation. To this fact is due a large part of the great nutritive value of the dried fruits, as raisins, dates, and figs, and the advantage of milk-sugar over cane-sugar for children or invalids.
Under certain conditions - weakened digestive power or excess of sugar - cane-sugar may remain so long in the stomach before the change takes place that fermentation sets in and a "sour stomach" results. This is one of the dangers of too much candy.
The chemical transformations of starch and sugar have been very carefully and scientifically studied with reference to brewing and wine-making. Several of the operations concerned necessitate great precision in respect to temperature and length of time, and these operations bear a close resemblance to the process of bread-making by means of yeast.
There are two distinct means known to the chemist by which starch is changed to sugar. One is by the use of acid and heat, which changes the starch into sugar, but can go no farther. The other is by the use of a class of substances called ferments, some of which have the power of changing starch into sugar, and others of changing the sugar into alcohol and carbon dioxide. These ferments are very important in all vegetable and animal life. Some are formed by small plants like yeast, which is often present in the air.
Fig. 15. Among the well known ferments is one formed in sprouting grain, which is called diastase or starch converter, and under the influence of warmth, changes the starch into a sugar. The starch first takes up water; then under the influence of the ferment, is changed into maltose, a form of sugar which is easily soluble in water. A similar process is carried on in the preparation of the malted foods on the market.
The same cycle of chemical changes goes on in the human body when starchy substances are taken as food. Such food is moistened with saliva and warmed in the mouth, becoming well mixed through mastication. It thereby becomes impregnated with ptyalin, a ferment in the saliva, which can change starch into sugar, as can the diastase of the malt. The mass then passes into the stomach and the change, once begun, goes on. In the intestines the sugar formed is absorbed, into the circulatory system and by the life processes, is oxidized, that is, united with more oxygen and changed finally into carbon dioxide and water, from which it was made by the help of plant life and sun light.
No starch is utilized in the human system as starch. It must undergo transformation before it can be absorbed. Therefore, starchy foods must not be given to children before the secretion of the starch converting ferments has begun, nor to any one in any disease where the normal action of the glands secreting these ferments is interrupted. Whatever starch passes out of the stomach unchanged, meets with a very active converter in the intestinal juice. If grains of starch escape these two agents, they leave the system in the same form as that in which they entered it.
Fig. 14. Starch Much Magnified a, Potato Starch; b, Corn Starch