Digestion begins in the mouth where the food is subjected to the mechanical process of grinding to break it up into smaller particles thus enabling the digestive juices to get at the food more readily. This also aids in mixing the salive of the mouth with the food. (Chewing or mastication is the only conscious work of digestion and all the subconscious processes depend upon how well this has been performed.)
Simultaneously with the chewing of the food the digestive juice of the mouth is poured out upon and thoroughly mixed with it. Saliva, as it is called, is a colorless, tasteless ropy fluid secreted chiefly by the parotoid, submaxillary and sublingual glands. Secretions from the bucal, palatine, lingual, molar and tonsillar glands also contribute to the saliva. In man it is normally alkaline in reaction, although, during fevers, while fasting, when there are digestive disturbances, and between midnight and morning it may become acid. About 1500 grams, or between one and two quarts are secreted in twenty-four hours.
Its secretion is not a simple filtration due to blood pressure but is accomplished by the action of the cells composing these glands. In common with all the cells of the body, these exercise a selective power by which they select from the blood stream the elements needed in the manufacture of saliva and reject the rest. The salivary glands are under nerve control which secures coordination.
The active principle in saliva is an enzyme known as pytalin which acts upon starches (polysaccharides), converting these into a form of sugar known as dextrines (disaccharides).
If saliva is put into a test tube with starch it will convert this into sugar. At low temperatures this process goes on slowly, the velocity increasing as the temperature increases until it reaches its maximum at about 37° C. Above this temperature the velocity again decreases, the enzyme being destroyed at about 70° C.
Pytalin is lacking in the saliva of all carnivorous and some other animals. In these the saliva is not a true digestive juice, but acts, solely to moisten the food thus enabling the animal to swallow it.
Pytalin is not present in the saliva when food that does not contain starch is taken into the mouth. The tongue contains various sets of taste buds among which are proteid and starch buds. The function of taste not only affords us pleasure, but is an all important element in the subconscious process of digestion. Particularly it serves to stimulate the flow of the digestive juices, especially those of the stomach, and to suit their character to the food eaten. The nerve impulses set into motion by the taste of foods set the mechanism into action necessary to digestion. The character of food eaten determines, through the taste buds, the character of the digestive juices released to act upon it. Saliva will be poured into the mouth but no pytalin will be present if the food eaten contains no starch. Even sugar, if put into the mouth will not occasion the release of ptyatin, although, the mouth will quickly fill with saliva.
Edinger showed by experiment that potassium rhodanate is the antiseptic principle in saliva. He found that three parts of saliva to the thousand will kill the "bacilli of cholera morbus" in one minute, while nine parts to the thousand will kill diptheria germs in the same time. Here is a constantly produced antiseptic powerful enough to destroy any germ, yet harmless to the body. Chew your food well and the saliva will aid in preventing decomposition. A pint of human saliva was collected and exposed in an open jar to the sunshine and heat of June, July and August, and at the end of the experiment showed no sign of infection or disintegration.
Pytalin is destroyed by acid in a minute percentage. Tannic acid in tea and coffee interferes with the digestion of starch. Drug acids do likewise. One half of one per cent of acid stops the action of pytalin.
Tart (acid) fruits taken with starches completely neutralize the alkalinity of the saliva, the only secretion in the body able to initiate the digestion of starches, and paralyzes the ptyalin. Besides being wet, they are also acid and thus there is both a mechanical and a chemical reason why they should not be taken with cereals or other starch.
After food is masticated it is swallowed and enters the stomach where the work of digesting the starch continues until sufficient gastric juice has been poured into the stomach cavity to render its contents acid.
Dr. Cannon of Harvard University Medical School, demonstrated that if starch is well-mixed with saliva, it will continue to digest in the stomach for up to two hours. If proteins, which require an acid stomach juice in which to digest, are eaten at the same meal, nature deluges the food in the stomach, including the starch, with acid gastric juice which neutralizes the alkaline saliva and destroys the ptyalin, and starch digestion ceases shortly.
When starches are soaked with any kind of fluid--water, milk, fruit juices, etc.--very little saliva is poured out, no matter how long one chews them. Dry starches excite a copious flow of saliva rich in ptyalin. Dry starch increases in bulk upon being masticated; soaked starch does not. Dry starches, taken into the mouth with fruits, milk, water, coffee, or tea, etc., do not excite the flow of saliva.
If starches are to digest, they must be eaten dry. Starches put into soup are never digested. When starches are not digested they lie in the stomach and produce much trouble. Soaked starches are also likely to be swallowed without chewing. Unmasticated starches, even if they were insalivated would not digest. Boiled starches do not digest.
Experiments carried on by the Defensive Diet League showed that oatmeal is never digested in the stomach. The same was shown to be true of every other cooked and soaked cereal. The stomach does not digest starches--does not secrete a starch digesting fluid--and when starches are soaked so that they do not receive saliva and ptyalin, they cannot be digested in the stomach.