When you take any food, the first thing that happens in the mouth is, that it is broken up by the act ofmastication, and intimately mixed with the saliva. The saliva is the mixed secretion of the salivary and mucous glands of the mouth, and is a viscid, ropy, alkaline, opalescent fluid. It contains about 50 per cent, of solids, viz. epithelium, mucin, ptyalin, albumin, and salts. Its ash shows the presence of potassium, chlorine, and phosphoric acid. Saliva appears to possess two functions, both of great importance.
It facilitates the act of swallowing, at the same time keeping the mouth clean from particles of food, which, it allowed to decompose, would injure the teeth. Its other action, and without doubt its most important one, is that of changing starch into maltose and grape-sugar. This action is due to a strong and most energetic ferment called ptyalin, or, better still, salivary diastase, which is able to act upon all starchy materials, not even excepting glycogen.
How Food is Digested in the Mouth?
By the action of ptyalin, starch is changed into maltose and a little grape-sugar. This change goes on very much quicker if the starch has been sufficiently cooked to rupture the envelopes of cellulose which enclose the starch granules, and in such case is very little slower than the action of the pancreatic juice in the same direction. The rapidity of the action, indeed, is such that after a very few seconds the mixture will begin to have the property of reducing iodine from a solution of iodine in iodide of potassium. However, before the end product maltose is reached various intermediate substances are formed from the starch, called respectively amylodextrin, erythrodextrin, and achroodextrin, which differ in their reducing power.
The first effect of the salivary or pancreatic ferment upon starch paste is to liquefy it. If you test as soon as liquefaction has taken place you will get a pure blue with a weak iodine solution, and a very slight reaction of sugar with Fehling’s solution. In a few more minutes you will get, by adding more iodine to the diluted blue solution, a deep violet colour, showing the presence of erythrodextrin mixed with starch. After a few more minutes you will be unable to get the blue reaction with iodine, showing that the starch has been completely converted, but the reddish erythrodextrin reaction will be very marked. After a further interval you will cease to obtain any reaction with iodine. All this time the sugar reaction goes on increasing, and continues to increase for some time after the iodine reaction has stopped.
It is extremely probable that one of the chief uses of the salivary ferment in digestion of food in the mouth is to liquefy the starch jelly into which we have converted our food by cooking processes. From the rapidity with which the diastatic ferment acts, one would on first thoughts naturally suppose that all the starch would be quickly converted into sugar. However, this is evidently not the case, as on examining the stomach contents of healthy people several hours after a meal you will still get the violet iodine reaction showing the presence of unaltered starch. The arrest of the diastatic action of the salivary ferment is due to the presence of free HCl in the stomach. It has been experimentally shown that free acids in small quantity will check, and in large quantity absolutely stop this action.
And since under normal conditions the percentage of HCl in the stomach during digestion is at least 0.2 per cent, the cause of the cessation of the amylolytic action is very clear. The saliva also probably acts by materially facilitating the precisely similar action of the pancreatic juice, when the food has left the stomach and passed into the small intestine. For one may readily imagine that it will be far easier for the alkaline pancreatic juice to continue an action which has been once started in the mouth, than to originate the same de novo in starch that has been thoroughly acidified by contact with the gastric juice.
During the process of digestion of food In the mouth, the saliva will also feebly emulsify a little of the fat contained in the food, and will probably dissolve certain soluble ingredients of the food such as salts and sugar. On the remaining constituents of the food, it has no action.