The process of digestion begins in the mouth. The saliva in the moth, besides helping tomasticate the food, carries an enzyme called ptyalin which begins the chemical action ofdigestion. It initiates the catabolism (breakdown) of carbohydrates by converting starches intosimple sugars. This explains the need for thorough mastication of starchy food in the mouth. Ifthis is not done the ptyalin cannot carry out its functions as it is active in an alkaline, neutral orslightly acid medium and is inactivated by the highly acid gastric juices in the stomach.
Although enzymatic action starts while food is being chewed, digestion moves into high gearonly when the chewed food has passed the esophagus and reached the stomach. While thephysical action of peristalsis churns and kneads solid food into a semi-solid amorphous mixturecalled chyme, this mixture undergoes chemical changes initiated by gastric juices secreted bythe walls of the stomach. These juices include mucus for lubricating the stomach, hydrochloricacid and gastric juice.
The enzyme or active principle of the gastric juice is pepsin. This enzyme in combination withhydrochloric acid starts the breakdown of proteins into absorbable amino acids calledpolypeptides. An additional enzyme, rennin, plays an important role in the stomach of theinfant. It curdles milk and allows the pepsin to work upon it. The gastric juice has no effectupon starches or fats.