What Are the Four Main Stages of Food Processing?

Food processing begins with the first bite. During ingestion, teeth break apart the food as you chew. Salivary glands, located under the tongue, secrete saliva to lubricate the food particles and the enzyme amylase to begin the chemical breakdown of starches in the food. The muscular action of the tongue forms the food into a smooth, rounded package called a bolus. Once the bolus is swallowed, involuntary muscular action called peristalsis moves the bolus down the esophagus to the stomach.

The stomach stores food as it awaits passage to the small intestine. The stomach mechanically breaks down food by churning. Glands in the stomach secrete acid to help break down the food into smaller particles. The glands also secrete the enzyme pepsin, which begins the process of breaking down the proteins in food. This watery mixture of food, saliva, stomach acid and enzymes is called chyme. Fats are broken down in the small intestine with the help of bile salts produced in the liver and secreted by the gall bladder. The small intestine secretes the enzyme trypsin to further break down proteins. Enzymes secreted by the pancreas break down the rest of the carbohydrates, proteins and fats in the small intestine.

After food particles and large nutrient molecules have been broken down in the stomach and small intestine, the cells in the lining of the small intestine absorb nutrients from the chyme. The lining is composed of folds and finger-like projections called villi that increase the surface area of the small intestine. Greater surface area allows for more absorption of nutrients. These nutrients include carbohydrates, proteins, fats and vitamins. Cells allow nutrients to cross from the intestinal lining into the blood stream. Blood carries the absorbed nutrients to other body tissues, where they are stored or used for energy or repair.

The liquid contents that enter the large intestine cannot be digested any further; no more nutrients are absorbed here. The waste products consist of fiber, dead cells from the lining of the intestines and other undigested materials. Fiber cannot be broken down during digestion. Soluble fiber dissolves in water, but insoluble fiber remains intact. The large intestine transforms the watery remains of digestion into solid stool, first by the absorption of water and then by the secretion of mucus, which moves the stool into the rectum, where it waits to be expelled from the body through the anus.