Pepsin is an endopeptidase that breaks down proteins into smaller peptides (protease). It is produced in the stomach and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. It is one of three principal proteases in the human digestive system, the other two being chymotrypsin and trypsin. Pepsin is most efficient in cleaving peptide bonds between hydrophobic and preferably aromatic amino acids such as phenylalanine, tryptophan, and tyrosine. Studies on gastric digestion from 1820-1840 led to the discovery of pepsin as the substance which, in the presence of stomach acid, causes nutrients including meat or coagulated egg whites to dissolve. Pepsin is used as a replacement enzyme for those with pancreatic insufficiency. Stimulation of the pancreas and therefore enzymatic digestion of food is a tightly controlled and is a hormonally mediated process. Any changes or conditions affecting metabolic steps for successful digestion and absorption negatively affect pancreatic enzymatic secretion, entry into the intestine, functionality once inside the intestine, and appropriate mixing with foods/nutrients. Many causes of pancreatic insufficiency require that enzyme replacement therapy is started, including cystic fibrosis, pancreatic cancer, acute and chronic pancreatitis, as well as pancreatic surgery. Commercial pepsin is extracted from the glandular layer of hog stomachs. It is a component of rennet used to curdle milk during the manufacture of cheese. Pepsin is used for a variety of applications in food manufacturing: to modify and provide whipping qualities to soy protein and gelatin, to modify vegetable proteins for use in nondairy snack items, to make precooked cereals into instant hot cereals, and to prepare animal and vegetable protein hydrolysates for use in flavoring foods and beverages. It is used in the leather industry to remove hair and residual tissue from hides and in the recovery of silver from discarded photographic films by digesting the gelatin layer that holds the silver.