Low pH of the stomach is due to hydrochloric acid content which is secreted by parietal cells into the lumen. The hydrochloric acid functions in activating enzymes that breaks down proteins. It activates pepsinogen into enzyme pepsin, that breaks the bond that links amino acids together, a process called proteolysis.
The acidic nature of the stomach also functions in killing or inhibiting harmful bacteria that is ingested with food. Most bacterial species cannot survive in an acidic environment.
