Stomach acid is a type of concentrated hydrochloric acid that is excreted by the stomach lining in response to the ingestion of food. The acid excreted is extremely strong – on par with the potency of acids used in a laboratory. The acid denatures the proteins in food and also acts to break down the chemical bonds in starches, fats and other foods. It also has the positive side effect of killing any bacteria or viruses that may be mixed in with the food. Once the food has reached the required acidity, the bottom of the stomach opens up into the duodenum where alkalines are added back to the food to bring its pH back towards something approximating pure water.