Putrefaction

Putrefaction is the part of decomposition in which microorganisms, above all bacteria from the gut, digest the soft tissues of a dead body. It begins alongside autolysis, the self-digestion of cells by their own enzymes, and soon becomes the main engine of decay. Bacterial metabolism releases gases such as hydrogen sulfide, methane and ammonia, along with foul-smelling compounds such as putrescine and cadaverine.

The first visible sign is usually a greenish patch on the lower right abdomen, where the cecum lies close to the skin, as hydrogen sulfide combines with hemoglobin. Bacteria then spread along the blood vessels, tracing veins in green and black, a pattern called marbling. Gas swells the face, abdomen and genitals (bloating), blisters form and the outer layer of skin slips off, and internal pressure pushes bloodstained fluid from the mouth and nose, which can be mistaken for injury. Over time the soft tissues liquefy until only the skeleton remains.

Warmth and moisture speed putrefaction, whereas cold, dryness, deep burial and water slow it, so its timetable varies widely. In dry places a body may mummify instead, and in wet conditions body fat can turn into a waxy substance called adipocere. Forensic scientists score the stage of decay against accumulated degree days, a measure of heat over time, to estimate long postmortem intervals.