From Lerner :
Also known as "grave wax," adipocere (from the Latin, adipo for fat and cera for wax) is a grayish-white postmortem (after death) matter caused by fat decomposition, which results from hydrolysis and hydrogenation of the lipids (fatty cells) that compose subcutaneous (under the skin) fat tissues.
Although decomposition of fatty tissues starts almost immediately after death, adipocere formation time may vary from two weeks to one or two months, on average, due to several factors, such as temperature, embalming and burial conditions, and materials surrounding the corpse. For instance, the subcutaneous adipose (fatty) tissue of corpses immersed in cold water or kept in plastic bags may undergo a uniform adipocere formation with the superficial layers of skin slipping off.
Several studies have been conducted in the last ten years to understand and determine the rate of adipocere formation under different conditions. Other studies also investigated the influence of some bacteria and chemicals, present in grave soils, in adipocere decomposition. Although this issue remains a challenging one, the purpose of such studies is to establish standard parameters for possible application in forensic analysis, such as the estimation of time elapsed since death when insect activity is not present. In forensics, adipocere is also important because preserved body remains may offer other clues associated either with the circumstances surrounding or the cause of death. The ability of adipocere to preserve a body has been well illustrated in exhumed corpses, even after a century.
Adipose cells are rich in glycerol molecules and are formed by triglycerols (or triglycerides). Bacterial activity releases enzymes that break these triglycerides into a mixture of saturated and unsaturated free fatty acids, a process known as hydrolysis. In the presence of enough water and enzymes, triglycerol hydrolysis will proceed until all molecules are reduced to free fatty acids. Unsaturated free fatty acids, such as palmitoleic and linoleic acids, react with hydrogen to form hydroxystearic, hydroxypalmitic acids and other stearic compounds, a process known as saponification, or turning into soap.
This final product of fat decomposition, or adipocere, can be stable for long periods of time due to its considerable resistance to bacterial action. This resistance allows for slower decomposition of those areas of a corpse where adipose tissues are present, such as cheeks, thighs, and buttocks. When a corpse is exposed to insects, however, adipocere probably will not be formed, as body decomposition will be much faster because of the insects' action. Animal scavenging of a dead body will also prevent adipocere formation.
Also worth a read is Ruttan and Marshall's 1917 piece,
The Composition of Adipocere.
1. "Adipocere." World of Forensic Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Gale Cengage, 2006. eNotes.com. 2006. 16 Sep, 2010