Decomposition in animals is a process that begins immediately after death and involves the destruction of soft tissue, leaving behind skeletonized remains. The chemical process of decomposition is complex and involves the breakdown of soft tissue, as the body passes through the sequential stages of decomposition. Autolysis and putrefaction also play major roles in the disintegration of cells and tissues.
The human body is composed of approximately: 64% water, 20% protein, 10% fat, 1% carbohydrate, 5% minerals. The decomposition of soft tissue is characterized by the breakdown of these macromolecules, and thus a large proportion of the decomposition products should reflect the amount of protein and fat content initially present in the body. As such, the chemical process of decomposition involves the breakdown of proteins, carbohydrates, lipids, nucleic acids, and bone.
Protein degradation
Proteins make up a variety of different tissues within the body, which may be classified as soft or hard tissue proteins. As such, proteins within the body are not degraded at a uniform rate.
Proteolysis
Proteolysis is the process that breaks down proteins. It is regulated by moisture, temperature, and bacteria. This process does not occur at a uniform rate and thus some proteins are degraded during early decomposition, while others are degraded during later stages of decomposition. During the early stages of decomposition, soft tissue proteins are broken down. These include proteins that: line the gastrointestinal tract and pancreatic epithelium form the brain, liver, and kidneys
During later stages of decomposition, more resistant tissue proteins are degraded by the effects of putrefaction. These include: reticulin muscle protein collagen (a hard tissue protein), which survives even longer than the former tissue proteins
Keratin is a protein which is found in skin, hair, and nails. It is most resistant to the enzymes involved in proteolysis and must be broken down by special keratinolytic microorganisms. This is the reason that hair and nails are commonly found with skeletal remains.
Proteolysis products
In general, proteolysis breaks down proteins into: proteoses peptones polypeptides amino acids
Continuing proteolysis leads to the production of phenolic substances. In addition, the following gases will also be produced: carbon dioxide hydrogen sulphide, which is highly toxic ammonia methane
The sulfur-containing amino acids cysteine and methionine undergo bacterial decomposition to yield: ammonia thiols (decomposition gases known for their foul odours) pyruvic acid sulphides hydrogen sulphide gas Ferrous sulphide will be produced if iron is present, which can be seen as a black precipitate
Two common decarboxylation products of protein associated with decomposition are putrescine and cadaverine. These compounds are toxic at high levels and have distinctive, foul odours. It is believed that they are components of the characteristic odours of decomposition commonly detected by cadaver dogs.
A summary of the protein degradation products can be found in Table 1 below.
Nitrogen release
Nitrogen is a component of amino acids and is released upon deamination. It is typically released in the form of ammonia, which may be used by plants or microbes in the surrounding environment, converted to nitrate, or can accumulate in soil (if the body is located on top of or within soil). It has been suggested that the presence of nitrogen in soil may enhance nearby plant growth.
In acidic soil conditions, ammonia will be converted to ammonium ions, which can be used by plants or microbes. Under alkaline conditions, some of the ammonium ions entering soil may be converted back to ammonia. Any remaining ammonium in the environment can undergo nitrification and denitrification to yield nitrate and nitrite. In the absence of nitrifying bacteria, or organisms capable of oxidizing ammonia, ammonia will accumulate in the soil.
Phosphorus release
Phosphorus can be released from various components of the body, including proteins (especially those making up nucleic acids), sugar phosphate, and phospholipids. The route phosphorus takes once it is released is complex and relies on the pH of the surrounding environment. In most soils, phosphorus exists as insoluble inorganic complexes, associated with iron, calcium, magnesium, and aluminum. Soil microorganisms can also transform insoluble organic complexes into soluble ones.