Have you ever noticed changes in food that have been left out for too long? It starts to smell bad and change colour. You might even see some mould growing over it. When this happens, we commonly say that the food has gone rotten. The term 'rot' is a lay form of describing decomposition or decay.
To decay may sound alarming, and often it is! After organisms die, their body decomposes and is broken down into small pieces. However, the process of decay is essential in recycling nutrients between the living and non-living components of the Earth. Without it, humans and wildlife would not have access to the elements we need to survive! In this article, we shall cover the stages of the decay process and some of the factors affecting the decay rate.
What is Decay in Biology?
Decomposition or decay is the process of breaking down dead organic materials into simpler organic or inorganic molecules such as carbon dioxide, water, simple sugars, and inorganic ions.
Decay is an essential part of the nutrient cycle responsible for recycling dead organic matter into nutrients that other living organisms, usually plants, can reuse. After an organism dies, its body starts to degrade quite rapidly. The dead cells release enzymes that break down the proteins and other biological molecules in tissues. In addition, animals, such as worms, bacteria, and fungi, come to help decompose the organic materials. These organisms that aid the process of decomposition are called decomposers.
It is important to remember that decay is not always a bad thing!
- Decay is essential in agriculture as farmers build up compost from decaying faecal matter. This compost can be used as environmentally friendly fertiliser for their crops and to cut down waste.
- Animal waste will also be used to produce renewable energy in the form of biogas. Decomposing bacteria and fungi are added to biogas generators and allowed to anaerobically digest the animal waste, releasing methane and carbon dioxide.
The Decay Process
All organic materials that are cut off from a living organism, whether plants or animals, will follow the same general steps when decomposing. This also includes the carcass of an organism after death. The process of decay, regardless of the original organic material, can be divided into several stages: fragmentation, leaching, catabolism, humification and mineralisation.
Fragmentation during Decay
Fragmentation is the initial stage of decay where larger pieces of organic matter are physically broken down into smaller fragments.
This process can occur through various mechanisms such as weathering, physical abrasion, or the actions of detritivores (organisms that feed on dead organic material). For example, fallen leaves may be broken into smaller pieces by insects or other organisms, exposing a larger surface area for further decomposition.
Leaching during Decay
During leaching, soluble substances are washed out from the decaying organic matter by water. This process is particularly important in environments with high rainfall or excessive moisture. Water carries away dissolved organic compounds and inorganic nutrients, such as nitrogen, phosphorus, and potassium, from the decaying material. These nutrients can then be transported to surrounding soil or water bodies.
Catabolism during Decay
Catabolism is the stage of decay where decomposers, such as bacteria and fungi, break down complex organic compounds into simpler molecules through the secretion of enzymes. During catabolism, carbohydrates, proteins, and lipids present in the decaying organic matter are broken into smaller components. This process provides a source of energy for decomposers, allowing them to grow and reproduce.
Humification during Decay
Humification is a stage where the remaining organic matter undergoes further decomposition, resulting in the formation of humus. Humus is a dark, stable, and nutrient-rich substance that contributes to soil fertility. It is highly resistant to further decomposition and can persist in the soil for long periods. Humification is mainly driven by fungi and bacteria, which continue to break down complex organic compounds into simpler, more stable forms.
Mineralisation during Decay
Mineralisation, also known as immobilization, is the final stage of decay where the remaining organic matter is completely broken down into inorganic nutrients. Decomposers convert the simpler organic molecules, such as carbon compounds, back into carbon dioxide (CO2) through respiration. In addition to CO2, mineralization also releases other inorganic nutrients, such as nitrogen, phosphorus, and sulfur, into the surrounding environment. These nutrients become available for uptake by plants and other organisms, completing the nutrient cycling process.
The Stages of Biological Decay in Animals
Although generally speaking all organic material will go through the same stages of decay we saw above, animal corpses react slightly differently than other organic matter when decomposing.
From the minute an organism dies, the process of decomposition begins. First, by the body's own digestive enzymes (autolysis), then by other organisms, mainly microorganisms (putrefaction).
Autolysis is the enzymatic digestion of cells by their own enzymes, usually occurring in dying or dead cells.
Putrefaction is the breakdown of organic matter, particularly the anaerobic breakdown of proteins by bacteria and fungi.
This process often results in the generation of foul-smelling, partially oxidized compounds.
The process of animal decomposition consists of five main stages; fresh, bloat, active decay, advanced decay and skeletonization.
Stages of Biological Decay in Animals: Fresh
After death, the process of thermoregulation stops. So, the dead body starts to cool down or get warmer to match the environment's temperature.
In animals with muscular tissues, the muscles become stiff and unable to relax. This is called rigor mortis.
Rigor mortis is the depletion of ATP in muscle cells since the regeneration of ATP cannot occur after death. Contraction in muscles is based on the sliding of myosin and actin filaments over each other. ATP is needed to detach myosin protein from actin filaments. So without ATP, the myosin proteins remain bound to actin filaments, causing the muscle to stay contracted and incapable of relaxing.
In organisms with systemic circulation, the heart stops beating after death. Therefore, the blood is no longer pumped around the body to provide oxygen and nutrients while removing waste products such as carbon dioxide. Accumulation of carbon dioxide in tissues increases acidity and lowers the pH. This triggers certain changes that lead to the loss of structural integrity in cells and the release of cellular digestive enzymes. Once released, these enzymes break down the neighbouring cells and tissues. This type of breakdown is called autolysis.
With no blood being pumped around to supply oxygen, the little oxygen that remains in the body is rapidly exhausted by cellular metabolism and aerobic microorganisms of the gut flora. This creates a perfect environment for the growth and proliferation of anaerobic microorganisms that thrive in the absence of oxygen.
Stages of Biological Decay in Animals: Bloat
As anaerobic microorganisms grow and undergo anaerobic metabolism, they produce high amounts of gases such as hydrogen sulphide, methane and carbon dioxide as by-products. These gases build up and collect within the abdomen, creating a bloated and distended (swollen, enlarged) appearance.
Stages of Biological Decay in Animals: Active decay
During active decay, the most significant amount of mass is lost from the carcass. This is due to bacteria and insects' degradation of bodily materials and liquids released into the surrounding environment. At this stage, the range of decomposition grows, and the highest number of insects are present to feed on the liquified and partially decomposed remains.
The insects and maggots leaving the decaying corpse would mark the end of active decay.
This stage is characterized by the release of gases, strong odours, and the formation of liquefied remains.
Stages of Biological Decay in Animals: Advanced decay
Decomposition is considerably slowed during advanced decay. This is because most soft tissues have decayed by this point, leaving primarily bones, hair, cartilage, ligaments, and tough sticky leftovers. At this stage, insects with chewing mouthparts, such as beetles and ants, are drawn in to chew on and digest what's left.
Stages of Biological Decay in Animals: Skeletonization
The last stage occurs after all by-products of decomposition have dried up, leaving just the skeleton and sometimes some hair. When all soft tissue is removed from a cadaver (corpse), it is said to be totally skeletonized; yet, when just sections of the bones are revealed, it is said to be partially skeletonized.
If any soft matter remains, the beetles and flies consume them. Meanwhile, mites and moth larvae break down the excess hair. Over time, the remaining skeleton is broken down physically and biochemically, and the minerals stored in the bones are returned to the soil.
Decomposers in the Ecosystem
Because of the way decomposing bacteria and fungi break down dead organic materials, they are classified as saprotrophic. They cannot produce their own food, so they feed on decaying matter.
During saprotrophic nutrition, bacteria and fungi release digestive enzymes into the soil or a dead organism. These enzymes digest the dead organic material. Since this process occurs outside the cells, it is called extracellular digestion. The bacteria and fungi then absorb the products of this digestion process and release inorganic ions into the soil (e.g., nitrogen, phosphorus).
These inorganic ions are taken up by plant roots and are circulated back into the ecosystem (plants are consumed by herbivores, which carnivores can then consume). Animals depend on access to carbon, nitrogen, and phosphorus-containing molecules for a wide range of metabolic and biochemical processes in the body.
Aerobic decomposers release ions into the ecosystem much more efficiently than anaerobic decomposers, so aerated soils are often more productive than saturated ones.
Saprotrophs are not the only decomposers acting on dead organic matter in the soil; there are saprophytic decomposers too! Saprophytes are often non-photosynthetic plants which cannot produce their own food, so they rely on their unusual ability of extracellular digestion. This process occurs similarly to saprotrophs, with the plant's excreting enzymes acting on the dead matter, breaking it down into smaller, inorganic molecules that can be absorbed.
The Rate of Decay
The rate of decay is the rate at which dead organic matter is decomposed. This rate can be determined by monitoring and measuring pH, mass, or temperature changes.
Choosing which to measure depends on the dead organic materials.
The rate of decay can be calculated using this formula:
\[\text{Rate of decay} = \frac{\text{Changes in the value of pH, mass or temperature}}{\text{Time passed}}\]
When plotting the rate of decay of organic matter, you must remember to plot time passed on the x-axis (the independent variable) and changes in pH, mass or temperature on the y-axis (dependent variable).
Time passed is independent as we change this variable while observing, whereas changes in values are dependent as they vary depending on how much time expired.
Once you have a smart-looking graph (like the one below), you can pinpoint different points over the observed period and calculate the rate of decay using the equation above.
How quickly a dead organic body decomposes depends on various factors in the physical environment. These include temperature, water, and oxygen availability.
Temperature and Rate of Decay
Decomposing organisms are less active at cooler temperatures. Therefore the rate of decomposition is slow at low temperatures.
This is precisely why food stored in the refrigerator and freezer last longer.
Decomposing organisms become more active as the temperatures rise, and so does the rate of decay. Although, if the temperature exceeds a certain amount, the heat can kill the decomposing organisms or any pathogenic microorganisms. This is why we cook things like meat and eggs before eating them.
Water and Rate of Decay
Water is essential for life, and ironically, it is also necessary for decomposition. Decomposers are living organisms, and they cannot survive without water.
This is why dried meat and fruits have a longer shelf life.
As the volume of water that is accessible increases, so does the decomposition rate. This is because, in addition to decomposers needing water to thrive, they release degrading enzymes onto organic substances to decompose the dead materials. For these enzymes to work, they need water. Most biological molecules are broken down via hydrolysis, a chemical reaction that uses water. In addition, simpler products produced by the breakdown of these molecules need to be dissolved in water to be taken up by decomposers.
The Egyptians used to mummify the body of their kings and queens after death. This was to preserve the corpses so that they would be able to transport them to the spiritual afterlife.
During the mummification process, the body's internal organs were removed, and all the water from the body was removed. This was to prevent decomposers from breaking down the dead tissue. The embalmers employed natron, a naturally formed salt with excellent drying characteristics, to eliminate all the moisture. They packed natron packets into the body, coated it entirely with salt, and eventually placed the mummy on an embalming table to dry.
Oxygen and Rate of Decay
Besides water, oxygen is another essential element for most life forms. Since decomposers are living organisms, they need oxygen to thrive. So without oxygen, there is little or no decomposition. Although there are some decomposing bacteria and microorganisms that are anaerobic and do not need oxygen, the majority of decomposers require oxygen to breathe, develop and proliferate. As the amount of oxygen accessible to decomposers increases, so does the rate of decomposition.
This is why we frequently wrap food in bags and cling film or put it in Tupperware before storing it in the fridge.
Practical Experiment: Milk Decay
In this practical experiment, we will investigate the process of decay using milk as a model organic substance. By observing and analyzing the changes that occur over time, we can gain a better understanding of the factors that contribute to decay and calculating the rate of decay.
Materials Needed:
- Fresh milk
- Clear, labelled containers (e.g., glass jars or plastic cups). You will need one container per temperature.
- Plastic wrap or lids for covering the containers
- pH measuring system, like a pH meter or a universal indicator paper
- Heating plates or baths
- Camera or smartphone (optional, for documentation purposes)
Experimental Steps:
- Fil the containers with 20 cm3 of fresh milk.
- Label each container with the temperature you will set it at, and cover the containers.
- Put each container on a heating plate or bath at the desired temperature after measuring the initial pH of the milk.
- Measure the pH of each container at different times: 24 h, 48 h and 72 h after the beginning of the experiment. Write down your measurements in a table like the following one.
| 0 h | 24 h | 48 h | 72 h |
| 15oC | | | | |
| 20oC | | | | |
| 30oC | | | | |
Once you have the pH values for each data point, you can compare the rate of change at each temperature and time by subtracting the pH values at different time points and dividing them by the elapsed time.
Look at the example table below.
If you want to calculate the rate of change of the pH at 15oC between the beginning and the end of the experiment, you would subtract the pH value at 72 h from the pH value at 0 h and divide by 72:
\(\text{Rate of change} = \frac{5.8 - 6.5}{72-0} = -0.01\)
If you want to calculate the rate of change at 20oC between 48 h and 24 h, though, you would subtract the pH value at 48 h from the one at 24 h, and divide by 24, since that is how much time has passed between those two data points.
\(\text{Rate of change} = \frac{4.6-6.5}{48-24} = -0.08\)
The minus sign means that the pH is decreasing, meaning that it is becoming more acidic.
| 0 h | 24 h | 48 h | 72 h |
| 15oC | 6.5 | 6.3 | 6.1 | 5.8 |
| 20oC | 6.5 | 5.9 | 4.6 | 4.6 |
| 30oC | 6.5 | 5.7 | 4.6 | 4.6 |
Decay - Key takeaways
- Decomposition or decay is the process of breaking down dead organic materials into simpler organic or inorganic molecules.
- Decay is an essential component of the nutrient cycle responsible for recycling dead organic matter into nutrients that other living organisms, usually plants, can reuse.
- The process of animal decomposition consists of five main stages; fresh, bloat, active decay, advanced decay and skeletonization.
- The rate of decay is the rate at which dead organic matter is decomposed.
- How quickly a dead organic body decomposes depends on various factors in the physical environment, such as temperature, water and oxygen availability.
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