ANAEROBIC LEATHER

Anaerobic leather - most people have heard of the leather find that was a shoe or a bag from antiquity, stored in a preservation condition that has allowed the relic to last several hundred years without being broken down. Is this the model of how, as a material, leather can be stored in a fossilised form taking carbon out of the atmosphere and putting it back into the ground? Or will a clever tannery come along and produce a material that does well in anaerobic conditions?
Much has been written in S&V African Leather about composting or the aerobic breakdown of leather. In summary, the bacteria and fungi, growing in an oxygen-rich environment, use their enzyme machinery to utilise sugars, amino acids, and triglycerides to fuel their metabolism.

The compost can often become anaerobic (oxygen depleted) and the microorganisms will turn into facultative anaerobes - organisms that can switch their machinery into a metabolism that does not need oxygen (continuously). In other words, the final stage of energy acquisition does not need oxygen. Oxidative phosphorylation is the primary means by which aerobic (oxygen-requiring) organisms take the agents that they get from the famous Krebs cycle and turn it into energy. In anaerobic metabolism a different mechanism is used.

Material breakdown (anaerobic)
When a microbe finds itself in an environment that is sufficient for water, pH, ionic strength, oxygen (or lack thereof), and an abundance of food, its survival instinct that starts its food gathering will kick in. The organism will spend a little energy to excrete some simple enzymes that could digest simple compounds surrounding it. The reward will come if sugars, amino acids, and triglycerides increase in number outside its cell. The organism has received confirmation that there is digestible food around it. Both aerobic and anaerobic organisms will do this (continuously). A “no-return” will mean the microbe will need to try other enzymes that increase in complexity (and energy cost) as it moves forward. A bacteria/fungus cannot digest everything so they may not be able to use the materials they have found they are surrounded by.

Pure anaerobic digestion is not the same as the facultative anaerobic system (briefly) mentioned above. Oxygen is toxic to a truly anaerobic microbe (largely bacteria and yeasts) and the system described below is for pure anaerobes. Anaerobic conditions appear in sludges, in deep soil/sediment, and in areas where an agent rapidly absorbs all the oxygen (especially in aquatic environments). Of course, the depleted area must not receive a replenishment of oxygen after depletion. In human anaerobic digesters, like sewage works, oxygen is purposely excluded.

Specialised microorganisms will break down materials in an anaerobic environment given the right conditions and if the materials they are surrounded with are conducive to digestion. These organisms break down materials through the following stages:

• Hydrolysis - larger molecules are broken down into smaller parts by specific enzymes that can then be absorbed by the cell.
• Acidogenesis - an optional stage, some compounds coming from hydrolysis can skip acidogenesis and go straight to acetogenesis. In acidogenesis, larger fatty acid chains are broken into smaller components.
• Acetogenesis - components from the proceeding stages are used to produce acetic acid. The stages proceeding and including acetogenesis are typified by low pH.
• Methanogenesis - the last stage of anaerobic digestion where acetic acid is altered into methane, carbon dioxide, and water.

The breakdown outlined above generates compounds that get carried to the energy powerhouses of the cell (usually the mitochondria). These reduced agents then donate their electrons to powerful electron acceptors (just not oxygen), like fumarate, sulfate, or nitrate (and others), generating adenosine triphosphate in the process.

Digestion rates
Anaerobic digestion (AD), in good conditions, will break down materials into almost nothing. Residues emanating from AD plants are minimal, whilst the production of biogas (methane/CO2) is high. The benefit of this conversion is why humans have used AD for the breakdown of wastes in general. The disadvantage is that methane is generated, which currently is the main villain in the climate change debate.

AD is split into three main groups, low solids, high solids (dry), high solids (wet) and the breakdown is wildly variable between these types. The low solids (<15% solid) can achieve very high breakdowns, if there is separation between the acidic parts or the digestion and then methanogenic part. Methanogenesis is sensitive to pH (ideally it should be between 6.5 and 8) so separation helps. The low solid content helps with hydrolysis as enzymes can move freely and find their substrates easier. Thermophilic AD (high temperature) assists with abiotic hydrolysis allowing faster breakdown.

High solids AD (both types) is a bit trickier, with the drier of the two being slower. In a natural environment, materials incorporated in a dry, anaerobic, sediment will find themselves preserved and this forms the basis of fossil carbon sources – where heat and pressure convert them into hydrocarbons.

Anaerobic digestion for leather
Scientists have demonstrated that solid leather wastes (sludges) perform well in waste management scenarios. However, a young scientist called Clive Jackson-Moss in his PhD showed that low solids AD of wastewater was a bit more problematic. Low-solid anaerobic systems are difficult, as there are many potentially toxic components in wastewater that can be a source of oxygen, but they can also directly affect the microbes (salt being a notable example). In marine environments, the presence of salt negatively affects AD in ocean sediments, allowing almost the perfect preservation of dead biological organisms.

Leather can be a difficult substrate for AD as the initial hydrolysis can pose problems for low solid systems. Collagen (and modified collagen) needs enzymes that require more energy to make, than simple enzymes used for something like glucose. Microbes do not normally choose to make these enzymes until they are starting to get starved. It can be that the breakdown is slow to begin and then increases over time. The more complex, the modification to the collagen, the more this will be a problem.

It is entirely feasible for low and high solid waste streams of leather to undergo effective AD, but one must always think of the storage conditions of those perfectly preserved archaeological leather items - normally oxygen starved, low pH, high solids boggy sediments that may have an agent that prevents the digestion. The industry should be looking to the tanneries that currently have effective AD plants for their leather waste for helping the design of certified anaerobic biodegradable leathers.

In the next issue: Traceability in Africa – is it possible? It seems like in a high-income country where the slaughter process is highly regulated (and centralised), and there appears to be veterinary and health restrictions that control the movement of animal by-products that assist a traceability system. However, in Africa, where rural slaughter and decentralised slaughter, coupled with hides and skins that are appearing in hide traders stocks many days after slaughter - that makes the accountability and information flow prohibitive.