PRESS SELF-DESTRUCT: Even biodegradables need the right environment to, well, degrade

Leather as a kill switch
A question commonly asked about leather biodegradability is ‘why doesn’t the leather break down during its working life?’ This is a good question. Why doesn’t the biodegradable wooden spoon break down during the cooking process or while sitting on the shelf? The answer for the wooden spoon is obvious - the cooking process is far from perfect for the biological breakdown mechanisms that the wood-degrading bugs will need to allow biodegradation. Similarly, the dry, room temperature conditions of the storage of that wooden spoon are also far from ideal. Do not be fooled however, there will be bacteria and fungi on that wooden spoon that given enough time will degrade that wooden spoon in the kitchen - it is just a significant amount of time - possibly hundreds, if not thousands of years.
Cattle hides and sheepskins, by-products of the meat industry, are highly putrescible. Left in their natural state, the wearing of a raw hide shoe would be an interesting endeavour - and maybe human ancestors started with this? The raw material is porous and there is no stabilisation of the collagen protein, especially when wet. Drying the raw hide out offers some protection to the material, because if the water activity of the material drops below 0.6 then the fungi or bacteria cannot get enough moisture to break the product down. A sweaty caveman would find that, given time, the raw material would break down as the sweat would increase the water activity and the material would decompose.
Early tanners could add ingredients that would slow down (despite a higher water activity) the decomposition of the raw material. Here is the key point: early tanners would have incrementally increased the resistance to decomposition. The challenge for taphonomists is to understand how that decomposition can be manipulated - in other words - how can a tanner engineer enough resistance to decomposition during the leather's working life and no more.

Collagen and its kill switch
It is actually quite hard for most bacteria and fungi to break down collagen. It is a material that only falls within the reach of organisms that can produce collagenases - a set of enzymes that specialise in breaking down collagen. The largest groups of collagenase enzymes are what are called the metalloproteases, these are enzymes that rely on a metal core inside their protein structure. Early preservation of collagen can centre around the use of chelators or sequestering agents that can bind the metal in the collagenase, rendering the enzyme useless. This was the principle of the LIRICURE method which used ethylenediaminetetraacetic acid (EDTA) to render collagenases useless.
Future in life stabilisations of leather could be as simple as protecting the collagen from collagenases. Of course, a dry leather makes this decay even harder, so a dry leather protected with EDTA could be the initial strategy. The next focus must revolve around the protection of the collagen peptide bond which is the key target of the metalloproteases. A big vegtan molecule sits on that peptide and stops decay et voila. However, the protection that vegtans give may be too permanent. Is there a chemical that can detach from the peptide given a trigger?
The increase in the collagen thermal stability may be the key. Vegetable tanning generally increases the wet collagen shrinkage temperature from 65°C to 75-85°C. The “half-tannages” (most of the chromium free tannages) increase that to low 70s. In other words, even if the peptide is “blocked”, raising the leather temperature (while wet), could cause the collagen to “implode” - the collagen melts and the chemicals between the fibres are released. So, science could focus on that slider between stable collagen and something that can be dissolved by heat in end-of-life.

In-life vs end-of-life conditions
Like the wooden spoon, the leather working parameters are very different to its end-of-life parameters. A wooden spoon is no pushover for a healthy forest ecosystem. Let’s say that the wooden spoon ends up under some leaf litter in the forest (damp, microbe-rich, neutral to alkaline pH, available to insects and small invertebrates eating attention). Within a very short space of time the surface of the wooden spoon would be populated by bacteria and fungi that would be eager to digest the cellulose, hemi-celluloses and lignins of the spoon. As these microbes infiltrate into the spoon the characteristic blocks of brown-rot digested wood, or the fibrous residue of the white-rot fungi become a further route of entry for micro- and macrobiology. Decomposition-friendly conditions in end-of-life can be summarised as follows and these apply for leather and wooden spoons:

• Neutral to high pH
• High moisture content
• Room or high temperatures
• High microbiology loading
• Mechanical or other “eating” damage
• Environmental chemistry amplifiers (abiotic degradation factors)
• It could be that the switch from the opposite of these factors to the ones listed above, if enough of a kill switch that doesn’t allow leather to break down during its working life, but does so in its end-of-life.

In the next issue: To simplify the allocation method in LCA, the analyst is encouraged to expand the system boundaries or by virtual subdivision. Multifunctional processes complicate LCA and the global default is to allocate using economic allocation - the lowest rank of the ISO 14040/44 recommendations. Why does the LCA persist with this lunacy? Is it laziness or the lack of creativity/knowledge of the system process to be able to do expansion or subdivision. In the next article the magazine will look at a modular approach to LCA.