The heart of leather biodegradability - Part 1 of 4
Updated: Nov 26, 2021
This is the first in a four-part series that takes a detailed look at the chemistry and biology of the biodegradability of leather. At Authenticae, we believe that biodegradability is an important toolkit in the global ambition for resource management. Crafting leathers that come from nature and return to nature are the ultimate in resource stewardship.
Sixteen to twenty-four hours is the textbook amount of time that is taught to tanners, when informing them of how little time they have, before obvious damage to the raw material is noticeable - if no form of preservation takes place.
In a biodegradability capacity, the work of breakdown is done by bacteria and fungi that may be found in the breakdown environment. Water is a key ingredient to both micro-organism life-cycles. In fact, not only must enough water be present, but particularly in the case of the fungi, too much water can also be a problem. Bacteria can cope a little better in a very waterlogged environment because they are mobile in such environments and can produce enzymes into their surroundings and can extract nutrient. Multicellular fungi like the Ascomycetes and Basidiomycetes generally need a solid structure for their cellular structure to be supported and to prevent damage. Unicellular fungi, like the yeasts can cope much better in a liquid surrounding.
Bacteria can thrive in an oxygen-rich environment but can also do well in an oxygen-free environment. The facultative anaerobes can switch between the two depending on what kind of oxygen level is present. Fungi generally prefer an oxygen containing environment and are not as versatile as the bacteria. Fungi however can operate at generally lower temperatures to do the work that they do. Both the bacteria and fungi are particularly good at reproducing in an environment, sensing their surroundings by releasing simple enzymes and then monitoring what chemicals begin to permeate through their cellular envelopes.
To produce an enzyme, feedback sensors (promotion or inhibition signals), are used by codon regions of the micro-organism deoxyribonucleic acid (DNA), to detect the presence or absence of available nutrients. The enzymes can then be produced if the organism triggers the reading of the DNA producing messenger ribonucleic acid (mRNA) segments. The mRNA can then, through the careful placement of transfer RNA (tRNA) onto the mRNA, resulting results in the build-up, piece-by-piece, of the enzyme protein.
In a freshly infected environment the initial growth of the micro-organisms is characterised by the lag phase, see Figure 1.
Figure 1. A schematic graph showing the predicted growth of most organisms following initial infection over time.
In the lag phase the growth pattern for the bacteria is typified by slow start, accompanied by the organism secreting enzymes into its surrounding matrix, whilst it investigates what the surroundings are composed of. During this phase the organism will manufacture a small number of low-cost enzymes that will feedback, by chemical signal, into the DNA, triggering the production of large amounts of enzymes. These enzymes will be typically simple sugar enzymes, simple protein enzymes, and simple fat enzymes. In a low competition environment, the bacteria will reap the benefits of their enzyme release and this will allow the organisms to begin exponential reproduction.
In the case of a leather example, a biodegradable hide and skin will be infected by the organism and it will enter lag phase. The surroundings of that raw material will contain a rich supply of globular proteins, glycosaminoglycans (the “sugar-proteins”) and triglyceride fats. A rapid assimilation of glucose, amino acids, and fatty acids from the breakdown of these surrounding compounds will cause log phase growth of the organisms. The enzyme machinery used to breakdown these hide/skin components are very low cost. The enzymes are easy to assemble, and the promotion regions of the coding DNA are easy to trigger. The corresponding flood of enzyme activity will result in increased respiration output from the organisms.
Respiration of bacteria and fungi can be monitored by ISO 20136 or IUC 37, an ultimate biodegradability test that monitors carbon dioxide (CO2) output. The enzymes themselves will carve away at the biodegradable hide/skin and the material will begin to disintegrate in the softer, easy to digest areas. Disintegration can be monitored using the disintegration test known as ISO 20200. Fragments of the material that cannot simply be digested by simple enzymes will remain.
After the log, exponential growth phase, and with depletion of easily digested surrounding chemistry, the simple enzymes no longer produce the feedback chemicals that will allow their continued expression. The organism then begins to slow down in growth. Tentative expression of some complex enzymes, that occurred during the log phase would have yielded results in chemistry that would be absorbed into the cell and would have triggered expression of the expensive-to-produce enzymes. In leather technology, the most significant is collagenase. Collagenase in bacterial terms is expensive to produce and the bacteria has to be sure of return-of-investment before it will commit to their production.