The meat processing industry generates hides of dead animals which would have caused environmental problems in disposal, had it not been for the leather industry. Fortunately the leather industry makes use of these hides to process it further and make leather. Although the leather industry takes care of this environmental problem and generates employment, the processing of hide to leather itself generates a fair amount of pollutants. That is because, the conventional processing of leather involves the use of chemicals and the maximum amount of solid wastes like lime and chrome sludge and noxious gases( like hydrogen sulphide )are generated during the leather making processes. It is in these areas that biotechnology through the use of enzymes has played a key role in refining the process of leather making.
Although attempts to use enzymes in the dehairing process of leather manufacturing started as early as 1913 by researchers Rohm and Haas (got the first patent for enzymes in bating), it was only in 1970 that researchers Folk, Cole and Chung unveiled basic research on the mechanisms of enzyme action in leather manufacturing.
Today several of the chemicals used in leather processing have been substituted with enzymes. This has made the entire process of tanning hides to become more efficient and quicker. So today proteases, lipases and amylases are used in leather manufacturing.
When enzymes are used in leather processing it conveys certain advantages such as:
• Water usage is high in conventional leather processing which is about 30 to 40 liters per kg of hide processed. The use of enzymes reduces this requirement considerably.
• The effluent discharges (both gaseous and aqueous) in leather processing using the conventional route (without using enzymes) contributes to dissolved solids (chromium, lime, sulphides and sulphates etc) and Biological Oxygen Demand (BOD), and Chemical Oxygen Demand (COD). However, using biotech processes helps in reducing COD by 80%, chromium by 85% and Total Dissolved Solids by 85%.
Different stages of leather processing
One has to understand the different steps in conventional leather processing and co-relate it to how biotechnology impacts each of these steps.
There are three distinct stages of leather processing, namely, preparation for tanning, tanning, and finishing. Each of these stages involves several other steps. Some of these steps like soaking, liming, bating and degreasing involving a biotech perspective will be discussed in detail in this article.
Curing the hide
This is the first step which entails treating the flayed hide with brine. If this is not done, then there is the chance of the flayed hide getting putrefied. Conventionally, the hide was soaked in brine to remove unwanted parts of the hide and the skin.
Now enzymes can do the job better as it can provide better soaking effect, because they can re-hydrate the hides better and quicken up the entire process. One example of such an enzyme is Specialty Enzyme's SEBsoak product.
Biocides have also been found useful in curing the hides but this is not environment friendly. Despite all this, salt curing is still the predominant way of curing and biotech hasn't made inroads for curing the hide process.
In this stage, the hides are washed and soaked in surfactants and other compounds mostly anti-microbial. The intention is to help in the further processing to leather. The conventional process of soaking uses sodium tetrasulphide plus a surfactant, in which case the soaking process will take nine hours. But proteases and lipases used along with surfactants can reduce the time required for soaking to five hours.
Examples of proprietory enzymes used in soaking process are:
a) Palkosoak™ which is a mixture of protease and lipase suitable for alkaline conditions
b) Palkosoak ACP™ which is again mixture of protease and lipase that suits acidic conditions.
After soaking, the next step is liming operation. It may be that soaking would not have made the skins swollen to the required degree, so liming is done precisely to achieve desired swelling of skin. Conventionally this is done with milk of lime, resulting in swelling of the collagen structure, so the fiber bundles can be opened up. The idea is to remove the keratinous matter and remove proteins like mucins and the ultimate quality of leather depends on this process.
Although not many enzymes are used in the liming process as of now, there are some proprietory enzymes that can stabilize the hide by removing all proteinous matter of non-leather origin from the hide. An example is SEBbate Acid ™ that makes the hide smoother for fabrication and dyeing by ensuring smoother grain and pliability of the hide.
This entails making the hides free of hairs and furs. The conventional method is to use sulphide to eliminate keratin but the problem is it produces effluents with high COD. Instead, proteolytic enzymes of bacterial and fungal origin can now be used that will do the job by attacking the protein matter at the hair base. This obviates the need for sodium sulfide, and the process does not produce toxic wastes. Moreover, enzymatic process is far quicker.
De-hairing can be done using extra cellular protease secreted by Bacillus isolate, by enzymes secreted by Rhizopus oryzae or by using alkaline protease from Alcaligenes faecalis.
An example of a proprietory enzyme used in de-hairing process is Palkodehair ™ which is a protease enzyme that works in alkaline conditions. Another example is SEB lime, which is biodegradable and eco friendly. The swell regulating properties of this enzyme results in better grain smoothness of leather.
The idea of bating is to make the leather soft and supple (to bring out the grain and give flexibility) suitable for tanning, normally achieved by striking the leather with metal and wooden rods so that residues of proteins and epidermis are removed, or sometimes by using the manure of pigeon or hen.
Bating increases the stretch of leather, removes swelling and produces silky grain. Now proteolytic bating enzymes of pancreatic or bacterial origin are used for bating under alkaline conditions. It works by diffusion of the enzymes into the hides.
Trypsin and alkaline proteases are commonly used.
An example of a proprietory enzyme suitable for bating is Palkobate™ that works best in alkaline conditions.
It is essential to remove the fatty substances that escape the liming and other processes. Else it will result in uneven dyeing and finishing as for example cause waxy patches in leathers. The conventional method is acqueous emulsification, solvent extraction and pressure degreasing. When enzymes of lipase type are used, they will rupture fat cell membranes and cause triglyceride splitting thereby facilitating degreasing process.
This is the last stage in making leather which involves introducing a tanning agent in the hides. Enzymes are not directly involved in this stage.
Trypsin and proteolytic enzymes are used in further processing chrome tanned waste from tanneries.
As you can see from this article, there is a paradigm shift in leather processing from chemical driven processes to enzyme driven processes.
The key to using enzymes in leather processing is that it shouldn't damage or dissolve the keratin in the hides, but it should have the ability to hydrolyze casein, elastin, albumin and other non-structured proteins which are not required in the hide for leather making.
Current biotech research in leather manufacturing has generated technologies for non-lime enzyme assisted de-hairing for cow hides, enzymatic dehairing for goatskin and sheepskin and a unique bio-driven three step tanning technique.
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