BIOLOGY-LECTURE 4: Leather Biotechnology

LECTURE 4: Leather Biotechnology

Hides are pelts of large animals while skins are pelts of smaller animals

Tanning is the process of treating hides or skins of animals into leather, which is more good looking, durable and less susceptible to decomposition.

Raw Leather composition

Water 60-65%, Protein 25-30% , Fats 5-10% , The protein is mainly collagen and other fibrous protein forms

USE OF CHEMICALS

Chrome-tanned leather, invented in 1858, is produced when skins and pelts are tanned using chromium sulfate and other salts of chromium.

Aldehyde-tanned leather is leather tanned using glutaraldehyde or oxazolidine compounds. This is the leather that most tanners refer to as wet-white leather due to its pale cream or white color.

It is the main type of "chrome-free" leather, often seen in automobiles and shoes for infants.

Formaldehyde tanning (being phased out due to its danger to workers and the sensitivity of many people to formaldehyde)

APPLICATION OF BIOLOGICAL AGENTS

Historical use of biological materials in Leather Processing

Traditionally tanners would soak the skins in water to clean and soften them. Then they would pound and scour the skin to remove any remaining flesh and fat. Next, the tanner needed to remove the hair fibers from the skin. This was done by either soaking the skin in urine, painting it with an alkaline lime mixture, or simply allowing the skin to putrefy for several months then dipping it in a salt solution. After the hair fibers were loosened, the tanners scraped them off with a knife.

Once the hair was removed, the tanners would bate the material by pounding dung into the skin or soaking the skin in a solution of animal brains. Among the kinds of dung commonly used were that of dogs or pigeons. Sometimes the dung was mixed with water in a large vat, and the prepared skins were kneaded in the dung water until they became supple, but not too soft. The ancient tanner might use his bare feet to knead the skins in the dung water, and the kneading could last two or three hours.

Soaking

Hides and skins received into a tannery are in the four conditions, green or fresh, wet salted, dry salted or as dried.

Soaking cleans hides and skins by removing dirt, blood, flesh, grease, dung etc. and most importantly, re-hydrates them to bring skins as far as possible back to state of green hides.

Soaking is done by a combination of lipase and protease to refresh the leather, once received it is the first important operation of leather processing. Palkosoak/AcP for alkaline/acidic

De hairing (Un-hairing)

The conventional and most wide spread way to remove hair from bovine hides is to use lime and sodium sulphide in a hair-burning process sulphide to eliminate keratin. They dissolve the hair and open up the fibre structure.

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. 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 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.

Advantages of enzymatic approach

(i) results in a cleaner grain surface and improved area yield and softness.

(ii) The sulphide and lime costs requirements can be reduced by as much as 40% while maintaining the same liming time.

(iii)  Alternatively tanners can shorten the liming time by at least half without any loss of quality.

Bating

To make leather pliable, the hides and skins require an enzymatic treatment before tanning know as bating.

During bating, scud is loosened and other unwanted proteins are removed. Bating de-swells swollen pelts and prepares leather for tanning. It makes the grain surface of the finished leather clean, smooth and fine. Bating with enzymes is an indispensable operation of leather processing to obtain best quality of leather and cannot be substituted with a chemical process.

Traditional methods for bating employed manure of dog, pigeon or hen. These were very unpleasant, unreliable and slow methods. Bio-technical developments in science have now completely replaced these methods with use of industrial enzymes.

Degreasing

Lipases are a type of enzyme that specifically degrades fat and so cannot damage the leather itself. Lipases hydrolyse not just the fat on the outside of the hides and skins, but also the fat inside the skin structure. Once most of the natural fat has been removed, subsequent chemical treatments such as tanning, re-tanning and dyeing have a better effect.

The main advantages of using lipases are a more uniform colour and a cleaner appearance. Lipases also improve the production of hydrophobic (waterproof) leather; makers of leather for car upholstery have commented that 'fogging' is reduced. This is the term for the build-up of a film of chemicals on the inside of car windscreens

Lipases represent a more environmentally sound method of removing fat. For bovine hides, lipases allow chemical fat removers called tensides to be replaced completely. For sheepskins, which contain up to 40% fat, the use of solvents is very common and these can also be replaced with lipases and surfactants. Solvents tend to dry out the skin and give it a pale colour.  Commercial Palkodegrease

ENVIRONMENTAL IMPACT

One ton of hide or skin generally leads to the production of 20 to 80 m³ of wastewater including chromium levels of 100–400 mg/L, sulfide levels of 200–800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport. With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.^[8]