R. Puvanakrishnan
Emeritus Scientist, Department of Biotechnology
CLRI, Chennai – 600 020
e-mail: puvanakrishnan@yahoo.com
*For correspondence
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Abstract
It is well known that environmental pollution has been a
major irritant to industrial development. Chemical and
chemical based industries are the prime targets of
environmentalists for their crusade against pollution and
leather industry is not left out of the reckoning. The chemicals
mainly responsible for pollution in pre-tanning processes are
lime, sulphide and caustic soda apart from common salt and
degreasing chemicals. In fact, one third of the pollution caused
by leather industries results from the wastes generated during
dehairing operations. The wastes are let out into the drains
which in turn empty into the main sewerage causing hazard to
those who use this water. Many tanneries have been forced to
close down because of their noncompliance with the standards
laid down. In a short span of time, Indian Leather Industry has
faced serious challenges such as German ban on
pentachlorophenate, certain azo dyes, formaldehyde etc. on
one hand and court orders imposing strict compliance of
environmental regulations on the other. The attention of
tanners is focused towards revamping the processing methods,
recovery systems and effluent treatment techniques to make
leather processing ecofriendly. Intensive efforts have been
taken up to use a viable alternative technology viz. microbial
enzyme technology in pre-tanning processes and this could be
one of the ways of solving the industrial pollution problems
caused by tannery effluents.
Introduction
The raw skin goes through a series of chemical treatments
before it turns into a flattering leather. This includes soaking,
liming, dehairing, deliming, degreasing and pickling. During
all these steps, the chemicals used are quite toxic. Naturally,
these pre-tanning operations make leather processing one of
the worst offenders of the environment.
Enzyme is a biological catalyst with clear cut specificity.
An important enzyme used in pre-tanning processes belongs to
the group of proteolytic enzymes, also known as proteases which act on proteins. Although enzymes from plant, animal
and microbial sources have been used, large scale use of
microbial enzymes received a boost only in 1960s following
the introduction of fermentation technology. The enzymes or
enzymatic formulations need not be pure but must be cheap as
compared to that of commercial chemicals used in leather
industry. Animal proteases and microbial proteases from
bacteria and fungi are used in the pre-tanning processes of
leather manufacture. The animal proteases are mixtures of
trypsin, chymotrypsin and various peptidases which may
contain amylase or lipase as secondary enzymes. Mainly for
economic reasons, enzymes from microorganisms have come
to play a significant role in recent years and enzymatic
products of microbial origin are already being produced on a
wide scale.
Use of microbial enzymes in pre-tanning processes
The important stages in which microbial enzymes used in
pre-tanning processes are: soaking, dehairing, bating and
degreasing. Soaking is the first operation in the tannery in
which the skins are treated with water for making them clean
and soft. Wet salted or freshly slaughtered skins do not require
any chemical agent for their proper soaking. Soaking is
necessary for solubilization and elimination of salts and
globular proteins contained within the fibrous structure of
skins. Soaking is carried out under alkaline conditions at low
temperature in water containing sodium hypochlorite, sodium
pentachlorophenate, formic acid etc. and it is accelerated by
some of the nonionic detergents and additives such as sodium
sulfide or sodium tetrasulfide.
The advantages of enzymatic soaking include loosening of
the scud, initiation of the opening up of the fibre structure,
production of leather with less wrinkled grain and a decrease in
soaking time. Enzymes from Aspergillus parasiticus, A. flavus,
A. oryzae and Bacillus subtilis are used alone or in mixtures.
Soaking is usually performed with a combination of proteolytic
enzymes that are optimally active in the neutral or alkaline pH
range.
Dehairing of skins and hides using microbial
enzymes
The most important operation in which enzyme is used in
leather processing is dehairing. Five methods of dehairing are
generally adopted viz.(i) Clipping process (ii) Scalding process
(iii) Chemical process (iv) Sweating process and (v) enzymatic process. The conventionally practiced method of dehairing is
the chemical process using lime and sodium sulfide. However,
the use of high concentration of lime and sodium sulphide
creates an extremely alkaline environment resulting in the
pulping of hair and its subsequent removal. While the efficacy
of this process cannot be questioned, it has some major
disadvantages. About 75% of the organic waste from a tannery
comes from the pretanning yard and 70% of this waste is from
hair which is rich in nitrogen. This illustrates the contribution
made by the lime and sulphide process towards pollution.
Sulphide is highly toxic and has an obnoxious odour and if left
untreated, it can cause major problems in the sewers. The
severe alkaline condition is a health hazard for the workers.
(Image Source: Conventional dehairing)
What are the advantages of enzymatic dehairing?
Enzymatic dehairing is suggested as an environmentally
friendly alternative to the conventional chemical process. Hair
of good quality and strength with a good saleable value is
recovered. An ecologically conducive atmosphere for the
workers is created. A significant nature of the enzymatic
dehairing process is the time factor involved. The lime-sulfide
process takes about 16h whereas the enzymatic dehairing
would be completed between 8h and 20h.
Proteolytic enzymes are of great commercial importance
contributing to more than 40% of the world‘s commercially
produced enzymes. Approximately 50% of the enzymes used
as industrial process aids are proteolytic enzymes. Proteolytic
enzymes are more efficient in enzymatic dehairing than
amylolytic enzymes. Proteolytic enzymes from plant, animal
and microbial sources are used for dehairing. Microbial
proteases, because of their inherent advantages, are preferred
in modern practice.
Three methods of application are commonly used in the
enzymatic dehairing process: a) Paint method, b) dip method
and c) drum method. In the paint method, the enzyme solution
is mixed with an inert material like kaolin, made into a thin
paste, adjusted to the required pH and applied on the flesh side
of skins. In the dip method of enzymatic dehairing, the skins
are kept immersed in the enzyme solution at the required pH in
a pit or tub. In addition, drum method is also followed.
(Application of enzymes by paint method)
(Application of enzymes by spraying technique)
Bating using microbial enzymes
Another application of Microbial enzyme technology in
pretanning process is known as bating. The concept of
softening skins by treating them in a warm infusion of animal
dung has been termed as ―bating‖ and the product used for
such process is known as a bate. The main object of bating is
to remove some of the non leather forming proteinous
materials such as albumins, globulins and mucoids from skins
and to allow splitting of collagen fibres.
The principal materials which a bate contains are
proteolytic enzymes, a suitable carrier for the enzyme and a
deliming agent such as ammonium chloride or sulphate.
Pancreatic enzymes are found to be the best for use in bating.
Alternatively, microbial enzymes could be used for bating.
Degreasing using microbial enzymes
Another major operation where enzyme is used is known as
degreasing and it is the process of removal of excess natural fat
from greasy skins. The presence of natural grease in certain
skins, especially woolly sheep skins, results in various defects.
During the degreasing operation in the pretanning process, the
fat or grease is removed from the interfibrillary spaces of the
skins to facilitate the uniform penetration of tanning materials,
fat liquors etc.
Degreasing is carried out after pickling and conventionally,
it is carried out by either aqueous emulsification using detergents
or by solvent extraction. It is well known that organic solvents
like kerosene, petrol, perchlorethylene and trichloroethylene are
highly unsafe and hazardous to the workers and heavily pollute
the environment. The detergents, though not hazardous while
handling and storing, cause serious pollution problems. These
detergents and solvents add to the BOD (Biological Oxygen
Demand) load of the pickling effluent and the chlorinated
hydrocarbons and solvents add to the toxicity of the effluent.
Enzymatic degreasing is suggested as a viable alternative to
combat the pollution problems caused by the use of solvents and
detergents. Lipases catalyse the breakdown of fats and can be
obtained from animal, microbial and plant sources. The
advantages of using enzymes for degreasing are the elimination
of solvents, reduction in surfactants and possible recovery of
valuable byproducts.
Byproducts utilization
Microbial enzymes have a role in byproducts utilization.
They could be used in the treatment of fleshings from the
tannery. A combination of hydrolytic enzymes viz. proteases,
carbohydrases and lipases might be ideal for this use.
Conclusion
Tanneries in future will use a combination of enzymatic
and chemical processes. The potential for use of microbial
enzyme technology in leather processing lies mainly in areas in
which pollution causing chemicals such as sodium sulfide, lime
and solvents are being used. Future might witness ecolabelled
leather products emerging as niche products by the use of
microbial enzyme technology and the experience gained by the
Indian Leather Industry in this area might greatly help to emerge
as a global leader
References
Sivasubramanian, S., Manohar, B.M. and Puvanakrishnan, R. (2008). Mechanism of enzymatic deharing of skins using
a bacterial alkaline protease. Chemosphere, 70: 1025-
1034.
Sivasubramanian, S., Manohar, B.M., Rajaram, A. and
Puvanakrishnan, R. (2008). Ecofriendly lime and sulfide
free enzymatic dehairing of skins and hides using a
bacterial alkaline protease. Chemosphere, 70: 1015-1024.
Puvanakrishnan, R., Sivasubramanian,S and Hemalatha, T.
(2015). Microbes and Enzymes Basics and Applied. MJP
Publishers, Chennai.
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