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Journal of Scientific & Industrial Research J SCI IND RES VOL 66 SEPTEMBER 2007
710
Vol. 66, September 2007, pp.710-715
Industrial applications of keratins – A review
R Karthikeyan, S Balaji and P K Sehgal*
Bio Products Laboratory, Central Leather Research Institute (CLRI), Adyar, Chennai 600 020
Received 20 October 2006; revised 02 April 2007; accepted 16 April 2007
Keratin, a fibrous protein forming main structural constituent of feather, hair, wool, horn, hoof etc., is abundantly available
as a by-product from poultry, slaughterhouse, tanning and fur processing industry. Keratins though find applications in food,
pharmaceutical, cosmetic and fertilizer industry, considerable amount of these products is wasted repeatedly. Keratins are
difficult to degradation and their disposal leads to environmental problems. Research is being done globally to utilize these
wastes. Keratin hydrolysates find potential application in leather tanning industry.
Keywords: Fibrous protein, Keratin, Leather tanning industry
Introduction
India has a large livestock population, which produce
annually: goatskins, 82; sheep skins, 30; cattle hides, 23;
and buffalo hides, 28 million1. World production of bovine
and ovine skins during 2000 was 1,192 million pieces2.
Feathers constitute up to 8-10% of total chicken weight
and it is estimated that several million tons of feathers
are produced annually. Bovine and ovine hair is obtained
as a by-product from the tanneries during hair-saving
unhairing process and it is estimated that about 5% of
dry hair is recovered based on the raw hide weight3,4.
But still most of the tanneries are following hair-burning
process, which destroy the hair completely and contribute
high amount of COD, BOD, TDS etc., to the effluent5-8.
Microbial proteases offer potential solution to remove
the hair completely from the raw skins9-12.
An enormous quantity of keratins in the form of hairs,
feathers, horns and hoofs are wasted each year13.
Keratins are broadly classified as hard (5% sulfur) and
soft (1% sulfur) keratins. Keratin is mechanically robust
and chemically unreactive due to tight packing of protein
chain in the form of α-helix or β-sheet into a super coiled
polypeptide chain crosslinked with disulfide bonds. In
horns, hoofs and hair, keratin is in the form of α-keratin,
whereas in feathers it is in the form of β-keratin14-17.
*Author for correspondence
E-mail: [email protected]
Feather (90% keratin) is used as animal feedstuff in the
form of feather meal18-21. Acid, alkali or enzymes
hydrolyze keratin and hydrolysates have number of
applications22-27. Cosmetics based on keratin preparations
have been reported for the treatment of human hair and
skin28-33. Keratinous materials are used as additive in
the preparation of concrete and ceramics34,35. Sulfur
bound amino acid solution is used to prepare organic
fertilizer, which enhances plant metabolism36. Fire
fighting composition is prepared from a solution of organic
colloid derived from the hydrolysis of horns and hoofs37.
Oxidization of keratinous materials cleaves and oxidizes
some of the disulfide linkages to form water-soluble
peptides and this material is used as a wound healing
agent38. An attempt has been made for the first time in
Central Leather Research Institute (CLRI), Chennai to
utilize keratin wastes successfully in leather tanning
processes39,40,92-95,102,103. Successful attempts have been
made to convert keratinous wastes like poultry feathers,
animal hair, horns and hoofs into keratin hydrolysate (KH)
by controlled alkali hydrolysis. KH has been successfully
employed in leather processing particularly during chrome
tanning and rechroming operations to enhance the uptake
of chromium salt by leather. KH is also successfully
employed in filling cum retanning operation in leather
processing. The process of KH preparation was upscaled
in CLRI pilot plant and technology was transferred to a
company for commercial exploitation.
KARTHIKEYAN et al : INDUSTRIAL APPLICATIONS OF KERATINS — A REVIEW
711
Fig. 1—Rendering plant located in Bioproducts Pilot Plant (CLRI)
Recently, authors at CLRI subjected raw horns and
hoofs of slaughtered cattle and buffaloes collected from
the local slaughterhouse at Chennai to high steam pressure
(40 psi) in a rendering plant (FMC, Australia) for 3 h
(raw horn: water ratio: 100:30 w/v). The resulting material
(Fig. 1) was dried in a dryer (BHL, Ahmedabad) and
pulverized in pulverizer (FMC, Australia) to get horn meal
(60%).
Approaches to Convert Keratin Protein into
Keratin Hydrolysate
Hydrothermal Treatment
Hydrothermal process usually employs high
steam pressure (10-15 psi) and/or high temperature
(80-140°C) in the presence of acid41 (HCl, H 2SO4,
HCOOH etc.) or alkali42,43 (NaOH, KOH, Na 2CO3,
K2CO3, NaSiO2 etc.). Treatment with acid or alkali at
the boiling temperature for over 2-3 h opens disulfide
linkages of keratin and yields water soluble polypeptides,
oligopeptides or even amino acids44-46. The effect of
change in pressure, temperature, chemicals and pH
during thermo-chemical treatment has been studied to
analyze the nutritional value of feather meal18,19,47. Main
drawback in hydrothermal process is that hydrolysis may
result in partial or even complete destruction of amino
acids, which contain peptides with varying molecular
weight and nutritional improvement. These methods leads
to losses of essential amino acids (lysine, methionine and
tryptophan) and causes the formation of non-nutritive
amino acids48,49 (lysinoalanine, lanthionine, etc). Merits
and demerits of alkali versus acid hydrolysis50 have been
reported.
Microbial Treatment
Microbial conversion of keratin wastes is a potential
technique for degradation and utilization of keratin as a
hydrolysate in terms of cost-effective and
environmentally benign processing 51-54. Presence of
disulfide bonds in keratins hinders their degradation by
proteolytic enzymes55-58 (trypsin, pepsin and papain),
which can be efficiently degraded by soil
microorganisms59,60, actinomycetes, bacteria and fungi
by synthesis of keratinolytic proteases-keratinases61-71.
Keratinases are robust enzymes with a wide temperature
and pH activity range and are largely serine or
metalloproteases. Keratinases attack keratin residues72,73
(feather, wool, steam hydrolyzed horn powder etc.) and
convert them into degradative and cost effective
products. Application of keratinase-producing
microorganisms is being explored in feed, fertilizer,
detergent, leather and pharmaceutical industries where
there is great need for materials derived from alternative
raw materials specifically animal wastes derived from
meat processing plants, poultry units, marine and slaughter
houses.
712
J SCI IND RES VOL 66 SEPTEMBER 2007
Applications of Keratin Hydrolysate in Leather
Processing Industry
Keratin Hydrolysate in Tanning
In leather processing, tanning converts putrescible skin
collagen into stable leather74. At present, chromium sulfate
is widely used world over75 as a tanning agent due to its
versatile nature to produce different types of leathers
with required properties and uses76,77. But this tanning
system is under increased pressure from the green groups
due to its polluting and toxic nature78-88. One of the
constraints reported in conventional chrome tanning
practices is of the exhaustion of Cr in the tanning bath,
which does not exceed 60-65% in commercial
tanneries89,90. Due to this, very large quantity of chromium
is discharged into the effluent causing environmental
pollution. The discharge limit of chromium in the waste
stream should not exceed 2.0 ppm in most of the
countries91 necessitating treatment of discharge in
Common Effluent Treatment Plant (CETP) before it let
out for usage. This leads to increase in cost of leather
processed in tanneries. Hence, CLRI studies conducted
to improve exhaustion of Cr in tanning bath by using KH
prepared from poultry feathers and tannery hairs92-94.
KH (2-3%) was used in chrome tanning (based on skin
weight) and the exhaustion of chromium in tanning bath
could be more than 90%. KH prepared from horn meal
by acid hydrolysis (using HCl) and microbial hydrolysis
(using Bacillus subtilis strain) could also be successfully
employed in chrome tanning process to improve
exhaustion95. The reaction of water-soluble keratin
peptides in chrome tanning involves two-step reaction.
Initially, low molecular weight peptides react with
chromium. In second stage, collagen in leather reacts
both with free chromium and chromium-keratin complex.
Fixation of keratin complex in leather enhances further
uptake of chromium resulting into enhanced uptake of
chromium by leather. Fixation of KH generates additional
carboxylic groups that have high affinity for chromium
used in tanning process.
Keratin Hydrolysate in Retanning
Fibre structure of hide/skin is not uniform throughout
the entire area and it is most common to fill the empty
nature of chrome tanned leathers by retanning to improve
the required properties of leathers96,97, which are intended
for making foot wear, garments, gloves, furniture and
automotive upholstery etc. Today several developments
are taking place in the field of retanning such as phenol
formaldehyde and naphthalene formaldehyde
condensates, melamine, dicyandiamide and carbodiimide
based syntans, polymers of various types, such as
acrylates, urethanes and melamine resins98-100. Most of
these retanning agents are still suspected in their
application due to release of high COD, TDS, free phenol
and free formaldehyde. Protein based retanning agents
offer better prospects as they fill loose areas such as
belly, flanks and poor substance materials without
contributing much load to tannery effluent. Chicken
feathers and hairs can be converted into water soluble
peptides using lime and sodium hydroxide at elevated
temperature and pressure in an autoclave and hydrolysate
has been successfully employed as a retanning-cum-filling
agent in leather processing102,103. KH has selective filling
action, upgrades the poor substance skins and it can be
used along with other retanning agents. Besides, the use
of KH in retanning process influences lubricating effect
that enhances grain smoothness and softness
characteristics of leathers.
Conclusions
This review provides base material towards the
establishment of environmentally friendly technology for
the treatment of keratin wastes and throws light on
conversion of non-edible slaughterhouse and tannery
waste (hair, feather, horn and wool) into value-added
products. The use of KH in leather processing has twofold advantage. Initially, bio waste is converted into KH,
which is used as exhaustive aid in chrome tanning to
reduce the pollution load, and finally it is used as fillercum-retanning agent to replace existing retanning-cumfilling material used by the leather industry.
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