Ph.D Thesis - Improvement of Conventional Leather Making Processes

IMPROVEMENT OF CONVENTIONAL
LEATHER MAKING PROCESSES TO
REDUCE THE ENVIRONMENTAL
IMPACT

Doctoral Thesis directed by

Dr. José Costa López
Dr. Jaume Cot Cosp

Eduard Hernàndez Balada

Barcelona, December 2008

Programa de doctorat d’Enginyeria del Medi Ambient i del Producte

Bienni 2006-2008

El Dr. JOSÉ COSTA LÓPEZ, Catedràtic del Departament d’Enginyeria Química de la
Universitat de Barcelona,

CERTIFICA QUE:

El treball d’investigació titulat “IMPROVEMENT OF CONVENTIONAL
LEATHER MAKING PROCESSES TO REDUCE THE ENVIRONMENTAL
IMPACT” constitueix la memòria que presenta l’Enginyer Químic EDUARD
HERNÀNDEZ BALADA per a aspirar al grau de doctor per la Universitat de
Barcelona. Aquesta tesi doctoral ha estat realitzada dins del programa de doctorat
“Enginyeria del Medi Ambient i del Producte”, bienni 2006-2008, en el Departament
d’Enginyeria Química de la Universitat de Barcelona.

I per què així consti als efectes oportuns, signa el present certificat a Barcelona, a 8 de
Desembre de 2008.

Dr. José Costa López
Codirector de la tesi doctoral

El Dr. JAUME COT COSP, professor de recerca del Departament d’Ecotecnologies
de l’Institut d’Investigacions Químiques i Ambientals de Barcelona (IIQAB),
pertanyent al Consell Superior d’Investigacions Científiques (CSIC)

CERTIFICA QUE:

El treball d’investigació titulat “IMPROVEMENT OF CONVENTIONAL
LEATHER MAKING PROCESSES TO REDUCE THE ENVIRONMENTAL
IMPACT” constitueix la memòria que presenta l’Enginyer Químic EDUARD
HERNÀNDEZ BALADA per a aspirar al grau de doctor per la Universitat de
Barcelona. Aquesta tesi doctoral ha estat realitzada dins del programa de doctorat
“Enginyeria del Medi Ambient i del Producte”, bienni 2006-2008, en el Departament
d’Enginyeria Química de la Universitat de Barcelona.

I per què així consti als efectes oportuns, signa el present certificat a Barcelona, a 8 de
Desembre de 2008.

Dr. Jaume Cot Cosp
Codirector de la tesi doctoral

Luck is what happens when preparation meets opportunity

Seneca

Life is what happens to you while you are busy making other plans

John Lennon

Acknowledgments

ACKNOWLEDGMENTS

Hi ha moltes persones a les quals dec un enorme reconeixement en la realització de la
present tesi doctoral. El suport incondicional i a distància de la meva família ha estat
sens dubte cabdal. Voldria destacar particularment als meus pares, avis, germà i tiets,
que em van fer sentir tan aprop quan ens separava tot un oceà. A tota la resta de
família, els dec un sentit agraïment per tots els àpats de rebuda i comiat cada cop que
tornava a Catalunya.

Al Professor Jaume Cot (o Jaume a seques, com em va fer acostumar a dir-li) li dono
les gràcies per obrir-me les portes del Consell Superior d’Investigacions Científiques
(CSIC) i haver fet possible la meva estada al Departament d’Agricultura dels Estats
Units (USDA). La seva bondat, humiltat, sentit de l’humor i capacitat científica són un
exemple a seguir.

No em voldria oblidar dels companys del CSIC que em van donar suport i ànims quan
vaig decidir fer les Amèriques. Especial menció mereixen el Drs. Albert Manich, Agustí
Marsal, Fernando Fernández i Merche Catalina. A tota la resta d’amics i companys
del CSIC, que són massa nombrosos per enumerar-los tots, moltes gràcies per les
enriquidores estones passades junts.

També voldria agraïr al codirector de tesi Dr. Costa, així com al Prof. Mans i Dra.
González, el suport rebut durant els darrers anys i la generositat per ajudar-me a fer
tràmits a distància.

I would like to acknowledge a lot of amazing people I met during my stay at the United
States of America. I must start thanking my three overseas “aunts”: Maryann Taylor,
Ellie Brown and Lorelie Bumanlag. Their kindness, generosity and wisdom were
absolutely priceless. With them, I felt home.

Special mention goes to my first office mate at the USDA, Brian Coll, who took the
patience to teach me proper English and “show me around”. I would also like to thank
Joe Lee, not only for sharing his endless knowledge in leather, but for his true
friendship during all this time. To all of you, I will truly miss you.

Acnowledgements

I would also like to extend a BIG thank-you to the Fats, Oils and Animal Coproducts
Research Unit staff, and to Dr. William Marmer in particular. Thanks for believing in
me and giving me the opportunity to stay at the USDA two more years beyond the initial
appointment.

Visit of my parents to the USDA.
Top – With Ms. Maryann Taylor
Bottom – With Dr. Ellie Brown

Table of contents

Table of Contents

SUMMARY .................................................................................................................. i

1. HIDES AND SKINS ................................................................................................ 1
1.1 History of leather processing ...................................................................... 2
1.2 Economics of hides and skins in the U.S. market ...................................... 3
1.3 Structure of hides and skins ........................................................................ 5
1.4 Conversion of hides and skins into leather ................................................. 9

2. PRESERVATION OF RAW HIDES AND SKINS ............................................... 14
2.1 Causes and signs of decay of hides and skins .......................................... 15
2.2 Preservation of raw hides and skins with common salt ............................ 18
2.2.1 Salt pack curing ......................................................................... 18
2.2.2 Brine raceways .......................................................................... 18
2.2.2.1 Brine curing in the industry ......................................... 20
2.2.2.2 Advantages and disadvantages of brine curing ........... 21
2.4 Alternatives to brine curing ...................................................................... 23
2.4.1 Alternative methods ................................................................... 24
2.4.2 Alternative chemicals ................................................................ 26

3. FILLERS IN THE LEATHER INDUSTRY .......................................................... 28
3.1 Looseness and veininess .......................................................................... 29
3.1.1 Veins ......................................................................................... 29
3.1.2 Grain break ................................................................................ 30
3.2 The upgrading of veiny or coarse break leather ……............................... 31
3.3 Whey and whey products ......................................................................... 32
3.3.1 Description and characterization ............................................... 32
3.3.2 Uses of whey ............................................................................. 35
3.4 Gelatin ...................................................................................................... 36
3.4.1 Description and characterization ............................................... 37
3.4.2 Uses of gelatin ........................................................................... 39
3.4.3 Gelatin in the leather industry ................................................... 39
3.5 Enzymes .................................................................................................... 40

Table of contents

3.5.1 Enzymes in the leather industry ................................................ 42
3.5.2 Microbial transglutaminase ……….......................................... 42
3.5.3 Reactivity of TGase with gelatin and whey proteins ................ 45

4. MATHEMATICAL MODEL OF RAW HIDE CURING WITH BRINE ............. 48
4.1 Letter of acceptance .................................................................................. 50
4.2 Abstract ..................................................................................................... 51
4.3 Resum ....................................................................................................... 52
4.4 Introduction .............................................................................................. 53
4.5 Theory ....................................................................................................... 53
4.6 Experimental ............................................................................................. 59
4.6.1 Materials .................................................................................... 59
4.6.2 Methods ..................................................................................... 59
4.6.3 Analyses ..................................................................................... 59
4.6.3.1 Chloride concentration determination ........................ 59
4.6.3.2 Fluorescence imaging ................................................. 59
4.7 Results and discussion .............................................................................. 60
4.7.1 Epifluorescence microscopy ...................................................... 60
4.7.2 Determination of diffusion coefficients …................................. 61
4.7.3 Determination of optimum brine curing conditions ….............. 63
4.8 Conclusions .............................................................................................. 64
4.9 Definition of terms ................................................................................... 65
4.10 References .............................................................................................. 66
4.11 Acknowledgments .................................................................................. 67

5. EVALUATION OF DEGREASERS AS BRINE CURING ADDITIVES ............... 68
5.1 Letter of acceptance ..................................................................................... 70
5.2 Abstract ........................................................................................................ 71
5.3 Resum .......................................................................................................... 72
5.4 Introduction ................................................................................................. 73
5.5 Experimental ................................................................................................ 74
5.5.1 Materials ....................................................................................... 74
5.5.2 Methods ........................................................................................ 74
5.5.2.1 Stratigraphic study ......................................................... 74

Table of contents

5.5.2.2 Degreaser study .......................................................... 75
5.5.3 Analyses ..................................................................................... 75
5.5.3.1 Determination of moisture and ash content ................ 75
5.5.3.2 Determination of fat content ....................................... 75
5.5.3.3 Determination of thermal stability .............................. 76
5.5.3.4 Back-scattered/Low Vacuum Scanning Electron
Microscopy (SEM-BSE) ......................................................... 76
5.5.3.5 Statistical analysis ....................................................... 77
5.6 Results and Discussion ............................................................................. 77
5.6.1 Stratigraphic study ..................................................................... 77
5.6.2 Degreasing study ....................................................................... 81
5.7 Conclusions .............................................................................................. 85
5.8 References ................................................................................................ 86
5.9 Acknowledgments .................................................................................... 87

6. PROPERTIES OF BIOPOLYMERS PRODUCED BY TRANSGLUTAMINASE
TREATMENT OF WHEY PROTEIN ISOLATE AND GELATIN ......................... 88
6.1 Letter of acceptance .................................................................................. 90
6.2 Abstract ..................................................................................................... 91
6.3 Resum …................................................................................................... 91
6.4 Introduction .............................................................................................. 92
6.5 Experimental ............................................................................................. 93
6.5.1 Materials .................................................................................... 93
6.5.2 Sample preparation .................................................................... 94
6.5.3 Analyses ..................................................................................... 95
6.5.3.1 Gel strength ................................................................. 95
6.5.3.2 Viscosity ..................................................................... 95
6.5.3.3 Rheology ..................................................................... 95
6.5.3.4 SDS-PAGE ................................................................. 96
6.5.3.5 Statistical modeling .................................................... 96
6.6 Results and discussion .............................................................................. 96
6.6.1 Gel strength ................................................................................ 96
6.6.2 Viscosity .................................................................................... 98
6.6.3 Rheological properties ............................................................. 100

Table of contents

6.6.4 SDS-PAGE .............................................................................. 101
6.7 Conclusions ............................................................................................ 103
6.8 References .............................................................................................. 104

7. WHEY PROTEIN ISOLATE: A POTENTIAL FILLER FOR THE LEATHER
INDUSTRY .............................................................................................................. 108
7.1 Letter of acceptance ................................................................................ 110
7.2 Abstract ................................................................................................... 111
7.3 Resum ..................................................................................................... 112
7.4 Introduction ............................................................................................ 113
7.5 Experimental ........................................................................................... 114
7.5.1 Materials .................................................................................. 114
7.5.2 Methods ................................................................................... 114
7.5.2.1 Preparation of WPI-Gelatin blends ........................... 114
7.5.2.2 Application of WPI-Gelatin blends to wet blue leather
................................................................................................ 115
7.5.2.3 Retan/Color/Fatliquor (RCF) .................................... 116
7.5.2.4 Drying ....................................................................... 116
7.5.3 Analyses ................................................................................... 116
7.5.3.1 Mechanical properties ............................................... 116
7.5.3.2 Subjective evaluation ................................................ 116
7.5.3.3 Protein concentration determination ......................... 117
7.6 Results and discussion ............................................................................ 117
7.6.1 Shoe upper wet blue ................................................................ 117
7.6.2 Upholstery wet blue ................................................................. 121
7.6.3 Mechanical properties .............................................................. 123
7.7 Conclusions ............................................................................................ 126
7.8 References .............................................................................................. 127

8. GENERAL CONCLUSIONS AND RECEOMMENDATION ........................... 129
8.1 Preservation of raw hides and skins with brine. Conclusions ................. 130
8.2 Preservation of raw hides and skins with brine. Recommendations ....... 131

Table of contents

8.3 Obtaining and characterization of potential fillers for leather.
Conclusions ................................................................................................... 132
8.4 Obtaining and characterization of potential fillers for leather.
Recommendations ......................................................................................... 133

9. REFERENCES ..................................................................................................... 135

10. NOTATION ........................................................................................................ 144

11. GLOSSARY OF TERMS ................................................................................... 148

12. RESUM EN CATALÀ ....................................................................................... 157

List of figures

List of Figures

FIGURES OF CHAPTER 1

Figure 1.1 – Leather articles …………………………………………....................... 3
Figure 1.2 – Typical distribution of a cow’s weight after the slaughtering .................. 5
Figure 1.3 – Composition of hide ................................................................................. 5
Figure 1.4 – Structure of collagen ................................................................................ 6
Figure 1.5 – Schematic cross-section of a bovine hide ................................................ 7
Figure 1.6 – Structural comparison of hides and skins ................................................. 8
Figure 1.7 – Unit processes and operations in leather processing ................................ 9
Figure 1.8 – Pile of wet blue leather ........................................................................... 11


Figure 2.1 – (a) Scratches on the grain of leather; (b) Leather damaged by bites of
insects ......................................................................................................................... 15
Figure 2.2 – Pinholes (open grain) in sheepskin ........................................................ 17
Figure 2.3 – (a) Process flow drawing of a typical brine curing system; (b) Raceway
brine cure tank ............................................................................................................ 19
Figure 2.4 – Red heat damage on salted skins ............................................................ 23


Figure 3.1 – Veininess in calfskin ......................................................................................... 29
Figure 3.2 – Scanning electron microscope images of blue stock without and with
visible veins ................................................................................................................ 30
Figure 3.3 – Samples of shoe upper leather showing coarse break and fine break .... 30
Figure 3.4 – Processing of whey protein isolate ......................................................... 32
Figure 3.5 – Snack fortified with whey protein isolate .............................................. 36
Figure 3.6 – Transformation of collagen into gelatin in the course of hydrolysis ..... 36
Figure 3.7 – Conversion of chrome shavings into gelatin .......................................... 40
Figure 3.8 – Role of an enzyme in a given reaction ................................................... 40

List of figures

Figure 3.9 – Lock and key analogy for enzymes and substrate .................................. 41
Figure 3.10 – Temperature and pH activity profiles of microbial transglutaminase .. 43
Figure 3.11 – Cross-linking of gelatin chains upon reaction with microbial
transglutaminase ......................................................................................................... 45
Figure 3.12 – Effect of mTGase concentration, incubation time, pH and incubation
temperature on breaking strength of a 10% (w/w) type A gelatin .............................. 46
Figure 3.13 – Reduction of a disulfide bond by two thiol-disulfide exchange reactions
involving DTT ............................................................................................................ 47


Figure 4.1 – Mathematical model of the curing process of a raw hide ..................... 54
Figure 4.2 – Dimensionless sodium chloride concentration field within the hide
during the curing process ............................................................................................ 56
Figure 4.3 – Dimensional sodium chloride concentration field within the hide during
the curing process for various soaking numbers ........................................................ 57
Figure 4.4 – Epifluorescent microscopic images of a cross section of a hide at
different stages of curing ............................................................................................ 60
Figure 4.5 – Determination of transport parameter from experimental data. The
graph corresponds to c0p = 30% (w/v) and Na = 3 ..................................................... 62


Figure 5.1 – Stratigraphic distribution of water, ash, and hide salt saturation in a hide
treated for various intervals of time with a 500% float of an initial 95 °SAL brine .. 79
Figure 5.2 – Denaturation temperature (TD) of the grain, middle, and flesh layers
of a hide treated for various intervals of time with a 500% float of an initial 95 °SAL
brine ............................................................................................................................ 80
Figure 5.3 – Composite images of hide samples, collected at different curing times
by low vacuum, mixed signal SEM imaging .............................................................. 81

List of figures


Figure 6.1 – Gel strengths of gels formed from (a) bovine type B gelatin, 1 to 10%
(w/w), and (b) WPI-gelatin blends, 10% WPI with 0 to 3% added gelatin ............... 98
Figure 6.2 – Viscosities of solutions of (a) gelatin, (b) WPI and (c) WPI-gelatin
blends ....................................................................................................................... 100
Figure 6.3 – Time sweep analysis of 10% (w/w) WPI and WPI-gelatin blends ...... 101
Figure 6.4 – SDS-PAGE gels of WPI, gelatin, and WPI-gelatin blends .................. 102


Figure 7.1 – Flow diagram for retan, color and fatliquor formulation of upholstery
and shoe upper wet blue ........................................................................................... 116
Figure 7.2 – mTGase and protein uptake profiles by shoe upper wet blue pretreated
with a solution containing 0, 2.5 or 5% mTGase and treated with a solution of
5% WPI + 0.5% gelatin ............................................................................................ 118
Figure 7.3 – mTGase and protein uptake profiles by upholstery wet blue pretreated
with a solution containing 0 or 2.5% mTGase and treated with a solution of
2.5% WPI + 0.25% gelatin ……………………………………………………....... 122
Figure 7.4 – Subjective properties of upholstery crust leather ................................. 123
Figure 7.5 – Effect of the various treatments of leather with WPI and mTGase on
the tensile strength of upholstery and shoe upper crust leather ................................ 124
the Young’s modulus of upholstery and shoe upper crust leather ............................ 125
the tear strength of upholstery and shoe upper crust leather .................................... 125


Figure 12.1 – Model matemàtic del procés de curat d’una pell crua ........................ 160
Figure 12.2 – Mostres de cuir amb un toc de flor gruixut o fi .................................. 163
Figure 12.3 – Reacció de crosslinking entre molècules proteiques amb l’enzim
mTGase ..................................................................................................................... 165

List of figures

Figure 12.4 – Força de gel, viscositat, i mòdul elàstic (G’) d’una barreja proteica
composta per 10% (w/w) WPI i diferents quantitats de gelatina (de 0.5 a 3%),
en un ambient reductor ............................................................................................. 166
Figura 12.5 – Imatges de microsopia epifluorescent del wet blue després d’haver
Estat tractat amb una solució etiquetada de WPI i gelatina ...................................... 167

List of tables

List of Tables

TABLES OF CHAPTER 1

Table 1.1 – U.S. hides and skins production ........................................................................... 4
Table 1.2 – U.S. hide and skin exports by country .................................................................. 4
Table 1.3 – Typical reagents needed and wastes produced during the manufacturing
of leather .................................................................................................................................. 13

TABLES OF CHAPTER 2

Table 2.1 – Typical range of emission factors for conventional leather processing .................. 22

TABLES OF CHAPTER 3

Table 3.1 – Composition of different whey products .............................................................. 33
Table 3.2 – Summary of properties of the basic constituent proteins in whey ....................... 34
Table 3.3 – Composition of whey protein isolate .................................................................... 35
Table 3.4 – Physicochemical properties of types A and B of gelatin ...................................... 37
Table 3.5 – Dependence of molecular weight on the bloom strength of type B gelatin ......... 38
Table 3.6 – List of proteinaceous substrates reported to be reactive towards TGase ............. 44

TABLES OF CHAPTER 4

Table 4.1 – Transport coefficient for various conditions of initial brine concentration (c0p)
and soaking Number (Na) ...................................................................................................... 62

TABLES OF CHAPTER 5

Table 5.1 – Effect of commercial degreasers on brine curing [0.5% w/w] ............................. 82
Table 5.2 – Effect of degreaser 1 on brine curing ................................................................... 83
Table 5.3 – Effect of sophorolipid on brine curing ................................................................. 84

TABLES OF CHAPTER 7

Table 7.1 – Uptake rate coefficient k for various treatments ................................................ 118
Table 7.2 – Subjective evaluation of shoe upper crust leather .............................................. 120

Summary

Summary
The research reported in the present PhD dissertation was developed in its totality at the
United States Department of Agriculture (USDA), Eastern Regional Research Center
(ERRC) located in Wyndmoor, Pennsylvania.

During my stay at the ERRC from July 2005 to October 2008, I was assigned to two
CRIS (Current Research Information System) projects. The main goals of each project
are shown below. Further information about them can be found at the ERRC website
(http://cris.csrees.usda.gov/).

1. New and efficient processes for making quality leather
Project number: 1935-41440-013-00D
Lead Scientists: Dr. William N. Marmer and Dr. Cheng-Kung Liu

Objectives: Develop new technology for preparing hides for tanning. Establish drying
and finishing processes and develop in-line nondestructive tests for improving the
quality and durability of leather. Additional funding was obtained to expand the scope
of hide preparation research by investigating ways to impart efficiency to short-term
hide preservation (brine-curing).

2. Sustainable technologies for processing of hides, leather, wool and associated
byproducts
Project number: 1935-41440-014-00D
Lead Scientist: Dr. Eleanor M. Brown

Objectives: 1. Functional modification, leather and leather byproducts. Develop a
foundation for the use of new chemical and biochemical technologies (a) in the
production of high quality chrome-free leathers; (b) in expanding the range of high

i

Summary

value biomaterial applications for solubilized proteins from leather byproducts. 2.
Functional modification, wool: modify wool to impart functionality for improved
performance and expanded uses of domestic wool.

Therefore, my assignments were divided into two different areas.

1. New and efficient processes for making quality leather
The preservation of raw hides and skins with a highly concentrated sodium chloride
solution (brine) is the most traditional and cost effective method. Nevertheless, it is a
lengthy process and detrimentally impacts the environment due to the disposal of salt
during the hide' conversion into leather. A mathematical model that described the
s
diffusion of sodium chloride in the hide during the curing process was developed in
order to search for the optimum brine curing conditions such as brine concentration and
float percentage. The diffusion of salt into the hide was characterized by the transport
coefficient , which was found to be in the order of 10-5 s-1. From the model it was
found that the use of an initially saturated brine (35.9 g NaCl/100 ml water) as well as a
minimum float of 500% yielded an optimal diffusion rate. These findings corroborated
the generally accepted rule which states that about five kg of brine per one kg of hide is
required for a proper curing. The model also revealed that hides that were cured with
diluted brines (e.g. 20 or 25 g NaCl/100 ml water) would not receive a proper cure
regardless of the float percentage used. Importantly, the model put in evidence that the
results obtained were highly dependent on the hide salt saturation level required to reach
a proper cure, which was targeted at 85%. Finally, the proposed mathematical model
may be used to optimize the curing process under any given conditions and thus
rationalize the amount of salt and time employed to properly preserve raw hides and
skins.

Another goal of the project was to find ways to accelerate the uptake of sodium chloride
by the hide during the cure. By accomplishing this, the turn-around times in raceways
would be reduced and thus additional curing capacity created. By means of a
stratigraphic analysis of the hide at varying curing time intervals, it was found that salt
entered the hide mainly from the flesh side whereas water was withdrawn from both
sides of the hide, with the epidermis acting as a semipermeable membrane. These
findings were supported by epifluorescence microscopy and scanning electron

ii

Summary

microscopy in back-scattered electron mode. Bearing in mind that the adipose tissue
that adheres to the flesh side of the hide is believed to slow down the penetration of salt,
three commercial degreasers as well as a glycolipid-based surfactant (sophorolipid, SL)
were tested as brine curing additives. One of the three commercial degreasing agents
was proved to significantly enhance the uptake of salt by the hide during the cure,
simultaneously as it decreased the fat content. Trials with the SL also turned out
successfully; when the SL was used above its solubility limit, the degreasing ability was
comparable to that of commercial degreasers. The usage of SL in the leather
manufacturing has not yet been attempted on a commercial scale, but the promising
results here presented as well as their antimicrobial properties make them a very
promising candidate.

2. Sustainable technologies for processing of hides, leather, wool and associated
byproducts
The usage of leather byproducts in later stages of tanning is established practice,
although not well publicized in the literature. One of the products tanners frequently
use as a post tanning agent is called filler. Fillers minimize the veiny, loose and pipey
areas of the hide to obtain a uniform leather product. Back in the 1970’s, extracts of
vegetable tannins were used for that purpose. Recently, leather industry suppliers offer
filling agents such as polymers, resins and proteins. One of the proteins more
commonly used in U.S. tanneries was casein. Current casein prices in the American
market ($5.8-5.9/lb) encouraged the search for cheaper sources of protein.

Whey protein isolate (WPI), a byproduct of the cheese industry ($0.9-1.2/lb), and
gelatin, a byproduct of the leather industry ($2.4/lb), were selected for that purpose.
Biopolymers formed by the enzymatic crosslinking of dissimilar proteins have the
potential for generating novel products. Thus, small amount of gelatin was added to the
less expensive WPI and reacted with the enzyme microbial transglutaminase (mTGase)
under reducing conditions. The improvement in physical properties over either protein
component, given the same treatment, suggested the possibility of greater utilization and
new products from these coproducts.

Next, a biofiller composed of WPI and small amounts of gelatin was evaluated as a
filling agent for shoe upper and upholstery wet blue. Also, the effect of pretreating the

iii

Summary

wet blue with mTGase was examined. The outcome of the process was assessed by
subjective properties of the crust leather such as grain break, fullness, handle or color.
The general appearance of both shoe upper and upholstery leather was markedly
improved upon treatment with the biofiller, yielding fuller crust leather with a tighter
grain break and enhanced color. Importantly, the proteins contained in the filler were
not considerably removed by further processing. Furthermore, although treated samples
were a little stiffer and presented slight lower tear strength than the untreated samples,
the various treatments did not negatively affect the mechanical properties of the crust
leather. The pretreatment of the wet blue leather with mTGase affected the kinetics of
protein uptake and also contributed in the further improvement of the grain break in
samples with very bad original break. Noteworthy, it was proved that a 200% float
satisfactorily enabled the proteins to be taken up by the wet blue. If this technology is
to be transferred to the industry, use of a shorter float could be feasible due to a stronger
mechanical action. Further research that explores the possibility of using even cheaper
sources of protein as a raw material for bio-based leather products is an interesting
option currently being examined at the ERRC laboratories.

iv

Chapter 1

1. HIDES AND SKINS
1.1 History of leather processing
From the earliest civilizations right to the present time, leather has been considered
nature’s product with inherent beauty and universal appeal. Prestige, durability,
physical properties and eye-appeal are some of the many natural qualities of this
product.

The process of leather making predates recorded history. The earliest record of the use
of leather dates from the Paleolithic period, when primitive man removed the hides and
skins from wild animals after having hunted them for food. They used those hides and
skins to make coats and footwear. Primitive man learned that skins rotted away after a
relatively short time. Throughout the following centuries, they discovered that if the
skins were stretched out and allowed to dry in the sun, they became stiff and hard but
also lasted much longer (Covington, 1997). Much more time had to pass until it was
eventually discovered that the bark of certain trees contained tannin or tannic acid
which could be used to convert raw skins into what we recognize today as leather.
Although it is hard to substantiate chronologically the exact time this tanning method
materialized, many claim that it had to be around 5,000 BC (Thomson, 1981).

Later on, wall paintings and artifacts in Egyptian tombs indicated that leather was used
for military equipment, sandals, clothes, gloves, buckets, bottles, etc. Also the Romans
used leather on a wide scale for footwear, clothes, and military equipment including
shields, saddles and harnesses. As a matter of fact, the manufacture of leather was
introduced to Britain by the Roman invaders.

Through the centuries, leather manufacture expanded steadily and by medieval times
most towns had a tannery located on the stream or river, which they used as a source of
water for processing and as a source of power for their water wheel driven machines.
During the middle age, leather was used for all kind or purposes: footwear, clothes,
bags, cases, upholstery of chairs and couches, book binding and military uses.

With the discovery and introduction of basic chemicals such as lime and sulfuric acid,
leather production slowly became a chemically based series of processes. Until the later
part of the 19th century, there were relatively few changes in the methods used to

2

Hides and skins

produce leather. However, the industrial revolution did not bypass tanning. A wider
range of dyestuffs, synthetic tanning agents and oils were introduced during that time.
Together with precision machinery, these changes and continued innovations to the
present day have combined to make tanning into a viable, modern manufacturing
industry.

Figure 1.1 – Leather articles. From left to right, a woman’s bag, leather seats in automobile and
leather jacket.

1.2 Economics of hides and skins in the U.S. market
The hides and meatpacking industries may be regarded as a bridge between production
of the hide as a byproduct of the food industry and its manufacture into leather goods,
for which it provides a basic raw material.

Hides need some form of preservation after being removed from the animal. Once this
is accomplished, they can be shipped great distances (e.g. overseas) or stored until used.
Thus, the preserved hide becomes an article of international commerce. The supply of
hides is determined by the amount and type of meat in people’s diet. For instance, the
United States is a hide exporting nation since its appetite for beef exceeds the capacity
for tanning of the industry. On the contrary, Japan has a strong demand for leather
goods but a limited hide supply, which turns it into a hide importing nation
(Thorstensen, 1993).

Table 1.1 shows the estimate values of total slaughtered cattle in the Unites States over
the last years. On average, about 60% of the hides produced in the U.S. from 2001 to
2005 were exported, in their majority to Asia (Table 1.2). Latest figures available from
the United States Hide Skin & Leather Association (USHSLA) stated that an 80% of the
hides produced in the U.S. in 2008 were exported, half of which were shipped to China

3

Chapter 1

and Hong Kong (Reddington, 2008). This trend concludes that the United States’
exporting character is currently growing even further.

Table 1.1 – U.S. hides and skins production (1,000 hides)

Year Total slaughter Exports Imports Net exports
2001 35,530 23,471 1,721 21,750
2002 35,734 20,783 1,299 19,484
2003 35,647 19,330 1,153 18,177
2004 32,880 18,704 1,316 17,388
2005 32,535 19,200 1,355 17,845
Source: U.S. Leather Industry Statistics (2006 edition)

Table 1.2 – U.S. hide and skin exports by country (1,000 hides)

Destination 2005 2003 2001
China 8,191 5,434 5,417
Korea 4,089 4,860 7,602
Taiwan 1,718 1,941 2,751
Hong Kong 1,331 2,534 1,382
Mexico 1,287 1,348 1,647
Thailand 651 819 888
Italy 594 826 920
Japan 333 475 1,343
Vietnam 165 15 1
Brazil 141 160 76
India 107 11 41
Source: U.S. Leather Industry Statistics (2006 edition)

At the present time it is not uncommon for cattle hides to be produced in the United
States, converted into leather in China and shipped back to the U.S. to make garments.
As a matter of fact, hide sales to China are soaring. By the end of April 2006, U.S.
exports sales to China amounted to 47% of all exports worldwide which compares with
36% a year earlier (Leather International, 2006).

4

Hides and skins

1.3 Structure of hides and skins
Raw hides and skins are byproducts of the meat industry, and in turn are the raw
material of the leather industry. Figure 1.2 depicts that only about one third of the
animal live weight is manufactured as edible meat. Hide, hair, bones and organs
account for an approximate 23% of the animal’s weight.

Figure 1.2 – Typical distribution of a cow’s weight after the slaughtering.

Hides and skins are removed from the carcass of the animal during the slaughtering
process. By convention, the tanner employs the word hide to refer to the skin covering
large animals, such as cows, steers, horses, buffalos, etc., and the word skin is mostly
used to refer to smaller animals such as calves, sheep, goats, pigs, etc. The term hide is
never applied to the small animals.

The approximate composition of a freshly flayed hide is as follows (Figure 1.3).

Figure 1.3 – Composition of hide (Sharphouse, 1971).

5

Chapter 1

Collagen, with a content of 29%, is the main structural protein in the hide. Up to twenty
eight different types of collagen are described in literature. Among them, fibril-forming
collagens types I, II, III, V and XI are the majority components of skin, cartilage and
bone (van der Rest and Garrone, 1991). Collagen molecules consist of three
polypeptide chains, each coiled in a left-handed helix. The three chains are thrown into
a right-handed triple superhelix stabilized by periodic hydrogen bonds (Rich and Crick,
1955; Ramachandran and Kartha, 1955). The triple helices, also known as
tropocollagen, associate laterally and longitudinally to form microfibrils. These, in turn,
form fibrils, aggregates of which constitute various forms of connective tissue (Figure
1.4).

Figure 1.4 – Structure of collagen.

Other structural proteins present in the hide are elastin (0.3%) and keratin (2%). The
former helps skin to return to its original position when it is poked or pinched, whereas
the latter is the constituent protein of the hair. The figure for keratin varies widely
depending on the amount of hair present. Among the non-structural proteins, albumens
and globulins account for 1% and mucins and mucoids 0.7%.

An interesting approach of seeing the structure of a hide is to examine a cross section
(Figure 1.5).

6

Hides and skins

Figure 1.5 – Schematic cross-section of a bovine hide
(Sharphouse, 1971).

Starting from the hair side the following elements are found:

• Hair. It is composed by an actively growing root zone embedded in the skin and
a visible dead hair shaft above the skin. Hair is primarily keratin, a sulfur-
bearing protein. The structure of a hair follicle is quite complex and up to five
concentric layers can be differentiated (Wagner and Bailey, 1999).

• Epidermis. It is the outermost layer of skin. It consists of a variety of cell layers
produced in the geminative basal cell region located at the base of the dermis. It
is hard, quite inert chemically, and constantly in the state of flaking.

• Sweat glands. They release sweat and undesirable body wastes through the
pores of the skin.

• Sebaceous glands. They release oil into the hair and onto the surface of the skin
in order to maintain a proper body temperature in warm-blooded animals.

• Corium. It consists of a network of collagen fibers intimately woven. In the
upper part of the corium, the fibers are very thin and tightly woven and towards
the center they are coarser and stronger. The orientation of the fibers and

7

Chapter 1

whether they are loosely or tightly-woven will determine some characteristics of
the resultant leather.

• Flesh. It is composed of varying amounts of fatty adipose tissue, blood vessels,
nerves and voluntary muscle.

Hides and skins differ in their structure, depending upon the habits of life of the animal,
season of the year, age, sex and breeding. Figure 1.6 depicts the different structure of a
goatskin, sheepskin and cattle hide. The amount of hair and fat present as well as the
tightness of the structure are three important parameters used to differentiate them. For
instance, goatskin presents less hair and fat and a more firm structure than sheepskin
and cattle hide. Cattle hide presents fat in both sides of the hide and has a tighter
structure than that of sheepskin but more open than goatskin.

Figure 1.6 – Structural comparison
of hides and skin (Thorstensen, 1993).

8

Hides and skins

1.4 Conversion of hides and skins into leather
The conversion of a raw hide or skin into leather requires numerous processing steps,
which may be grouped in the following five categories (Figure 1.7).

Post tanning Finishing
Stages Preservation Pretanning Tanning Operations Operations

Trimming
Soaking Wringing Conditioning
Unhairing Splitting Staking
Liming Retanning Toggling
Unit Dying Buffing
Deliming
Operations Bating Fatliquoring Spraying
Scudding Setting Plating
Pickling Drying

Preserved Hides Pelt Wet Blue Crust Finished
Outcome and Skins Leather Leather Leather

Figure 1.7 – Unit processes and operations in leather processing (Saravanabhavan et al., 2003).

1. Hide preservation. Treatment given to raw hides or skins just removed from
the carcass of the animal to minimize putrefaction and bacterial action, but
enabling the skins to be rehydrated conveniently in preparation for tanning.
Sometimes the preservation of hides is carried out after cutting away the
subcutaneous tissues or flesh, which accounts for about 20% of the weight of the
hide. This process is known as fleshing.

2. Beamhouse (Pretanning). It refers to the processes in the tannery between the
removal of the skins or hides from storage and their preparation for tanning.
Despite this definition, hide preservation is typically not considered a
beamhouse process. Beamhouse operations are of tremendous importance in the
ultimate quality of the leather. The following operations are considered
beamhouse:

a. Trimming. Cut away useless or unwanted material from the edges of raw
hides or skins to give them a better shape.

9

Chapter 1

b. Soaking. Treat hides or skins with water, sometimes with the addition of
a disinfectant, to cleanse them, remove salt and other soluble matter, and
to rehydrate and soften them.
c. Unhairing. Removal of hair or wool from hides or skins. Various
chemicals may be used for this purpose: lime-sulfide, an oxidizing agent
in acid solution, or an enzyme preparation.

d. Liming. Treatment of hides or skins with lime intended to loosen hair,
fat, flesh, etc. Sometimes it is done simultaneously to the unhairing.

e. Deliming. Removal of alkali and pH adjustment for bating.

f. Bating. Enzymatic action for the removal of unwanted inter-fibrillary
proteins, as well as any remaining hair roots, epidermal structure and
fatty cells.

g. Scudding. Working over the grain surface of limed, or bated, pelt with a
blunt-bladed tool, by hand or machine, to eliminate hair fragments,
pigment granules, lime soaps and other impurities.

h. Pickling. Treatment of pelts with an acid liquor, such as a solution of
sulfuric acid and sodium chloride, to preserve them or to prepare them
for tanning, especially chrome tanning.

3. Tanning. Tanning is defined as a process by which putrescible biological
material is converted into a stable material which is resistant to microbial attack
and has enhanced resistance to wet and dry heat (Gustavson, 1956). Chrome
tanning is the most frequently used method to tan hides, mainly due to the short
time it takes (4 to 6 hours) and because it produces leather that combines both
the best chemical and physical properties sought after in the majority of leather
uses (Leather Facts, 1973). A hide or a skin that has been subjected to the usual
beamhouse processes, chrome-tanned and left wet receives the name of wet blue
(Figure 1.8). Wet blue may be stored or exported in this state.

10

Hides and skins

Figure 1.8 – Pile of wet blue leather.

Alternatives to chrome tanning include vegetable tanning, carried out by means
of tanning agents contained in the barks, woods, fruits, leaves, etc., of plants.
Although they have not been implemented in the industry, natural products like
genipin (Ding et al., 2007) or salts of metals such as aluminum (Brown and
Dudley, 2005) or titanium (Peng et al., 2007) were reported to be successfully
tan the skins and hides.

4. Post tanning operations. The order of processes varies considerably for
different leathers. The control and choice of these operations will determine the
characteristics of the leather made.

a. Wringing. Removal of excess moisture from the stock for a proper
splitting.

b. Splitting. Adjustment of the thickness of the hide to the specified
requirements. The cut off layer, named split, is still a valuable raw
material for making into sueded type of leathers. Some hides may need
further thickness adjustment in a process called shaving.

c. Retanning, Coloring and Fatliquoring (RCF). These three operations
have vastly different purposes, but they are considered as a unit because
one follows the other without interruption. To retan is to subject an
already tanned leather to a further tanning treatment to modify its
properties. Coloring, also called dyeing, gives the required color to the

11

Chapter 1

leather. The aim of fatliquoring is to soften the leather by the use of oils
that lubricate the fibers.

d. Setting out. Multiple purpose operation, which smoothes and stretches
the leather while squeezing out excess moisture from it.

e. Drying. Removal of all but equilibrium water from the stock. Moisture
content after this step should be around 10-12%.

5. Finishing operations. The act of making completely tanned leather more
attractive, serviceable and durable. It can include one or more of the following
processes:

a. Conditioning. Application of a fine mist of water to raise the water
content to about 25%.

b. Staking. Mechanical operation that softens and flexes the leather.

c. Toggling. The straining and fixing of leather onto frames with toggles.
The purpose is to dry leather keeping it under tension.

d. Buffing. Abrade or grind a leather surface, especially the grain surface,
by a moving band of abrasive paper or cloth.

e. Spraying. Apply a liquid in the form of very fine droplets designed to
enhance the appearance and/or give the grain or flesh surface special
properties.

f. Plating. Mechanical finishing process used to subject the surface finish
of leather to a high pressure from a heated, polished plate or cylinder to
obtain desired smoothness, flow-out, gloss and film formation.

12

Hides and skins

Table 1.3 summarizes the reagents typically used in the tannery for the conversion of
hides and skins into leather, as well as the different wastes generated at each one of the
processing stages.

Table 1.3 – Typical reagents needed and wastes produced during the manufacturing of leather
(Rao et al., 2003)

Reagents Liquid Waste Solid Waste Air pollutants
Water
Dusted salt
Lime Salt
Beamhouse Trimming
Sulfide Protein Sulfide
operations Fleshing
Enzymes Lime
Hair
Salt & Acid
Water
Salt Splits
Chrome
Tanning Chrome Shavings
Vegetable
Tannins Vegetable bark
tannins
Water
Dyes
Chrome
Post tanning Greases
Syntans
operations Syntans
Dyes
Chrome
Fatliquors
Binders
Pigments Organic solvents
Tanned wastes
Finishing Organic solvents Buffing dust
Buffing dust
Formaldehyde Formaldehyde
Lacquers

13

CHAPTER 2

Preservation of raw hides and skins

Chapter 2

2. PRESERVATION OF RAW HIDES AND SKINS
2.1 Causes and signs of decay of hides and skins
The purpose of preserving hides and skins is to temporarily prevent deterioration from
the time they are removed from the animal until they are processed into a product that is
no longer susceptible to putrefaction or rotting. A hide that has not been properly cured
will yield a finished leather of poor quality. Therefore, this very first step plays an
essential role in the quality of the finished product.

Hide damage can be classified into two categories: physical damage and putrefaction.

• Physical damage. Occurs before the slaughter of the animal. It becomes an
important issue because these defects will have an adverse effect on the finished
leather even if the hides receive a proper cure. It includes tears, scratches, cuts,
hook marks, contamination with dirt (manure), insect attack, etc (Figure 2.1).

a b

Figure 2.1 – (a) Scratches on the grain of leather; (b) Leather damaged by bites of insects
(Tancous et al., 1959).

• Putrefaction. The main object of preservation is to fight the damage caused by
bacteria and the proteolytic enzymes produced by them. Bacteria are one-celled
microorganisms that multiply very rapidly when they feel comfortable in the
surrounding environment. These conditions are summarized in the following
list:

a) Food. Bacteria secrete a digestive juice that contains enzymes. The
function of these enzymes is to nourish bacteria by breaking down
molecules of the substrate (hide). Three different kinds of bacteria are

15


involved in the degradation of skin proteins: anaerobic, aerobic and
facultative (which can grow with or without oxygen). Anaerobic
bacteria deteriorates the proteins into the stage of amino acids and
therefore it is one of the most dangerous. A species named Clostridium
histolyticum was the first to be reported to produce the enzyme
responsible for collagen degradation, collagenase (Mandl et al., 1958).

b) Water. Bacteria need water to live, grow, multiply and produce
enzymes. Nevertheless, there are some bacteria that have become
accustomed to dry conditions that form dormant spores which are
capable of reproducing when moisture is available again, sometimes
even after years. The critical moisture, below which level the skin is not
conducive for bacterial attack was found to be at around 50% (Stuart and
Frey, 1938).

c) pH. Bacteria require a near neutral or slightly alkaline environment to
survive. Generally, microorganisms can not survive in an environment
with a pH very much lower than 5 or higher than 8.

d) Temperature. Bacteria are more likely to reproduce in a warm
surrounding, leading to poor cure efficiency and loss of hide substance.
Literature reported that optimum temperature for bacteria was between
15 to 37 °C and that a better preservation was attained if curing was
carried out at a temperature between 10 to 18 °C (McLaughlin and
Rockwell, 1922).

e) Pre-curing period. This term defines the time comprised between the
slaughtering of the animal and the commencement of the curing process.
It was reported that a five hours period led to degenerative changes in the
cells lying around the sweat glands, and that another 6 hours led to
structural damage of the skin and the breaking down of the polypeptide
chains into dipeptides (Chattopadhyay, 1998).

16

Chapter 2

f) Presence of certain chemicals. Microorganisms can be killed by a
variety of chemical substances. These chemical poisons may be used as
short term preserving agents or along with salt in order to obtain hides
that will remain preserved in a long term period. A bacteriostat is a
chemical agent that stops or inhibits the activity of bacteria at whichever
state they are in. A bactericide is a chemical agent that kills bacteria
outright. They used to be based on mercury compounds or chlorinated
phenols, but they were ruled out with the enforcement of more strict
environmental laws.

Typically, preserved hides may be graded according to the following classification:

• Very good. No sign of putrefaction, raw skin smell, fresh appearance.
• Good. No sign of putrefaction, smells good, not so fresh as raw skin, appearance
good but not so fresh.
• Fair. Signs of putrefaction present, slight putrid smell, dull appearance.
• Poor. Several bacterial damage, strong putrid smell, slimy appearance.

Hair slippage, bad odor, the presence of mold and grain damage (e.g. discoloration) are
amongst the most common signs of a hide decay. Hair slippage has been used as an
indicator by packers and tanners that hides or skins have not been preserved to the
fullest degree. It is a sign that either autolytic or bacterial degeneration has occurred to
the extent that the hair and epidermis is loosened and can be easily removed. This
phenomenon may be accompanied by degradation of the hair follicle leaving a hole in
the grain (Didato et al., 1999) (Figure 2.2).

Figure 2.2 – Pinholes (open grain) in sheepskin.

17


2.2 Preservation of raw hides and skins with common salt
Preservation of hides with sodium chloride (NaCl) is the most frequent curing method
used currently. The hygroscopic nature of sodium chloride reduces the water content of
the hide as well as lowers the water activity of the remaining moisture (Bailey, 2003).
Sodium chloride takes the water away from the bacteria that inhabit the surface of the
hide. Yet an exception to this statement are the halophilic bacteria, which grow in a
concentrated salty environment (see Section 2.3.).

There are two distinct processes that use sodium chloride as a preserving agent: salt
pack curing and brine curing.

2.2.1 Salt pack curing
Also known as green salting, is a more antique method of salt preservation. It is not
common in the United States or Europe but it is extensively applied in warm countries
like India. It consists of sprinkling solid salt onto the flesh surface of the hide, usually
about one kilogram of salt per kilogram of hide. As the solid salt slowly diffuses into
the hide, the water inside migrates out. Successive layers of salt and hides are added to
the pack until a height of four to five feet. Because of the slow diffusion of salt into the
hide, a considerable time (approximately 30 days) is needed to insure a satisfactory and
uniform curing.

2.2.2 Brine raceways
This method is extensively used in American and European hide processing facilities. It
consists of a huge vat containing a very concentrated or saturated solution of sodium
chloride (brine) where hides are suspended for a minimum of 18 h. The brine is kept
close to saturation by circulating it through a salt-box or Lixator, or by having excess
salt in the brining vat. Brine curers also add bactericide in the brine to keep low the
presence of bacteria both in the hide and in the vat.

18

Chapter 2

a

b

Figure 2.3 – (a) process flow drawing of a typical brine curing system (Thorstensen, 1993)
(b) raceway brine cure tank, provided with two small paddles to keep the solution agitated
(courtesy of Diamond Crystal Salt Co.).

Tanners and meatpackers monitor the concentration of brine in the raceway with an
instrument called salometer. This hydrometer is a long, narrow, graduated stem
attached to a weighted bulb. It is more buoyant the higher the salinity of the liquid and
therefore sinks less deeply into the liquid. The salometer scale ranges from 0 °SAL
(pure water) to 100 °SAL (saturated brine).

There are other hydrometers that can be used for the same purpose, which measure the
specific gravity or the sodium chloride percentage by weight. The specific gravity is the
relation between the weight of a certain volume of any substance and the weight of the
same volume of water. This value rises from 1.000 (pure water) to 1.198 (saturated
brine at 25 °C).

At 25 °C, 100 ml of saturated brine hold 31.7 g of salt, or likewise 100 g of saturated
brine holds 26.5 g of sodium chloride. Saturation levels can also be referred as g of salt
per 100 ml of water. In this case, 35.9 g of salt saturates 100 ml of pure water, at 25 °C.
The following equation relates brine concentration (g NaCl/100 g brine) and salometer
degrees (°SAL), at 15 °C.

Salometer (°SAL ) = 3.789 ⋅ [NaCl ] − 0.0004

19


It is important to note than the solubility of sodium chloride is slightly dependent on
temperature, with values ranging from 35.7 to 39.8 g/100 g water, at 0°C and 100 °C
respectively.

Despite the low knowledge of technology involved, there are a few aspects that need to
be taken into consideration when curing hides and skins in a raceway.

• Brine needs to be kept always in fast motion. Otherwise the hides settle to the
bottom and will not receive a proper cure.
• Brine needs to be changed regularly due to the accumulation of dirt and manure
in the bottom of the vat. The removal of dirt, clay, manure and urine during
brining results in hides cleaner than those obtained in salt pack curing. If no
action is taken, the actual float decreases and the hides are more likely to receive
a poor cure. Float (or float percentage) stands for the ratio between the volume
of brine and the volume of hides.
• A large float and a high concentration of brine are essential to attain a proper
cure of the hides. A generally accepted rule stipulates a minimum 500% float
(about 5 m3 of brine per m3 of hide) in order to reach a good cure.

2.2.2.1 Brine curing in the industry
Brine curing is performed in conjunction with one of two hide processing methods:
green fleshing or cured fleshing.

1. Green fleshing + Brine curing. It consists of fleshing the hides first and then
brine cure. This is the option of choice for packers that flesh in operations
adjacent to their kill floors. There are three main reasons that justify this modus
operandi:

• Green fleshing has allows the removed flesh to be processed in a
rendering plant, since it is not contaminated with salt, thus preserving
some value in the flesh. In fact, production of biodiesel from solid
tannery waste is being currently considered (Kolomaznik, 2008).

20

Chapter 2

• With the adipose tissue mostly removed, salt penetrates more quickly
and to a greater extent into the hide. This fact also leads to an additional
weight increase in the hides fleshed before cure (Meat Industry, 1979).
• Hides fleshed before the cure show significantly lower bacterial levels
than hides that have not been prefleshed.

2. Brine curing + cured fleshing. In this case, the hide is cured first and then
fleshed. Packers who do not have adjacent hide processing facilities or hide
processors that are located at some distance from their clients, usually choose
this method. Similarly, this method presents two advantages.

• The 18 h curing time in the raceway gives some time to soften and
remove the feedlot mud that adheres to the hair of cattle (especially from
January through April).
• From an operational point of view, the hide has to be handled twice: put
it in the raceway, cured and fleshed. Conversely, green fleshing requires
the hide to be washed in a fresh water tank first, fleshed, returned to the
brine raceway, cured and then removed. Therefore, the hide needs to be
handled three times and more wash tank/raceway capacity is needed.

2.2.2.2 Advantages and disadvantages of brine curing
Brine curing of hides and skins presents the following advantages and disadvantages.
The most relevant advantages are the following:

• It is relatively inexpensive in comparison to the other preservation methods or
curing materials (current commodity prices for sodium chloride are around
$0.11/kg).
• A raceway can process a high volume of hides and skins (various thousands per
day).
• There is not high tech knowledge involved.
• It involves the usage of safe chemicals.

The most important disadvantages of the brine-curing method are detailed next.

21


• Water pollution. Production of leather entails a high consumption of water. In
fact, the conversion of 1 ton of raw hides into leather requires between 20 and
40 m3 of water (Ramasami and Prasad, 1991), which will be almost fully
discharged upon completion of the process.

Soaking of brine cured hides releases about 40-50% of the total dissolved solids
(TDS) of the whole leather making process (Table 2.1). Technology to treat a
stream heavily contaminated with TDS and chlorides (e.g. membrane processes)
is not cost effective. In addition, the presence of common salt in irrigation water
can be extremely adverse, leading to an increase in soil salinity and reduction of
crop yields (Daniels, 1998).

Table 2.1 – Typical range of emission factors for conventional leather processing
Parameters Soaking
Volume of effluent 6-9
Biological oxygen demand (BOD) 6 - 24
Chemical oxygen demand (COD) 18 - 60

Total solids 200 - 500

Dissolved solids 190 - 400
Suspended solids 15 - 60
-
Chloride as Cl 90 - 250
Total chromium as Cr -

Note: all values expressed in kg/ton of hide or skin processed. They were obtained from the
formula (concentration x volume of effluent)/ton of leather processed (Ramasami et al., 1998).

• The large amount of salt required. Most industrial curing operations try to
maximize salt concentration in the brine to 97 °SAL (25.6 g NaCl/100 g brine).
• Despite the historically low cost of common salt, economics of brine curing of
hides and skins has been affected recently by increasing commodity prices for
sodium chloride (a 10-15% increase over the past few years) (Godsalve, 2007).
• The enormous amount of hides being cured at once makes it difficult to ensure
that they remain in the vat a minimum of 18 hours. Brining in a raceway is a
batch process and hides are not tagged. It is probable that hides are removed

22

Chapter 2

from the vat in a different order that they were put in. These hides will be
undercured and are likely to present deterioration in long term storage.
• Overloading the vat with hides causes the float to fall below the recommended
500%. With a lower float, the concentration of salt in the brine decreases
rapidly, which also leads to a slower rate of diffusion of salt into the hide.
Under this scenario, it is likely that a hide removed from the raceway after 18
hours may not be fully cured.
• Red heat damage. Hides that have received a proper cure are more susceptible
to be attacked by bacteria that grow in a concentrated salt environment.

Figure 2.4 – Red heat damage on salted skins
(Source: BLC Leather Technology).

Archaea, an extremely halophilic bacteria belonging to the family
Halobacteriaceae, was demonstrated to cause significant damage on brine cured
hides (Bailey and Birbir, 1993). A concentration of at least 1.5 to 2 M NaCl was
needed for growth and optimally most species required 2 – 4 M NaCl (Grant et
al., 2001). A study showed that 98% of brine cured American hides were
contaminated with those microorganisms (Bailey and Birbir, 1993).
Furthermore, a total of 332 extremely halophilic Archaeal strains were isolated
and 94% of these strains were protease positive (Bailey and Birbir, 1993).

2.4 Alternatives to brine curing
Issues associated to the preservation of raw hides and skins with common salt
encouraged the search for alternatives. Over the last 30 years, different substances were
assayed as preserving agents, in either salt-less or less salt methods. Unfortunately for

23


the majority of cases, they were not applied in the industry due to the high cost
involved. Also, new proposed techniques that efficiently preserved the hides were
studied, but again the high capital investment ruled them out. A quick summary of
these new methods and substances is listed next.

2.4.1 Alternative methods

• Drying. It is the earliest method of preservation of raw hides and skins. Even
though this method is still used by primitive men to cover huts or making tents,
it presents a few disadvantages:

a) If the drying is too slow, which is likely to happen in a cold and wet
climate, putrefaction may occur before the moisture is low enough to
prevent the bacterial attack.

b) If the drying is too fast and the temperature too high, it is likely that
some areas of the hide start to become gluey, thus preventing inner layers
from drying.

c) Hides are thicker than skins and therefore more difficult to dry out.

d) Increase of slipperiness of the pelt during fleshing.

e) The dried hides may be subject to insect attack, which is as bad as the
damage caused by bacteria.

• Cooling and chilling. Some countries use cooling systems to preserve skins.
The preservation time depends on the temperature, going from 3 weeks to 2 days
for temperatures of 0 and 15 °C, respectively. There are three methods to cool
raw hides and skins (Kanagaraj and Chandra Babu, 2002).

24

Chapter 2

a) Cooled air treatment. Just flayed hides are cooled at 3 to 5 °C for about
one hour, which preserves them in a sufficient extent to confront the
transport from the slaughterhouse to the tannery without decaying
(approximately 12 hours).

b) Addition of ice. The addition of ice cubes or flakes in a container that
encloses hides cause a drop of about 20 °C and then can be stored for 24
hours without further treatment. This technique was improved by the use
of a preservative solution to produce ice.

c) Use of dry ice. The use of dry ice cools hides to temperatures of -35 °C
and effectively preserves them for a minimum of 48 hours. The use of
carbon dioxide, however, entails risk of suffocation.

• Freezing. It is a medium term preservation technique that consists of storing
hides at temperatures around -10 to -20 °C. Freezing is rarely used nowadays
due to the high investment required and to the micro-distortion within the fiber
structure caused by the formation of ice crystals.

• Freeze drying. In this case, water is removed by means of applying low
pressure at low temperatures, which gives a flexible and porous cured hide. This
preservation technique cuts back money on shipping and makes the soaking
process easier. However, the operational costs are too high for common use.

• Gamma irradiation. The irradiation of hides with gamma rays along with the
application of a bactericide was reported as a successful salt-less preservation
technique. However, the high cost capital investment required and the difficulty
to control the minimum amount of radiation needed to attain a proper cure
impeded its commercial adoption (Bailey, 1999).

• Electron beam irradiation. The application of electron-beam-irradiation to
cattle hides, also known as EvergreenTM technique, yields a preservation level

25


comparable to that of brine. This high tech method, however, presented the
same disadvantages as the irradiation with gamma rays (Bailey et al., 2001).

2.4.2 Alternative chemicals

• Formaldehyde (H2CO). The use of low amounts of this aldehyde was proved to
efficiently preserve the hide but its usage was ruled out due to its toxicity
(Sharphouse and Kinweri, 1978).

• Soda ash (Na2CO3). Even though soda ash is not a true preservative, its high
alkalinity created an environment where bacteria and proteolytic enzyme activity
was inhibited. This method was only analyzed as a short term preservation
technique (storage time up to 8 days) (Rao and Henrickson, 1983).

• Potassium chloride (KCl). It could be effectively used as a curing agent.
Furthermore, this substance presented a few advantages over the traditional
brine curing:

a) Production of less excess salt.

b) Possible effective use of the water stream as a plant fertilizer.

c) More rapid kinetics on uptake of salt.

d) Elimination of halophilic bacteria.

However, preservation with potash did not take off due to the more expensive
cost of potash over brine. In addition, the low solubility of potassium chloride at
low temperatures made this technique unviable in cold climate tanneries (Bailey
and Gosselin, 1996).

• Boric acid (H3BO3). This substance, used either in a salt-less or less-salt
method, effectively cured skins for storage times up to two weeks. The main

26

Chapter 2

advantage is the reduction by more than 80% of the TDS and chlorides in the
effluents. However, the cost for that new susbtance was about 15-20% higher
than for the traditional brine curing (Kanagaraj et al., 2005).

• Silica gel. This powerful dehydrating agent was proved to be effective in short
and long term preservation of raw hides and skins (Kanagaraj et al., 2001;
Munz, 2007). The advantages of this method are various:

a) Reduction of TDS and chlorides content in soaking liquors.
Furthermore, these liquors can replace pure water for irrigation
purposes.

b) Stronger dewatering of the hide which does not affect the subsequent
rehydration in the soaking step.

c) No impact on the quality of the final product.

However, the production cost of silicate was double that of common salt, and
therefore this method has not been extensively used yet.

• Azardirachta Indica. This herbal preservation agent, commonly named Neem
tree, was demonstrated to be an efficient agent in the curing of goatskins. By
using it instead of sodium chloride, both TDS and solid waste discharge were
reduced, and the quality of the finished crust leather was not adversely affected
(Preethi et al., 2006).

27

CHAPTER 3

Fillers in the leather industry

Chapter 3

3. FILLERS IN THE LEATHER INDUSTRY
3.1 Looseness and veininess
The presence of veins and loose or pipey areas in finished leather figure amongst the
most important concerns that tanners are facing in today’s leather processing. Thus, the
upgrading of leather that presents these defects is one of the most value adding
opportunities for a tanner.

3.1.1 Veins
Although the term veiny is widely accepted among tanners, the term “prominent blood
vessels”, which includes veins, arteries and their branches is more appropriate. A vein
can be compared to a tube which is either empty or full of blood and which is located in
the deeper or very superficial layers of the dermis. Veininess was firstly believed to be
caused by an improper bleeding of the animal after death, followed by coagulation of
animal blood in the blood vessels of the skin (Orthmann and Higby, 1929). Later on,
many more causes were believed to exert an influence on the appearance of veins: the
age, diet and breed of the animal, the climate, the period of slaughter, the preservation
method, the manufacturing process, etc. Belly and neck areas of the hide are most
likely to suffer from veininess.

Veins are very easily recognizable; they are very apparent in stock that is inspected
visually (Figure 3.1).

Figure 3.1 – Veininess in calfskin.

A significant difference in the structure of veiny and non-veiny leather can also be
observed when looking them under a scanning electron microscopy (Figure 3.2).

29


a b

Figure 3.2 – Scanning electron microscope images of blue stock (a) without and (b) with visible
veins. The flesh side of the hide lies on the bottom of the picture, and the grain side on the top.

3.1.2 Grain break
The grain break, also known as break of leather, is characterized by the wrinkles formed
on the surface of leather when it is bent grain inward. A fine break, characterized by the
presence of many fine wrinkles per linear inch, is more pleasing to the eye and thus
more desirable than a coarse break (Figure 3.3). Because no objective standard has
been established to evaluate the break of hides, it remains a matter of subjective
personal inspection.

a b

Figure 3.3 – Samples of shoe upper leather showing (a) coarse break, and (b) fine break.

The break is a naturally occurring characteristic of the skin or hide, although it can be
influenced by processing. Typically, the butt has a finer break than the shoulder and
belly areas.

Frequently, grain break, pipiness and looseness are used interchangeably. These three
conditions have in common that the grain separates to some degree from the corium.

30

Chapter 3

The grain break, pipiness and looseness can be caused by inherent condition of the hide
or conditions associated to tannery processing.

3.2 The upgrading of veiny or coarse break leather
One common way to address this issue is by introducing a substance into the voids that
exist between the fibers of the leather. This substance is called filler or filling agent.
Filler’s objective is to give more body and substance to the leather by reducing
looseness and diminishing the appearance of veininess.

The nature of fillers has changed over time. At first, tanners used extracts of vegetable
tanning agents, barium compounds, glucose, flour and gum as fillers (Harris, 1974).
More recently, filling agents were made from conventional petroleum feedstocks, which
are becoming increasingly expensive. Therefore, the utilization of products from
renewable resources, and particularly those from waste proteins, became highly
interesting. Gelatin chemically crosslinked with glutaraldehyde was demonstrated to be
an effective filling agent, entering the loose areas of the hide and remaining attached to
the leather upon further processing (Chen et al., 2001). However, the potential toxicity
of glutaraldehyde advised against this method.

Our laboratory at the Eastern Regional Research Center (ERRC) has been working over
the last several years in the modification of various waste proteins with the enzyme
microbial transglutaminase (mTGase), with the goal of obtaining a product that could be
used as a filler. Unlike glutaraldehyde, mTGase is a nontoxic and food grade protein
crosslinker (see section 3.5.2). Gelatin, either commercial or experimental obtained
from tannery waste, whey, whey protein isolate (WPI) and sodium caseinate were
demonstrated to be reactive substrates towards the enzyme mTGase, and their reacted
products could be effectively used as potential fillers for leather (Taylor et al., 2001;
2002; 2003; 2004; 2005; 2006a; 2006b; 2007). These proteins, used alone or in
combination, were effectively bound to the leather and not significantly removed upon
further processing. In addition, the mechanical properties of the finished leather were
not adversely affected by the filling treatment (Taylor et al., 2008). Current prices of
sodium caseinate ($5.8/lb) and gelatin ($2.6/lb) emphasized the need for further
research into cheaper sources of protein to generate fillers for leather, like the less
expensive whey ($0.31/lb) or WPI ($1.05/lb) (USDA Agricultural Marketing Service).

31


Other technologies to upgrade the quality of finished leather have also been
investigated. A recent publication reported the use of polymeric micro-spheres that
penetrate selectively into the loose areas of the leather to subsequently expand by the
application of saturated steam (Tegtmeyer et al., 2007).

Next, a brief description of the products used in the preparation of new fillers for leather
(whey protein isolate, gelatin and microbial transglutaminase) is given (sections 3.3 to
3.5).

3.3 Whey and whey products
3.3.1 Description and characterization
Whey is a liquid that separates from clotted milk during manufacture of cheese and after
coagulation of the caseins at pH 4.6 and 20 °C (Eigel et al., 1984).

The majority component in whey is lactose (70-72%), followed by minerals (12-15%)
and whey proteins (8-10%). The whey protein fraction can be selectively or totally
removed from raw whey and concentrated by using various membrane processes (e.g.
diafiltration, electrodialysis, nanofiltration) or ion exchange columns (Figure 3.4).
Membrane processing of whey is more cost effective than ion exchange technology, but
it also yields a whey concentrate with a higher content of fat and less heat stability than
that obtained using the ion exchange columns.

Figure 3.4 – Processing of whey protein isolate (U.S. Dairy Export Council).

32

Chapter 3

Depending on the removal of non-protein constituents, whey products may be classified
as whey protein concentrate (WPC) or whey protein isolate (WPI). The composition of
these products is shown in Table 3.1.

Table 3.1 – Composition of different whey products (Jelen, 2002)

Product type Total protein (%) Lactose (%) Minerals (%)
Whey powder 12.5 73.5 8.5
Whey protein concentrate (WPC) 65.0-80.0 4.0-21.0 3.0-5.0
Whey protein isolate (WPI) 88.0-92.0 <1 2.0-3.0

Next, a brief description of the main constituent proteins in whey and whey products is
given.

Beta-lactoglobulin ( -Lg) is the most abundant of the whey proteins. It comprises 10%
of the total milk protein or about 58% of the whey protein. It has about 15, 43 and 47%
-helix, -sheet, and unordered structure, respectively (Kinsella, 1984).

The structure of -Lg is pH and temperature dependant. Above its isoelectric point (pH
= 5.2), the protein exists as a dimer with a molecular weight of 36.7 kDa. However,
above pH 7.5 and below pH 3.5, the dimer dissociates to its monomeric form. Between
pH 3.5 and 5.2 the dimer polymerizes to a 147 kDa octomer (Cayot and Lorient, 1997).
The conformation of -Lg can also be modified by temperatures above 65 °C, at which
the protein undergoes denaturation accompanied by conformational transitions that
expose highly reactive -NH2 groups (Kinsella, 1984).

Alpha-lactalbumin ( -La) is the second most abundant protein in whey. It comprises
2% of the total milk protein which is about 13% of the total whey protein. A single
polypeptide chain contains 123 amino acid residues, which include eight cysteines
covalently linked by four disulfide bonds. The protein is arranged in a spherical and
highly compact structure that undergoes thermal denaturation at approximately 62 to 65
°C, depending on the pH (Rüegg et al., 1977; de Wit and Swinkels, 1980). Due to an
unusually high apparent heat resistance, -La is extensively renatured upon cooling.

33

Ph.D Thesis - Improvement of Conventional Leather Making Processes

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