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In: Natural Polymers: Derivatives, Blends and Composites Vol. I ISBN: 978-1-63485-831-1
Editors: Saiqa Ikram and Shakeel Ahmed © 2016 Nova Science Publishers, Inc.
Chapter 10
Wahid Ul Rehman*, Aasim Majeed, Richa Mehra, Satej Bhushan,
Pooja Rani, Khem Chand Saini and Felix Bast
Molecular Genetics Laboratory, Central University of Punjab, Bathinda, Punjab, India
ABSTRACT
Gelatin is a collagen derived product, obtained by incomplete hydrolysis of collagen
procured from skin, bones and connective tissues of animals and exhibits flavourlessness,
colourlessness and translucency. It is commonly utilised as a gelling agent and also as
additive in food, drugs, cosmetics, paints, matches, photographic films and foam
stabilizer. The overall amino acid composition and proportion of gelatin varies according
to the source of raw material, however glycine, proline and hyrdoxyproline constitute
almost 60% of the total amino acid residues while cysteine is absent. Besides being used
in food industries, gelatin based composites and blends are used in pharmacy for
manufacturing biocompatible gelatin scraps, tissue engineering films and controlled drug
delivery systems. This chapter focuses on the physio-chemical properties of gelatin, its
extraction, composites and blends.
Keywords: gelatin, collagen, food industries, hydrogels, composites
1. INTRODUCTION
Gelatin (Latin: gelatos
colourless biopolymer obtained from collagenous animal products. It is commonly utilised as
a gelling agent and is also used as additive in food, drugs, cosmetics, paints, matches,
photographic films and foam stabilizer. Gelatin gels have a lower melting temperature (below
35°C) than those obtained from starch, alginate, pectin and agar, thus, making it suitable for
* Corresponding Author address: Molecular Genetics Laboratory, Central University of Punjab, Bathinda-151001,
Punjab, India; Email: wahidulrehman@gmail.com.
Wahid Ul Rehman, Aasim Majeed, Richa Mehra et al.
210
food industry where flavour release is desirable (Djagny et al., 2001). The process of
extraction of gelatin from collagen has been developing for over 150 years and has taken
advantage in food technology and food engineering (Gómez and Montero, 2001;
Mariod and Adam, 2013). It is obtained by incomplete hydrolysis of collagen procured from
skin, bones and connective tissues of animals. As it is derived from a complex molecule, its
degradation is totally arbitrary and generates a variety of peptide chain species. Hence, most
gelatin arrangements are non-homogenous leading to varied molecular weight s (Liu et al.,
2015). Gelatin has a very wide range of functional properties which promotes its utility as a
major ingredient in many food items (Gibbs, 1999; Karim and Bhat, 2009). Gelatin is used in
desserts, ice creams, yoghurt, confectionary juices, jellies and marshmallow. There have been
various important new product coming into the market that are developed from the gelatin
like low fat products, spreads, micro particulation of proteins, microencapsulation of proteins,
fining of wines, fruit containing beverages (Gibbs, 1999; Gómez-Guillén et al., 2007). Global
gelatin demand has been increasing over the years recently and so are the research initiatives
for gelatin manufacturing. Some hydrolyzed gelatins are non-gelling proteins. Now a days
instant gelatins, which are soluble in cold water, are gaining attention in food industry. In all
of these applications the chemistry and functionality of gelatin is a key factor (Karim and
Bhat, 2009). So a better understanding of structure and chemistry gelatin is required to
understand its functionality and its utilization in various industrial products.
2. PROPERTIES OF GELATIN
2.1. Gelatin Structure
Gelatin is very similar to synthetic high polymers, shows a rather wide molecular weight
distribution (Ward, 1954). It is interesting to note that gelatin can form a large variety of
structures ranging from the simplest globular structure which is typical of amorphous
polymers, to supermolecular structures and well-developed fibrillary structure with various
intermediate states as well. The differences in supermolecular structures is reflected in the
physicomechanical properties of the gelatin materials (Courts, 1959).
The Structural diversity of gelatin chains consequently affects its properties. Among the
unique features of gelatin, the presence of acidic and basic functional groups in the
macromolecules configuration, its ability to form a triple-stranded helical structure not
observed in other synthetic polymers, the rate of which depends on many factors such as the
presence of iminoacids, gelatin molecular weight, the presence of covalent cross-bonds, and
the gelatin concentration in the solution (Harrington and Von Hippel, 1961; Pchelin,
Izmailova, and Merzlov, 1964) and its interaction with water which is different to that
observed with synthetic hydrophilic polymers.
Gelatin is manufactured by denaturation of native structure of collagen. The collagen
conformation is distorted on heating and partially recovered during cooling process. Thus, the
amino acid composition of gelatin remains largely close to collagen with a few changes due
to manufacturing process. For instance, alkaline pre-treatment process deaminates glutamine
to glutamic acid and asparagine to aspartic acid; consequently the proportion of glutamic acid
and aspartic acid is more in Type B gelatin (Boran and Regenstein, 2010). Upon gelation,
Gelatin: A Comprehensive Report Covering Its Indispensable Aspects 211
water gets trapped in the mesh of helix fibres and the structure of gelatin changes. Helix
chains undergo different space rearrangements and interactions depending upon the state of
gel. For example, in a second order reaction, one double stranded structure can be formed
- -chain forming loop. Likewise, in a third order reaction,
- -chain
with one forming loop or by si -chain forming two loops (Duconseille et al., 2015).
2.2. Amino Acid Composition
Though definite amino acid composition of gelatin is not clearly known, it is claimed that
glycine, proline and hyrdoxyproline constitute almost 60% of the total amino acid residues in
collagen as well as gelatin. The basic amino acid sequence of collagen as well as gelatin is
-X-
However, the overall amino acid composition and proportion varies depending upon the
source of raw material (Eastoe, 1955).
2.3. Molecular Weight
The molecular weight and other physical properties of gelatin not only depend on its
amino acid composition
weight fragments
effects on the functional properties of the gelatin viz. lowering melting and setting points and
lowering viscosity (Olijve et al., 2001). The severity of extraction treatment affects the
molecular weight profile of gelatin. Gelatin obtained from high temperature treatment
exhibits lower molecular weight profiles than that obtained from lower temperature. Thus, the
molecular weight profile of gelatin primarily depends on the extraction process.
2.4. Chemical Interactions
Glycine molecules are mainly stabilized by hydrogen bonds, covalent bonds,
hydrophobic interactions and electrostatic interactions. The double stranded or triple helices
are stabilized by hydrogen bonding between glycine residues occurring after every third
amino acid residue -chains. The H-atoms of glycine are situated inside the triple helix
and form a weak interaction with the O-atom of the carboxyl group. Water molecules are also
known to be involved in hydrogen bonding of the gelatin network (Vaca Chávez et al., 2006).
It has been reported that gelatin gels in deuterium oxide are stabilized by both hydrogen
bonds of NH group of one amino gelatin chain with CO group of other chain and hydrogen
bonds of water molecules with gelatin chains (Oakenfull and Scott, 2003). Likewise,
hyrdoxyproline also forms hydrogen bonds with water molecules. However, the exact number
and types of hydrogen bonds is still not clearly defined.
Wahid Ul Rehman, Aasim Majeed, Richa Mehra et al.
212
Figure 1. Chemical (configuration) interactions of gelatin Adopted from (Ofori R. A. 1999).
Despite the thermal and chemical treatments, covalent bonds still remain within
gelatin molecules and impart it mechanical properties. These cross-links are favoured by high
temperature, humidity, UV-light and some chemical compounds like formaldehyde and
reducing sugars (Bhat and Karim, 2009; Singh et al., 2002). The types of cross links reported
in gelatin include pentosidine, desmosine, methlylene, aminal and aminoglycoside bonds
(Haug et al., 2009). Hydrophobic interactions have an insignificant role in triple helix
structures, but they are proposed to have a major impact in -sheet structures. Xu et al.,
(2012) demonstrated using UV analysis that hydrophobic interactions have a positive relation
with increasing gelatin concentration and outcompete hydrogen bonds (Xu et al., 2012).
Similarly electrostatic interactions between cationic and anionic groups of protein molecules
contribute in gelatin stability. These interactions are, however, influenced by pH and salt
concentrations. A study revealed that degree of swelling of gelatin gels was influenced by the
degree of ionization of the solution which was attributed to the generation of ion pairs
between counter ions and network charges (Miyawaki et al., 2003).
Other types of cross links have also been found and characterized in the gelatin. For
example, free amine group of any lysine residue may react with an aldehyde group forming a
hydroxymethylamine that yields a water molecule to generate a secondary aldimine. This
imine group further reacts with another lysine residue and generates dimethylene ether which
undergoes rearrangements and links two lysine residues with methylene bond (Haug et al.,
2009). Thus, the cross links in gelatin involves multiple interactions which occur intra-
molecular or inter-molecular regions of the helices. However, there are a few cross links
which are still under discussion viz. disulphide linkages and pyridinoline.
2.5. Gelatin Derived Peptides
An extensive enzymatic hydrolysis of gelatin leads to the formation of gelatin
hydrolysates. It is commonly used to improve the nutritional and functional properties of food
proteins. Commercial proteases like pepsin, alcalase, trypsin -chmyotrypsin,
neutrase, properase E, savinase, protamex, NS37005 and endogenous fish proteases have
been utilized for the production of fish gelatin hydrolysates (Himaya et al., 2012; Zhang et al.,
2012). The molecular weights of gelatin hydrolysates also varies with gelatin sources,
enzymes used and hydrolysis conditions. A few gelatin derived peptides have been identified
Gelatin: A Comprehensive Report Covering Its Indispensable Aspects 213
and sequenced. For example, Alaska Pollock skin gelatin derived peptide contains glycine at
every third position (Gly-Glu-Hyp-Gly-Pro-Hyp-Gly-Pro-Hyp-Gly-Pro-Hyp-Gly) like that
from gelatin (Kim et al., 2001). Another peptide derived from hoki skin gelatin contains
typical Gly-X-Y repeat unit where X is mostly Proline and Y is leucine or histidine (Mendis
et al., 2005). A few peptides without the Gly-X-Y repeats were also identified viz. Japanese
flounder skins (Gly-Gly-Phe-Asp-Met-Gly), Pacific cod skins (Thr-Gly-Gly-Gly-Asn-Val)
and Alaska Pollock skins (Ser-Cys-His) (Himaya et al., 2012; Kim et al., 2001). However,
there are chances that some of these peptides were actually impurities which were not
properly removed during extraction.
3. SOURCES OF GELATIN
Gelatin can be obtained from any collagen containing tissue. It was first time extracted
from the pig skin and is still being used widely at commercial scale. The most commonly
used raw materials for gelatin are skins, bones and cartilages of mammals like porcine and
bovine, however, alternative sources include marine sources, especially, fishes and fowls.
Around 45% of global gelatin production is procured from pork skin, followed by bovine
hides (30%) and bones of bovine and porcine (23%) (Baziwane and He, 2003). The two types
of mammalian gelatins i.e., type A (from bovine) and type B (from porcine) contain different
components with molecular weight varying from 10 to 400 kDa. A strong correlation between
average molecular weight and gel strength is also suggested. Despite being the major
contributor in gelatin industry, mammalian gelatin has been facing scepticism amongst
consumers due to health-related and socio-cultural concerns. Gelatin market crippled after the
outbreaks of bovine spongiform encephalopathy (BSE) and foot mouth diseases in light of
public health concerns (Mariod, 2016).
Recently, fish gelatin gained great interest as an alternative to mammalian gelatin,
eliminating the risk of bovine and porcine diseases. Another advantage of fish gelatin is that it
is acceptable in Islam and faces minimal restrictions in Hinduism and Judaism. Furthermore,
it can be obtained from the by-products of fishing industries and reduces the pollution burden.
Post-filleting waste from fish industries accounts for almost 75% of the catch weight, 30% of
which comprises skin and bones that can be utilized for gelatin production (Boran and
Regenstein, 2010). In light of sustainable development, the productive use of fish waste is
necessary and is expected to increase in near future (Karim and Bhat, 2009). Many fish
species like Atlantic salmon, Cod, Hake, Sin Croaker, Lumpfish, Megrium, Tilapia, Allaska
pollock, Yellow fish tuna, Nile perch, Rainbow trout, Black and red tilapia have been
investigated for gelatin extraction and many more are still being investigated (Feng et al.,
2014; Niu et al., 2013; Wu et al., 2013).
Insect gelatin is yet another alternative for mammalian gelatin. The watermelon
bug, Coridius viduatus (formerly Aspongopus viduatus) and sorghum bug, Agonoscelis
versicoloratus (formerly Agnoscelis pubescens), are commonly used as a source of edible oil
and medicinal products in Sudan. The biochemical analyses revealed that crude protein
content of these bugs is 27.0 and 28.2%, respectively and both contained 16 essential amino
acids (Mariod and Fadul, 2014). However, as per the recommended amino acid profiles by
FAO and WHO, these proteins are of medium quality. SDS-PAGE patterns of both the insect
Wahid Ul Rehman, Aasim Majeed, Richa Mehra et al.
214
gelatins contained 40 kDa protein as the main component. Scanning electron microscope
images showed that gelatin network of melon bug is finest with smaller voids than sorghum
bug. Apart from gelatin, the edible oils obtained from these bugs can also be used to prepare
biodiesel with H2SO4 catalysed reactions (Mariod, 2013).
4. EXTRACTION OF GELATIN
Gelatin is obtained from the hydrolysis of collagen. There are several processes for the
gelatin extraction depending up on the source and type of tissue. The principle of gelatin
production is to remove interfering moieties and to convert water-insoluble collagen to
soluble gelatin form. The net yield of gelatin depends on the parameters of extraction protocol
like pH, temperature, pressure and treatment time. The general layout is discussed below:
Figure 2. General layout of gelatin production.
4.1. Pre-Treatment
Raw materials are thoroughly washed to remove surface contaminants. Bones are
processed differently as after washing, grinding and rewashing, crushed bone chips are
subjected to acid solutions (4-7% HCl) for atleast two days. The samples are, later, treated
with either acid or alkali to weaken the collagen framework by breaking intramolecular cross-
linkages. Skins are preferably treated with acid while bones are treated with alkali. Gelatin
produced by acid treatment leads to Type-A gelatin and that by alkali treatment becomes
Type-B gelatin. In acid treatment, clean and hydrated raw material is sopped in cold dilute
Gelatin: A Comprehensive Report Covering Its Indispensable Aspects 215
acid solution (pH 1.5-3.0) for 18-24 hrs as per the sample size and thickness. The commonly
preferred choice of acid is citric acid, as it does not impart objectionable colour and odour to
gelatin unlike acetic acid. After acid treatment, samples are washed under running water and
neutralised (Boran and Regenstein, 2010; Haug et al., 2009). While in alkali treatment, bones
are first demineralised with a mild acid to get rid of non-collagenous components and placed
in liming pits or vats and sopped in hydrated lime solution for at least 20 days that can extend
up to 60 months depending on the sample properties. For hides and skins caustic soda
solution is more suitable for shorter time period. After the treatment samples are thoroughly
washed under running water and neutralised for subsequent steps. Isoelectric point of Type- A
gelatin (9.4) is higher as compared to Type-B (4.8) as mild acid treatments do not remove
amide nitrogen of asparagine and glutamine (Mariod and Adam, 2013).
4.2. Extraction
The extraction procedure for both acid and alkali treated samples is similar and involves a
series of steps of water extraction at controlled temperatures. It is the most important step in
gelatin production and determines the yield of gelatin. Mostly, multiple extractions are done
using water with progressively increasing temperatures (5-10° rise) ranging from 50-100°C.
Gelatin fractions obtained from the low temperature extraction have minimal degradation
while subsequent fractions have more variable degradation products with varying molecular
weights. Different combinations of pre-treatments and extractions result in final gelatin
product which is a mixture of polypeptide chains of different compositions and molecular
weights (Mariod, 2016). Good quality gelatin with standard average molecular weight and gel
strength is obtained from low temperature extractions. High temperature extractions generate
more coloured product containing depolymerised gelatin. This coloured product is due to the
-amino groups of amino acids of gelatin and carbohydrate
residues in the sample (Hoque et al., 2010).
4.3. Recovery and Refinement
The gelatin solution obtained after extraction is clarified using lamellar clarifier, an
inclined plate clarifier to remove particulate matter, and filtered. The filtrate is then deionized
using ion exchangers and concentrated to attain standard viscosity. The concentrate is
sterilized by plate heat exchangers and steam sterilization. This sterilized gelatin is then
cooled to form a gel
and crushing. The final powdered gelatin is, thus, obtained with moisture content varying
from 8-12% (Haug et al., 2009).
Several research groups have investigated design of experiments to optimise
gelatin production and quality, including high pressure treatment, radiation exposure ,
different organic acids, protease inhibitors, pre-treatment time, extraction temperatures and
water/sample ratio (Bhat and Karim, 2009; Chen et al., 2014; Khiari et al., 2015; Nalinanon et
al., 2008).
Wahid Ul Rehman, Aasim Majeed, Richa Mehra et al.
216
5. TYPES OF GELATIN
There are mainly two types of gelatin, Gelatin A and gelatin B, which differ in the way
of preparation. Gelatin A is processed by an acidic pretreatment before thermal denaturation,
while gelatin B is processed by an alkaline pretreatment. The alkaline pretreatment is thought
to convert amide residues of glutamine and asparagines into glutamic and aspartic acid, which
leads to a 25% higher carboxylic acid content for gelatin B than for gelatin A. These gelatin
gels were chemically crosslinked with N,N-(3-(dimethylamino)propyl)-N¢-ethyl carbodiimide
(EDC) and N-hydroxysuccinimide (NHS). The chemical cross-linking of gelatin gels resulted
in chemically cross-linked physical gelatin networks that probably forms network structure
and called chemical gelatin gels (Fernandez- . It was
shown that the properties of these chemical gelatin A and gelatin B gels differed with respect
to initial lysozyme uptake from solution by the gels and the total release time. These
differences are caused either by differences in lysozyme interaction with gelatin A or B or by
differences in the network structures of both gelatin types (Ross-Murphy, 1992).
6. GELATIN BASED COMPOSITES AND BLENDS
Gelatin, a collagen derivative renewable resource, forms a key raw material in several
industries. It can be used as such or in the form of hydrogels, blends and composites.
Different manufacturing industries in the field of pharmacy, cosmetic, food etc. generate
substantial quantity of gelatin scraps whose disposal is an environmental concern. Being high
in carbon and nitrogen and due to their high swelling property in water, they create greater
oxygen demand after reaching the drainage systems or treatment plants which imposes labor
intensive and more expensive disposal management (Chiellini et al. 2001). Such waste gelatin
can be used to form blends and composites for further use. The cast films produced from the
blends and composites of the poly vinyl alcohol and sugarcane bagasse with pharmaceutical
waste gelatin possess good mechanical and thermal characteristics thereby making them
favorable to be used as biodegradable mulching films (Chiellini et al. 2001). The edible films
generated at low temperature from chitosan and gelatin possess enhanced tensile strength and
low rate of transmission for water vapors and gas (Arvanitoyannis et al. 1998a). Similar
results were obtained by using hydroxypropyl starch and gelatin (Arvanitoyannis et al.
1998b). Using electrospinning technique and fluorinated alcohol of 2,2,2-trifluoroethanol
(TFE) as a solvent, the superfine gelatin fibers generated showed varying morphology
depending upon gelatin concentration. Further 10% v/w of gelatin/TFE solution after co-
composite membranes which possess enhanced mechanical characteristics and wettability .
Preliminary tests of these membranes in bone marrow stromal cell cultures showed that they
have excellent scaffolding properties where the cells not only attach to their surface but also
migrate through it. Hence they act as a good candidate to be be used in tissue engineering
applications (Zhang et al. 2005). Nanobiocomposites generated from gelatin plus bimetallic
silver gold nanoparticles exhibited biodegradable properties plus enhanced tensile strength
. Further these nanocomposites showed positive results for fibroblast
cell tissue culture and are claimed to be used as scaffold in tissue engineering experiments
Gelatin: A Comprehensive Report Covering Its Indispensable Aspects 217
(Mandal and Sastry 2014). Bone replacement therapy is in practice nowadays. For a material
to be used in such applications, it should be biocompatible and dissolved or reabsorbed during
the growth of the bone. Gelatin being less antigenic than collagen, biocompatible and
biodegradable is also used as a scaffold for bone replacing experiments (Chen et al. 2011).
Composites of gelatin and Cu2S and CdS nanoparticles show sensitivity to different gasses
and can be used as gas sensors and detectors (Murdov et al. 2007). Gelatin based composites
are also used in drug delivery systems. Genipin crosslinked gelatin composite gel loaded with
indomethacin, a hydrophobic anti-inflammatory drug, showed controlled release of the drug
and biocompatibility with tissue culture experiments (Thakur et al. 2011). Chitosan/gelatin
composite films are claimed to suitable for contact lenses because they possess greater
transparency, flexibility, biocompatibility, and increased solute and oxygen permeability
(Xin-Yuan et al. 2004).
7. GELATIN BASED HYDROGEL
Gelatin hydrogels are formed by physical crosslinking in water above a certain
concentration (around 2% w/v), and below 30-350C. Gelatin molecules during the process
undergo aggregate, and conformational changes from a random coil to form a triple helix, and
intermolecular hydrogen bonds form between the large fractions of gelatin chains. The non-
covalent associations are broken easily at higher temperatures more than 30-35 0C, thereby
destroying the physical network (Zhao et al. 2006; Bode et al. 2011; Peña et al. 2010). The
gelatin hydrogels have poor mechanical strength, low elasticity, and low shape stability (Dash
et al. 2013), which limit their biomedical applications at physiological temperatures (37 0C).
To increases its stability and mechanical properties, the gelatin gel is covalently crosslinked
by small chemicals such as glutaraldehyde, carbodiimides, and formaldehyde, which couple
the carboxyl groups with amino groups, and formed stable amide bonds (Kuijpers et al.
1999). The cell attachment was observed by culturing human mesenchymal stem cells, which
are obtained adult stem cells that show extensive and significant use in biomedical
applications (Xing et al. 2010).
CONCLUSION
Although gelatin has wide range of applications in cosmetic and food industries, being
used in in desserts, ice creams, yoghurt, confectionary juices, jellies and marshmallow, low
fat products, spreads, micro particulation of proteins, microencapsulation of proteins, fining
of wines, fruit containing beverages, yet it raises certain ethical concerns in some sections of
the society. Being obtained from the animal origin products, it raises concern in vegetarian
sect of the society and when obtained from pig products, it certainly a concern in the Muslim
sect.
Wahid Ul Rehman, Aasim Majeed, Richa Mehra et al.
218
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