Literature Review 2: Collagen

 Literature Review 2: Collagen
1. Introduction
Collagen is the main protein of connective tissue in animals and fish, where it is
the most abundant protein in mammals, constituting up to 25-30% of the total protein
content of the body. On the other hand, collagen constitutes 1-2% of muscle tissue
besides contributing to the tune of 6% of the weight of strong and tendinous muscles.
Even the people of early civilization discovered collagen's multiple utility value, such as
waterproofing, adhesive, and decoration.
In the modern era collagen has become an inseparable instrument in both bio
medical and non bio medical industries, with an extended range of usage. For example, it
provides structure additive to food besides providing food supplements of collagen;
enables the pharmaceutical industry to produce hard and soft capsules, artificial skins,
injections of regeneration of cells, provides cosmetic industry for the beautification and
elimination of age-related skin wrinkles, acts as a source of glue in its hydroslated form,
helps in films in the photographic industry
The main sources of collagen are pig and bovine skins due to their easy
availability, though fish collagen is also making its mark and the researchers are
contemplating with the potential of chicken skin. The major advantages of marine collage
sources are that they are free from the risk of BSE (bovine spongiform encephalopathy),
culturally more acceptable across the world, more suitable to the human skin than its
counterparts, and are available in abundance, since fish skin is a major by-product of the
fish-processing industry.
2. Background
In the period between 1920-1935 Nageotte (1927a, b, 1928, 1930, 1933),
Nageotte & Guyon (1933), Leplat (1933), and Faure-Fremiet (1933) had studied and
observed the partial dissolution of collagen in dilute solutions of weak acids such as
formic and acetic acid. Renewed interest in this soluble protein was generated in the
period between 1940-1955 with the works of Orekhovich and his colleagues in the
U.S.S.R., who reported the extraction of a soluble collagenous-type protein from the skin
of various animals using dilute citrate buffers, which they suggested as a soluble
precursor of collagen (Plotnikova, 1947; Tustanovskii, 1947; Orekhovich et al. 1948;
Chernikov, 1949; Orekhovich, 1950, 1952). Such findings on soluble collagens were
further reviewed by Harkness et al. (1954), who also reported the presence of a small
amount of a protein of collagen type which was extracted from skin by dilute phosphate,
pH 9-0 (alkali-soluble collagen).
Later Harkness et al. (1954) experimented on the feeding of labeled glycine to
rabbits to arrive at a conclusion that this is a true precursor of collagen, whereas the
metabolic role of the acid-soluble collagen described by Orekhovich is less certain, and it
is not necessarily an intermediate in the formation of all insoluble collagens of the skin.
Harkness et al. (1954) determined the hydroxyproline and tyrosine content of the alkalisoluble and acid-soluble collagen, and also of the gelatin obtained from the remaining
insoluble collagen.
Both soluble collagens contained less tyrosine and more hydroxyproline than the
insoluble collagen, and the acid-soluble had a higher hydroxyproline and tyrosine content
than the alkali-soluble collagen. Bowes et al. (1953) also observed similar differences
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between the hydroxyproline and tyrosine content of the acid soluble collagen of calf skin
and the adult collagen of ox hide and between the acetic acid-soluble and insoluble
fractions of tendon collagen.
3. Methods
It requires solvents like salt, dilute acid, alcohol, detergents and H2O2 for
removing non-collagenous proteins, as well as for removing fat and odors. Alongside it
requires chemicals (EDTA) for deashing. It is after that the solvents like acetic acid,
lactic acid, pepsin enzyme, Bacillus bacteria, and yeast are used to extract collagen under
the temperature range of 0-40C. Altogether three steps are involved in the extraction of
collagen, such as pre-treatment steps, extraction of acid-soluble collagen, and purification
of collagen. Apart from the above, there is also super-critical extraction method where
CO2 is used as solvent, which has some advantages like ease of controlling the dissolving
power of supercritical fluid (SCF) by controlling temperature and pressure. It is also easy
to recover SCF by decompressing pressure, and it is impossible to separate the precipitate
from extracts by centrifugation. SCF is a non-toxic solvent and is applicable to extract the
thermally decomposed compounds. However, this method is costly and requires expertise
to handle the proceedings under high pressure.
3.1. A Typical Process
In a typical extraction process, at least eight steps are taken (Mingyan et al. 2009).
For example, in process of squid (Ommastrephes bartrami) collagen extraction, the steps
could be like below