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