Food Biochemistry and Food Processing (2 edition)

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20 Fish Collagen 369

Figure 20.2.Contribution of hydroxyproline in hydrogen bonding between collagen polypeptides.

Generally, collagen can be extracted from fish skin, bone, scale,

and so on. Those raw materials have been prepared by cleaning,

size reduction, followed by appropriate pretreatment, prior to

extraction. The extraction is carried out by the aid of acid, in

which the resulting collagen is termed “acid-soluble collagen,

ASC.” Because of the lower yield of collagen obtained from

this process, pepsin-aided process has been developed to max-

imize the extraction yield and the resultant collagen is named

“pepsin-soluble collagen, PSC.” Flow chart for the preparation

of ASC and PSC is shown in Figure 20.3. All procedures are

performed at low temperature (4◦C) to avoid the thermal denatu-

ration. All processes used for collagen extraction directly affect

the extraction yield and properties of collagen obtained. The

conditions for the extraction of collagen from different sources

can be summarized in Table 20.4.

Preparation of Raw Materials

In general, the raw material for collagen preparation contains

a number of contaminants including noncollagenous proteins,

lipids, and pigments, and so on. Calcium or other inorganic mat-

ters are found in scale and bone. One or several pretreatments

are used to remove the contaminants and increase the purity of

extracted collagen. The removal of residual meats and clean-

ing are performed before further chemical pretreatments. Size

reduction of raw material is also useful to facilitate the contam-

inant removal and collagen extraction. To remove noncollage-

nous proteins and pigments, alkaline pretreatment is used (Nagai

and Suzuki 2002, Kittiphattanabawon et al. 2005, Hwang et al.

2007, Duan et al. 2009, Zhang et al. 2009, Benjakul et al. 2010).

NaOH at a concentration of 0.1 M is widely used. However,

NaCl and H 2 O 2 have also been used to remove noncollagenous

proteins and pigments, respectively (Wang et al. 2007, Zhuang

et al. 2009). Some raw materials such as bones from carp (Duan

et al. 2009) and bigeye snapper (Kittiphattanabawon et al. 2005)

and scale from carp (Duan et al. 2009) contain high amount of

calcium. Those raw materials are effectively decalcified by using

ethylenediaminetetraacetic acid (EDTA) because of its chelat-

ing function. The decalcification can be also achieved by using

inorganic acid, especially hydrochloric acid. As a consequence,

the porous decalcified raw material with increased surface area

can be readily subjected to collagen extraction.

The yield and properties of collagen would be influenced by

the source of the raw material, the nature and concentration of

acid or alkali used during pretreatment and the temperature and

time of pretreatment and extraction (Regenstein and Zhou 2007).

Prior to collagen extraction, skin, scale, and bone are generally

subjected to noncollagenous protein removal. Thereafter, pre-

treated raw material is defatted or the pigments in the skin can

be bleached to lower the color. In shark skin, different process

was proposed by Yoshimura et al. (2000). For the alkali–acid ex-

traction of collagen from the skin of great blue shark (Prionace

glauca), a pretreatment with 0.5 or 1 M NaOH containing 15%

Na 2 SO 4 for 5 days at 20◦C was required prior to acid extraction.

For the alkaline direct extraction, a treatment with 0.5 M NaOH

containing 10% NaCl at 4◦C for 20–30 days was implemented.

Extraction

Acid Solubilization Process

The acid solubilization process has been widely used for col-

lagen preparation. The collagen can be extracted using acid

solution and the collagen obtained is referred to as “acid-soluble

collagen, ASC.” Extraction is conducted using acidic condition,

in which the positive charge of collagen polypeptides becomes

dominant. As a consequence, the enhanced repulsion among

tropocollagen can be achieved, leading to the increased solu-

bilization. Generally, tropocollagen is still in the form of triple

helix with negligible changes. Fourier transform infrared (FTIR)

spectra of collagens from the skins of carp and brownbanded

bamboo shark and arabesque greenling extracted with acetic

acid apparently revealed the triple-helical structure of collagen