Food Biochemistry and Food Processing (2 edition)

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21 Fish Gelatin 391

Levine 1987, Galea et al. 2000). Generally, gelatin quality is

determined by the length of collagen chains: longer chains pro-

duce higher quality gelatin (Tomka 1982, Galea et al. 2000). To

obtain the high-quality gelatin, acid process should be used for

gelatin production since this process disrupts acid-labile colla-

gen cross-links with negligible peptide bond hydrolysis and less

amino acids are destroyed (Slade and Levine 1987, Galea et al.

2000). However, raw materials with high amount of molecu-

lar cross-links result in the lower extraction yield (Galea et al.

2000). To increase the extraction yield, some proteases capable

of solubilizing the collagen or cleaving the cross-links have been

applied during acid process. In general, triple-helical domain of

collagen is relatively resistant to proteolytic attack, while the ter-

minal telopeptide regions, which contain inter- and intramolecu-

lar cross-links, are readily cleaved (Galea et al. 2000). Chomarat

et al. (1994) prepared bovine skin gelatin using pepsin or proc-

tase (isolated fromAspergillus niger) to solublize the collagen

prior to extraction at 90◦C. Both enzymes solubilized collagen

with a yield of 75–76% as calculated from hydroxyproline con-

tent. After heating, collagen was converted to gelatin with yields

of 32% and 39% when pepsin and proctase were used as the ex-

traction aid, respectively. Nalinanon et al. (2008) reported that

the extraction yield of gelatin from bigeye snapper skin treated

with pepsin during swelling process was approximately two fold

higher than that of gelatin from skin without pepsin treatment.

Extraction of gelatin

When the heat is applied, collagen fibrils shrink to less than one-

third of their original length at a critical temperature known as the

shrinkage temperature (Foegeding et al. 1996). This shrinkage

involves a disassembly of fibers and a collapse of the triple-

helical arrangement of polypeptide subunits in the collagen

molecule. During the collagen to gelatin transition, many non-

covalent bonds are broken along with some covalent inter- and

intramolecular bonds (Schiff base and aldo-condensation bonds)

and a few peptide bonds are cleaved. Heat applied at tempera-

ture higher than transition temperature (Tmax) is able to disrupt

the bonds, mainly H-bond, which stabilizes collagen structure

(Fig. 21.2). This results in conversion of the helical collagen

structure to a more amorphous form known as gelatin. Never-

theless, when the collagen molecule is completely destructured,

glue is produced instead of gelatin (Foegeding et al. 1996).

The conversion of collagen to gelatin yields molecules of vary-

ing mass, depending on the temperatures used, indigenous pro-

teinases, and so on. Therefore, gelatin is a mixture of fractions

varying in molecular weight from 15 to 400 kDa (Gelatin Man-

ufacturers Institute of America 1993).

The maximum yield together with the desirable physical prop-

erties is the main objective for the processor. The pH of extrac-

tion can be selected either for the maximum extraction rate

Figure 21.2.Gelatin network associated with hydrogen bond, hydrophobic interaction, and ionic interaction.