Food Biochemistry and Food Processing (2 edition)

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396 Part 3: Meat, Poultry and Seafoods

brownbanded bamboo shark and blacktip shark extracted at

45 ◦C for 6 hours exhibited higher bloom strength as compared to

that of bovine gelatin. Additionally, gelatin from brownbanded

bamboo shark can be set at room temperature within 3 hours

(Kittiphattanabawon et al. 2010). This superior gelling ability is

mainly attributed to the high imino acid content of gelatin from

the skin of both sharks.

Presence of Endogenous Protease in Raw Material

Proteinases are enzymes inducing the cleavage of peptides.

Collagenolytic enzyme is another proteinase, which catalyzes

the hydrolysis of collagen and gelatin (Sikorski et al. 1990,

Kristjansson et al. 1995, Sovik and Rustad 2006). The major ́

collagenolytic enzyme is collagenase, a group of matrix metal-

loproteinase in a family of zinc-dependent enzyme. Collagenase

can cleave triple-helical collagen at a single site, resulting in the

formation of fragments corresponding to one-fourth and three-

fourth of its initial length (Sano et al. 2004). Metallocollagenase

obtained from fibroblastic cells of rainbow trout was classified

as MMP-13 (collagenase-3, EC 3.4.24) (Saito et al. 2000).

Recently, the presence of an endogenous protease—serine

protease—in bigeye snapper skin has been reported (Jongjare-

onrak et al. 2006a, Intarasirisawat et al. 2007, Nalinanon et al.

2008). Intarasirisawat et al. (2007) reported that endogenous ser-

ine protease contributing to the autolysis of the skin of bigeye

snapper had the maximal activity at 60◦C, which is within the

temperature range used for gelatin extraction. Nalinanon et al.

(2008) reported that the gelatin extracted by the typical process

in the absence of protease inhibitor—soybean trypsin inhibitor

(SBTI)—showed lower gel strength as compared to that ex-

tracted in the presence of SBTI. When gelatin was extracted

from the skin of bigeye snapper skin in the presence of SBTI,

β-andα-chains along with the polymerized components were

retained (Fig. 21.3). Gelatin molecules with the shorter chains

generated by serine protease are not able to form the strong in-

terjunction zone, especially via hydrogen bond or other weak

bonds such as hydrophobic interaction or ionic interaction. As

a consequence, a weaker network develops, in contrast to the

gelatin molecules with longer chain length, which are capable

of alignment or self-aggregation more effectively (Fig. 21.4).

Extraction Conditions

Most of the processes involved in gelatin extraction directly or

partially affect the functional properties of gelatin. Pretreatment

of fish skin has been reported to have an impact on gel-forming

ability of gelatin. Gomez-Guill ́ ́en and Montero (2001) extracted

the gelatin from megrim skins pretreated with different organic

acids including formic acid, acetic acid, propionic acid, lactic

acid, malic acid, tartaric acid, and citric acid at a concentration

of 0.05 M. Gelatin extracted from the skin pretreated with acetic

acid and propionic acid exhibited the highest gel strength. Fur-

thermore, the skin from dover sole swollen with 50-mM acetic

acid produced gelatin with the highest gel strength as compared

to that swollen in 25- and 50-mM lactic acid (Gimenez et al. ́

2005). Ahmad and Benjakul (2011) reported that gelatin from

I

α 1

β

α 2

C 0.1 μM 1 μM 10 μM

SBTI

Figure 21.3.Protein patterns of gelatin extracted from bigeye

snapper skin in the absence and the presence of soybean trypsin

inhibitor (SBTI) at different concentrations. I and C denote calfskin

collagen type I and gelatin extracted without protease inhibitor.

(Adapted from Nalinanon et al. (2008).)

the skin of unicorn leatherjacket swollen with 0.2 M phospho-

ric acid (GPA) resulted in the decreasing major band intensity

(α-,β-, andγ-chains) and the increasing low-molecular weight

components occurred in comparison with that swollen with 0.2

M acetic acid (GAA). However, GPA exhibited higher bloom

strength than GAA since some phosphate groups might attach

with some amino acids during swelling process, leading to phos-

phorylation of GPA. Phosphorylation may introduce ionic inter-

action between phosphate groups and –NH+proton of amino

acids, increasing the cross-links of proteins (Guo et al. 2005).

Extraction temperature and time are two other important fac-

tors affecting the gelation of gelatin. Generally, bloom strength

of gelatin gel decreases as the extraction temperature and time

increase. Normand et al. (2000) reported that higher extraction

temperature caused protein degradation, producing protein frag-

ments and lowering gelling ability. A weak gelatin gel was as-

sociated with the formation of small fragments (Gomez-Guill ́ ́en

et al. 2002). Degradedα-chains of gelatin are not able to anneal

correctly during overnight stabilization by hindering the growth

of the existing nucleation sites (Ledward 1986). Gomez-Guill ́ ́en

et al. (2002) found that squid gelatin extracted at 80◦C showed

very weak gel. For gelatin from Nile perch skin and bone, the

gelatin extracted at 50◦C exhibited higher gel strength than the

corresponding bone gelatin. Gelatin extracted from skin of Nile

perch at higher temperature exhibited lower gel strength, but

temperature had no effect on gel strength of bone gelatin (Muy-

onga et al. 2004). Bloom strength of gelatin from brownbanded

bamboo shark and blacktip shark decreased with increasing ex-

traction temperature and time. The marked decrease in bloom