Food Biochemistry and Food Processing (2 edition)

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392 Part 3: Meat, Poultry and Seafoods

(low pH) or for the maximum physical properties (neutral pH)

(Johnston-Banks 1990). Basically, more efficient pretreatment

conditions allow the manufacturer to use lower extraction tem-

peratures, and still obtain gelatins with high bloom strength

(Johnston-Banks 1990). Shorter time is generally associated

with higher extraction temperatures if neutral pH levels are cho-

sen. However, lower gel strength is obtained (Johnston-Banks

1990). Muyonga et al. (2004) reported that Nile perch skin ex-

tracted at higher temperature yielded lower gel strength, melt-

ing point, setting temperature, and longer setting time. Yang

et al. (2007) also reported that gelatin from channel catfish skin

showed lower gel strength as extraction temperature was in-

creased from 60◦Cto75◦C. Additionally, gelatin extracted from

the skin of brownbanded bamboo shark and blacktip shark us-

ing higher temperature had decreasing band intensity of major

components (α-,β-andγ-chains) as well as increasing low-

molecular weight components. Such changes correlated well

with lowered bloom strength. Shorter chain fragments of gelatin

could not form the junction zone, in which the strong network

could be developed (Kittiphattanabawon et al. 2010) (Fig. 21.2).

Drying

After extraction, the gelatins are filtered to remove suspended or

insoluble matters including fat, unextracted collagen fibers, and

other residues. Diatomaceous earth or activated carbon can be

used to make gelatin solution clear. The final stage is evapora-

tion, sterilization, and drying. These are performed as quickly as

possible to minimize loss of properties (Johnston-Banks 1990).

Kwak et al. (2009) extracted gelatin from shark cartilage using

three drying methods, (1) freeze drying, (2) hot-air drying, and

(3) spray drying. Freeze-dried gelatin showed the highest gel

strength and foam formation ability, but its foam stability was

the lowest. Nevertheless, spray-dried gelatin exhibited the best

emulsion capacities.

FISH GELATIN

Fish gelatin from both cold- and warm-water fish has been ex-

tracted under different conditions (Table 21.2) (Montero and

Gomez-Guill ́ en 2000, Jamilah and Harvinder 2002, Cho et al. ́

2004, Muyonga et al. 2004, Zhou and Regenstein 2004, Cho

et al. 2005, Jongjareonrak et al. 2006a, Arnesen and Gildberg

2007, Cheow et al. 2007, Yang et al. 2007, Nalinanon et al. 2008,

Aewsiri et al. 2008, Gimenez et al. 2009, Aewsiri et al. 2009a, ́

Jongjareonrak et al. 2010, Kittiphattanabawon et al. 2010). How-

ever, gelatins from these sources have limited application as they

have lower gel strength and their gels are less stable, compared

with those from their mammalian counterparts. The differences

between fish and mammalian gelatin mainly depend on their

molecular weight distribution and amino acid content, espe-

cially the content of imino acid (proline and hydroxyproline)

(Johnston-Banks 1990). However, gelatin from skin of brown-

banded bamboo shark extracted under the appropriate conditions

had properties similar to those of mammalian gelatin. The gel

could be set at room temperature and had the bloom strength

comparable to commercial bovine gelatin (Kittiphattanabawon

et al. 2010). This shark skin gelatin might contain similar amino

acids, especially imino acids, as well as could form gel compa-

rable to that from mammalian source.

FUNCTIONAL PROPERTIES OF GELATIN

Gelatin is a gelling protein, which has widely been applied in

the food and pharmaceutical industries (Cho et al. 2005). Gelatin

gel has "melt-in-the-mouth" characteristic and shows an excel-

lent release of flavor (Choi and Regenstein 2000). Moreover, its

functional properties, including gelation, emulsifying proper-

ties, foam-forming properties, and film formation, are important

for the food industry as it enhances the elasticity, consistency,

and stability of food products, and it is also used as an outer

film to protect foods against light and oxygen (Montero and

Gomez-Guill ́ en 2000). ́

Gelation

An aqueous solution of gelatin becomes slightly viscous at tem-

perature above its melting temperature. On cooling, the gelatin

solution starts to form transparent elastic thermoreversible

gels when the temperature is below the setting temperature

(Normand et al. 2000, Babin and Dickinson 2001). The interac-

tion initiates a disorder-to-order transition, as the random coil

gelatin molecules seek to return to the ordered triple helix con-

formation. Gelatin gel is a reversibly cross-linked biopolymer

network stabilized mainly by hydrogen-bonded junction zones

(Fig. 21.2). Furthermore, hydrophobic and ionic interactions are

also involved in the gelation of gelatin (Fig. 21.2). The gelation

of gelatin is dependent on many factors such as source of raw

material for gelatin extraction, presence of endogenous protease

in raw material, and the conditions for gelatin extraction, partic-

ularly temperature, play a role in gelation of resulting gelatin.

High temperature generally renders high yield, the gelatin so

obtained has shorter chains (Kittiphattanabawon et al. 2010).

These fragments cannot form the junction zone effectively. As a

result, a poor gel is formed or gel of these fragments cannot set,

especially at room temperature (25◦C) (Fig. 21.2).

Source of Raw Material

The sources of raw materials influence the composition of

gelatin, especially amino acid composition of gelatin, which

affect gelling properties. Generally, the imino acid content of

gelatin obtained from mammals is higher than that of gelatin

from both warm-water and cold-water fish (Table 21.3). Imino

acid content correlates with gel strength of gelatin. Imino acid,

especially hydroxyproline, is involved in gel formation by act-

ing as a H-donor, in which hydrogen bond can be formed with

an adjacent chain possessing H-acceptor (Fig. 21.2). The hy-

droxyl group of the hydroxyproline plays a part in the stability

of the helix by interchain hydrogen bonding via a bridging water

molecule as well as direct hydrogen bonding to a carbonyl group

(Wong 1989).

Fish gelatin has been recognized to exhibit poorer gelling

properties than its mammalian counterpart. Gelatin from