Food Biochemistry and Food Processing (2 edition)

BLBS102-c21 BLBS102-Simpson March 21, 2012 13:39 Trim: 276mm X 219mm Printer Name: Yet to Come

398 Part 3: Meat, Poultry and Seafoods

1.89× 10 −^8 gmmh−^1 cm−^2 Pa−^1 , respectively. Zhang et al.

(2007) characterized edible film from channel catfish gelatin ex-

tracted using different pretreatment methods. The pretreatment

method with 0.25 M NaOH and 0.09 M acetic acid, followed by

extraction at 50◦C for 3 hours was selected as the optimum ex-

traction method. The resultant film had tensile strength (TS), per-

centage elongation (%E), and water vapor permeability (WVP)

comparable to the film obtained from commercial mammalian

gelatin. Additionally, Nile perch skin gelatin films were also

found to exhibit film strength (stress at break) and percentage of

strain (elongation at break) similar to that of bovine bone gelatin

(Muyonga et al. 2004).

TS and %E of catfish gelatin films significantly increased

from 27.1 MPa to 61.7 MPa and from 105.1% to 115.1%, re-

spectively, when the gelatin contents were increased from 0.5%

to 1%. WVP also increased with the increase in gelatin con-

tents from 0.5% to 2.5% (Zhang et al. 2007). TS and %E of

skin-gelatin films of bigeye snapper increased with increasing

protein concentration from 2% to 3%, but the increase in protein

concentration did not affect WVP (Jongjareonrak et al. 2006a).

Additionally, the decrease in TS and increase in %E and WVP

were observed in the gelatin-based film from catfish, bigeye

snapper, and brownstripe red snapper, when the concentration

of glycerol was increased (Jongjareonrak et al. 2006a, Zhang

et al. 2007). Generally, less stiff and rigid, more extensible films

may be obtained by increasing the plasticizer concentration in

the film, due to the reduction in interactions between the biopoly-

mer chains (Arvanitoyannis 2002). Hoque et al. (2010) studied

the effect of heat treatment, at different temperatures (40–90◦C),

of film-forming solution (FFS) containing 3% gelatin from cut-

tlefish ventral skin and 25% glycerol (based on protein) on film

properties. The gelatin film prepared from FFS heated at 60◦C

and 70◦C showed the highest TS (p<0.05), while that from FFS

heated at 90◦C had the highest elongation at break (EAB). Addi-

tionally, increasing heating temperatures resulted in the decrease

in WVP of films, while L∗-value and transparency value were not

different. However, an increase in b∗-value was observed with

increasing heating temperature, mainly mediated by Maillard re-

action at higher temperature (Manzoccu et al. 2000, Chinabhark

et al. 2007).

IMPROVEMENT OF FUNCTIONAL

PROPERTIES OF FISH GELATIN

Because of the inferior gel-forming ability of fish gelatin to its

mammalian counterpart, fish gelatin can be subjected to mod-

ifications for gel improvement or be used for other functional

properties. To increase or maximize its functional properties,

the modification of gelatin by several methods or the addition of

selected compounds has been implemented.

Use of protein cross-linkers

Aldehydes

The reaction of glutaraldehyde with an amine group involves

the formation of an imine followed (or preceded) by aldol

reactions (Monsan et al. 1975). Subsequent dehydration leads

to stable unsaturated cross-links. The sugar-based counterpart

of this process is known as the Maillard or browning reaction

(Ellis 1959, Ledl and Schleicher 1990), the reaction between

aldose sugars and amino groups. The first step in the Maillard

reaction is the reversible formation of a glycosylamine that can

undergo a so-called Amadori rearrangement (Hodge 1955) to

1-amino-1-deoxy-2-ketose, which will not hydrolyze under am-

bient conditions. A sugar aldehyde, therefore, can act as a pro-

tein cross-linker through glycation and Amadori rearrangement

reactions (Schoevaart and Kieboom 2002). The aldehydes com-

monly used include glutaraldehyde, formaldehyde, and glyoxal

(Marquie et al. 1995). However, there are indications of possi-

ble toxicity, which make it questionable to use as the modifying

agents in foods (Lam et al. 1986, Galietta et al. 1998).

Phenolic Compounds

Phenolic compounds as food components represent more than

6000 identified substances and have been known as the largest

group of secondary metabolites in plant foods (Rawel et al.

2007). They generally possess an aromatic ring bearing one

or more hydroxy substituents (Robards et al. 1999). They are

usually found in plants and are bound to sugars as glycosides

(Hollman and Arts 2000). Phenolic compounds can interact with

proteins in two different ways: via noncovalent (reversible) in-

teractions and via covalent interactions, which in most cases are

irreversible (Prigent 2005). Hydrogen bonds may involve the

interactions between hydroxyl groups of phenolic compounds

and the nitrogen or oxygen of lysine, arginine, histidine, as-

paragine, glutamine, serine, threonine, aspartic acid, glutamic

acid, tyrosine, cysteine, and tryptophan. Hydrophobic interac-

tions may occur between phenolic compounds and amino acids

such as alanine, valine, isoleucine, leucine, methionine, pheny-

lalanine, tyrosine, tryptophan, cysteine, and glycine residues

(Prigent 2005).

Covalent interactions between phenolic compounds and pro-

teins can occur via oxidation of phenolic compounds to rad-

icals or quinones (Balange and Benjakul 2009). Monophenol

and polyphenol are readily oxidized toortho-quinone, either

enzymatically as in plant tissues, or by molecular oxygen. The

quinone forms a dimer through a secondary reaction, or reacts

with amino or sulfhydryl side chains of polypeptides to form

covalent C–N or C–S bonds with the phenolic ring (Fig. 21.5)

(Strauss and Gibson 2004). However, gelatin contains a negligi-

ble content of cysteine. Thus, the cross-links formed are mainly

via amino groups, in which covalent bonds can be formed. Ly-

sine in gelatin molecule could provideε-NH 3 as the binding

site in this reaction. Fish gelatin had lysine ranging from 12 to

32 residues/1000 residues (Table 21.3). Polyphenols can be reox-

idized and bind a second polypeptide. As a consequence, protein

cross-links can be formed (Fig. 21.5).

Strauss and Gibson (2004) prepared oxidized phenolic

compounds–gelatin mixtures by adjusting pH to 8 followed by

mixing with gelatin in the presence of oxygen at 40◦C. Gels

so obtained had greater mechanical strength, reduced swelling,