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

21 Fish Gelatin 399

Figure 21.5.Reaction between phenolic compounds and side chain of gelatin.

and fewer free amino groups. Dynamic light scattering analyses

showed that such cross-linking results in denser polymeric net-

works. Aewsiri et al. (2009b) reported that cuttlefish skin gelatin

modified with oxidized phenolic compounds, including ferulic

acid and tannic acid, at the level above 5% (w/w of protein) ex-

hibited a decrease in the emulsion activity index (EAI). This was

possibly associated with the decrease in surface hydrophobicity

of the modified gelatin.

Genipin

Genipin is a hydrolytic product of geniposide and its related iri-

doid glucosides, which are extracted from the fruits ofGardenia

jasminoides Ellis(Yao et al. 2004) via enzymatic hydrolysis with

β-glucosidase (Butler et al. 2003). Genipin has been known as a

naturally occurring cross-linking agent (Nickerson et al. 2006b).

Genipin participates in both short- and long-range covalent

cross-linking ofε-amino groups in amine-containing polymers

(Nickerson et al. 2006a). It can react with amino acid or proteins

in the presence of oxygen to form dark blue pigments used in

the fabrication of food dyes (Touyama et al. 1994). Additionally,

it can form stable cross-linked products with resistance against

enzymatic degradation, comparable to that of glutaraldehyde-

fixed tissue (Huang et al. 1998, Sung et al. 1998). Further-

more, gelatin-derived bioadhesives display higher biocompat-

ibility and less cytotoxicity when cross-linked with genipin than

with other agents, such as formaldehyde, glutaraldehyde, and

epoxy compounds (Sung et al. 1999). The mechanism of the

reaction of amino acids or proteins with genipin is proposed by

Touyama’s group (Touyama et al. 1994a, 1994b). The formation

of the genipin–methylamine monomer is through a nucleophilic

attack by methylamine on the olefinic carbon at C-3 in genipin,

followed by the opening of the dihydropyran ring and an attack

by the secondary amino group on the resulting aldehyde group.

The blue-pigment polymers are presumably formed through the

oxygen radical-induced polymerization and dehydrogenation of

several intermediary pigments.

Yao et al. (2004) reported that genipin is a good cross-linker

for the gelatin. The rate of degradation and the degree of cross-

linking of the genipin-fixed gelatin depend upon genipin con-

centration. Moreover, increasing genipin concentration makes

the gelatin network gradually shift from being dominated by hy-

drogen bonds to covalent cross-linking (Nickerson et al. 2006b).

However, the concentration of genipin should exceed 0.5% (w/v)

of the overall weight of the gelatin-based material (25% (w/v))