Cuttlefish skin gelatin gels with EKWE or ECBE oxygenated
at pH 9 had the fewest free amino groups, suggesting the
greater cross-linking of gelatin via the amino groups. Phenolic compounds at higher pH could be most completely oxidized to quinone and then the quinone further reacted with amines on the proteins (Cao et al., 2007). A reaction of proteins with oxidized phenolic compounds led to a decrease in free amino groups (Rawel et al., 2000). Under alkaline conditions, the hydroxyl groups are readily oxidized to hydroxyl radicals (Rawel et al., 2000). Those radicals would react with the neutral molecules and the benzene ring to cause oxidation with subsequent polymerization. Increasing pH induces the deprotonation of the phenolic hydroxyl group, eventually leading to the formation of quinones (Prigent,2005). The decrease in free amino groups of gelatin gels paralleled the increased gel strength.