The effects of caffeic acid and tannic acid on physicochemical, morphological and hydrogel properties of cold-water fish gelatin

Gelatin is mainly produced by collagen denaturation. Gelatin obtained from cold-water fish has low sol-gel transition temperatures. Chemical and physical treatments can be used to modify the gelatin network by establishing cross-links between the gelatin chains to improve the properties of the gel. In this study, cross-linking in cold-water fish gelatin-based hydrogels was established by tannic acid (TA) and caffeic acid (CA), each at concentrations of 1, 3 and 5%. The effect of CA and TA concentrations on the physicochemical properties of gelatin hydrogels was investigated. The strength of the gel and the degree of crosslinking increased with increasing the concentration of tannic acid from 1 to 3%, which increased the strength of the gel from 325.00 to 343.62 N/mm2 and the degree of crosslinking from 82.01 to 84.99%. At higher levels of tannic acid, a decrease in gel strength and degree of crosslinking was observed 301.90 N/mm2 and 75.48%, respectively. However, these properties of hydrogels increased steadily with increasing levels of caffeic acid (p <0.05). The swelling rate also decreased due to the combination of different levels of tannic acid and caffeic acid. The maximum swelling rate for the control was 1732.30% and the minimum swelling rate for 3% tannic acid was 594.79%. The crosslinked gelatins by tannic acid significantly improved the denaturation temperature and their thermal stability was higher than that of caffeic acid. This temperature was 89° C in untreated hydrogels and increased to 94 °C and 98 °C in caffeic acid and tannic acid treated hydrogels, respectively. Scanning electron microscopy images of the hydrogel samples showed that the structure of the hydrogels based on cold-water fish gelatin was spongy. The addition of crosslinking agents only slightly reduced the pore size of the gelatin and had no significant effect.