Extraction and Characterisation of New Zealand Hoki (Macruronus novaezelandiae) Skin Gelatine

Abstract

The extraction of gelatine from the skins of New Zealand cold water fish species, hoki (Macruronus novaezelandiae), was investigated. Response Surface Methodology (RSM) was adopted to optimise the extraction conditions and its physico-chemical properties were compared to commercial mammalian gelatines. The optimum conditions for extraction of gelatine were pre-treatment of hoki skins in 0.75 M of NaCl for 9 minutes, followed by extraction of the gelatine in Milli-Q water at 49.3 °C for 60 minutes. Hoki gelatine exhibited relatively poor properties (low gel strength and low melting point) compared to bovine and porcine gelatines. However, it showed superior properties to other cold water fish species conducted in previous studies. Therefore, modifications through chemical (glutaraldehyde, genipin, caffeic acid) and enzymatic (transglutaminase, TGase) cross-linkings were conducted to improve the functional properties of hoki gelatine. Gelatine with added TGase exhibited the highest gel strength followed by those with added glutaraldehyde, caffeic acid, and genipin. Gelatine gels prepared at an optimum concentration of 3.33 mg of TGase per g of gelatine, incubated at 37 °C for 30 minutes, gave an optimum gel strength of 278.2 ± 0.19 g, comparable to the bovine gelatine (273 ± 16.1 g) studied. The gel strength of gelatine with added TGase at optimum conditions, increased by approximately 41 % (197 g to 279 g). The uncross-linked and cross-linked gelatine gels demonstrated distinctive differences in the molecular weight distribution, microstructure, and degree of cross-linkings. This study also revealed the rheological behaviour of enzyme cross-linked gelatines from three other New Zealand fish species, namely dory, ling, and salmon. Rheological measurements have demonstrated hoki gelatine to have the greatest gel strength compared to salmon, dory, and ling (hoki>salmon>dory>ling). The high gel strength observed in hoki gelatine was due to its higher content of imino acids and higher number of large molecular weight subunits compared to those of the other fish species studied. The current findings have successfully proven that modified fish gelatines, especially those of hoki, are able to replace the gelatines from land animals. These improved functional properties could also create a significant demand for fish gelatine in food applications and in meeting the global requirements for Halal and Kosher markets.