Abstract
Fish skin, a by-product of filleting could be upgraded to value-added products for human consumption, thereby preventing waste and pollution. The aims were to prepare gelatin from skin of Nile or black tilapia (Oreochromis nilotica); characterize its physical-chemical, rheological and thermal properties and those of fish gelatin-whey protein mixtures; prepare hydrolysates and assess their foaming properties in the presence and absence of whey protein and green tea. Gelatin extracted successfully from tilapia skins, gave a 19-26% yield and comprised high protein (90.2 + 0.68 %) and low fat (0.62 + 0.03 %), moisture (5.2 + 0.62 %) and ash content (0.08 + 0.05 %). Amino acid profile included mainly glycine (33.94%), alanine (26.1%) and glutamic acid, proline, arginine, aspartic acid and imino acids. The Bloom strength of 6.67% (w/v) tilapia gelatin in distilled water was higher (301 + 11.6 g) than commercial tilapia (207 + 5.6 g), bovine (~ 225 g), and porcine (~ 300 g) gelatins. The rheological properties of 3,5 and 10% (w/v) tilapia gelatin indicated G’ (elastic modulus) values 1,135 and 720 Pa, respectively, and gelling points of 16.2°C, 5% 17.2°C and 19.8°C respectively whereas melting temperatures were 14.9°C, 14.9°C and 15.0°C respectively. The gels were highly stable over a frequency range of 0-100 rad/sec. DSC results indicated high melting temperatures (Tm) 22oC, compared to bovine (26.2°C) and porcine (30.7°C) gelatin. Cold water fish gelatin did not gel. However, bovine and porcine gelatins had lower enthalpy change (∆H) values 0.58 J/g and 0.45 J/g respectively, compared with tilapia gelatin (2.51 J/g), probably due to protein denaturation of collagen. In tilapia gelatin-whey protein mixed gels, whey proteins were dominant. Gelling temperatures of 10% WPI with 5% gelatin (88.9°C) or 10% gelatin (87.4°C) mixtures were higher than for WPI (80.3°C) or gelatin at 3% (16.2°C), 5% (17.2°C) and 10% (19.8°C). Whey protein (10%) was unstable but formed a good stable gel network with gelatin at 5 % (597 Pa) and 10% (26974 Pa). Whey (10 %) and gelatin (10%) also gave a strong gel by large deformation analysis. Tm and ∆H for 10% whey protein and 5 or 10% gelatin/ mixtures were higher (67.2°C, 68.8°C and 68.2°C respectively) than for gelatin or whey protein alone. The ∆H values of gelatins increased when combined with the WPI. Three % gelatin (0.03 J/g), 10% WPI + 3% gelatin (0.44 J/g), 5 % gelatin (0.18 J/g), 10% WPI + 5% gelatin (0.56), 10% gelatin (1.3 J/g) and 10% WPI + 10% gelatin (0.59). Phase contrast microscopy of gelatin showed a fine uniform and homogeneous network with small particles whereas the whey protein structure had larger aggregates. The gelatin-whey mixtures formed a compatible network. Gelatin produced the highest volume of foam FE (933%), followed by gelatin + whey protein (853%), gelatin hydrolysate + whey protein (767%); these were not significantly different (p>0.05). This was followed by a mixture of gelatin + whey protein + green tea (575%) that was significantly lower (p< 0.05). Foaming stability of mixtures of gelatin + whey protein + green tea was significantly higher at 47%, than gelatin + whey protein (34%) and gelatin + green tea (25%) (p<0.05). Tilapia gelatin was significantly higher than that of whey protein alone (p<0.05) but the combination of whey protein with gelatin resulted in a significant increase (P<0.05) compared to WP or TG alone. Whey protein at FS of 8% was not as stable as the gelatin (18%) or green tea (21%). Gelatin hydrolysate prepared with alcalase had poor FE and FS which improved with added green tea and whey protein due to protein-polyphenol interactions. Tilapia skin gelatin displayed very good nutritional and rheological properties and can be used as an alternative to mammalian gelatins if food products.