Development and characterisation of blend films based on fish protein isolate and fish skin gelatin

Blend films based on fish protein isolate (FPI) from yellow stripe trevally muscle and commercial fish skin gelatin (FSG) at different blend ratios (FPI/FSG=10:0, 8:2, 6:4, 5:5 and 0:10, w/w) prepared at pH 3 and 11 in the presence of glycerol at 30 and 50% (based on total protein) were characterised. At the same pH, tensile strength (TS) decreased, whilst elongation at break (EAB), water vapour permeability (WVP) and solubility increased as glycerol content increased (p<0.05). However, glycerol content had no marked impact on colour and transparency of resulting films (p>0.05). FPI films generally had the lower TS, EAB and higher WVP than FSG films (p<0.05). Both TS and EAB of blend films increased with increasing FSG content (p<0.05), especially at pH 11. Decreased WVP was obtained for blend films having the increasing proportion of FSG (p<0.05). FPI/FSG blend films with higher FSG proportion had higher film solubility, L*-values (lightness), a*-values (redness) but lower b*-values (yellowness) and δ. E*-values (total colour difference) than FPI films (p<0.05). At all FPI/FSG ratios, films prepared at pH 11 were less transparent than those prepared at pH 3 (p<0.05). FTIR spectroscopic analysis revealed that hydrogen bonds in FPI/FSG blend film were dominant. Thermogravimetric analysis indicated that films prepared at pH 11 showed higher thermal stability than those prepared at pH 3. Scanning electron microscopic study revealed no distinct phase separation in the matrix of FPI/FSG blend film. Thus, the incorporation of FSG into FPI film up to 50%, in conjunction with lowering glycerol content (30%) could improve the mechanical and water vapour barrier properties of resulting blend film.