الملخص الإنجليزي
In this research gelatin was extracted from yellowfin tuna skin by alkali washing followed by heating in acid medium. The extracted gelatin was air dried and then ground into powder. The yield of extracted soluble solids was 57.8% based on the total solids in the skin. Thermal characteristics of gelatin extracted were determined from the thermograms measured by Differential Scanning Calorimetry (DSC) and modulated DSC (MDSC). Initially thermal characteristics (glass transition, melting and deterioration) were determined for tuna and different commercial gelatins. It was difficult to compare the thermal transitions of the gelatin of different sources with varied moisture content or water activity, thus gelatin from different sources were equilibrated at relative humidity of 11.3, 52.9 and 75.3% and then thermal transitions were measured. Tuna gelatin was more hygroscopic compared to mammalian source (bovine and porcine) since tuna gelatin adsorbed more moisture compared to others when equilibrated ať same level of relative humidity. DSC results indicated that tuna gelatin equilibrated at a constant relative humidity of 11.3, 52.9 and 75.3% showed lower glass transition compared to mammalian source gelatin (p<0.05). The glass transition of tuna gelatin equilibrated at 11.3% relative humidity was 30°C, which decreased to - 19°C when equilibrated at 52.9% relative humidity (p<0.05), however further increase of relative humidity to 75.3% showed insignificant change (p>0.05). Bovine and porcine samples equilibrated at 11.3 and 75.3% relative humidity showed significant differences in glass transition (p<0.05). However samples equilibrated at 52.9% relative humidity showed insignificant difference from the samples equilibrated at 11.3 and 75.3% relative humidity (p<0.05). In general, gelatin from different sources equilibrated with different water activities showed insignificant effect on the melting points (p>0.05). Glass transition was also determined from reversible heat flow thermogram of MDSC, which is a more sensitive and accurate method. The glass transition of tuna gelatin equilibrated at 11.3% relative humidity was 23°C, which decreased to -3°C when equilibrated at 52.9% relative humidity (p<0.05), however further increase of relative humidity to 75.3% showed significant increase to 12°C (p<0.05). In case of bovine gelatin, glass transition of samples equilibrated at 11.3% RH was 75°C, and then decreased for the samples equilibrated at 52.9% RH (p<0.05). Further increase in water activity had no effect on glass transition. However in the case of porcine gelatin water activity showed no effect on the glass transition. The results in this study showed that the degree of plasticization varied with the source of gelatin as well as their extraction methods.
