Effect of Deamination and Agar Addition on Physical Properties of Gelatin Extracted from Thunnus tonggol Skin

Background: In the past decade, gelatin extraction from fish skin has been intensively investigated. In comparison to mammalian gelatin, fish gelatin has weaker gelatinous and rheological properties, which limits its widespread application. Deamidation and addition of agar hydrocolloid performed to improve the physical and functional properties of fish gelatin. Methods: Gelatin extraction from fish skin was done using acid pretreatment and applying heat. Agar was also extracted from Gracilaria persica algae by alkaline method and after adding it to gelatin, the values of melting point, gel strength, isoelectric point, and type of constituent peptides and amide bands of gelatin were determined. Results: Gel strength and melting point in unmodified treatment (control) were 92.65g, and 18.5 oC, respectively. These values were increased to (170.85 g: 24.7 oC), and (108.78 g: 21.8 oC), respectively, through deamination process with 1N NaOH solution for 12 h and Agar addition (2% w/v agar extracted from Gracilaria persica). The isoelectric point of the deaminated sample decreased from 8.94 to 5.90, but no noticeable change in the isoelectric point was seen in the modified sample with agar (8.25). Fourier Transform Infrared Spectroscopy spectra showed that the deamination and agar addition to gelatin caused changes in the amide bonds, covalent bonding sites, and crosslinking of gelatin powder; consequently, increasing gel strength and melting point. Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis results showed that the two methods of deamination and addition of agar led to an increase in the molecular weight of gelatin. Conclusion: The findings indicated that gelatin and agar interacted successfully. The gelatin/agar mixture exhibited the highest gel strength and melting point and in the deaminated sample, the increase in gel strength and melting point was attributed to the reduction in the isoelectric point of the modified gelatin, which caused the gelatin strands to come closer together and form stronger hydrogen bonds.