Effect of acidic and alkaline pretreatment on functional, structural and thermal properties of gelatin from waste fish scales

Gelatin (G) is a thermoreversible polymer produced by hydrolysis of a structural protein called collagen. Fish scales (mixed freshwater species) were subjected to hydrochloric acid (HCl), sodium hydroxide (NaOH) and both HCl and NaOH pretreatment individually, and gelatin extraction with distilled water was labeled as GA, GB and GC, respectively. The percentage yield of gelatin GB (9.6 ± 0.5%) was higher than GA (7 ± 0.2%) and GC (9 ± 1.4%). Amino acids and imino acids (proline and hydroxyproline) composition confirmed the characteristic nature of collagen. The gel strength of gelatin was affected by the pH of the extraction medium rather than the presence of imino acid composition. Extracted gelatin GA, GB and GC showed ʎ_max at 230 nm indicating the peptide bond of gelatin and the presence of five amide bands I, II, III, A and B revealing the intrinsic nature of functional groups C=O, C–N, C–N, N–H and –NH₃ stretching vibrations of collagen, respectively. Two diffraction peaks of 2ɵ, one sharp peak around 10° and one broad peak around 20° represent the partially crystalline nature of fish gelatin. The presence of β, α₁ and α₂ chain at around 200 KDa, 110 KDa and 100 KDa, respectively, confirmed the triple helix nature of the gelatin. Glass transition temperature (T_g) of gelatin GA, GB and GC at 76.3 °C, 72.9 °C and 71.4 °C, respectively, and melting temperature (T_m) of GA, GB and GC at 406 °C, 295.2 °C and 291.7 °C, respectively, were evident through thermal study. The effect of acidic and alkaline pretreatment of fish scale had displayed fibrillar pattern, evenly spaced porous nature and network-like structure of gelatin which was thermally stable up to ~ 300 °C. Hence, waste fish scales have proved to be a cost-effective and environmentally friendly alternative raw material for gelatin production paving the way for safe alternatives for biomedical application.