Tuna skin as bioresource for gelatine: Extraction with ionic liquid

Abstract

Unlocking the ocean’s full potential requires converting fish waste into materials and products of interest, thus improving the circularity and sustainability of the fish processing industry. Fish skin is an excellent bioresource for obtaining gelatine, a product of high interest in many industries due to its multifunctionality. The current extraction methods involve several stages with harsh chemicals, which pose environmental concerns. In this work, an innovative method consisting of maceration with ionic liquid and extraction with water is proposed. Unlike other methods based on ionic liquids or deep eutectic solvents that focus on dissolving the fish by-product, and therefore require an energy-intensive stage to recover the protein, a non-dissolving treatment is proposed to facilitate protein extraction with water. The gelatine is precipitated from the aqueous solution by adding acetone. The method was tested obtaining gelatine from tuna skin and using 1-ethyl-3-methylimidazolium acetate, a low toxicity ionic liquid able to disrupt the skin through hydrogen bonding. Optimised conditions led to a recovery yield of 0.188 g gelatine/g wet skin. FTIR showed the characteristic bands corresponding to molecular vibrations and functional groups of the gelatine structure. α- and β-chains constitute approximately the 43 % of the product, with the remainder being peptides with an average molecular weight of 44 kDa. The pyrrolidine amino acids content was 21 %. The gelatine showed good thermal stability. Melting and gelling temperature (20.6 and 12.9 °C, respectively) fall within the typical range for warm water fish. As an advantage in comparison to classical methods, the proposed method avoids the need of multiple treatment steps with strong acids and alkalis.