Effects of agar films incorporated with fish protein hydrolysate or clove essential oil on flounder (Paralichthys orbignyanus) fillets shelf-life

This study evaluated the effect of protein hydrolysate (PH) and clove essential oil (CEO) on agar film properties and the shelf-life improvement of flounder fillets. Firstly, Argentine croaker muscle protein was alkali-solubilised and recovered to obtain a stable substrate. This substrate was hydrolysed to 20% degree of hydrolysis, by Alcalase (A20) and Protamex (P20). The molecular weight (MW) distribution, amino acid and in vitro antimicrobial activity of these hydrolysates were tested. Agar films were prepared with and without (control) incorporation of PH (A20 hydrolysate) or CEO and characterised by mechanical properties, water vapour permeability, solubility and optical properties. The microbiological, physical and chemical properties of flounder fillets coated with these films were monitored during storage at 5 °C for 15 days. Compared to P20, A20 hydrolysate possessed lower MW, higher hydrophobic amino acid content and the most effective antimicrobial activity against selected pathogens, such as Staphylococcus aureus, Yersinia enterocolitica, Aeromonas hydrophila, Debaryomyces hansenii and Listeria innocua. The presence of PH increased the water solubility, water vapour permeability, elongation at break and yellowness of the films. The CEO films possessed less transparency than PH films and control films, due to the presence of oil in the formulations. Fillets covered with PH and CEO films, especially the latter, presented lower total volatile bases and pH values, and delayed considerably the growth of HS-producing microorganisms compared to the control films. The CEO might be used as a natural biopreservative to extend the flounder shelf-life. However, further studies are necessary, to assess the antimicrobial effect of PH when used as food packaging.