Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
Splendid squid (Loligo formosana) is the species available in Thailand and used for local consumption and export. The squid ink is discarded as by-product which can be used as a source of active compounds. Therefore, the aim of this study was to investigate the antioxidant activity of squid ink after melanin removal. The antioxidative activities of melanin-free ink (MFI) from splendid squid were investigated using different in vitro assays. Antioxidative activity of MFI was also determined in different model systems. Thermal stability at 90°C was tested. MFI was subjected to ultrafiltration, and fractions with various molecular weight (MW) ranges were analyzed for antioxidative activities. MFI had DPPH, ABTS radical scavenging activities, and ferric reducing antioxidant power (FRAP) of 179.6 ± 2.1, 957.8 ± 89.3, and 171.2 ± 7.3 μmol TE/g protein, respectively. It also possessed metal chelating activity of 4.0 ± 1.2 μmol EE/g protein. MFI at 500 mg/L could prevent the oxidation of β-carotene-linoleic acid system, but its efficacy was lower than butylated hydroxyl anisole (BHA) at 200 mg/L (p < 0.05). The MFI (100 to 500 mg/L) showed the preventive effect on lipid peroxidation of lecithin liposome system in a dose-dependent manner. When mackerel mince was added with MFI at levels of 100 and 200 mg/kg, lipid oxidation was retarded during ice storage for 15 days as evidenced by the lower peroxide value and thiobarbituric acid-reactive substances, as compared with the control. However, its effectiveness was lower than 200 mg/kg BHA. When MFI was subjected to heat treatment at 90°C for up to 30 min, its DPPH and ABTS radical scavenging activities remained constant, but FRAP decreased within the first 5 min without subsequent changes. When the MFI was separated using ultrafiltration into different fractions (< 3 KDa, 3 to 10 KDa, and >10 KDa), the fractions with MW less than 3 KDa showed the highest antioxidative activities (p < 0.05). MFI, especially its fraction with MW < 3 KDa, from splendid squid could therefore serve as natural antioxidant to retard lipid oxidation in food products.