Enhancing the quality of seafood products through new preservation techniques and seaweed-based antioxidants. Algal polyphenols as novel natural antioxidants (2009)

Novel preservation techniques and addition of antioxidative and functional ingredients need to be implemented to maintain the prime quality and ensure the storage stability, to meet the demand of the global market for health-promoting functional seafood. The objective of this thesis was to investigate the effect of packaging and chilling preservation techniques on quality changes of fresh lean fish and study the potential application of algal polyphenols as novel natural antioxidants to prevent lipid oxidation of fish muscle and fish protein based products. \nThe key reasons for quality deterioration of perishable seafood products are bacterial growth and changes caused by oxidative and enzymatic reactions. Modified atmosphere packaging (MAP) in combination with superchilled storage was effective in retarding microbial growth and extending the prime quality shelf life of fresh cod loins. However, lipid oxidation may not be effectively inhibited and contributes to quality changes. The extraction and characterisation of antioxidant activity of algal polyphenols and their inhibitory effect on lipid oxidation in fish model systems was therefore the main focus of the studies. Among the ten species screened by in vitro antioxidant tests, Fucus vesiculosus were found to possess the highest total phlorotannin content (TPC) and the greatest radical scavenging activities. Subfractions rich in oligomeric and polymeric phlorotannins exhibited potent DPPH radical scavenging activity, comparable to or even higher than several commercial antioxidants. Successive ultra-filtration showed that phlorotannins in F. vesiculosus are mainly composed of high molecular weight polymers. But no clear correlations were observed between degree of polymerisation, molecular size and chemical antioxidant activities. The chemical antioxidant activities of F. vesiculosus extract/fractions did not adequately reflect their ability to prevent lipid oxidation in fish model systems. The oligomeric subfractions at 300 mg/kg were highly effective in inhibiting lipid oxidation while polymeric subfractions only had slight retarding effect. Additionally, all the subfractions separated by Sephadex LH-20 column and ultra-filtration showed potent in vitro ROS (Reactive Oxygen Species) scavenging ability in a mononuclear cell-based bioassay as well as high ACE (angiotensin converting enzyme) inhibitory activities, demonstrating the multiple bioactive properties of phlorotannins. The efficiency of enzyme-assisted extraction of algal polyphenols is species and enzyme-dependent. Protease treatment enhanced the extraction of polyphenols and other hydrophilic antioxidant components from Palmaria palmata but negative effects were observed for F. vesiculosus and Laminaria hyperborea. The studies provide first evidence on the inhibitory effects of phlorotannin-rich extract and fractions on lipid oxidation in fish model systems. Valuable information gained on antioxidant properties and possible health promoting effects of phlorotannins derived from marine algae, are not only of academic interest but also of great practical value for the food industry.