Antioxidants delay or inhibit lipid oxidation at low concentration. Tocopherols, ascorbic acid, carotenoids, flavonoids, amino acids, phospholipids, and sterols are natural antioxidants in foods. Antioxidants inhibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and photosensitizers, and inactivating lipoxygenase. Antioxidants show interactions, such as synergism (tocopherols and ascorbic acids), antagonism (α-tocopherol and caffeic acid), and simple addition. Synergism occurs when one antioxidant is regenerated by others, when one antioxidant protects another antioxidant by its sacrificial oxidation, and when 2 or more antioxidants show different antioxidant mechanisms.

Mechanisms of Antioxidants in the Oxidation of Foods

Fresh fish and shellfish are highly perishable products due to their biological composition. Under normal refrigerated storage conditions, the shelf life of these products is limited by enzymatic and microbiological spoilage. However, with increasing consumer demands for fresh products with extended shelf life and increasing energy costs associated with freezing and frozen storage, the fish-processing industry is actively seeking alternative methods of shelf life preservation and marketability of fresh, refrigerated fish and at the same time economizing on energy costs. Additional methods that could fulfill these objectives include chemical decontamination, low-dose irradiation, ultra-high pressure, and modified atmosphere packaging (MAP). This review focuses on the biochemical and microbiological composition of fresh fish/shellfish, the spoilage patterns in these products, factors influencing spoilage, and the combination treatments that can be used in conjunction with refrigeration to extend the shelf life and keeping quality of fresh fish/shellfish. The safety concerns of minimally processed/MAP fish, specifically with respect to the growth of Clostridium botulinum type E, is also addressed.

Spoilage and shelf-life extension of fresh fish and shellfish.

Publisher Summary This chapter presents the ubiquitous nature and pervasiveness of lipid oxidation ex vivo in foods and in vivo , and demonstrates that oxidation generally has deleterious results in both systems. This dramatizes the need for more effective strategies for controlling lipid oxidation, both in food materials and in tissues in vivo . The chapter discusses the existing practices using antioxidants, chelators, enzyme inactivation, and anoxic and low-temperature storage conditions. The oxidative degradation of the unsaturated fatty acid components of food lipids may be beneficial in some foods in generating low levels of desirable flavorful carbonyl compounds. However, in general, oxidation causes deleterious changes in flavor, taste, color, texture, and possibly safety of foods. The polyunsaturated fatty acids (PUFA)—particularly the trienoic, pentaenoic, and hexaenoic PUFA commonly found in oilseeds and seafoods—render these foods, which are particularly sensitive to oxidative changes which limit their self-life.

Oxidation of polyunsaturated fatty acids: mechanisms, products, and inhibition with emphasis on fish.

The inhibition of lipid peroxidation and radical scavenging effects were studied to evaluate the antioxidant activity for extracts of 17 species of seaweed The antioxidant effect was evaluated by determination of lipoxygenase activity and by α, α-diphenyl-β-picrylhydrazyl (DPPH) decolorization Lipoxygenase activity was depressed in the presence of aqueous and ethanol extracts of 4 algal species; Sargassum species had the highest antioxidant activity of all the species examined The ethanol extracts of one Sargassum species showed competitive inhibition with the substrate The same species also showed radical scavenging activity in the DPPH decolorization test Comparison of these results shows no relationship between enzyme inhibition and radical scavenging activity

A comparison of screening methods for antioxidant activity in seaweeds

Lipid oxidation is an important cause of quality deterioration in seafood, particularly those products which contain high concentrations of lipid. Although this is a primary deteriorative reaction in many types of food, it is particularly acute in fish because of the high content of highly polyunsaturated fatty acids found in organisms from the marine environment (Ackman, 1980). The free radical mechanism of lipid oxidation involving initiation, propagation, and termination steps is familiar (Khayat and Schwall, 1983;Frankel, 1985). Recent kinetic models for lipid oxidation in a variety of food products and biological tissue have shown excellent agreement with chemical measurements of oxidation based on this free radical mechanism (Tappel et al.,1989;Ozligen and Ozligen, 1990). In this paper inhibitors and activators of lipid oxidation will be discussed, with their likely contribution to seafood oxidation. Some general methods for limiting the rate and/or extent of lipid oxidation will also be considered.

Oxidation of lipids in seafoods

Lipid peroxidation is an important process responsible for the flavor deterioration of many plant and animal foods. In order to understand possible mechanisms of initiation, a soybean lipoxygenase-polyunsaturated fatty acid model system was studied. The rate of enzyme-initiated oxidation was monitored by both oxygen consumption and diene conjugation. The hydroperoxides from an n-6 fatty acid, i.e. arachidonic, and n-3 fatty acids, i.e. linolenic, eicosapentaenoic, and docosahexaenoic acids were reduced and analyzed by reverse-phase high pressure liquid chromatography and gas chromatography/mass spectrometry. The lipoxygenase specifically catalyzed oxygenation at the n-6 carbon of both n-3 and n-6 fatty acids. Lipoxygenase activity was demonstrated in skin homogenate from rainbow trout and its role in off-flavor development is discussed.

R. J. Hsieh

Papers

Lipoxygenase‐Catalyzed Oxidation of N‐6 and N‐3 Polyunsaturated Fatty Acids: Relevance to and Activity in Fish Tissue