Antioxidative activity of chitosans of different viscosity in cooked comminuted flesh of herring (Clupea harengus)

Abstract

Antioxidant efficacy of chitosans of different viscosity (14 cP, 57 cP and 360 cP) in cooked, comminuted flesh of herring (Clupea harengus), was investigated. The oxidative stability of treated fish flesh was determined and compared with those treated with conventional antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ) at a level of 200 ppm. The progress of oxidation was monitored by employing the peroxide value, 2-thiobarbituric acid-reactive substances (TBARS) and static headspace gas chromatographic analysis. In general, all chitosans exhibited varying antioxidant activities in a fish flesh model system. The formation of hydroperoxides and TBARS, in herring samples containing 200 ppm 14 cP chitosan, was reduced after 8 days of storage by 61 and 52%, respectively. Among the different viscosity chitosans, 14 cP chitosan was more effective than the higher viscosity chitosans in preventing lipid oxidation in the herring flesh model system.

Introduction

The highly unsaturated fatty acids commonly found in seafoods are particularly sensitive to oxidative change during storage (Hsieh & Kinsella, 1989b, Shahidi, 1998). Tichivangana and Morrissey (1985) have shown that the oxidation of muscle foods occurs in the order of fish>poultry>pork>lamb. Although the process of lipid oxidation is thermodynamically favourable, the direct reaction between oxygen and highly unsaturated lipids is kinetically hindered (German & Kinsella, 1985, Hsieh & Kinsella, 1989b). Hence, an activating factor is necessary to initiate free radical chain reactions followed by their self-propagation (German & Kinsella, 1985, Shahidi, 1998). It has been proposed that lipid oxidation in fish may be initiated and promoted by a number of mechanisms involving autoxidation, photosensitized oxidation, lipoxygenase, peroxidase, and microsomal enzymes (Slabyj & Hultin, 1982, Frankel, 1985, Josephson et al., 1987, Hsieh & Kinsella, 1989a).

Decker and Hultin (1992) identified several sources of protein-bound iron that exist in biological tissues, namely myoglobin, haemoglobin, ferritin, transferrin, and haemosiderin. St. Angelo (1996) reported that iron bound to these proteins may be released during post-harvest storage and cooking, activating oxygen and initiating lipid oxidation. There is a range of concentrations of haematin compounds in muscles from different species of fish and these are present in relatively large concentrations in the muscle of most fatty fish, especially their lateral band dark muscle (Castell & Bishop, 1969). Autoxidation of oxymyoglobin and oxyhaemoglobin (both in the Fe²⁺ oxidation state) may also result in the formation of superoxide anion, metmyoglobin and methaemoglobin (both in the Fe³⁺ oxidation state), respectively. The formation of superoxide anion from oxymyoglobin/oxyhaemoglobin may be catalyzed by anions such as SCN⁻, OCN⁻, F⁻ and Cl⁻ (Satoh & Shikama, 1981). Flick, Hong, and Knobl (1992) reported that increased oxidation of seafoods at lower humidities may be attributed to the concentration of prooxidants such as metal ions and haemoglobin. The main source of free iron or non-haem iron in cells is ferritin, which is a soluble iron storage protein found in liver, spleen and skeletal muscle and has a molecular mass of 450 kDa and contains 4500 iron atoms when fully loaded (Decker & Welch, 1990). Decker and Hultin (1990) observed that storage of unfrozen mackerel ordinary muscle at 4C for 7 days resulted in a 1.4-fold increase in low molecular weight iron-containing compounds from 0.16 to 0.23 μg Fe/g muscle. A small amount of iron is also bound to molecules such as ATP, ADP, organic acids and DNA. These compounds are capable of decomposing hydroperoxides (ROOH) in order to form free radicals (Kanner & Doll, 1991). Shahidi and Hong (1991) reported that metal ions such as those of copper and iron, can enhance lipid autoxidation to a greater extent in their lower valency states.

Tichivangana & Morrissey, 1982, Tichivangana & Morrissey, 1985 reported that ferrous ion at 1–10 ppm levels acts as a strong pro-oxidant in cooked fish muscles. Castell, Maclean, and Moore (1965) observed that the relative prooxidant activity of ions in fish muscle decreased in the order of Cu²⁺>Fe²⁺>Co²⁺>Cd²⁺>Li⁺>Ni²⁺>Mg²⁺>Zn²⁺>Ca²⁺>Ba²⁺. Superoxide anion may be dismutated to form hydrogen peroxide, resulting in the formation of hydroxyl radicals via the reaction of H₂O₂ with Fe²⁺ (Frankel, 1980, Yen et al., 1999).

Chitosan, which is the deacetylated form of chitin, has been identified as a versatile biopolymer for a broad range of food applications (Shahidi, Kamil, & Jeon, 1999). Both chitin and chitosan have unusual multifunctional properties, including high tensile strength, bioactivity, and biodegradability which makes them an attractive speciality materials (Berkeley, 1979, Ikejima & Inoue, 2000). Furthermore, these polymers have been identified as being biocompatible, non-antigenic, non-toxic, and biofunctional (Hirano et al., 1990, Li et al., 1992). Recently, Rao and Sharma (1997) reported that acute systemic toxicity tests in mice did not show any toxic effect of chitosan; all mice injected with the test material lived during the entire period (72 h) of observation. These authors further observed that eye irritation tests in rabbits and skin irritation tests in guinea pigs did not produce any undesirable toxic effect due to chitosan.

Both chitin and chitosan are able to form complexes with many of the transition metals, as well as some of those from groups 3–7 of the periodic table (Muzzarelli, 1973). The heavy metal-polymer complexes are believed to form as a result of dative bonding with chitosan. This involves the donation of nonbonding pairs of electrons from the nitrogen, and/or the oxygen of the hydroxyl groups, to a heavy metal ion (Winterowd & Sandford, 1995). N,O-Carboxymethyl chitosan has been found to bond chemically with ions of numerous heavy metals, such as iron, copper, mercury and zinc, thus binding or sequestering them when present in even 10–1000 ppm (Hayes, 1986). The cupric ion seems to form one of the strongest complexes with chitosan in the solid state (Chuti, Mok, Nag, Luong, & Ma, 1996).

Synthetic antioxidants and chelating agents may be added to food products in order to prevent lipid oxidation. However, the growing consumer demand for food devoid of synthetic antioxidants has focused efforts on the discovery of new natural preservatives (Madsen & Bertelsen, 1995). Several sources of natural antioxidants are known (Shahidi, 1997), and some of them, such as those of rosemary and sage, are currently used in a variety of food products. However, fundamental studies on chitosan as a natural antioxidative agent in fish and seafood are lacking. Therefore, the objective of this study was to examine the effect of chitosans of different viscosity on lipid autoxidation in a fish model system.