New antioxidative strategies needed for preventing rancidity in fish muscle.

It's been thought in the past that rapid oxidation in the muscle of fatty fish is attributable to the product's high total lipid content, such as its triacylglycerol (TG) levels. However, recent research using low-lipid washed cod muscle systems shows that strong rancidity can develop in the presence of hemoglobin (Hb).

Researchers in Sweden found that the rate and extent of rancidity do not increase with the presence of six times more membrane lipids and 15% more TG. Varying the Hb content and pH levels, however, strongly controls the rate and extent of oxidation. Adding Hb to TG, then mixing it into an inert matrix--cellulose--also caused the rapid onset of rancidity, compared to samples lacking additional Hb.

Data indicate that the amount of lipid and the lipid fraction itself seems to be of less importance for the oxidation reaction than the pro-oxidants present and the environment in which the lipids are distributed. This is interesting, according to the scientists, considering that most mechanisms concerning lipid oxidation are the outcome of research in pure TG systems, such as bulk oils.

Classic tools that inhibit lipid oxidation in fish include freezing the uncooked product at -30 C, applying synthetic antioxidants, and reducing the product's exposure to light and oxygen through the use of light-impermeable packaging and vacuum systems. Such tools do not fit with demands of the modern consumer, who is looking for pre-cooked, low-fat, natural, and non-frozen products in visually appealing, transparent packaging materials.

Thus, more modern antioxidative strategies are needed. Promising advances in this field involve: making adjustments to the pH of fish muscle to prevent Hb from becoming deoxygenated or oxidized; heating the fish under mild conditions from 55 C to 60 C to inhibit the development of pro-oxidative enzymes while minimizing the loss of heme or iron from Hb; adding polar polyphenols extracted from fruits and plants; and adding aqueous extracts from fish or chicken muscle--press juice. Testing herring press juice in a single cell system has shown in vivo radical scavenging capacity.

Further information. I. Undeland, Department of Food Science, Chalmers University of Technology, Chemistry and Bioscience-Food Science, SE-412 96 Goteborg, Sweden; phone: [phone]; URL: www.chalmers.se.