Aquaculture—farming fish for our dinner tables—is a big and growing international industry. Because many of the tastiest and most-profitable farmed fish are carnivores, their prepared diets usually include flakes or powders made from low-value fish, from fish processed for their oil, or from scraps of fish prepared for restaurants and supermarkets. Fish farmers happily bulk up their products with this fishmeal.
One problem: Fishmeal can contain significant quantities of dioxins, which are toxic industrial pollutants. That fact is widely blamed for farmed fish carrying more dioxinlike contaminants than wild members of their species do. Indeed, an international team of scientists has just reported that dioxinlike pollutants "may reduce the net health benefits derived from the consumption of farmed salmon." Their data on these pollutants in farmed fish will appear in an upcoming issue of Environmental Science & Technology.
Danish scientists think they may have a solution, however: Irradiate the fishmeal with sun lamps. Their tests show that shining ultraviolet (UV) light on fishmeal degrades its dioxin.
The process still requires substantial tinkering before it's ready to go commercial, notes Caroline P. Baron of the Danish Institute for Fisheries Research in Lyngby. However, the food chemist adds, it holds promise for minimizing the risk that farmed fish will serve as a substantial source of dioxins in people's diets. Some of these chemicals are established carcinogens (SN: 5/15/99, p. 309).
The fish link to dioxins
The incomplete combustion of plastics, certain chemicals, or other hydrocarbon-based products can create pollutants with a distinct double-ring structure to which chlorine atoms are attached. These chemicals are often referred to collectively as dioxins or dioxinlike compounds. They include not only true dioxins, but also furans, and polychlorinated biphenyls (PCBs). These toxic chemicals have become ubiquitous in air, water, and soils around the globe. Because they're fat soluble, dioxins tend to build up in the body fat of animals and can travel from there throughout the body via the blood system.
Once in the body, dioxins can pose a host of health risks. At high concentrations, not only have the most toxic dioxins been designated carcinogens, but these pollutants have also been linked to bad teeth (SN: 2/20/99, p. 119), to reproductive impairments (SN: 6/3/00, p. 358: Available to subscribers at Dioxin cuts the chance of fathering a boy), and even to a reduction in the chance that parents will produce male children (SN: 4/4/98, p. 212). Children exposed to PCBs in the womb can have neurological impairments and diminished IQs (see Because We Eat PCBs).
Creatures highest on the food chain—people and carnivorous fish, for example—tend to acquire the highest concentrations of dioxins, mostly through their diets. Indeed, people who fish for a living tend to have especially high buildups of true dioxins in their bodies, presumably from eating lots of the fish they catch.
Each time we eat a fish, we incorporate its lifetime accumulation of pollutants. And that's what makes the new and remarkably simple Danish treatment so intriguing. It holds out the prospect of reducing the dioxin accumulation of farmed, carnivorous fish—potentially to concentrations lower than those in wild-reared kin.
Why sun lamps?
Previous studies have shown that to some extent, dioxinlike compounds degrade in the environment, provided they have access to sunlight. The Danish scientists reasoned that solar ultraviolet rays might be the triggers, so they monitored dioxin in fishmeal before and after they irradiated it with high-intensity lightbulbs designed to emit wavelengths in either the UV-A spectrum (320 to 400 nanometers) or UV-B spectrum (280 to 320 nm). Some portion of each fishmeal sample was prevented from getting any UV exposure but was otherwise handled normally.
To imitate outdoor conditions, the UV tests lasted several days. Five-day exposures to UV-B lamps reduced dioxin concentrations in the fishmeal by 70 percent, the Danish scientists report in the Sept. 7 Journal of Agricultural and Food Chemistry. Comparable irradiation under UV-A lamps cut dioxins by only 10 percent. Irradiating the fishmeal for more days with either UV type cut the dioxin concentrations a little more in each category. Dioxin concentrations of fishmeal kept in the dark remained unchanged throughout the test period.
"UV light did not seem to significantly affect the amino acid composition of the fishmeal, inferring preservation of its full nutritional value," the scientists report. However, both types of UV light triggered a comparable and potentially detrimental oxidation of fatty substances in the fishmeal—chemical changes that give rise to rancidity and off flavors. In tests where Baron's team added a chemical antioxidant to the fishmeal prior to irradiation, UV-triggered oxidation of fat was greatly diminished. The degree to which the oxidation was inhibited correlated with the amount of antioxidant added.
Unfortunately, Baron adds, dioxin degradation also fell as the antioxidant concentration increased. This suggests that some amount of oxidation may be needed to foster the contaminants' breakdown, she says.
And there's the rub, she notes: How much oxidation must be tolerated to keep dioxin at acceptably low levels? Her team will now begin tweaking the process, increasing the light's intensity to cut the dioxin-breakdown time. The researchers will also begin tests aimed at controlling fat oxidation during the light regime by limiting the fishmeal's exposure to air during treatment.
Currently, the most-effective techniques for removing dioxins from fish flesh have been to extract the toxic chemicals from the animals' fat using solvents, to press the fat from the tissues, or to broil fish so that its pollutant-laden fat can drip out. The first two techniques alter the physical properties of the fish tissues, Baron's group notes. The latter places the onus on the cook to reduce the food's toxic-chemical threat. Moreover, broiling doesn't remove nearly as much of the pollutants as the other techniques do. For instance, Japanese researchers report in an upcoming issue of the Journal of Agricultural and Food Chemistry that broiling diminished a mackerel steak's dioxinlike chemicals by only about one-third.
The new UV strategy would instead limit the entry of contaminants into farmed fish in the first place.
Caroline P. Baron
Department of Seafood Research
Danish Institute for Fisheries Research
Technical University of Denmark
DK-2800 Kgs. Lyngby
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