Science News Online - Food for Thought

March 8, 1997

'Sea'-soning inland fish

Salmon harvested at sea taste quite different from those -- even of the same species -- caught while traversing freshwater runs. Indeed, most oceanic seafood possesses a slightly iodinelike flavor, typical of Gulf shrimp, that distinguishes them from freshwater fare.

Ever wonder what it is? Robert C. Lindsay did. A flavor chemist, he has traced this marine essence to a class of chemicals known as bromophenols.

Lately, he has begun lacing the diets of freshwater shelled and finned fish with four of these compounds -- to the delight of taste panels. Trout fillets took on a rich, sea-like flavor that most diners prefer, while crayfish that were reared on bromophenols acquired a flavor resembling that of lobster or crab.

Lindsay believes that as additives for aquaculture operations, bromophenols hold out the prospect of inexpensively enhancing the flavor -- and marketability -- of freshwater fish, and restoring an oceanic taste to farmed salmon and shellfish.

Though 2,6-dibromophenol appears to be the single most important of these candidate flavorants, Lindsay says, "Our studies have pretty clearly shown that you need to add combinations [of the bromophenols] to duplicate flavors that develop in nature." When added to plain vegetable oil, he found, these compounds imparted a flavor reminiscent of herring.

Only trace quantities are necessary to impart this oceanic essence -- perhaps just 10 nanograms (billionths of a gram) per milligram of fish feed, he says.

Moreover, unlike many contaminants and flavor compounds that preferentially store in a fish's fat, the water-soluble bromophenols quickly move into the blood to perfuse the flesh. What this means, Lindsay notes, is that you may achieve the full flavor benefit of these compounds by feeding them to fish on just the 2 days immediately prior to harvesting.

Worming them into the diet

Currently, Lindsay's team is looking for natural sources of the bromophenols. They could be leached from fishmeal or crabmeal during processing, he notes, without detracting from the nutritional value of the source protein. Or it's possible that marine organisms that naturally produce these compounds might be farmed.

Among these organisms are sand worms. They apparently take ubiquitous phenols from sea water, and through an enzyme-driven process, attach ambient bromine onto it. The worms "produce very high amounts of these bromophenols naturally, apparently as a defense mechanism," he says.

"We've been obtaining these worms from biological specimen collectors to characterize them in terms of their flavor." It's possible that future farmed worms might be used as fish food directly or as the raw material from which any bromophenols would be chemically extracted.

Lindsay suspects that shrimp aquaculturists may be among the first to experiment with bromophenol additives.

"The average pond-cultured shrimp -- that's the dominant one on the market now -- comes out of Central America and other aquaculture centers," he notes. While these look and have the texture of wild-caught shrimp, he notes they have "a very bland taste" -- largely owing to their diet, usually low-cost protein derived from the terrestrial environment. With bromophenol additives, Lindsay says, seafood farmers might be able to market their product as tasting like its wild briny brethren.

Farmed trout and salmon represent another segment of the aquaculture industry that may pursue use of these flavorants. The one fish that doesn't seem to derive any notable benefit is catfish -- probably, Lindsay says, because it filters out the bromophenols instead of incorporating them.

How soon might flavor-enhanced fish find their way to your local grocery market or onto restaurant menus? "I tend to be conservative," Lindsay says. Though it's technologically feasible to employ these additives today, "we don't have the funding to prepare large amounts for commercial-scale testing" -- despite acknowledged interest by several aquaculture operations in pursuing such trials.

However, he adds, "I bet you that within 5 years, products [intentionally] flavored with bromophenols will be out there."

While these compounds may improve the flavor of certain fishy products, they can have the opposite effect in cheese, as dairy researchers in New Zealand will attest. Investigating the source of an objectionable taste in some locally produced Gouda, they traced the unwanted "chemical" flavor to 2-bromo-4-methylphenol. As with the fish, just trace quantities -- 25 parts per billion in the cheese -- imparted the flavor.

Their forensic analyses, described in the just published February Journal of Agricultural and Food Chemistry, indicate that brine derived from sea salt, and used during the latter stages of the Gouda's production, contained substantial quantities of bromine. The 4-methylphenol occurs naturally in New Zealand cow's milk. They now speculate that the ultraviolet light used to control microbial contamination during the brine stage catalyzed the formation of the unwanted fishy flavor compound.

How bad was it? At the bottom of their paper, the authors actually published a note of thanks to "those colleagues who carried out the unpleasant task of tasting the tainted cheeses."

March 8, 1997

'Sea'-soning inland fish

Worming them into the diet

This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.

Illustration: Wendy Temple.

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