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Source Of Fishy Odors Identified

ACS Meeting News: When omega-3 fatty acids are oxidized, a mixture of 11 of the degradation products causes unpleasant smells, flavors

by Celia Henry Arnaud
September 11, 2013

These omega-3 fatty acids form the same degradation products but not in the same amounts.
Structures of alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid.
These omega-3 fatty acids form the same degradation products but not in the same amounts.

Food scientists would like to add omega-3 fatty acids like eicosapentaenoic acid and docosahexaenoic acid to foods because they’re believed to have health benefits and humans have trouble making them. The problem is that these unstable fatty acids can degrade via chemical or enzymatic oxidation to form compounds that give foods a fishy smell.

Scientists haven’t known what compounds cause the unpleasant odor—until now. Peter H. Schieberle of the Technical University of Munich, reported that his team has identified the chemical culprits. He announced the findings at the ACS national meeting in Indianapolis in the Division of Agricultural & Food Chemistry

“We wanted to find the aroma compounds that interact with receptors in our nose,” Schieberle told C&EN, and are responsible for the fishy off-odors and flavors people experience.

For their search, the scientists separately reacted α-linolenic acid (a plant-derived omega-3 fatty acid), eicosapentaenoic acid, and docosahexaenoic acid with oxygen in the presence of lipoxygenase or copper(II). By using gas chromatography and mass spectrometry, they found that all six reactions produced the same mixture of 11 compounds, but in different ratios. Most of the compounds were unsaturated aldehydes formed via hydroperoxides. One compound, an epoxy-decadienal, had never been seen in foods before.

“None of these compounds smells like fish, but in a mix, if you have the right concentrations, it has a terribly fishy odor,” Schieberle said.

The fishiest odor was produced by eicosapentaenoic acid oxidized by Cu(II); α-linolenic acid didn’t produce a fishy smell at all. Schieberle and his coworkers reproduced the fishy smell by mixing the 11 compounds at the concentrations found in the fishiest mixture in a neutral oil matrix.

“The study was very elegant and well thought out,” said session organizer Karen M. Schaich, a professor of food science at Rutgers University. “It provides an inkling of how to try to go about controlling the production of the fishy flavor.” She notes that moving the oxidation to a different double bond or converting a cis to a trans gives a different product and that if you do that to enough different positions, it’s possible to eliminate compounds contributing to the smell.

That’s precisely what Schieberle hopes to do next. He is looking for ways to minimize the fishy taint and extend the shelf life of nutritional supplements and other foods to which fatty acids have been added.

“It’s possible to change the overall aroma attribute” and increase the storage time of foods at the same time, he said. “I cannot say that we will be able to completely avoid this degradation for a year or more, but we can at least increase the storage time.”



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