Food scientists would like to add omega-3 fatty acids found in fish oil to foods because they’re believed to have health benefits and humans have trouble biosynthesizing them. But these unstable fatty acids can degrade via enzymatic or chemical oxidation to form compounds that give foods a fishy smell. Peter H. Schieberle of the Technical University of Munich reported that his team has identified the compounds responsible for the unwanted odors. The scientists separately reacted α-linolenic acid (a plant-derived omega-3 fatty acid) and eicosapentaenoic acid and docosahexaenoic acid (both derived from fish) with oxygen in the presence of lipoxygenase or a copper(II) salt. By using gas chromatography and mass spectrometry, they found that all six reactions produced the same mixture of 11 compounds, but in different ratios. One compound, an epoxydecadienal, had never been identified in foods before. The fishiest odor was produced by eicosapentaenoic acid oxidized by copper, whereas α-linolenic acid didn’t produce a fishy smell at all. Schieberle’s team re-created the fishy smell by mixing the 11 compounds at the concentrations found in the fishiest mixture in a neutral oil.