As unsaturated lipids oxidize, they form hydroperoxides, which are susceptible to further oxidation or decomposition to secondary reaction products including aldehydes, ketones, acids, and alcohols. While oxidation reactions of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are responsible for fishy off-flavors in marine oils, gas chromatography-olfactometry (GC-O) and other types of analytical studies have failed to reveal which specific oxidation products are involved. Previous research (Marsili, R.T.; Laskonis, C. The importance of odourant synergy effects in understanding malodour problems in DHA and EPA products. Lipid Technol. 2014, 26 (2), 31-34) has indicated that fishy malodor may be caused by the presence of two lipid oxidation products, heptanal and (E,Z)-3,5-octadien-2-one. The aims of the present study are to provide experimental method details and offer further evidence that these two oxidation products are indeed the cause of fishy malodors. Initial GC-MS-O studies of marine oils with fishy malodors revealed numerous oxidation products, but none were characterized as fishy. However, when all sample volatiles were captured together and then desorbed simultaneously in GC-O experiments, the fishy malodor was evident, indicating odorant synergy effects were responsible. A simple, novel method was developed using an olfactometry detector as a fraction collector to trap various peaks in marine oil chromatograms. The nose cone of the olfactometry detector was replaced with a PDMS foam absorption tube at various times during GC analysis. Combinations of GC peaks were trapped on PDMS tubes, desorbed in a Gerstel thermal extractor (off-line), and sniffed. The combination of two analytes was found to cause fishy malodors: heptanal and (E,Z)-3,5-octadien-2-one. Purge-and-trap, solid phase microextraction (SPME), and headspace stir bar sorptive extraction (HSSE) sample preparation methods prior to GC-MS were investigated. All methods confirmed the combination of heptanal and (E,Z)-3,5-octadien-2-one as the cause of fishy odor.
Keywords: DHA; EPA; algal marine oils; fishy; lipid oxidation products; olfactometry; synergy.