Do you ever experience an undesirable fishy aftertaste when you consumed a fish oil supplement? The fact is that whether you are ingesting fish oils in capsule form or liquid form, the oil can be difficult to digest in your stomach and produced excess gas. This is why you are burping after you consumed a fish oil supplement.
What actually causes the smelly fishy odor of fish oils?
It was initially suspected that trimethylamine (TMA) 1 was the culprit for fishy odor in fish oils. There might be some other chemical compounds that contribute to the odor also. In order to prove this statement and identify the actual source of fishy odor, analysis was carried out by ChemoPower on some fish oil samples provided by a very well reputed fish oil supplier in Singapore. The volatile components of the fish oil samples were being analyzed by using static headspace coupled with gas chromatography/mass spectrometry (GC/MS) 2. Following the analysis, TMA was not found in any fish oil samples even in an expired and smelly fish oil, implying that TMA is not responsible for the fishy odor in fish oils. So what exactly are the chemicals in fish oils that causes the odor?
Well, since ChemoPower has the ability to determine minor and trace components easily, we can discover larger amounts of components within one sample as compared to other technology. With this, ChemoPower would be able to obtain more information to find out which compounds are contributing to the smelly odor of fish oil. Despite TMA that was not found in any samples, there are a total of 31 other chemical compounds discovered from fish oil samples. Among the components discovered, most of them are with peak areas of less than 0.2% of the total area percentage within one sample data, and it was found that there are a group of small molecular weight chemical compounds has been increased drastically in composition in expired fish oil samples. This group of compounds are volatile and mostly comprises of oxygenated compounds (e.g. 1-penten-3-ol and 2-methylpentanal) and hydrocarbons (e.g. isobutane, 2-pentene and (E)-2-dodecen-4-yne), which give off rancid smell and petroleum-like odor respectively.
Fig. 1: Chemical structures of some examples of oxidation products
This phenomenon can be explained by oxidation process. Fish oils, which made up mostly of polyunsaturated fatty acids, DHA and EPA 3, will be oxidized over time and decomposed into smaller molecular weight compounds. These decomposition products of fatty acids are contributing to the fishy odor or rancid smells in fish oils.
Fig. 2: Illustration of oxidative degradation of fish oils
Our findings have proved that fishy odor that you experienced from fish oils comes from the decomposition of the oil itself over time, but not TMA. Besides, it is believed that some off-odor might be produced even when you consumed fresh fish oils as the oil being digested in the acidic environment of stomach (pH 1.5 to 3.5) and broken down into smaller molecular weight compounds, as suggested in our study.
A lot of studies have shown that rancid fish oils are harmful to our health such as increasing risk of cardiovascular diseases. Therefore, it is very important for consumers to always check the product labels and stop consuming when the oil is found rancid.
All GC/MS data have been analyzed by ChemoPower’s software, SmartDalton and MoleculeDB, to give a list of chemical compounds found in each sample within hours.
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1 Trimethylamine, or TMA, is an active chemical compound that is formed from the decomposition of plants and animals. It gives off a characteristic fishy odor which is commonly found in fish. Usually it is not found in a freshly caught fish, but after a day or two, the fishy smell starts to develop and this is what the compound that is responsible for stinky fish.
2 Static headspace coupled with gas chromatography/mass spectrometry (GC/MS) is an analysis tool capable to concentrate and analyze volatile organic compounds in a wide range of samples, such as food, beverages, perfumes and cosmetics.
3 Fish oil is an excellent dietary source of omega-3 fatty acids – docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) – substances that are derived from food and cannot be produced in our body. Fish oil supplements have gained some popularity due to several important health benefits of omega-3 fatty acids, such as improve cardiovascular health, lower cholesterol levels and promote healthy brain function.
Written by Zhi Kheng LIM
Chemist