This article provides an overview of the use of solid-phase microextraction (SPME) in the fish oil industry to monitor oxidation products in marine oils. SPME was developed by Janusz Pawliszyn et al.¹ in the 1990s and its applications in the field of pharmaceutical analysis, food analysis and environmental analysis have been extensively reviewed.² By combining sampling and sample preparation into one step, SPME has been shown to provide many benefits, including speed, simplicity of operation, ease of automation, as well providing a solvent-free sample preparation method.

The fish oil industry is developing food supplements and functional food products that contain omega-3 fatty acids. The use of SPME to measure the quality of fish oil will be assessed in this review.

Oxidation of Fish Oil

The hydroperoxides are called primary oxidation products and are unstable. They decompose eventually into secondary oxidation products, which are volatile and have a strong and unpleasant smell and taste, characteristic of rancid marine oils.⁷ The smell and taste of rancid fish affects the consumption of marine oil food supplements, which is something the industry obviously wants to avoid. The problem of rancidity can be solved by presenting the oil in a capsule or by adding a masking agent, but for functional food products only non-oxidized marine oils are acceptable.

Determining Fish Oil Quality

The industry is developing new methods of refining oils that have superior quality and are testing the smell/taste profile using trained and qualified taste panels. This is an expensive and time-consuming process and the need for a laboratory method to identify and quantify molecules that taste panel members can detect is required. Classical lipid chemistry methods to measure oxidation have been investigated, however, it has been found that peroxide value,⁸ anisidine value, 2-thiobarbituric acid value and conjugated dienes do not correlate the oxidation status with the sensory perception of a taste panel.⁹ Lipid peroxides form a range of breakdown products that result in a highly complex headspace. It comprises of unsaturated and saturated compounds such as aldehydes, ketones, alcohols and hydrocarbons.¹⁰ Some of the compounds have no flavour, others have a distinct smell and taste of, for example, rancid fish, metal, paint, cucumber or plastic. The odour threshold value ranges from 90–2000 μg/g for hydrocarbons to 0.002–7.000 ng/g for unsaturated ketones.¹¹ Macfarlane et al. identified three key components that were responsible for the fishy smell and taste, namely 2,6-nonadienal, 4-heptenal and 3,6-nonadienal. Based on these observations a model for the fishy taste was developed that was called the fatty acid smell and taste (FAST) index. Jacobsen et al. claim that the FAST index is insufficient to describe the fishy flavour¹² because it does not consider sensory descriptors such as metallic, rancid and "paint-like". It has been shown that at least 60 volatiles are present in fish oil-enriched emulsion and that the most potent odorants were 1-penten-3-one, (Z)-4-heptenal, 1-octen-3-one, 1,5-octadiene-3-one, (E,E)-2,4-heptadienal and (E,Z)-2,6-nonadienal.¹³ Using partial least-squares regression and multiple linear regression, Jacobsen et al. found the importance of (E,Z)-2,6-nonadineal and 1-pentene-3-one for causing off-flavours. Furthermore, they also suggested that the compounds could be useful markers for fishy and metallic off-flavours in fish and fish oil-enriched food products. As the volatiles responsible for causing off-flavours have been identified, there is a potential for an instrumental method that can replace the human taste panel. Measurements of volatiles present in olive oils for quality control (QC) purposes have been performed for a number of years.¹⁴ Cavalli et al.¹⁵ compared static headspace, headspace SPME and headspace sorptive extraction and direct thermal desorption techniques to determine the chemical composition of different olive oils. They concluded that for the quality control of olive oil, SPME is the preferred method because of its operational simplicity, repeatability and low cost. Headspace SPME is more selective than direct SPME and is easier to automate, faster than dynamic headspace and has better precision and linear response.²⁰

To determine the sensory profile of olive oil, electronic sensing techniques based on gas sensors have been tested and shown to correlate very well with a human taste panel.¹⁶ Unfortunately, this is not the case for marine oils according to the observations performed by others.⁹ The reason may be that in fish oil there are volatiles present with very low flavour threshold levels that are not detected by an electronic nose. Detection limits (DLs) in the low ng/g range are required for compounds such as (E,Z)-2,6-nonadienal, (Z)-4-heptanal and 3,6-nonadienal.