Using Infrared Spectroscopy to Assess Fish Oil Quality

By Stefano Tommasone Ph.D.Reviewed by Danielle Ellis, B.Sc.

Fish oil presents several health benefits, and it is used in dietary supplements and food products. Assessing its quality is key to ensuring effectiveness and safety. Traditional methods of analysis present drawbacks that could be overcome by introducing new protocols that use infrared spectroscopy.

The compounds associated with fish quality are normally analyzed with colorimetric or chromatographic techniques requiring extensive sample preparation and chemical manipulations. Infrared spectroscopy has already been used to investigate fish components such as protein and lipid content and can provide a strong alternative to assess fish oil quality.

Understanding Infrared Spectroscopy

Infrared spectroscopy (IR) is an analytical technique based on the interaction of molecules with infrared light. The absorption of radiation by molecules causes a change in their vibrational state, with different functional groups absorbing specific wavelengths, creating a unique spectral fingerprint.

For instance, carbonyl groups show characteristic signals between 1,800-1,600 cm^-1, whereas aliphatic C-H vibrations appear between 3,000 and 2,800 cm^-1. IR spectroscopy is fast, non-destructive, and allows the simultaneous determination of several analytes from a single measurement. Additionally, samples can be measured inside closed glass vials, thus avoiding sample alteration and cross-contamination.

Parameters of Fish Oil Quality

Free fatty acids (FFA) – produced by the hydrolysis of fish lipids – are often used as a quality index for fish. Frozen storage leads to an accumulation of FFA. In addition, FFA leads to fish muscle toughness and may also cause nutritional deterioration.

Fish oil supplements are rich in omega-3 long-chain polyunsaturated fatty acids (n-3 PUFAs) and are known to have essential roles in the prevention and treatment of hypertension, coronary heart disease, inflammatory diseases, arthritis, etc. Oxidation levels and the presence of contaminants like heavy metals and pesticides are also other important parameters that reflect fish oil quality.

Fish Oil Applications of Infrared Spectroscopy & Commercial Landscape

The nutraceutical industry must follow good manufacturing practice standards, with quality control procedures in place to prevent adulteration. Consequently, adequate analytical techniques and validated methods are necessary. Being a rapid and non-invasive technique, infrared spectroscopy, particularly near-infrared (NIR) methods, can determine various constituents in food and other solid samples, including fish oil.

The FFA content of fish samples (mackerel) at different storage times was determined by NIR spectroscopy. The results showed that FFA increased with storage time and temperature. Commercial menhaden oil was used as a model to prepare the calibration curves of FFA contents.

The mackerel stored at 4 °C showed an increase in FFA content at a moderate rate. When instead the fish was stored at 24 °C, the FFA levels increased much more rapidly. The trend observed was in agreement with the Hypoxanthine (Hx) content (an accurate marker of the degree of fish freshness) determined on the same samples, highlighting how NIR spectroscopy can be a useful marker to assess mackerel quality.

Whilst omega-3 polyunsaturated fatty acids have benefits for human health, a high intake of omega-6 polyunsaturated fatty acids can have detrimental effects. Hence having knowledge of the presence and concentration of lipids in fish oil is crucial.

NIR spectroscopy, in combination with a partial least squares (PLS) regression model, was explored as a rapid method for the determination of fatty acids and lipid classes (including oleic, palmitic, linoleic, and linolenic acids, as well as omega-3 and omega-6) in salmon oil.

NIR spectra (between 14,000 and 4500 cm^-1) showed characteristic bands at 5792 and 5676 cm^-1 related to C-H stretching vibrations of -CH₃, -CH_2, and -HC=CH groups and small peaks at 4654 cm^-1 associated with combination bands of C-H and C-O stretching vibrations.

The method showed outstanding predictive capability for ergosterol, FFA, triglycerides, diglycerides, omega-3, and PUFAs, and proved to be a valuable screening tool for the characterization of salmon fish oil samples. However, only species with relatively high concentration levels could be determined, whereas there was low sensitivity towards fatty acids with lower abundance.

A method for the quantification of phosphatidylcholine (PC) and total phospholipid (PL) in krill oil using Fourier-Transform Infrared (FT-IR) spectroscopy was also recently reported. Krill oil is an attractive source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which, unlike other fish oils exist in the form of phospholipids (PLs). Most of the total PL is in the form of phosphatidylcholine (PC).

The official analysis method for total PL is ³¹P nuclear magnetic resonance (³¹P NMR) spectroscopy, although it is an expensive technique, and not all laboratories can be equipped with it. FT-IR spectra between 4000–400 cm⁻¹ were acquired on 12 krill oil test samples, 7 commercial supplement samples, and 5 mixture samples of krill oil raw material with arbitrary ratios.

Characteristic signals at 970 cm^-1 (asymmetric stretch in –N–(CH₃)₃) and 1236 cm^-1 (asymmetric phosphate diester stretch in PO₂^- were chosen as indicators to determine PC and total PL content. The results agreed with those obtained by the 31P NMR method, evidencing the potential of infrared spectroscopy as a convenient and inexpensive alternative to ³¹P NMR techniques in the analysis of krill oil.

Conclusion

Infrared spectroscopy can be a suitable technique for the assessment of fish oil. The methods described highlight the potential for useful implementations in the process industries as cost-effective alternatives to the current destructive, expensive, and time-consuming methods. In particular, FT-IR and NIR spectroscopy could be implemented in quality control laboratories and industrial process monitoring, where, thanks to the possibility of performing on-line analysis without destructing samples, they could contribute to a better understanding of fish oil quality and support the development of safer fish oil products across industries.