Continuous gradient temperature Raman spectroscopy of fish oils provides detailed vibrational analysis and rapid, nondestructive graphical product authentication

Authors: BROADHURST, C - University Of Maryland; Schmidt, Walter; Qin, Jianwei - Tony Qin; Chao, Kuanglin - Kevin Chao; Kim, Moon

Submitted to: Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2018
Publication Date: 12/12/2018
Citation: Broadhurst, C.L., Schmidt, W.F., Qin, J., Chao, K., Kim, M.S. 2018. Continuous gradient temperature Raman spectroscopy of fish oils provides detailed vibrational analysis and rapid, nondestructive graphical product authentication. Molecules. 23(12):3293. https://doi.org/10.3390/molecules23123293.
DOI: https://doi.org/10.3390/molecules23123293

Interpretive Summary: The global market for fish oils is expected to reach $4.08 billion by 2020. Fish oils provide the essential omega-3 polyunsaturated fats eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and are utilized for direct human consumption, animal/livestock feeds and aquaculture. Fish oils for human consumption are mainly in the form of encapsulated and liquid supplements, with a growing market for functional foods. Current fish oil authentication methods are labor intensive, expensive, highly technical, and use toxic solvents. At ARS we have developed the technique of continuous gradient temperature Raman spectroscopy (GTRS). GTRS data are unique in that they are provided in a three-dimensional, color-coded contour plot which is essentially a picture. We applied GTRS to five commercial fish oils from leading manufacturers. We found each product has a distinctive spectroscopic response to the continuous heating process that results in very different contour plots. The entire data set or any subset can be utilized to create a graphical standard to quickly authenticate a given source: the test dataset is simply graphically overlaid on the first set. This benefits ARS and the public because we have provided a rapid, inexpensive and environmentally safe means to authenticate fish oils and provide industrial quality control. Further, the technique is applicable to a wide variety of edible fats and oils, and only limited personnel training is required to understand and utilize the graphical analysis.

Technical Abstract: Gradient temperature Raman spectroscopy (GTRS) applies the continuous temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy. 20 Mb three-dimensional data arrays with 0.2°C increments and first/second derivatives allow complete assignment of solid, liquid and transition state vibrational modes, including low intensity/frequency vibrations that cannot be readily analyzed with conventional Raman. GTRS also provides a new means for rapid high throughput material identification and quality control. The entire set or any subset of the any of the contour plots, first derivatives or second derivatives can be utilized to create a graphical standard to quickly authenticate a given source. In addition, a temperature range can be specified that maximizes the information content, and every line spectra within the range can be retrieved if desired. Herein we compare GTRS and DSC data for five commercial fish oils that are considered excellent sources of docosahexaenoic acid (DHA; 22:6n-3) and eicosapentaenoic acid (EPA; 20:5n-3). Complex lipids with a variety of fatty acids and isomers with inter- and intra-molecularly repeating component moieties have three dimensional structures based mainly on how structurally similar sites pack. Any localized non-uniformity in packing results in discrete “fingerprint” molecular sites due to both increased elasticity and decreased torsion. Consequently, each of the products has a unique, distinctive response to a thermal gradient, which GTRS records to clearly differentiate the products. We also present detailed Raman data and full vibrational mode assignments for EPA and DHA.