Abstract
In the present study, the possibility of employing spatially offset Raman spectroscopy
(SORS) in the qualitative and quantitative characterization of quality parameters of salmon
through the skin has been explored. A laboratory-based SORS setup comprising an 830 nm laser
was employed, and intact samples and model samples made of salmon tissue constituents were
used to investigate the penetration of Raman signals through the dark and light part of salmon
skin. Intact salmon samples with both dark and light skin were measured at different spatial
offsets. When using spatial offsets in the range of 5-6 mm, the results clearly show that
information regarding fatty acid composition and carotenoid content could be obtained from
both dark and light parts of the skin. Similar information could not be obtained using
conventional backscattering Raman spectroscopy. Model samples of ground salmon spiked with
either solutions of carotenoids or a range of vegetable oils were also measured, and at a
spatial offset of 5 mm, a clear relationship between Raman carotenoid band intensities and
carotenoid concentrations in the model samples was revealed. In addition, high correlations
for the estimation of iodine values (i.e., fatty acid unsaturation) could be obtained for SORS
measurements through light and dark parts of the salmon skin. A crude estimate suggested that
information from around 5 mm beneath the surface area of the salmon skin could be obtained.
The choice of a laser line in the near-infrared region is a major prerequisite for successful
through-skin analysis of salmon. This feasibility study could pave the way for future Raman
analysis of intact salmon.
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