Where do we draw the line on depuration of RAS salmon?
A study involving blind tastings shows that many consumers aren't particularly sensitive to geosmin, and may help farmers decide how long to purge their fish
More than half of 85 self-identified salmon consumers who took part in blind taste tests were unable to identify an off-flavour in fillets containing up to five and half times the level of geosmin as fillets from a depurated control group, a study shows.
Microbially derived geosmin causes an earthy off-flavour in Atlantic salmon produced in land-based systems, requiring the fish to be depurated (purged) without food in fresh water for several days before harvest.
It’s an added expense for fish farmers, who must build depuration tanks, use a lot of fresh water, and may see their valuable stock lose weight. Knowing what level of geosmin is acceptable to consumers can therefore be useful for production.
Sensory threshold
With this in mind, researchers at The Freshwater Institute in West Virginia, United States, conducted a study to investigate the geosmin sensory threshold in market-size Atlantic salmon.
When natural waterborne geosmin levels peaked in a partial reuse system, some fish were moved to depuration tanks, while others remained in water containing 12–25 ng/L (nanograms per litre) geosmin.
This approach yielded five groups of fish with average geosmin levels of 87 (Control), 181, 282, 366, and 482 ng/kg.
Three microwaved fillet samples (triads) - two control fillets, and one from another category - were presented to salmon consumers with ascending geosmin concentrations.
Best estimate
Panellists were asked to identify the higher geosmin sample in each triad. The taste test determined that the best-estimate geosmin threshold of all panellists was 381 ± 153 ng/kg. Only 44% of participants accurately detected geosmin within the tested range.
However, 13% identified all samples correctly, demonstrating that highly sensitive consumers can detect a difference between salmon fillets with 87 and 181 ng/kg geosmin. The study’s authors point out that the finding must be interpreted very carefully, because these very sensitive consumers merely discerned a difference between salmon fillets with average geosmin levels of 87 and 181 ng/kg.
“The lack of geosmin detection at 87 ng/kg is not confirmed by this result, as these very sensitive panellists may have only detected a difference in geosmin intensity. Moreover, this sensory panel did not evaluate whether geosmin was objectionable.”
Narrowing the gap
Nonetheless, they say, the results of the study provide new insights that could help land-based Atlantic salmon farmers narrow the required duration of the depuration period, which is typically dictated by the moment fish have eliminated geosmin and/or MIB (2-Methylisoborneol) to an extent that the product is perceived as being “free of objectionable off-flavour”. No MIB was present in the experiment.
“The results confirmed that consumers have different inherent abilities to detect geosmin and that a low percentage of salmon purchasers can taste earthy geosmin at concentrations as low as 181 ng/kg in salmon fillets,” conclude the authors.
“These research findings should be considered with specificity to the study conditions, including the analytical techniques used for off-flavour analysis, which ultimately dictated the resulting sensory detection thresholds, the food preparation methods and form of the product presented for tasting, the lipid levels of the tested fillets, and the methods by which off-flavour was imparted including system brushing to create a sudden spike in geosmin just before fish were harvested.
Guiding decisions
“Nevertheless, this research expands our understanding of the geosmin detection threshold in Atlantic salmon, and it is therefore expected to guide risk management decisions surrounding earthy off-flavour detection at the end of the fish production cycle, namely, optimising the timing of fish harvests from depuration systems to ensure residual geosmin levels are undetectable.”
The open access research paper, “Utilizing a consumer taste panel to assess the geosmin sensory detection threshold in market-size Atlantic salmon, Salmo salar, produced in land-based aquaculture systems”, is published in the Journal of the World Aquaculture Society, 57(2), e70080.
The authors are John Davidson, Curtis Crouse, and Travis May, all from The Conservation Fund, Freshwater Institute, Shepherdstown, West Virginia, US, and Ran Tao and Ashley M. Soldavini, both from Ohio State University’s Department of Food Science & Technology.
Read the full paper here.