Pedro Martínez Noguera

The impact of a commercial microbial population (product name “RAS Right”) on the off-flavours geosmin and 2-methylisoborneol (2-MIB) in water and fish of a recirculating aquaculture system (RAS) for Nile tilapia production was studied over five months. The “RAS Right” product contains a microbiome that is reported to reduce geosmin. In the system that received “RAS Right”, geosmin ranged from 1.6 to 171.2 ng/L, while 2.4 to 89.3 ng/L occurred in the control RAS. After fluctuations in the first two months, water in the control RAS had lower geosmin concentrations (mean of 8.8 ng/L) than the treated RAS (mean of 16.6 ng/L). 2-MIB was low (<4.3 ng/L) or undetectable in both the control and treated systems. In the fish, geosmin varied from 112 to 3683 ng/kg, with the highest levels measured in the treated RAS during three of eight samplings. 2-MIB in the fish ranged from 11.6 to 136 ng/kg and peaked in the treated RAS in one sampling. The results indicate that “RAS Right” did not produce a significant reduction of geosmin or 2-MIB in water or fish. However, optimisation of the treatment with respect to dose or addition frequency may improve its effect, though this remains to be examined.

Rationale: Geosmin and 2‐methylisoborneol (2‐MIB) are odor‐active compounds responsible for earthy off‐flavors in fish, creating significant quality concerns in recirculating aquaculture systems (RAS). Their extremely low odor thresholds make rapid and highly sensitive detection essential for maintaining product quality and consumer acceptance in aquaculture production. Methods: A novel analytical method was developed using static headspace sampling combined with proton transfer reaction‐mass spectrometry (PTR‐MS) for the detection of geosmin and 2‐MIB in aquaculture water. Sample preparation was optimized by sodium chloride addition to enhance salting out , incubation at 90 °C, and maximization of headspace injection speed into the PTR‐MS system to improve analyte sensitivity. Results: The optimized method achieved detection limits of 19 ng·L⁻¹ for geosmin and 4 ng·L⁻¹ for 2‐MIB with a total sample screening time of less than 10 minutes. Detection limits were within the same low ng·L⁻¹ range as the odor thresholds of these compounds, enabling quantification at sensorily relevant concentrations associated with off‐flavor perception. Conclusions: This PTR‐MS‐based approach provides a rapid and sensitive method for objective detection of geosmin and 2‐MIB in aquaculture water at concentrations relevant to sensory off‐odor impairment. The method shows strong potential for routine monitoring in RAS and may be extended to other volatile compounds and aqueous analytical applications.

Geosmin is a volatile sesquiterpenoid with an ‘earthy’ odour that negatively affects the flavour of aquaculture fish. While bioconcentration models for this compound in fish exist, there is an absence of studies that account for physiological kinetics across different organs and stress‐related biochemical variables. A deeper understanding of geosmin kinetics is crucial for refining models of its absorption, elimination and metabolic transformation in fish to improve predictions of its impact on flavour quality and clarify its effect on fish stress response and welfare. This study examines the waterborne absorption and distribution of geosmin in rainbow trout ( Oncorhynchus mykiss ) in blood, liver and fillet, as well as in aquaculture water, and assesses stress responses via biochemical markers. Trout were exposed to a single dose of 0.4 ng/mL geosmin ( n = 28; plus no exposure, n = 28, as a control) under static water conditions. The concentrations of geosmin in water and fish tissues were measured at five intervals over 144 h (all fish in one exposure and one control tank for each interval) using gas chromatography‐mass spectrometry (GC‐MS) analysis. Concomitant evaluations of eight biochemical stress indicators were undertaken. No significant stress response to geosmin exposure was observed, but biological tissues exhibited varying concentrations of geosmin, characterised by an initial increase followed by a decline. Strong effects of tissue‐specific properties and exposure duration on distribution kinetics and concentrations were observed. Second‐order polynomial regressions effectively captured the concentration dynamics of geosmin across all fish tissues and water. Cumulatively, over 88% of the geosmin dose remained unaccounted for, with only minor proportions of geosmin detected in water and animal biomass. This suggests primarily hepatic metabolic biotransformation in the fish, with a possible contribution from unsampled biomass and minor analytical/compartmental losses. These results advance the understanding of geosmin distribution dynamics in fish, with implications for aquaculture practices.

Despite advances in the operation of recirculating aquaculture systems (RAS), accumulation of earthy-muddy off-flavors in the fish remains a potential risk. Myxobacteria (Myxococcota) are reported to be among the most abundant geosmin synthase-harboring groups in RAS, but previous isolation attempts have been unsuccessful, limiting the knowledge of their role in off-flavor production. For the first time, we successfully isolated two geosmin-producing myxobacteria from RAS: Myxococcus virescens AT3 and Corallococcus exiguus AT4. Cell-specific geosmin production varied with the nutrient content in different media but was highest in a low-nutrient medium and when cultivated in water from RAS. Cultivation in RAS water also stimulated the production of other volatile organic compounds (VOCs). Newly identified potential off-flavor compounds included 4-methyl-2-heptanone (“forest” odor), 3-methyl-1-butanol (“medicinal” and “chemical”), and a presumptive sesquiterpenoid described as “musty,” “earthy,” and “flowery.” The previously known off-flavor compound dimethyl sulfide was also detected. Myxobacteria have previously been proposed as keystone bacteria in the environment due to their predatory lifestyle. In predation assays using isolated bacteria from RAS, M. virescens AT3 and C. exiguus AT4 could successfully feed on 15 of 16 tested strains, suggesting a large influence on the biology of RAS microbiomes. The combination of predatory behavior and potent production of geosmin and other VOCs underscores the ecological and sensory impact of these bacteria in RAS. Understanding their behavior and metabolic outputs is critical to developing strategies for mitigating off-flavors in RAS. IMPORTANCE: Issues with off-flavored fish in recirculating aquaculture systems (RAS) due to the presence of the earthy-musty smelling compounds geosmin and 2-MIB are considered as one of the industry’s most economically significant challenges. Knowledge of conditions that affect off-flavor production is essential information in the development of viable solutions for its mitigation. Little is known about the function of these microbially produced compounds or the conditions that trigger their production, especially in the underexplored myxobacteria. Investigation of natural isolates is crucial to determine the function of the genes involved and their differential expression in response to environmental cues. While myxobacteria in RAS have been previously shown to harbor the geosmin synthase gene in molecular studies, the present study is the first attempt to isolate these bacteria from RAS and quantify their geosmin production under various nutrient conditions. Through cultivation-based methods, we demonstrate their production of both known and novel compounds with earthy attributes.

Despite advances in the operation of recirculating aquaculture systems (RAS), accumulation of earthy-muddy off-flavors in the fish remains a potential risk. Myxobacteria (Myxococcota) are reported to be among the most abundant geosmin synthase-harboring groups in RAS, but previous isolation attempts have been unsuccessful, limiting the knowledge of their role in off-flavor production. For the first time, we successfully isolated two geosmin-producing myxobacteria from RAS: Myxococcus virescens AT3 and Corallococcus exiguus AT4. Cell-specific geosmin production varied with the nutrient content in different media but was highest in a low-nutrient medium and when cultivated in water from RAS. Cultivation in RAS water also stimulated the production of other volatile organic compounds (VOCs). Newly identified potential off-flavor compounds included 4-methyl-2-heptanone (“forest” odor), 3-methyl-1-butanol (“medicinal” and “chemical”), and a presumptive sesquiterpenoid described as “musty,” “earthy,” and “flowery.” The previously known off-flavor compound dimethyl sulfide was also detected. Myxobacteria have previously been proposed as keystone bacteria in the environment due to their predatory lifestyle. In predation assays using isolated bacteria from RAS, M. virescens AT3 and C. exiguus AT4 could successfully feed on 15 of 16 tested strains, suggesting a large influence on the biology of RAS microbiomes. The combination of predatory behavior and potent production of geosmin and other VOCs underscores the ecological and sensory impact of these bacteria in RAS. Understanding their behavior and metabolic outputs is critical to developing strategies for mitigating off-flavors in RAS. IMPORTANCE: Issues with off-flavored fish in recirculating aquaculture systems (RAS) due to the presence of the earthy-musty smelling compounds geosmin and 2-MIB is considered one of the industry’s most economically significant challenges. Knowledge of conditions that affect off-flavor production is essential information in the development of viable solutions for its mitigation. Little is known about the function of these microbially produced compounds or the conditions that trigger their production, especially in the underexplored myxobacteria. Investigation of natural isolates is crucial to determine the function of the genes involved and their differential expression in response to environmental cues. While myxobacteria in RAS have been previously shown to harbor the geosmin synthase gene through molecular studies, the present study is the first attempt to isolate these bacteria from RAS and quantify their geosmin production under different nutrient conditions. Through cultivation-based methods, we demonstrate their production of both known and novel compounds with earthy attributes.

This study investigated the distribution of odorant compounds in water collected from the tank, drum filter, biological filter, and UV system outlets and the sludge inside the drum filter across two sturgeon RAS farms. A combinatory sensory‐analytical approach was applied to assess the aroma profiles via descriptive sensory analysis and the odorant composition by gas chromatography–olfactometry and gas chromatography–mass spectrometry. All water samples were described as fishy, moldy, earthy, algae‐like, and sea‐water‐like notes. Eighty‐six odorants were detected olfactorily, of which 44 were identified. Notably, several compounds are reported here for the first time in RAS water, including 1‐butanol, myrcene, 1‐tetradecene, γ‐crotonolactone, 2,6‐di‐tert‐butyl‐4‐methylphenol, 2‐phenoxyethanol, ethyl palmitate, methyl stearate, and methyl dihydrojasmonate. Geosmin and 2‐MIB were quantified in all samples. Their concentrations varied from 85 to 432 and from 40 to 43 ng/L for RAS 1 and from 70 to 178 and from 60 to 82 ng/L for RAS 2, respectively. The concentration of both compounds did not vary through the compartments due to water recirculation, and a high concentration was only detected in water from the sludge.

The consumption of seafood is driven by flavour, yet achieving its high quality remains a challenge for many species reared in recirculating aquaculture systems (RAS). A comprehensive knowledge of off‐odour sources in aquatic foods is indispensable in ensuring flavour quality standards. At the beginning of the production chain, early post‐harvest lipid oxidation products develop into endogenous off‐odours and accumulate over time. These malodours add to those already absorbed exogenously, namely from the water and feeds, although the information on the interactions between these sources is currently scarce. Despite geosmin and 2‐methylisoborneol receiving significant attention in relation to fish off‐flavour, only limited knowledge on the molecular and ecological mechanisms driving their production in aquatic environments has been reported. Moreover, RAS‐hosted bacteria have been associated with a wide range of other odour‐active compounds, such as pyrazines, terpenoids, and other degradation byproducts that are frequently overlooked when studying flavour taint in fish. The influence of aquaculture feeds on the flavour of fish flesh has been underestimated, too, both as a source of off‐odours but also as a novel modulator strategy to achieve desirable aquatic food flavours. Finally, the influence of water treatment processes widely used in RAS operations, such as advance oxidation process, ozone, ultraviolet and hydrogen peroxide disinfections, is greatly underexplored with respect to odour quality. This article reviews the current scientific evidence with supporting data on the chemical diversity of off‐odours found in aquaculture fish worldwide and their potential sources and highlights knowledge gaps that should be addressed in future research.

Pea-based ingredients are increasingly being used in foods because of their nutritional, functional and environmental benefits. However, their bitter taste is not appreciated by consumers. Saponins have been reported to be bitter in whole pea flour (PF) but not in the purified ingredients obtained from it, such as pea protein isolate (PPI) and pea starch (PS). In addition, the evolution of saponins in cooked foods made from these ingredients and their relationship to bitter flavor has not been investigated. This study, therefore, explored the presence of two bitter saponins, βg and Bb, in whole pea flour (PF) and a composite flour reconstructed from the two main fractions (PS + PPI). In addition, it investigated the impact of baking on the chemical state of these compounds in a sponge cake. Finally, the sensory impact of the baking process on the perceived bitterness of cakes made with these two pea flours was also evaluated. High-Performance Liquid Chromatography–High-Resolution Mass Spectrometry (HPLC-HRMS) was used to identify and quantify pea saponins in the flours and cakes, and a descriptive sensory analysis was obtained by a trained panel to assess sensory differences in bitterness. Our results showed marked differences in saponin concentration and composition among the pea ingredients studied. Concentrations were highest in PPI (1.497 mg·g⁻¹ dry matter), with 98% of saponin Bb. PS had the lowest saponin concentration (0.039 mg·g⁻¹ dry matter, with 83% Bb), while 0.988 mg·g⁻¹ dry matter was quantified in PF, with only 20% Bb and 80% βg. This research also highlighted the thermal degradation of saponin βg to Bb in sponge cakes during baking at 170 °C. However, at a sensory level, these chemical changes were insufficient for the impact on bitterness to be perceived in cakes made with pea flour. Moreover, baking time significantly reduced the bitter flavor in cakes made with the composite flour (PS + PPI).