New research: how the biofilter affects off-taste in RAS fish
There are no established methods to address geosmin problems in food fish production in recirculating aquaculture systems. Therefore, researchers have conducted a study to investigate whether microbial maturity can limit the production of so-called off-flavour.
American researchers from, among others, the Freshwater Institute, together with Canadian researchers, have recently published a scientific article in the Journal of World Aquaculture Society in which they have compared off-flavour trends in freshwater-based RAS systems with microbially mature or immature biofilters during Atlantic salmon production.
The researchers write in the article that the microbial metabolites geosmin (GSM) and 2-methylisoborneol (MIB) sometimes cause earthy and musty off-flavours in fatty fish such as salmon produced in RAS.
There is a severe risk of negative consumer response, reduced price and reduced profitability if fish with measurable off-flavour reach the market. A final process known as purging, or depuration, is therefore usually used to eliminate GSM and MIB from RAS-produced fish before harvest.
Disadvantages of purging
In this process, the fish are often moved to separate systems where feeding is stopped and large amounts of clean water are used in the flow-through. This allows the fish to get rid of off-flavours through their gills.
"Although purging is effective, the approach has several disadvantages: increased labour, handling of fish, need for dedicated purification tanks and associated investments," the researchers point out.
The significant water consumption also results in increased pumping and energy costs, as well as fasting periods with some weight loss and thereby reduced sales revenue.
"Methods that can reduce or eliminate the need for purging will therefore be able to improve fish welfare and profitability in RAS operations," the researchers write in the article.
Alternative required
Despite the disadvantages, alternative methods that are both cost-effective and commercially viable have not been fully developed.
"Physical and chemical methods for off-flavour removal have been tested in RAS with varying success. For example, it has been shown that water flow through an anaerobic sludge digestion basin limited off-flavour via adsorption and microbial degradation," report the researchers.
Various oxidants, advanced oxidation processes and electrochemical techniques have also shown potential, including frequent addition of peracetic acid (PAA), UV photocatalysis with titanium dioxide, combinations of hydrogen peroxide, ozone and/or PAA, as well as electro-oxidation.
"Nevertheless, commercial use has been limited due to operational complexity, safety challenges, scaling issues, and incompatibility with fish or nitrifying bacteria."
The experiment
The experiment was designed so that triplicate RAS with newly established nitrification ("immature" RAS) were compared with "mature" RAS that had been in continuous operation for 2.5 years, while producing ready-to-harvest Atlantic salmon.
Average levels of GSM and MIB in the water peaked at 35–40 ng/L in the immature systems but were maintained at lower levels in the mature systems.
"Time series for varying analysis parameters showed clear flavour profiles, indicating that production was linked to the RAS environment," the authors write.
Total ammonia nitrogen (TAN), nitrite nitrogen, total suspended solids (TSS), heterotrophic bacteria count and true colour (the colour of water after all suspended particles have been removed through filtration) were significantly higher in immature systems, while the amount of nitrifying bacteria was generally lower and less stable.
Of these, machine learning identified “true colour” (colour number) as the most important factor affecting the GSM level.
Environmental parameters important
The researchers conclude that the study showed that microbially matured RAS minimised off-flavour in both water and fish fillets.
"This study provides new insight into off-flavour production in RAS by showing that recently disinfected and microbially immature systems are significantly more susceptible to the production of GSM and MIB," they write.
Microbially mature systems, on the other hand, appeared to inhibit these compounds.
"Contrary to the hypothesis, the findings were not related to the amount of off-flavour-producing bacteria (OFPB) in the analysed biofilms, but rather to environmental parameters in the system."
They write that mature RAS showed efficient nitrification, documented through lower TAN and nitrite levels and more stable abundance of nitrifying bacteria.
Biofilter design
"This underscores the importance of proper biofilter design and operation. Mature systems also had better water quality, including significantly lower true colour, lower heterotrophic bacteria counts, and lower TSS."
Furthermore, off-flavour levels began to fall in both treatments after feeding, while nutrient concentrations stabilised.
"Maintaining such conditions can help commercial RAS facilities limit earthy and musty off-flavours, reduce the risk of product rejection, and cut down on depuration time, which in turn reduces water and energy use as well as costs associated with weight loss during fasting."
However, they emphasise that further research is needed to understand how sudden off-flavour episodes can occur in continuously operated, microbially mature systems.
"Larger microbiome studies with multiple replicated RAS may be needed to uncover how specific microbial groups – particularly producers, decomposers and competitive nitrifiers – contribute to the production or inhibition of off-flavour."
According to the researchers, such knowledge may eventually make it possible to develop optimised microbial consortia that can be added to RAS to limit off-flavour production.
Read the article in full here.
