You could have the best husbandry skills in the world, the best fish entering the system, and the best feed. You could get the best FCR, and the best growth rates, and have the lowest cost per kg of production. But if you can’t sell what you produce, you don’t have a business.
Much work has been done in past years to address the issue of off-flavors in aquaculture products. Much is known about the problem. But many of the solutions are expensive, and therefore much work is left to be done.
CAUSE OF OFF-FLAVORS
Off-flavors are cause by the presence of bacteria in the RAS which give off geosmin and.or 2-MIB. These chemicals have detectable odor and taste at very low levels-the parts per trillion range and are often described as “earthy” and/or “musty”.
Perhaps it’s no consolation, but of possible interest is that RAS is not the only industry to be negatively impacted by this phenomenon. The battle is also being fought in the drinking water, coffee, and brewing industries, where microbiological action producing geosmin and 2-MIB is also the principle culprit.
The drinking water industry deals with the issue mainly with chlorination, although activated carbon, ozonation, and UV treatment have been presented as possible solutions. To date they are not practical, especially for larger utilities. The coffee and brewing industry tend to focus on sanitation and disinfection to overcome the off-flavor issue.
BAD WATER, BAD FISH
For some species earthiness occurs because of the environment in which the fish are raised. A waterway with high organic loading, leads to the earth taste. A fish from a different waterway with low organic loading may have a very different taste void of earthiness. Often times, cooking consumers will bathe the meat in acidic solutions such as buttermilk to break down the compounds and remove the off-flavor.
While mainly a problem for RAS operations, the off-flavors could conceivably be seen in any operation whose water source contains these compounds. Any water source experiencing an algae bloom may have sufficient concentrations to cause a problem for the farmer who is sending fish to market at the time.
The off-flavor compounds are not limited to any certain temperature or salinity. Their presence has been documented in cold water, warm water, freshwater and marine environments. There have also been reports that while the bacteria which produce the compounds are present in all RAS, the off-flavors are not always found in the product.
As with most obstacles, one of the best courses of action is prevention.
It is well documented that the bacteria in question require a substrate on which to establish their population.
In RAS, this substrate is likely to be any spot in the system where solids are permitted to settle and remain. The substrate could also be high solids that have been allowed to remain in the system. A treatment systems that removes solids quickly is a high priority for prevention of these off-flavors.
Treatments by ozonation or UV on the recirc water can help to keep bacterial populations from thriving.
Ozonation has some drawbacks, and most significantly, the ability to kill the fish itself should they be exposed to the ozone. One study found that ozonation to 248mV ORP, had no affect on off-flavors in RAS. In marine RAS, ozonation to levels required to deal with off-flavors will also create bromate at levels toxic to fish, and is not recommended for this purpose.
If using ozone in a freshwater system, it is possible that a high dosage treatment of about 10% of recirc flow may be adequate for off-flavor treatment while managing the ozone risk. Perhaps the simplest solution is the addition of sufficient new water to flush out the off-flavor causing bacteria. It may be the simplest solution, but not always physically or fiscally feasible, and often contrary to the purpose of RAS.
TREATMENT BY DEPURATION
The most widely accepted treatment to remove off-flavor compounds is depuration. Depuration is done by removing fish that are ready for market from the main RAS system and putting them in a system with a higher proportion of new water.
For depuration to be effective the bacteria need to be managed. It is almost impossible to get the fish into the depurating system without also introducing the bacteria that cause the off-flavor. Therefore the depuration system has to be even less likely to have opportunities for solids to settle out and remain in the system. It has to have a higher degree of new water. It should have as little surface area as possible. Media used to create air-water interface should be avoided as they are difficult to keep clean and to disinfect.
Depuration time will vary between species but is typically 10-20+ days. A higher fat content in the fish will require a longer depuration period. Ina cooler temperature species, a long depuration period is required. It may be necessary to test a few fish from each batch to ensure the off-flavors have been purged before the final harvest is done.