The presence of bacterial metabolites in aquaculture production facilities and in fish products is a globally occurring issue. Geosmin and 2-MIB (2-methylisoborneol) are among the most common metabolites that cause off-flavours by imparting muddy and musty taste and odour to the cultured fish. Occurrence of off-flavours spoils the fish and the taste makes the fish unsuitable for human consumption. Removal off-flavour from fish is long-term process (requires depuration in clean water for several days) and it increase the production costs. Geosmin and 2-MIB are secondary metabolite produced by Actinomycetales, especially Streptomyces, myxobacteria and Cyanobacteria.
The purpose of this PhD study was to investigate the occurrence of off-flavours in Danish recirculated aquaculture systems (RAS) and in Brazilian Nile tilapia cage-farms. Specific topics were to localize hotspots for off-flavour production in studied aquaculture systems, to identify microorganisms involved in off-flavour production and the conditions influencing this production and furthermore, to develop depuration practices to remove off-flavours from fish.
In this study, I examined the occurrence of geosmin in Danish RAS farms producing pikeperch and Atlantic salmon, and found that biofilter and denitrification filter were hotspots for the geosmin production. I showed that effective management of water treatment system reduces the risk of potential occurrence of geosmin in RAS farms. I also investigated occurrence of off-flavours in Brazilian Nile tilapia farms by an instrumental method (GC-MS) and had the results confirmed by sensory perception of the tilapia meat, and I found a good correlation between the chemical and sensory data.
A functional gene-based (the geoA gene involved in geosmin biosynthesis) amplicon sequencing method was developed to examine the diversity of geosmin-producing organisms in aquatic environments. My study revealed occurrence of structurally diverse prokaryotic geosmin-producers in different aquatic environments. Sorangium-related geosmin-producing bacteria were dominant in Danish RAS system, while an unknown geosmin-producing bacterial group was abundant in the Brazilian fish farms. Application of the developed universal geoA probe to water and sediment in Brisbane River, Australia, showed dominance of geosmin- producing cyanobacteria in both water and sediment.
Removal of geosmin from RAS-farmed pikeperch by depuration in clean water showed that depuration of pikeperch for eight days could significantly reduce the geosmin content.