Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2009
Publication Date: 10/29/2009
Citation: Schrader, K., Summerfelt, S.T. Distribution of Off-Flavor Compounds and Isolation of Geosmin-Producing Bacteria in a Series of Water Recirculating Systems for Rainbow Trout Culture. North American Journal of Aquaculture. 72:1-9.
Interpretive Summary: The “earthy” off-flavor present in the flesh of rainbow trout raised in recirculating aquaculture systems (RAS) was determined to be due to the presence of the compound geosmin. The microbial sources of geosmin within the RAS were investigated. Four different species of bacteria were isolated and identified to be producers of geosmin. In addition, the highest numbers of these geosmin-producing bacteria were present in the drum filters and heat exchangers of the RAS. These geosmin-producing bacteria were determined to be contributors to the geosmin in the flesh of the RAS-cultured rainbow trout.
Technical Abstract: Pre-harvest “off-flavor” in aquaculture products results in large economic losses to producers due to delayed harvest. The common off-flavors “earthy” and “musty” are due to the presence of geosmin and 2-methylisoborneol (MIB), respectively. Although certain species of cyanobacteria are responsible for earthy-musty problems in pond-cultured fish, the microbial sources of geosmin and MIB in recirculating aquaculture systems (RAS) are still being explored. In this study, we investigated the following: 1) distribution of geosmin and MIB within six replicated RAS producing rainbow trout (Onchorhynchus mykiss); and 2) the microbial sources of earthy off-flavor in the flesh of RAS-cultured trout. Water, biosolids, and fish samples were collected when fish were at maximum feed levels and prior to harvest. Each RAS contained a fluidized-sand biofilter, a cascade aeration column, a low head oxygenation (LHO) unit, a LHO sump, a single 5.3 m3 culture tank, a microscreen drum filter, a pump sump, and a heat exchanger. Solid phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) was used to determine the levels of geosmin and MIB in collected samples. Water and biosolids samples were used to inoculate media for culturing microorganisms capable of producing geosmin and/or MIB (e.g., actinomycetes, cyanobacteria, fungi, and myxobacteria). Microbial isolates producing an earthy-musty odor were subjected to SPME-GC-MS analysis to verify geosmin and/or MIB production. Geosmin levels were higher and more prevalent than MIB in water, biosolids, and trout fillets, with highest geosmin levels in biosolids samples compared to water samples. The earthy-odor producing actinomycetes Nocardia cummidelens, Nocardia fluminea, Streptomyces luridiscabiei, and Streptomyces cf. albidoflavus were isolated from biosolids contained within the RAS drum filters and heat exchangers, and these isolates were subsequently confirmed to be geosmin producers. No other geosmin-producing microorganisms were isolated. These geosmin-producing isolates were the likely source of geosmin in the RAS water and contributors to the earthy off-flavor in the trout.