Distribution of Off‐Flavor Compounds and Isolation of Geosmin‐Producing Bacteria in a Series of Water Recirculating Systems for Rainbow Trout Culture

Abstract

AbstractPreharvest 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 these problems in pond‐cultured fish, the microbial sources of geosmin and MIB in recirculating aquaculture systems (RASs) are still being explored. In this study, we investigated (1) the distribution of geosmin and MIB within six replicated RASs producing rainbow trout Oncorhynchus mykiss and (2) the microorganisms responsible for 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 before harvest. Each RAS contained a fluidized‐sand biofilter, a cascade aeration column, a low‐head oxygenation (LHO) unit, an LHO sump, a 5.3‐m3 culture tank, a 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 the collected samples. Water and biosolid samples were used to inoculate media for culturing microorganisms capable of producing geosmin, MIB, or both (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 MIB production. Geosmin levels were higher and more prevalent than MIB in water, biosolids, and trout fillets. In addition, geosmin levels were higher in biosolids than in water. The earthy‐odor‐producing actinomycetes Nocardia cummidelens, N. 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 isolates were the likely source of the geosmin in the RAS water and contributors to the earthy off‐flavor in the trout.