Earthy Off-flavors Research Articles

Quality of Rainbow Trout (Oncorhynchus mykiss) Reared in Recirculating Aquaculture System and during Depuration Based on Chemical and Sensory Analysis

In recirculating aquaculture systems (RAS), off-flavors can accumulate in fish muscle tissue. They are problematic for consumer acceptance and the reputation of farmed fish products. Although off-flavors are not toxic at low concentrations, they often give fish muscle earthy, muddy, or other unwanted flavors. Traditionally, the study of off-flavors in fish focused on muddy and earthy off-flavors caused by geosmin (GSM) and 2-methylisoborneol (MIB), but other unwanted flavors and compounds have also been identified. In this study, the selected off-flavors were chemically quantified in fish from a RAS-rearing rainbow trout (Oncorhynchus mykiss) and in different stages of depuration. A group of trained panelists with experience in sensory evaluation was specifically trained in analyzing rainbow trout samples. The panelists evaluated the fish with a sensory profile of 29 sensory attributes (12 odor, 5 taste, and 12 flavor properties). Overall, the concentrations of all the studied off-flavor compounds decreased, some to below the limit of detection and others (e.g., octanal, octanoic acid, phenylacetaldehyde, and acetoin) to a certain low level. Moldy, earthy, and musty odors and flavors especially decreased during depuration compared to fish in RAS. This study shows the consistency of the chemical analysis and sensory profiling. It also provides important information about the effects of the depuration period in RAS and on the chemical and sensorial quality of rainbow trout.

Biocontrol of Geosmin Production by Inoculation of Native Microbiota during the Daqu-Making Process

Geosmin produced by Streptomyces can cause an earthy off-flavor at trace levels, seriously deteriorating the quality of Chinese liquor. Geosmin was detected during the Daqu (Chinese liquor fermentation starter)-making process, which is a multi-species fermentation process in an open system. Here, biocontrol, using the native microbiota present in Daqu making, was used to control the geosmin contamination. Six native strains were obtained according to their inhibitory effects on Streptomyces and then were inoculated into the Daqu fermentation. After inoculation, the content of geosmin decreased by 34.40% (from 7.18 ± 0.13 μg/kg to 4.71 ± 0.30 μg/kg) in the early stage and by 55.20% (from 8.86 ± 1.54 μg/kg to 3.97 ± 0.78 μg/kg) in the late stage. High-throughput sequencing combined with an interaction network revealed that the fungal community played an important role in the early stage and the correlation between Pichia and Streptomyces changed from the original indirect promotion to direct inhibition after inoculation. This study provides an effective strategy for controlling geosmin contamination in Daqu via precisely regulating microbial communities, as well as highlights the potential of biocontrol for controlling off-flavor chemicals at trace levels in complex fermentation systems.

Biocontrol of geosmin-producing Streptomyces spp. by two Bacillus strains from Chinese liquor

Streptomyces spp. producing geosmin have been regarded as the most frequent and serious microbial contamination causing earthy off-flavor in Chinese liquor. It is therefore necessary to control the Streptomyces community during liquor fermentation. Biological control, using the native microbiota present in liquor making, appears to be a better solution than chemical methods. The objective of this study was to isolate native microbiota antagonistic toward Streptomyces spp. and then to evaluate the possible action mode of the antagonists. Fourteen Bacillus strains isolated from different Daqu (the fermentation starter) showed antagonistic activity against Streptomyces sampsonii, which is one of the dominant geosmin producers. Bacillus subtilis 2–16 and Bacillus amyloliquefaciens 1–45 from Maotai Daqu significantly inhibited the growth of S. sampsonii by 57.8% and 84.3% respectively, and effectively prevented the geosmin production in the simulated fermentation experiments (inoculation ratio 1:1). To probe the biocontrol mode, the ability of strain 2–16 and 1–45 to produce antimicrobial metabolites and to reduce geosmin in the fermentation system was investigated. Antimicrobial substances were identified as lipopeptides by ultra-performance liquid chromatography tandem electrospray ionization/quadrupole-time-of-flight mass spectrometry (UPLC-ESI/Q-TOF MS) and in vitro antibiotic assay. In addition, strains 2–16 and 1–45 were able to remove 45% and 15% of the geosmin respectively in the simulated solid-state fermentation. This study highlighted the potential of biocontrol, and how the use of native Bacillus species in Daqu could provide an eco-friendly method to prevent growth of Streptomyces spp. and geosmin contamination in Chinese liquor fermentation.