Off-flavor Compounds Research Articles

Microcystis aeruginosa/Pseudomonas pseudoalcaligenes interaction effects on off-flavors in algae/bacteria co-culture system under different temperatures

We conducted an experiment to study the interaction effects of Microcystis aeruginosa and Pseudomonas pseudoalcaligenes on off-flavors in an algae/bacteria co-culture system at three temperatures (24, 28 and 32°C). Gas chromatography–mass spectrometry was applied to measure off-flavor compounds dimethyl sulfide (DMS), dimethyl trisulfide (DMTS), 2-methylisoborneol, geosmin (GEO) and β-cyclocitral. During the lag phase of co-cultured M. aeruginosa (first 15days), P. pseudoalcaligenes significantly increased the production of DMS, DMTS and β-cyclocitral at all three temperatures. In the exponential phase of co-cultured M. aeruginosa (after 15days), M. aeruginosa became the main factor on off-flavors in the co-culture system, and β-cyclocitral turned to the highest off-flavor compound. These results also indicated that DMS, DMTS and β-cyclocitral were the main off-flavor compounds in our M. aeruginosa/P. pseudoalcaligenes co-culture system. Univariate analysis was applied to investigate the effects of M. aeruginosa and P. pseudoalcaligenes on the production of off-flavors. The results demonstrated that both M. aeruginosa and P. pseudoalcaligenes could increase the production of DMS and DMTS, while β-cyclocitral was mainly determined by M. aeruginosa. Our results also provide some insights into understanding the relationship between cyanobacteria and heterotrophic bacteria.

Effect of Stocking Large Channel Catfish in a Biofloc Technology Production System on Production and Incidence of Common Microbial Off-Flavor Compounds

Density-dependent production and incidence of common microbial off-flavors caused by geosmin and 2-methylisoborneol were investigated in an outdoor biofloc technology production system stocked with stockersize (217 g/fish) channel catfish at 1.4, 2.1, or 2.8 kg/m3. Individual weight at harvest ranged from 658-829 g/fish and was inversely related to stocking density. Net fish yield ranged from 3.8-5.4 kg/m3, and increased linearly as stocking density increased. The percentage of sub-marketable fish (<0.57 kg/fish) increased linearly with increasing stocking rate. Mean total feed consumption increased linearly with stocking density, but feed consumed per fish was inversely related to stocking density. Feed conversion ratio did not differ significantly among treatments. Concentrations of geosmin and 2-methylisoborneol in biofloc water were low throughout the study. All sampled fillets contained low concentrations of geosmin and 2-methylisobornel, but these fillets likely would not be deemed as having objectionable “earthy” or “musty” off-flavors when evaluated by trained processing plant flavor testers because of the low concentrations present. Data from this study combined with data from our two previous studies provide strong evidence that the incidence of geosmin- and 2-methylisoborneol-related off-flavor episodes is low in the BFT production system.

Removal of Methyl Orange in Aquaculture Water by Spray-Coated Tio2 Photocatalysis

A major challenge for aquaculturists using recirculating aquaculture systems is the accumulation of off-flavor compounds, such as geosmin and 2-methylisoborneol (MIB), in the farmed fish. This work investigates the removal of off-flavor compounds in aquaculture systems using titanium dioxide (TiO2) photocatalysis. Due to concerns of TiO2 nanoparticles on fish, TiO2 was immobilized onto glass plates. A photodegradation reactor that incorporated the TiO2 coated plates was designed and operated as a recirculating system. Methyl orange was used as a surrogate compound for geosmin and MIB. In parameter optimization experiments, a flow rate of 2.5 mL/s, a water depth of 2 mm and a TiO2 dosage of 0.25 mg/cm3 were found to provide a high methyl orange removal rate constant of 0.77/h, when the compound was supplied in a solution made in deionized (DI) water. Due to the presence of natural organic matter (NOM) in the RAS water, the methyl orange removal rate constant decreased to 0.44/h. The results indicate that although NOM interferes with the photocatalysis process, if an extended reaction time is applied acceptable removal efficiencies can be achieved. This work is currently being applied in a photocatalysis study on removal of off-flavor compounds in RAS.

전자코와 푸리에 변환 근적외선 분광기를 사용한 사골 추출물의 향기 성분 변화 분석

This study was conducted to investigate whether four hot-water extraction steps could effectively remove offflavor from bovine bone extracts and produce compounds with pleasant aroma. Experiments were performed using a mass spectrometry-electronic nose and Fourier transform-near infrared spectrometer (FT-NIR). Off-flavor compounds were removed by washing and extraction with hot water. Steaming treatment produced compounds with a better aroma, such as 4-methylthiazole. In addition, a change in flavor compounds was observed in treated samples.

Reducing geosmin off-flavor compounds and waste outputs through dietary phosphorus management in rainbow trout aquaculture

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 6:105-117 (2014) - DOI: https://doi.org/10.3354/aei00119 Reducing geosmin off-flavor compounds and waste outputs through dietary phosphorus management in rainbow trout aquaculture Pallab Sarker1,3,*, Alexandre Pilote1, Marc Auffret2, Émilie Proulx1, Richard Villemur2, Marie-Hélène Deschamps1, Grant Vandenberg1 1Département des sciences animales, Université Laval, Québec City, Québec G1V 0A6, Canada 2INRS-Institut Armand-Frappier, 531 Boulevard des Prairies, Laval, Québec H7V 1B7, Canada 3Present address: Environmental Studies Program, Dartmouth College, Hanover, New Hampshire 03755, USA *Corresponding author: pallab.k.sarker@dartmouth.edu ABSTRACT: The aquaculture industry has long recognized the need to reduce phosphorus (P) waste outputs associated with environmental impact, and reduce off-flavor producing compounds, which can impact the quality of the fish product. This study was undertaken to investigate the effects of dietary high P (HP) and low P (LP) on growth, nutrient digestibility, P retention, and P loading as well as their correlation to the synthesis of geosmin-associated off-flavor in a recirculating aquaculture system of rainbow trout Oncorhynchus mykiss. The above diets were fed to quadruplicate tanks of rainbow trout (average mass ± SD: 127.4 ± 3.1 g) for 84 d. Results showed that the effects of the HP and LP diets on growth and P retention were not significantly different. While the apparent digestibility of P and other nutrients were higher in fish fed the LP diet, P waste outputs and geosmin levels in the fillets of fish were higher in fish fed the HP diet. Magnesium (Mg2+), potassium (K+) and zinc (Zn2+) concentrations in tank water were significantly lower in fish fed the HP diet than the LP diet in most of the sampling events. Furthermore, the tank water geosmin concentration was not strongly proportionally correlated with tank water-soluble P concentration for both the LP and HP diets. There was a strong proportional linear relationship between the geosmin concentration in tank water and in trout fillet for both the LP and HP diets. Results suggest that off-flavor contents in fish fillets and water were related to the dietary P level and metabolic P waste outputs into the system, findings that have implications for the formulation of sustainable diets for rainbow trout. KEY WORDS: Recirculating aquaculture system · Metabolic phosphorus waste · Off‑flavor · Geosmin · Oncorhynchus mykiss · Growth · Nutrient digestibility · Nutrient retention Full text in pdf format PreviousCite this article as: Sarker P, Pilote A, Auffret M, Proulx É, Villemur R, Deschamps MH, Vandenberg G (2014) Reducing geosmin off-flavor compounds and waste outputs through dietary phosphorus management in rainbow trout aquaculture. Aquacult Environ Interact 6:105-117. https://doi.org/10.3354/aei00119 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 6, No. 1. Online publication date: December 17, 2014 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2014 Inter-Research.

Detection of medicinal off-flavor in apple juice with artificial sensing system and comparison with test panel evaluation and GC–MS

Guaiacol, a potent volatile compound, has proven to be responsible for the distinct medicinal off-flavor in spoiled apple juice produced by Alicyclobacillus acidoterrestris. In this study, the artificial sensing system electronic nose (e-nose) was employed to detect this off-flavor compound in apple juice, using GC–MS and test panel evaluation as reference, as well as water as a comparative medium. Principle component analysis and linear discriminant analysis were employed to evaluate the capacity of e-nose for guaiacol detection in both media. A loadings analysis was used to examine the sensors' chemometric discrimination. Multiple regression analysis was applied to investigate the potential relationship between the concentration of guaiacol and the e-nose response. The detection limit based on e-nose for guaiacol in apple juice was 0.25–0.5 mg/L. For comparison, a 12-member test panel evaluation found an aroma detection threshold of 1.50 mg/L in apple juice, and the GC–MS detection limit for guaiacol in apple juice was determined to be 0.4 μg/L. In summary, the e-nose was capable of detecting guaiacol in apple juice, with significantly higher sensitivity than the test panel evaluation. This information should be useful for developing instrumental sensory detection techniques for rapid scanning of potential medicinal off-flavor in Alicyclobacillus-spoiled apple juice prior to consumer perception.

Nationwide survey of organism-related off-flavor problems in Japanese drinking water treatment plants (2010–2012)

Recent trends in off-flavor problems were investigated by a nationwide questionnaire survey targeting 239 drinking water treatment plants (DWTPs) maintained by 79 regional water suppliers (RWS) in Japan. The results indicated that 43 of 79 (54%) RWS and 89 of 239 (36%) DWTP were affected by off-flavor problems in a recent 2-year period (October 2010–September 2012). Off-flavor incidents occurred in all regions in Japan. Most DWTPs affected by off-flavor incidents take in water from closed water areas, such as dams and lakes, where phototrophic microorganisms, such as cyanobacteria and algae, can grow easily. The most common off-flavor was an earthy–musty odor, while a fishy smell was also reported in some cases. The common producers of geosmin were Anabaena spp., while those responsible for 2-methylisoborneol were Phormidium spp. and Oscillatoria spp. The off-flavor events frequently occurred during summer and early fall months (June–September). The water temperature and pH of raw water for the production of drinking water when off-flavor problems occurred were higher than those during normal periods. To eliminate off-flavor compounds, most DWTPs (about 58%) adopt powdered-activated carbon treatment, which considerably increases operational costs.

Gas Chromatographic Mass Spectrometry Determination of Geosmin and 2-methylisoborneol Off-Flavor in Mugil cephalus Roe

Botargo is the product of the industrial management of fresh roe of the grey mullet (Mugil cephalus). This food product is actually obtained from wild fish caught in different marine areas around the word. The increasing commercial demand of botargo has brought to carry out different fishing systems, gathering the wild mullet in restricted areas, lagoons, to be caught if necessary. This operating mode has caused the appearance of new problems related to the organoleptic approval by the consumers of fish and derived products, following off-flavor contamination derived from different microorganisms present in the lagoons. The most important off-flavor compounds are represented by 2-methylisoborneol (2-MIB) and geosmin (GSM) which give a muddy/musty smell and flavor to the fish. A simple and easy-to-use GC/MS method for the determination of 2-MIB and GSM has been developed. The method has been validated with the following parameters: instrument limit of detection (LOD) and of quantification (LOQ), precision, recovery, and linearity. The method carried out showed good LOD of 1.0 μg/kg and LOQ 2.0 and 5.0 μg/kg for GSM and 2-MIB, respectively. Validation parameters were in close agreement with those reported in the EC SANCO/12571/2013. Recoveries ranged from 80.2 to 121.4 %, with percent relative standard deviation (RSD%) ranging from 0.6 to 18.6 % in the most unfavorable case. Real sample analysis showed the presence of residues of both off-flavor in all botargo samples analyzed, except for sample S0, while in fresh roe samples, only S7 showed important residues of 2-MIB.

Benchtop fluorometry of phycocyanin as a rapid approach for estimating cyanobacterial biovolume

Cyanobacteria are the primary taxa responsible for freshwater harmful algal blooms (HABs), with several genera capable of producing potent intracellular toxins and off-flavor compounds. There is considerable growing interest in methods to rapidly quantify cyanobacteria in water samples. Past studies have demonstrated poor correlations between phycocyanin in vivo fluorescence and cyanobacterial cell densities. We conducted a series of laboratory experiments aimed at refining a protocol that uses benchtop fluorometry to measure the cyanobacterial pigment, phycocyanin, to accurately estimate cyanobacterial biovolume. In our study, we found strong correlations between phycocyanin concentration and cyanobacterial biovolume (but not for cell densities) both within and across ponds, which varied widely in productivity and algal diversity. Thus, benchtop fluorometry of phycocyanin is a viable method for water resource managers to quickly estimate cyanobacterial biovolume.

Identification of geosmin and 2-methylisoborneol in cyanobacteria and molecular detection methods for the producers of these compounds

Geosmin and 2-methylisoborneol (MIB) are muddy/earthy off-flavor metabolites produced by a range of bacteria. Cyanobacteria are the major producers of the volatile metabolites geosmin and MIB which produce taste and odor problems in drinking water and fish worldwide. Here we detected geosmin and MIB by studying 100 cyanobacteria strains using solid phase microextraction gas chromatography mass spectrometry (SPME GC–MS). A total of 21 geosmin producers were identified from six cyanobacteria genera. Two of the geosmin producers also produced MIB. A PCR protocol for the detection of geoA and MIB synthase genes involved in the biosynthesis of geosmin and MIB was developed. The geoA and MIB synthase genes were detected in all strains shown to produce geosmin and MIB, respectively. Cyanobacterial geoA and MIB synthase sequences showed homology to terpene synthases genes of actinobacteria and proteobacteria. Additional off-flavor compounds, nor-carotenoids β-ionone and β-cyclocitral, were found from 55 strains among the 100 cyanobacterial strains studied; β-ionone was present in 45 and β-cyclocitral in 10 strains. Six of the cyanobacteria which contain off-flavor compounds also produced toxins, anatoxin-a or microcystins. The molecular method developed is a useful tool in monitoring potential cyanobacterial producers of geosmin and MIB.

Characterization and analysis of structural isomers of dimethyl methoxypyrazines in cork stoppers and ladybugs (Harmonia axyridis and Coccinella septempunctata).

The three constitutional isomers of dimethyl-substituted methoxypyrazines: 3,5-dimethyl-2-methoxypyrazine 1; 2,5-dimethyl-3-methoxypyrazine 2; and 2,3-dimethyl-5-methoxypyrazine 3 are potent flavor compounds with similar mass spectrometric, gas chromatographic, and nuclear magnetic resonance spectroscopic behavior. Therefore, unambiguous analytical determination is critical, particularly in complex matrices. The unequivocal identification of 1-3 could be achieved by homo- and heteronuclear NMR correlation experiments. The observed mass fragmentation for 1-3 is proposed and discussed, benefitting from synthesized partially deuterated 1 and 2. On common polar and apolar stationary phases used in gas chromatography (GC) 1 and 2 show similar behavior whereas 3 can be separated. In our focus on off-flavor analysis with respect to wine aroma, 1 has been described as a "moldy" off-flavor compound in cork and 2 as a constituent in Harmonia axyridis contributing to the so-called "ladybug taint," whereas 3 has not yet been described as a constituent of wine aroma. A successful separation of 1 and 2 could be achieved on octakis-(2,3-di-O-pentyl-6-O-methyl)-γ-cyclodextrin as stationary phase in GC. Applying heart-cut multidimensional GC analysis with tandem mass spectrometric detection we could confirm the presence of 1 as a "moldy" off-flavor compound in cork. However, in the case of Harmonia axyridis, a previous identification of 2 has to be reconsidered. In our experiments we identified the constitutional isomer 1, which was also found in Coccinella septempunctata, another species discussed with respect to the "ladybug taint." The analysis of such structurally related compounds is a demonstrative example for the importance of a chromatographic separation, as mass spectrometric data by itself could not guarantee the unequivocal identification.

Evaluation of depuration procedures to mitigate the off-flavor compounds geosmin and 2-methylisoborneol from Atlantic salmon Salmo salar raised to market-size in recirculating aquaculture systems

Fish cultured within water recirculating aquaculture systems (RAS) can acquire “earthy” or “musty” off-flavors due to bioaccumulation of the compounds geosmin and 2-methylisoborneol (MIB), respectively, which are produced by certain bacterial species present in RAS biosolids and microbial biofilms. Fish cultured in RAS are generally transferred to separate depuration systems that are flushed with water in a single pass or operated with limited water recirculation (with no biofilter), in order to purge these unpalatable flavors. Technologies and standard operating practices that optimize purging kinetics for Atlantic salmon Salmo salar and other species cultured in RAS are needed to improve the consistency and efficacy of depuration. A 2×2 factorial trial was conducted to evaluate techniques to mitigate off-flavor from Atlantic salmon cultured to 3–5kg in a semi-commercial scale freshwater RAS. Twelve replicated depuration systems (0.5m3) were used to evaluate four combinations (n=3) of the following standard operating procedure and system design parameters: (1) disinfection of depuration systems as a 1h static treatment using 250mg/L hydrogen peroxide (H2O2) prior to stocking fish, (2) no disinfection prior to stocking fish, (3) presence of water aeration media within gas transfer columns of depuration systems, and (4) absence of water aeration media within gas transfer columns of depuration systems. Food-size Atlantic salmon were stocked within the depuration systems and kept off feed for 10 days. Six salmon were harvested from the original RAS on Day 0 and fileted for baseline assessment of off-flavor concentrations. Thereafter, filet samples (n=3–4) were taken on Days 3, 6, and 10 to evaluate off-flavor kinetics. Hydrogen peroxide disinfection of depuration systems resulted in significantly reduced off-flavor in salmon filets during the depuration period. Results also indicated that the presence of high-surface-area water aeration media shielded biofilms from complete disinfection, resulting in less and slower off-flavor removal from Atlantic salmon filets; while depuration systems void of media resulted in greater and more rapid off-flavor reduction. Thus, water aeration media should not be used in depuration systems because of the challenges posed for effective cleaning, disinfection, and inactivation of off-flavor producing bacteria that may be present, and unit processes and locations that are difficult-to-clean should be excluded. In addition, a wide range of off-flavor concentrations were measured within individual salmon, indicating that one salmon is not a representative sample size to determine market suitability.

Effects of high-pressure processing on the quality of chopped raw octopus

This study evaluated the effects of high-pressure treatment on microbial growth and production of off-flavor compounds in raw octopus during 16 days of refrigerated storage. Chopped raw octopus samples were treated at 150, 300, 450, and 600 MPa for 6 min using high-pressure laboratory food processing equipment. The number of psychrotrophic bacteria on day 16 was reduced by 0.1, 0.5, 1.3, and 2.8 CFU/g after pressure treatment at 150, 300, 450, and 600 MPa, respectively, as compared with control group. The amounts of trimethylamine (TMA) and dimethylamine (DMA) produced in the chopped raw octopus treated at 600 MPa was significantly reduced by 42.5% and 62.2%, respectively, as compared to the levels in the control. The production of biogenic amines (BAs) increased up to 1.82 mg/g in the control after 12 days of refrigerated storage, while the BA levels in the 450 MPa- and 600 MPa-treated octopus were 1.40 and 1.35 mg/g, respectively. Thus, high-pressure treatment effectively inhibited microbial growth and suppressed the formation of off-flavor compounds in raw octopus. High-pressure treatment is a promising alternative technology for extending the shelf life of raw octopus.

Determination of off-flavor compounds, 2-methylisoborneol and geosmin, in salmon fillets using stir bar sorptive extraction–thermal desorption coupled with gas chromatography–mass spectrometry

A sensitive and solvent-less method for the determination of musty and earthy off-flavor compounds, 2-methylisoborneol (MIB) and geosmin (GSM), in salmon tissue was developed using stir bar sorptive extraction–thermal desorption coupled with gas chromatography–mass spectrometry (SBSE–TD–GCMS). MIB and GSM were solid phase extracted using polydimethylsiloxane (PDMS) coated stir bars, analyzed by gas chromatography, and detected in full scan mode of mass selective detector (MSD). Using this method, the calibration curves of MIB and GSM were linear in the range of 0.3–100ng/L, with a correlation coefficient above 0.999 and RSDs less than 4% (n=4). The limit of detection (LOD, S/N=3, n=6) and limit of quantification (LOQ, S/N=10, n=6) of MIB and GSM were both ∼0.3 and 1ng/L, respectively. The recoveries of MIB and GSM were 22% and 29% by spike in 30ng/L standard compounds, 23% and 30% by spike-in 100ng/L standard compounds in salmon tissue samples with good precision (<8% of RSDs, n=6), respectively. The recoveries of MIB and GSM were better than reported methodologies using SPME fibres (<10%) in fish tissue samples. This method was successfully applied to monitor and characterize depurated salmon fillet samples (0, 3, 6 and 10 days).

Evaluation of the impact of nitrate-nitrogen levels in recirculating aquaculture systems on concentrations of the off-flavor compounds geosmin and 2-methylisoborneol in water and rainbow trout (Oncorhynchus mykiss)

Aquatic animals raised in recirculating aquaculture systems (RAS) can develop preharvest “off-flavors” such as “earthy” or “musty” which are caused by the bioaccumulation of the odorous compounds geosmin or 2-methylisoborneol (MIB), respectively, in their flesh. Tainted aquatic products cause large economic losses to producers due to the inability to market them. Certain species of actinomycetes, a group of filamentous bacteria, have been attributed as the main sources of geosmin and MIB in RAS. Previous studies have demonstrated that certain nutritional factors can stimulate or inhibit bacterial biomass and geosmin production by certain actinomycetes. In the current study, the effects of two nitrate-nitrogen (NO3--N) levels (20–40mg/L and 80–100mg/L) on geosmin and MIB levels in culture water and the flesh of rainbow trout (Oncorhynchus mykiss) raised in RAS were monitored. Water and fish tissue samples were collected over an approximately nine-week period from six RAS, three replicates each of low and high NO3--N, and analyzed for geosmin concentrations using solid phase microextraction–gas chromatography–mass spectrometry. Results indicated no significant difference in geosmin concentrations in water or fish flesh between the low and high NO3--N RAS. Therefore, higher NO3--N levels that may occur in RAS will not adversely or beneficially impact geosmin-related off-flavor problems.

Recent trends in microorganism-related off-flavor problems in drinking water treatment systems in Japan

In this study, recent trends in off-flavor problems were investigated by a nationwide questionnaire targeting 142 drinking water treatment plants (DWTPs) in Japan. The results indicated that 46 of 142 DWTPs in Japan had experienced off-flavor problems in a recent 5-year period. Cyanobacteria such as Anabaena spp., which produce musty/earthy odors, are the most common microorganisms that were presumed by DWTPs to be responsible for producing off-flavor compounds. Other microorganisms, such as diatoms, chrysophytes, and actinomycetes, are also involved in producing off-flavors. To eliminate off-flavor compounds, most facilities use powdered activated carbon treatment, which greatly increases operational costs. In addition, an interview survey targeting one typical DWTP affected by off-flavor problems was performed to investigate operational costs increased by off-flavor problems and the effects of water temperature on the occurrence of off-flavors. The operational costs when an off-flavor problem occurred were 2.9 times those when the problem did not occur, mainly due to the cost of powdered activated carbon. Therefore, off-flavors are a very serious problem in water treatment systems in Japan. The interview survey suggested that water temperature would be one of the important factors influencing the occurrence of off-flavor compounds in water sources.

Effect of Ubiquinol-10 on Citral Stability and Off-Flavor Formation in Oil-in-Water (O/W) Nanoemulsions

The effects of different concentrations of ubiquinol-10 (Q10H2) on citral's stability were systematically investigated and compared in citral-loaded oil-in-water (O/W) nanoemulsions. Solid phase microextraction gas chromatography (SPME-GC) was employed to monitor the degradation of citral and the formation of off-flavor compounds throughout storage at 25 and 45 °C. The optimum concentration of Q10H2 in the current formulation was determined to be around 0.10 wt % in the system (Q10H2/citral ratio 1:1), which can effectively protect citral from chemical degradation and oxidation. Results suggested, however, that a low concentration of Q10H2 may induce the majority of the ubisemiquinone (Q10(•-))/ubiquinone (Q10) redox transition, which possibly endowed Q10H2 with pro-oxidant properties. Further increase in Q10H2 concentration beyond a certain value also hindered its effect due to the complex properties of radicals involved and the overall environment encountered. With appropriate concentrations of Q10H2 presented in the system, major citral oxidation off-flavor compounds (p-cresol, α,p-dimethylstyrene, p-methylacetophenone), and some of the lipid degradation products can be inhibited to lower levels. In contrast, ubiquinone-10 (Q10) had a negligible effect on citral's chemical stability and off-flavor generation.

Aspergillus oryzae strains isolated from traditional Korean Nuruk: Fermentation properties and influence on rice wine quality

Forty-seven strains of Aspergillus oryzae isolates from Korean nuruks were compared for their brewing characteristics. A. oryzae YI-A6 and YI-A7 showed the highest acid α-amylase, glucoamylase, and carboxypeptidase activities, respectively. Sixteen isolates with high amylolytic or proteolytic enzyme activities were selected for investigation of their rice wine fermentation characteristics. After 12 days of brewing at 15°C, ethanol concentrations were 10.2–14.3% for A. oryzae strains. Fermentation rates were the highest for YI-A7. Most rice wine samples fermented with nuruk strains had lower concentrations of off-flavor compounds than the control did. All mean sensory attribute values significantly differed among samples. Pearson correlation coefficients showed that glucoamylase activity was positively correlated to both ethanol productivity and overall harmony (p<0.01). Thus, glucoamylase activity was identified as the best factor for screening Aspergillus strains for use in rice wine brewing.

Off-flavor compounds from decaying cyanobacterial blooms of Lake Taihu

The effect of cyanobacterial bloom decay on water quality and the complete degradation of cyanobacterial blooms in a short period were examined by an enclosure experiment in Gonghu Bay of Lake Taihu, China. Water quality parameters as well as taste and odor compounds during the breakdown of cyanobacterial blooms were measured. Results showed that the decay of cyanobacterial blooms caused anoxic water conditions, decreased pH, and increased nutrient loading to the lake water. The highest concentrations of dimethyl sulfide (DMS), dimethyl trisulfide (DMTS), and β-cyclocitral were observed in the anoxic water, at 62331.8, 12413.3, and 1374.9 ng/L, respectively. 2-Methylisoborneol was dominant during the live growth phase of cyanobacterial blooms, whereas DMS and DMTS were dominant during the decomposition phase. Dissolved oxygen, pH, and chlorophyll a were negatively correlated with DMS, DMTS, and β-cyclocitral, whereas total phosphorus, total nitrogen, and ammonium (NH4+-N) were positively correlated with DMS, DMTS, β-cyclocitral, and β-ionone. The experimental results suggested that preventing the anaerobic decomposition of cyanobacterial blooms is an important strategy against the recurrence of a malodor crisis in Lake Taihu.

In Vivo Solid-Phase Microextraction with in Vitro Calibration: Determination of Off-Flavor Components in Live Fish

The presence of off-flavor compounds in fish represents a significant economic problem encountered in aquaculture production. The off-flavor compounds are due to the absorption of substances produced by a range of microorganisms. Currently, a number of strategies have been used to prevent or limit the growth of these microorganisms. Therefore, it is important to evaluate the effectiveness of strategies via monitoring the concentrations of off-flavor compounds in the recirculating aquaculture system. In vivo solid-phase microextraction (SPME), a rapid and simple sample preparation method, will allow monitoring the concentration of off-flavor compounds in live fish. In this research, geosmin and 2-methylisoborneol (2-MIB) produced by cyanobacteria and actinomycetes, which are the major sources for "earthy" and "muddy" flavors in fish, were selected as representatives. In order to accurately quantify these compounds in fish muscle, two kinetic calibration methods, on-fiber standardization and measurement using predetermined sampling rate, were used as quantification methods, which were both validated by traditional methods. The detection limit of in vivo SPME in fish muscle was 0.12 ng/g for geosmin and 0.21 ng/g for 2-MIB, which are both below the human sensory thresholds.