Geosmin and Off-Flavor in Nile Tilapia (Oreochromis niloticus)

Abstract

The concentration of geosmin (1,10-trans-dimethyl-trans-9-decaol) in fresh tilapia were analyzed by applying high-vacuum distillation, extraction and gas chromatography techniques. The distribution of geosmin in the fish was determined by examining tissues from various parts of tilapia, along with the rates of geosmin absorption and purging. Analysis showed that when geosmin was added to fish flesh at concentrations ranging from 2.8 to 390 μg per kg of flesh, the rate of recovery was 51 to 89%. Sensory evaluation detected muddy flavor in freshwater tilapia with geosmin content ranging from 7.55 to 9.85 μg/kg of fish flesh. However, panelists failed to detect muddy flavor in brackish water tilapia that contained geosmin only in the range of 1.5 to 2.6 μg/kg. Among various tissues of the fish, the intestines contained the highest geosmin concentration, and appeared in descending order in the abdominal, skin and muscle tissues. After holding for 2 h in water with 5 and 50 μg/l geosmin, tilapia absorbed 17.6 and 42.2 μg/kg geosmin in the flesh, respectively. Holding the fish over longer periods in water with higher geosmin content resulted in a greater amount of absorption. In contrast, transferring muddy-flavored fish to static clean water purged geosmin from the fish but it required at least 16 days to eliminate the muddy flavor.     

渔业水体中土腥异味物质的研究进展

综述了国内外渔业水体中土腥异味物质——2-甲基异莰醇（2-methylisoborneol,MIB）和土臭味素（Geosmin）的产源、发生特点、产生的环境影响因素、检测以及预测方法等。     

A review of cyanobacterial odorous and bioactive metabolites: Impacts and management alternatives in aquaculture

Increased demand has pushed extensive aquaculture towards intensively operated production systems, commonly resulting in eutrophic conditions and cyanobacterial blooms. This review summarizes those cyanobacterial secondary metabolites that can cause undesirable tastes and odors (odorous metabolites) or are biochemically active (bioactive metabolites) in marine and freshwater, extensive and intensive aquaculture systems. For the scope of this paper, biochemically active metabolites include (1) toxins that can cause mortality in aquaculture organisms or have the potential to harm consumers via accumulation in the product (hepatotoxins, cytotoxins, neurotoxins, dermatoxins, and brine shrimp/molluskal toxins), (2) metabolites that may degrade the nutritional status of aquaculture species (inhibitors of proteases and grazer deterrents) or (3) metabolites that have the potential to negatively affect the general health of aquaculture species or aquaculture laborers (dermatoxins, irritant toxins, hepatotoxins, cytotoxins). Suggestions are made as to future management practices in intensive and extensive aquaculture and the potential exposure pathways to aquaculture species and human consumers are identified.     

顶空固相微萃取-气质联用测定水中嗅味化合物

[目的]建立顶空固相微萃取-气质联用测定水中痕量嗅味化合物的方法。[方法]水样中加入氯化钠,顶空固相微萃取出水中2-甲基异冰片、土臭素两种嗅味化合物,气质联用测定其含量。[结果]建立的2-甲基异冰片、土臭素标准曲线的回归方程分别为y=5 923.0x-199.5、y=21 346x+5 851.8,相对标准偏差分别为8.9%、8.7%,相关系数均大于0.99;加标回收率在90%~110%,表明方法准确、可靠。2012年7月初、8月初测定杭州饮用水原水中2-甲基异冰片浓度较高,达60 ng/L以上,土臭素浓度都较低。[结论]固相微萃取具有简单、快速、萃取效率高、灵敏度高等特点。该方法线性良好,最大偏差10%,符合水中痕量2-甲基异冰片、土臭素的测定要求。     

Effects of swimming speed and dissolved oxygen on geosmin depuration from market-size Atlantic salmon Salmo salar

Common off-flavor compounds, including geosmin (GSM) and 2-methylisoborneol (MIB), bioaccumulate in Atlantic salmon Salmo salar cultured in recirculating aquaculture systems (RAS) resulting in earthy and musty taints that are unacceptable to consumers. To remediate off-flavor from market-ready salmon, RAS facilities generally relocate fish to separate finishing systems where feed is withheld and makeup water with very low to nondetectable GSM and MIB levels is rapidly exchanged, a process known as depuration. Several procedural aspects that affect salmon metabolism and the associated rate of off-flavor elimination, however, have not been fully evaluated. To this end, a study was carried out to assess the effects of swimming speed and dissolved oxygen (DO) concentration on GSM levels in water and fish flesh during a 10-day depuration period. Atlantic salmon (5–8 kg) originally cultured in a semi-commercial-scale RAS (150 m³ tank) were exposed to a concentrated GSM bath before being transferred to 12 replicated partial reuse depuration systems (5.4 m³ total volume). Two swimming speeds (0.3 and 0.6 body lengths/sec) and two DO levels (90% and 100% O₂ saturation) were applied using a 2 × 2 factorial design (N = 3), and each system was operated with a 5-h hydraulic retention time, creating a water flushing to biomass ratio of 151 L/kg fish biomass/day. Geosmin was assessed at Days 0, 3, 6, and 10 in system water and salmon flesh. A borderline effect (P = 0.064; 0.068) of swimming speed was measured for water and fish, respectively, at Day 3, where slightly lower GSM was associated with low swimming speed (0.3 body lengths/sec); however, differences were not detected at Days 6 or 10 when salmon are commonly removed for slaughter. Overall, this research indicates that significant improvements in GSM depuration from RAS-produced Atlantic salmon are not expected when purging with swimming speeds and DO concentrations similar to those tested during this trial.     

炉渣污泥合成陶瓷吸附剂去除饮用水中土臭素的研究

探究了炉渣和污泥高温合成陶瓷吸附剂去除水中土臭素(geosmin,GSM)的能力,结果表明,陶瓷吸附剂对GSM的吸附行为可采用准一级动力学方程来拟合。GSM去除率随着反应时间增加而提高,在10 h接触时间内,陶瓷吸附剂对浓度200 ng L~(-1)和600 ng L~(-1)的GSM去除率分别是81.5%和76.4%。当pH=7,合成陶瓷吸附剂对GSM的去除效果最佳,去除率为82.3%。随着合成陶瓷吸附剂剂量增加,GSM的去除率呈现明显上升的趋势,当合成陶瓷吸附剂的投加量达到2 g L~(-1)时,GSM的去除率最大。Freundlich等温吸附方程模拟合成陶瓷吸附剂的GSM吸附性能优于Langmuir模型。陶瓷吸附剂具有较好pH缓冲性能力和再生性能,经5次再生的陶瓷吸附剂GSM去除率为76.9%。综上所述,合成陶瓷吸附剂GSM去除率高,生产成本低,是一种高效的、可循环使用的绿色GSM吸附剂。     

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 (20–40 mg/L and 80–100 mg/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.     

非均相UV/Fenton光催化降解土霉异味研究

[目的]探索非均相UV/Fenton光催化降解土霉异味的效果。[方法]利用离子交换方法将Fe2+负载在NaY分子筛载体上,制得催化剂FeY。在不同紫外波长照射下,利用Fenton反应降解2种土霉异味物质2-甲基异莰醇(MIB)和土腥素(Geosmin),优化pH和H2O2等降解条件,并将MIB和Geosmin添加到东湖本底湖水中进行降解。[结果]FeY的负载量为352.8 mg/g,Fe2+脱附率为5.7%。在FeY为28 mg/L,pH 6.5,H2O220 mg/L和反应60min的试验条件下,非均相UVB/Fenton体系对MIB和Geosmin的降解率分别为80.2%和84.9%。在UVA、UVB和UVC紫外光(波长分别为365、312和256 nm)条件下Photo-Fenton体系对MIB和Geosmin的降解率,随着紫外波长的降低而增大,且Geosmin降解速率常数高于MIB。湖泊水样中加入MIB和Geosmin降解表明,降解效率明显低于纯水样品。[结论]该研究制得的催化剂应用于非均相光催化体系,不仅可循环使用,而且还可扩大反应体系的pH应用范围。     

Development of phytoplankton communities and common off-flavors in a biofloc technology system used for the culture of channel catfish (Ictalurus punctatus)

The use of biofloc technology production systems continues to increase in the aquaculture industry worldwide. Recent research demonstrated that outdoor biofloc systems can be used to produce high yields of channel catfish (Ictalurus punctatus). However, studies have not yet been performed to determine the development and composition of phytoplankton communities and related off-flavor problems in these biofloc production systems. In this study, water samples were collected biweekly from May to November and channel catfish samples were collected during harvest in November from nine 18.6 m² biofloc culture tanks. Water and fillet samples were analyzed for levels of the common off-flavor compounds geosmin and 2-methylisoborneol (MIB). The development and composition of phytoplankton communities in each culture tank was also monitored. In addition, water and biofloc samples were evaluated to assess the microbial sources of geosmin and MIB within the culture tanks. Phytoplankton (including algae and cyanobacteria attached to bioflocs) biomass, as determined by concentrations of chlorophyll a in the water, gradually increased in all tanks over time. Phytoplankton communities that developed in the culture tanks were dominated by fast-growing, unicellular and small colonial types of green algae (chlorophytes) and diatoms (bacillariophytes) and slower growing, small colonial types of cyanobacteria (cyanophytes). A positive correlation (p < 0.05) between cumulative feed addition and chlorophyll a concentration was found. Although geosmin and MIB were present in the culture water of each tank during most of the study, levels were typically low and only one tank yielded catfish with geosmin and MIB in their flesh at levels high enough to be designated as off-flavor. A positive correlation (p < 0.05) between cumulative feed addition and MIB concentrations in the water of culture tanks indicates a greater potential for MIB-related off-flavor problems at high feed application rates. The microbial sources responsible for production of geosmin and MIB in the culture tanks remain unknown.