| As the living standards improving continuously, human demands for aquatic products arealso increasing worldwide. At the same time, consumers pay more attention to the qualityof the aquatic products, especially the taste of fish. The off-flavour of fresh fish isincreasingly becoming the concern of farmers and aquatic products processing plant. Theoff-flavour problem is linked to earthy/musty odours and tastes, due to significantconcentrations of geosmin or2-methyl-isoborneol (2-MIB) in fish that produced in thealgal metabolic processes, such as Oscillatoria, Anabaena and Lyngbya in cyanophyta.In this study, the tilapia cultured in Zhangjiang city were used as raw material, thesampling time began when the tilapia juveniles were put into pond and ended until thespecifications achieved market standards. The samples of water quality, planktonic algae,tilapia were collected from plastic film substrate, soil substrate and cement substrate pondrespectively. The correlation was studied between the analysis of the types and number ofphytoplankton, the changes in water quality and the content of off-flavour compounds infarmed tilapia. Homemade microwave-assisted distillation with open-loop strippingequipment was used to extract the earthy/musty substances in the flesh, K-D concentrationdevice was used to concentrate the extract. To achieve the best results, four parameters onthe extraction device were set. Gas chromatography-mass spectrometry measure (GC/MS)was used for qualitative and quantitative analysis of Geosmin and2-MIB.Experimental results show that:1. The set of parameters on microwave-assisted distillation with open-loop strippingequipment: Microwave heating power is400w, heating time is7min, nitrogen purge flowrate is100mL/min.2. Elution factor selection: The amount of powder activated carbon is4.5g, elution time is40min, eluent is ether.3. The correlation between changes in water quality and content of earthy/mustysubstances in tilapia flesh: As the contents of ammonia nitrogen, nitrite nitrogen achieved the highest levels, the growth rate of tilapia was also the fastest. During this period, theamount of Geosmin and2-MIB per unit mass of tilapia flesh achieved maximum level.4. The correlation between algal species, number changes and earthy/musty substances intilapia flesh: The species and amount of phytoplankton achieved maximum levels fromJuly to September when the growth rate of tilapia was also fast. During this period, thealgae that can produce Geosmin and2-MIB in its own metabolic processes appeared mostin the water bodies, such as Anabaena spiroides, Anabaena circinalis, Anabaenopsis,Oscillatoria tenuis, Oscillatoria agardhii and Lyngbya. All these algae that could prodeceearthy/musty substances were found commonly in a large number during this period,especially in soil substrate pond. This was regarded as the main reason why the tilapiacultured in soil substrate pond smells most unpleasant.5. The correlation between the substrate of pond, the specifications of tilapia and thecontents of earthy/musty substances in tilapia flesh: As the body weight increasing, thechanges of earthy/musty substances’ contents in tilapia farmed in three different sedimentpond, reflected the same trend: the contents of Geosmin and2-MIB increased continuously.This trend indicated that earthy/musty substances were enriched by tilapia through gillsand skin in water environment. In different stages, significant differences were foundamong the contents of earthy/musty substances in tilapia farmed in three different sedimentpond. The Geosmin and2-MIB contents in tilapia polycultured in brackish water withshrimp in cement pond were significantly lower than the other two ponds with differentsediment. The Geosmin and2-MIB contents of tilapia cultured in plastic film substratepond were significantly lower than that in soil substrate pond. This result can be explainedby the significant difference among the species and quantities of phytoplankton in differentsediment ponds.6. Optimal holding conditions to remove earthy/musty substances in tilapia flesh: Thesalinity of holding water was3‰, water exchange capacity was15.886L/h, holdingtemperature was33℃and holding time was24h.7. Artificial sensory evaluation to tilapia flesh treated by holding: Artificial sensoryevaluation value to the cooked fish showed that tilapia treated in optimal holdingenvironment had reached the consumers’ acceptable range. This result indicated that theexperimental design of holding factors had significant effects on the removal ofearthy/musty substances in tilapia flesh. |
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