Effects Of Salinity And Alkalinity On Flesh Quality And Metabolism Of Related Substances Of Oreochromis Niloticus

Tilapia is one of the world culture fish.China is a major tilapia farming country.Currently in China,tilapia is mainly cultured in freshwater in the south.However,tilapia cultured in freshwater generally have shortcomings,such as soft flesh,poor taste,and strong soil odor,which restrict their consumption and export.In order to understand the effect of water salinity and alkalinity on flesh quality of tilapia,the difference of the muscle quality of the tilapia in the fresh water culture and the saline-alkaline water culture is analyzed from two aspects of the sensory and the nutrition.The comparative effects of salinity and alkalinity on the nutrition and flavor quality of Nile tilapia were compared.The effects of high unsaturated fatty acid metabolism and proline metabolism-related enzymes on the activity and gene expression of Nile tilapia were compared.The main results are as follows:(1)Flesh quality of tilapia in Maoming freshwater culture(FW)and saline-alkaline water culture(SAW)in Cangzhou were compared and analyzed from sensory and nutritional aspects.The muscle hardness,chewiness,elasticity and adhesion of tilapia cultured in saline-alkaline water were significantly higher than those in fresh water culture group(p < 0.05),which was consistent with the results of sensory evaluation that muscle elasticity of tilapia was stronger than that of fresh water group.The total content of umami taste amino acid(19.44mg/100g)and sweet taste amino acid(121.40mg/100g)and the total of free amino acids(175.13mg/100g)of tilapia muscle in SAW group were significantly higher than FW group(p < 0.05).The contents of GSM(geosmin)and 2-MIB(2-methylisoborneol)in tilapia muscle and pond in SAW group were significantly lower than those in FW group(p < 0.05).The crude protein content(19.74%)of muscle of tilapia in SAW group was significantly higher than that in FW group(p < 0.05),other basic nutritional parameters were not significantly different.The ratio of the amount of essential amino acids to the total of amino acids and the ratio of essential amino acids to non-essential amino acids of tilapia cultured in SAW group and in FW group respectively were 41.76 and 71.69% and 41.52% and 70.99%.The amino acid composition of the muscle of tilapia cultured in two water environments all accord with the ideal model of FAO/WHO,It belongs to high quality protein source.The contents of eicosapentaenoic acid and docosahexaenoic acid(EPA+DHA,5.77%)and fatty acid n-3/n-6 ratio(0.57)were significantly higher than those of fresh water culture(p < 0.05).Therefore,the nutritional value and sensory characteristics of tilapia muscle cultured in saline-alkaline water were obviously better than those in fresh water culture.(2)The effects of salinity and alkalinity on the nutrition and flavor quality of Nile tilapia were compared.esult show that Compared with fresh water group,salt and alkalinity had no significant effect on specific growth rate(SGR)and feed coefficient(FCR)of body weight gain rate(WGR)of fish(p > 0.05),but significantly affected the content of muscle nutrients and flavor substances in fish body(p < 0.05).The content of crude protein in muscle of FW group was the highest,that of SAW group was the lowest,and that of SAW group was significantly lower than that of other groups(p < 0.05).The index of essential amino acids in FW group was significantly lower than that in other groups(p < 0.05).The index of essential amino acid in SW group and AW group was significantly higher than that in SW group and AW group.The total fresh amino acids in AW group,SAW group and SW group were 1.78 times,1.74 times and 1.63 times higher than those in FW group,respectively.The total contents of sweet amino acids and free amino acids in SAW group were significantly higher than those in SW and AW groups(p < 0.05).The saturated fatty acid(SFA)and monounsaturated fatty acid(MUFA)in AW and saw groups were significantly higher than those in SW group(p < 0.05),but there was no significant difference in polyunsaturated fatty acid(PUFA)between treatment groups(p > 0.05).But the total amount of EPA+DHA in SAW group was significantly higher than that in SW and AW group(p < 0.05).The results showed that water salt and alkalinity could effectively improve the flesh quality of tilapia,and the effect of alkalinity on the flavor taste of fish was more obvious.(3)In order to investigate the effects of water saline and alkaline stress on the expression and enzyme activity of metabolism-related genes of in high unsaturated fatty acids and proline.The gene expression and enzyme activity of ELOVL5,?6 FAD,?-OAT,P5 CS and PDH were determined.The results showed that the activity of ELOVL5 and?6 FAD enzyme and the expression of mRNA in fish liver of SAW group were significantly higher than those of AW and SW groups(p < 0 05).There was no significant difference between SW and AW groups(p < 0 05).The activity enzyme and mRNA expression of P5 CS in SW group were significantly higher than those in SAW and AW groups,while the enzyme activity and mRNA expression level of PDH in SAW and AW groups were significantly lower than those in SW group(p < 0.05).It can be seen that the difference of the influence of salinity and alkalinity on the quality of tilapia may be due to the salt in water.Alkalinity can improve the expression and enzyme activity of metabolism-related genes and increase the synthesis and accumulation of related substances through different ways of regulating and controlling the quality of tilapia.