The Key Water Quality Factors Affecting The Flavor Quality Of Grass Carp(ctenopharyngodon Idella)and Its Influencing Mechanism

The aquaculture sector is undergoing a change from a single increase in production in the past to a sort of improving quality and efficiency with the continual improvement of China's economic level and the development needs of fishery transformation.Consumers are concerned about the rise in the quality of farmed fish.Intensive culture based on high-density stocking is an essential method of pond fish production in China,however it will degrade water quality and have an impact on the quality of farmed fish.Currently,most research on improving fish quality is focused on fish nutrition and feed,but there are few studies on the influence of water quality factors on fish quality.Grass carp(Ctenopharyngodon idella),as the largest freshwater fish in China,the water body is easy to deteriorate in the process of culture,affecting the nutritional value,flavor and other quality of grass carp,thus affecting the market economy value of grass carp and restricting the sustainable development of the industry.Therefore,this study intends to screen the key water factors that affect the flavor and nutritional value of grass carp and explore their influencing mechanism.Two-stage experiments were designed in this study using grass carp as the research object: in the first stage,four different culture models were designed to establish different culture water environments.Six water quality parameters(nitrate,nitrite,ammonia nitrogen,phosphate,total nitrogen,total phosphorus)were evaluated on a regular basis,and grass carp muscle nutrients and volatile taste chemicals were also found.Correlation analysis was used to screen the key ecological factors affecting the nutritional value and flavor quality of grass carp.In the second stage,on the basis of the results of the previous stage,the selected key ecological factors were used in the culture experiment with gradient treatment.after treatment,the growth performance and muscle nutritional composition of grass carp were determined,and metabonomics was used to explore the different metabolites and metabolic pathways of grass carp under different treatments,and the possible influence mechanism was analyzed.The main findings are as follows:(1)In the first stage,four experimental groups were designed,which were commercial feed group(control),faba bean treatment group(FBT),grass powder treatment group(GPT)and waving water with commercial feed treatment(WWT).The effects of water quality and feed(eaten and uneaten)on the meat quality and nutrition of grass carp were studied.During the culture period(90 days),the water quality factors of each group(nitrate,nitrite,ammonia,phosphate,total nitrogen,total phosphorus)were measured every 15 days,and the flavor quality was evaluated by volatile flavor compounds detection and electronic nose analyzer.Meanwhile,the contents of crude protein,crude fat,free fatty acids and free amino acids in grass carp muscle were determined.The results showed that in the later stage of culture period,the water quality of FBT model was the worst,the concentration of nitrite and nitrate accumulation was the highest,while the water quality of GPT model was the best.The volatile compounds with high activity detected in the control,GPT and WWT models include 1-octene-3-ol,nonanal,hexanal,decanal,etc,and the flavor properties of these volatile compounds are pleasant.On the other hand,in the FBT model with the worst water quality,compared with the other three models,the high activity volatile compounds are mainly E-2-octenal,E-2-decenal,E-butanol,E-2-nonenal,(E,E)-2,4-nonadienal,E-2,4-decadienal,and most of these substances are fishy and fatty.The water content of FBT group was higher,while the contents of crude protein,crude fat,free fatty acid and free amino acid were lower.Correlation analysis showed that in addition to edible feed and uneaten feed,nitrite and nitrate may be the key water quality factors affecting flavor quality and nutritional value.(2)Water quality factors are closely related to the quality of grass carp.In the previous stage,nitrite and nitrate were selected as important water quality factors that may affect the quality of grass carp.The aim of this study was to investigate the effects of nitrite and nitrateon the growth performance,nutritional composition and flavor metabolism of grass carp under different concentration gradients,and to analyze the potential mechanism of the effects of nitrite and nitrate on the growth performance,nutritional composition and flavor metabolism of grass carp(Ctenopharyngodon idellus)under different concentration gradients.The results showed that low concentration of nitrite(0.1?0.5mg/L)had no effect on the growth performance of grass carp,but under the treatment of nitrite of 1mg/L,the weight gain rate and fatness were the lowest,and the feed coefficient was the highest.The growth performance of grass carp decreased under different concentrations of nitrate treatment.Nitrite and nitrate treatments decreased the contents of crude protein,crude fat,carbohydrates and amino acids in grass carp muscle(P < 0.05),and significantly increased water content.The concentrations of smelly and 2-methylisoborneol in muscle of grass carp treated with 1mg/L nitrite were the highest.The results of metabonomic analysis showed that both nitrite and nitrate had effects on metabolic process,and principal component analysis showed that nitrite was a more important water quality factor than nitrate.Nitrite treatment may lead to more significant changes in metabolic pathways.Nitrite treatment mainly affects amino acid metabolism and protein digestion and absorption pathway.The mechanism of nitrite affecting flavor quality may be through down-regulating the synthesis of arginine and the metabolic pathway of glutamic acid and proline.Overall,nitrite and nitrate in water are the key water quality factors affecting the nutritional value and flavor quality of grass carp,among which nitrite can cause more significant metabolic changes.The mechanism of nitrite affecting flavor may be to accelerate grass carp to absorb soil odor substances from the outside and affect arginine biosynthesis metabolic pathway in grass carp.In the past,there are few studies on the effects of water ecological factors on fish quality,and the findings of this paper can provide reference materials for improving the quality of freshwater farmed fish from the source of culture.