Physicochemical Properties And Flavor Variation Of Grass Carp During Chilled Storage

Grass carp is the main freshwater fish in China.Owing to well taste,riched in unsaturated fatty acid,high-quality protein,vitamins and a variety of minerals,grass carp is one of the favorite kinds of fish in China.But grass carp is easy to be corrupted after slaughtered,which increases the certain difficulties to the process of handling and processing because of its facial mucus,strong protease activities,high moisture and protein contents and so on.Freshness and flavor are important indices to evaluate the quality of fish products,a series of physiological changes of grass carp muscle after the slaughter is the main cause of the change of fish products freshness and flavor.Therefore,research on physiological changes of grass carp muscle during storage is particularly important to provide theoretical basis for flavor changes of grass carp during storage.This study on the change of physical and chemical index and flavor of grass carp dorsal and abdominal muscles under the chilled storage.Water content,water loss rate,meat color,quality and structure properties(elastic,hardness,cohesiveness,viscous,chewiness,resilience),p H,glycogen content,lactic acid content,structural protein content,cathepsin and correlative ATP content were analyzed and flavor was tracked by the electronic nose at each time point in order to clarify the connection between physiological change rule and flavor deterioration of grass carp during storage.To further clarify the physiological changes of grass carp muscle during storage,the specific research results are as follows:(1)With the extension of storage time(0 to 8 d),the moisture content was on the decline from 81.13%,80.43% of fresh grass carp dorsal,abdominal muscle to 79.02%,79.02% 8 d after slaughter.The filtration rate of the dorsal and abdominal muscles of fresh grass carp were 4.85% and 4.85% respectively.With the extension of storage time,the filtration rate rise 19.14% and 19.31%8 d after respectively.Whiteness value changed obviously in dorsal muscles of grass carp but not for abdominal muscles,and the value was increased from 29.76%,55.25% of fresh grass carp dorsal,abdominal meat to 57.24%,59.83% 8 d after,slaughter,with the rise of 48.01%,7.66% respectively.Structure features of elasticity,hardness,stringiness,cohesiveness,chewiness and resilience were declining,and declined fast after 1 d,then changed slowly.In Conclusion,the change of physical index cannot be a better judge of the improvement of the grass carp muscle changes,but it canspeculated 1 d and 5 d may be as two important time points of the improvement of the grass carp muscle changes.(2)For chemical indicators,the p H of dorsal muscle in the fresh grass carp was 6.58 and the value of p H is 6.61 in abdominal muscle.When grass carp stored up to 6 days,the p H reached the lowest value.The p H of dorsal muscle was 6.37 and the value of p H is 6.34 in abdominal muscle.Then,the p H increased slowly.When grass carp stored up to 8 days,the p H of dorsal muscle was 6.40 and the value of p H is 6.37 in abdominal muscle.The glycogen content of dorsal muscle in the fresh grass carp was 12.24 mg/g and the value of glycogen content is12.27 mg/g in abdominal muscle,which decreased quickly within 1 day.dorsal muscle declined by up to 99.9% and abdominal muscle was 99.8%.The lactic acid content of dorsal muscle was 28.47mg/g and then increased to a maximum of 83.59mg/g within 1 day.The lactic acid content of abdominal muscle was 30.85mg/g and then increased to a maximum of 81.95mg/g within 1 day.Increasing amplitude of dorsal muscle was 66.74 percent and abdominal muscle was 64.92%.However,lactic acid content in dorsal muscle and abdominal muscle decreased to 40.97mg/g and 43.67mg/g quickly after 1 day,which declined by up to 38.61% and 32.73%.With the increase of storage time,the contents of three kinds of cathepsin(A,B,L)were all increased.The contents of fibril protein indorsal muscle of fresh grass carp was 68.28mg/g and abdominal muscle was 81.39 mg/g.The contents of plasma protein in dorsal muscle and abdominal muscle were 24.31mg/g and 22.28mg/g.The contents of Muscle matrix were 16.23 mg/g?15.04 mg/g.With the increase of storage time,three kinds of Structural protein were decreased gradually.(3)The ATP contents in dorsal muscle and abdominal muscle of the grass carp were444.92 mg/kg and 344.79 mg/kg.Then they increased to 479.34% and 383.56% within 1 day,which decomposed rapidly.The contents of adenosine triphosphate(ATP)were decreased quickly and remain at a low value subsequently,which declined by up to 65.87% and 48.61%.The contents of adenosine(AMP)increased rapidly within 2 days,which can suppress the bitter taste.Subsequently,it decreased quickly.When grass carp stored up to 8 days,AMP in dorsal muscle and abdominal muscle declined by up to 98.81% and 98.41%.Umami substance content of inosinic acid(IMP)increased gradually from 0 day to 5 days.5 days latter,it decreased sharply.Dorsal muscle and abdominal muscle declined by up to 93.44% and88.93%.The contents of bitter substance like Hx and Hx R in dorsal muscle and abdominalmuscle increased with the extension of storage time.Through calculating K value of freshness,we can conclude that the value of K in dorsal muscle and abdominal muscle were 49.71% and49.28% on the sixth day.During these days,grass carp hadn't rotted yet.Until the seventh day,the value of K in dorsal muscle was 66.49% and abdominal muscle was 71.17%.At this time,the color of the fish was beginning to change and produce bad odor.Thus,it can be concluded that the cold storage shelf life of grass carp is 6 days.(4)By using electronic nose to analysis fish samples,it can be concluded that sensor response signal of alkanes,aromatics,ammonia and alkanes are zero in both dorsal and abdominal muscle during cold storage.However,the content of the organic sulfur,nitrogen oxides,sulfides and methane are changed steady within 6 days.Then they had a sharply increase 6 days later.It shows that the sixth day is the key time in protein degradation and lipid oxidation.The rate of contribution of PC1 and PC2 account for 97.74% in dorsal muscle,which are 98.74% in abdominal muscle.So,both of two principal components can reflect the sample's information accurately.During cold storage process,different parts of grass carp basically have the same volatile substances,which mainly are organic sulfide,nitrogen oxides,sulfides and methane classes.The rate of contribution of LD1 account for 66.08% in dorsal muscle while LD2 is 18.65%.The total rate of contribution account for 84.73%.The rate of contribution of LD1 account for 58.82% in abdominal muscle while LD2 is37.53%.The total rate of contribution account for 96.35%.Thus,through the analysis of electronic nose,it can be concluded that grass carp's shelf life is 6 days under the condition of cold storage.