| Chinese grass turtle is a kind of turtle that can be used as both medicine and food.Its shell has high medicinal value and is the raw material for making tortoisesheles glue,while its meat is the by-product of tortoisesheles processing.In this paper,ultrasonic brine was used to produce instant turtle meat.Response surface optimization was used to obtain the optimal ultrasonic brine process,and the protein digestibility was discussed.The research results could provide a basis for the utilization of turtle meat by-products.The effects of ultrasonic time,power and temperature on the sensory quality of ready-to-eat turtle meat were investigated.With the increase of ultrasonic time and power,the hardness,chewability and L value of ready-to-eat turtle meat showed a decreasing trend,while the elasticity and sensory scores increased.With the increase of marinating temperature,the hardness and adhesiveness of turtle meat gradually decreased,and the values of a*and b*increased.Principal component analysis was carried out on the sensory quality indexes of ready-to-eat turtle meat,and it was concluded that the correlation between the sensory scores and the texture and color indexes was the greatest.Therefore,the sensory scores were selected as the response values to optimize the process.Quadratic multinomial regression equation is obtained as follows:sensory score=88.38+A+B+0.25*0.35*0.38*0.23*A*B-C-0.025*A*C+0.32 to 1.43*A*B*C-^ 2-1.63*B ^ 2-1.98*C ^ 2.The order of influence of each factor on sensory score was as follows:A ultrasonic power>C temperature>B Ultrasonic time,the optimal process is:ultrasonic power 150 W,ultrasonic time 110 min,temperature 90℃.The effect of ultrasonic brine on the flavor of instant turtle meat was studied.The content of free fatty acids in the three groups of fresh sample,no ultrasonic brine and ultrasonic brine was determined.It was found that the type and content of free fatty acids in the ultrasonic brine group were the highest.Headspace solid phase microextraction was used to analyze the flavor substances produced by turtle meat.After ROAV screening,97 kinds of volatile flavor substances were found.In the ultrasonic group,the number of volatile flavor substances was the largest in the ultrasonic halogen-free and halogenated turtle meat,and the increased flavor substances were mainly aldehydes,hydrocarbons and ketones.It can improve the overall flavor of turtle meat.In the fresh sample group,the four substances with the highest ROAV value were 1-octene-3-ol,trans-2-decenoaldehyde,nononal and 1-octenol,which mainly presented the unpleasant odor of turtle and soil.Non-ultrasonic halogen-free group:nonaldehyde,2-n-amylfuran,decylaldehyde,citronella aldehyde,contributing oil scent,green scent,etc.Ultrasonic halogen-free group:trans-2-nonenal,nonal,decaldehyde,2-n-amyl furan,mainly aldehyde,presented oil fragrance and grass smell;No ultrasonic halogen group:anisole,p-methoxybenzaldehyde,citral,nonaldehyde,showing spices in the halogen,as well as the green flavor of turtle meat itself;Ultrasonic halogen group:p-methoxybenzaldehyde,anisole,citral,linalool,all from the halogen,showing halogen flavor;The results indicated that ultrasonic brine could promote the infiltration of the brine into the turtle meat,improve the flavor of instant turtle meat and enrich the volatile flavor of instant turtle meat in the ultrasonic group.It was found that using ultrasonic brine to prepare instant turtle meat could reduce the oxidation and hardness of myofibrillar protein of turtle meat and increase the yield of turtle meat.Compared with the control group,the microstructures of muscle fibrillar tissue of ultrasonic brined instant turtle meat showed increased fracture,and a large number of gaps appeared in the muscle fibrillar bundle,which was more conducive to the penetration and retention of the brine,and thus improved the yield,texture and flavor of instant turtle meat.Ultrasonic brining can improve the in vitro digestibility of ready-to-eat turtle meat protein.Compared with the control group,it was found that the digestibility of pepsin treatment in the ultrasonic group was significantly increased(P<0.05),increased by 6.70%,and the digestibility of gastric+trypsin treatment was also significantly increased(P<0.05),increased by 6.64%.At the same time,the degree of hydrolysis of pepsin digestion products of turtle meat after ultrasonic brine was increased by 23.95%compared with the control group,and the degree of hydrolysis of protein digestion products of stomach+trypsin was increased by 41.07%compared with the control group.The concentrations of peptides in pepsin digestion products and gastric trypsin digestion products in the ultrasonic group were also significantly higher than those in the control group(P<0.05),up to 6.50 mg/mL and 9.12 mg/mL respectively.SDS-PAGE of turtle meat digestion products showed that the ultrasonic histone bands were mainly clustered below 40 kDa in the protein bands digested by pepsin.After further digestion by trypsin,the ultrasonic protein bands almost disappeared in the protein bands above 35 kDa,and the color was lighter than that in the control group.The protein digestibility of ultrasonic group was higher than that of control group.The analysis of amino acid composition and content of turtle meat digestive products showed that the amino acid content of pepsin digestive products and gastric+trypsin digestive products in ultrasonic group was higher than that in control group,and the content of antioxidant amino acids in ultrasonic group was also higher than that in control group.It was found that the DPPH· clearance rate,ABTS+scavenging ability and IC50 value of Fe2+ chelating ability of extracorporeal digestive products in the ultrasonic group were lower than those in the control group,indicating that ultrasonic treatment had an enhanced effect on the antioxidant activity of extracorporeal digestive products of turtles.According to the nutritional evaluation of protein amino acids of turtle meat,the essential amino acid index of instant turtle meat protein produced by ultrasonic brine is up to 98.97,which is a high quality protein source with high nutritional value.To investigate the quality and microbial changes of instant turtle meat under refrigeration conditions,it was found that the changes of water retention,pH value,volatile base nitrogen content and thiobarbituric acid content of instant turtle meat in the ultrasonic group could maintain a stable state compared with the control group,indicating that ultrasonic brine can effectively delay the occurrence of rashidity and bad flavor of instant turtle meat in the storage process.The results showed that the total number of colonies in the instant turtle meat in the control group reached 5.09 lg CFU/g on the 16th day of refrigeration,while the total number of colonies in the ultrasonic group reached 5.08 Ig CFU/g on the 24th day of refrigeration,exceeding the national standard limit of 5 lg CFU/g.Compared with the control group,the total number of colonies,coliform and chillophilic bacteria in the ultrasound group were significantly reduced(P<0.05),indicating that ultrasonic treatment can effectively prolong the cold storage period of instant turtle meat.The correlation analysis of the cold storage time,quality and microbial changes of the instant turtle meat during the cold storage period showed that the cold storage time of the instant turtle meat in the control group and the ultrasonic group was significantly positively correlated with volatile base nitrogen,thiobarbituric acid,total number of colonies,coliform,mold and chillophilic bacteria(P<0.05),were not significantly correlated with water retention and pH(P<0.05),indicating that the cold storage time significantly affected the quality and microbial status of ready-to-eat turtle meat.At the same time,cluster analysis showed that the quality and microorganism of instant turtle meat in each group began to change significantly from the 12th day of refrigeration to the 8th day of refrigeration in the ultrasonic group,which further proved that instant turtle meat after ultrasonic brine was beneficial to prolong its storage period. |
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