Research On The Quality Characteristics And Storage And Preservation Of Bullfrogs

The bullfrog belongs to the genus Amphibia,order Anuranidae,family Frogidae,and is one of the most important aquatic products in China because of its excellent reproductive performance,fast growth rate,strong adaptability and low cost of breeding.Bullfrog meat is white and tender,tasty and nutritious,with high water content,high protein content and low fat content,and contains essential amino acids and polyunsaturated fatty acids,making it very popular among consumers.As consumers and companies are very interested in the quality characteristics of bullfrogs,and day-old is an important factor affecting the quality of bullfrogs,there is little research on the quality characteristics of bullfrogs,so it is important to study the quality characteristics of bullfrogs at different ages.In addition,bullfrogs contain a large number of microorganisms and endogenous enzymes,which are prone to spoilage during storage and can cause huge economic losses.There is also little research on bullfrog storage and preservation,and it is vital to study preservation techniques to effectively maintain the quality of bullfrogs.This thesis firstly investigated the food quality and nutritional quality characteristics of bullfrogs at different day old;secondly,the storage and preservation effects of different packaging methods（tray packaging,vacuum packaging and gas conditioning packaging）on bullfrogs were investigated to obtain the best packaging method;secondly,the storage and preservation effects of different preservation agents（sodium D-isoascorbic acid,Nisin and chitosan）on bullfrogs were investigated to screen out the best concentration of a single preservation agent.Subsequently,a response surface optimization design was used to further investigate the optimal compounding ratio of the three preservation agents.Finally,a comprehensive preservation technology combining the composite preservatives and packaging was developed to maintain the good quality and extend the shelf life of the bullfrogs.The specific research contents and results are as follows:（1）The food quality and nutritional quality characteristics of fresh bullfrogs at different ages were investigated.The results showed that the moisture content of bullfrog was 80.62±0.04%～81.43±0.06%,protein content was 16.82±0.14%～17.74±0.18%,and fat content was 0.77±0.03%～1.07±0.06%,bullfrog is a kind of aquatic product with high moisture,high protein and low fat.There were some differences in the quality characteristics of bullfrogs at different days of age.With the increase of days of age,the hardness and masticability of bullfrogs increased gradually,the cooking loss rate increased,the water retention decreased,the p H value decreased gradually,the protein content increased and decreased first,the fat content increased and then decreased,the total sugar content increased first and then decreased,and the protein and total sugar content of bullfrogs at D2 days of age were the highest.The protein of bullfrog is high quality protein and contains many essential amino acids required by human body,among which the protein quality of bullfrog at the age of D2 is the best.Fifteen kinds of free amino acids were detected in bullfrogs,among which the richer free amino acids were lysine,threonine,arginine,threonine and alanine.A total of23 fatty acids were detected in bullfrogs,and the percentage of unsaturated fatty acids in bullfrogs was higher than that of saturated fatty acids.The percentage of polyunsaturated fatty acids in bullfrogs increased and then decreased as the day age increased,with the highest percentage of polyunsaturated fatty acids in bullfrogs of D2day age.Compared with other day ages,the food value of D1-day bullfrogs was higher,and the protein content and quality,amino acid content,and percentage of polyunsaturated fatty acids and other nutrients of D2-day bullfrogs were higher.The quality of bullfrogs is better until 100～115 d.Continued breeding will increase the cost of breeding,so it is recommended to market bullfrogs at 100～115 d of breeding.（2）Taking bullfrog as the experimental object,the effects of different packaging methods such as pallet packaging,vacuum packaging and modified atmosphere packaging on the quality of bullfrog during storage were studied.The results showed that the total number of bacterium colonies,TVB-N value,TBARS value and juice loss rate of bullfrogs in different packaging methods showed an increasing trend with the extension of storage time.Among them,the total bacterial colony,TVB-N value,TBARS value and juice loss rate of bullfrogs in Control group increased the fastest,while tray packaging had a general inhibitory effect on the total bacterial colony and TVB-N value of bullfrogs,while vacuum packaging and air conditioning packaging could effectively inhibit the growth of microorganisms and lipid oxidation of bullfrogs,and delay the increase of total bacterial colony,TVB-N value,TBARS value and juice loss rate.It can also reduce the change of bullfrog p H value,brightness L^*value,whiteness value and sensory quality.The optimal packaging preservation method was60%CO₂/10%O₂/30%N₂in gas-conditioned packaging,which could extend the shelf life of bullfrogs from 2 d to about 5 d.（3）Taking bullfrog as the experimental object,under the condition of soaking in a given antistaling agent,the storage and fresh-keeping effect of different concentrations of sodium D-isoascorbate,Nisin and chitosan on bullfrog and the optimization of compound antistaling agent were studied.The results showed that different concentrations of sodium D-isoascorbate,Nisin and chitosan preservatives inhibited the growth of total bacterial colony,TVB-N value,TBARS value and juice loss rate of bullfrogs during storage,and delayed the decline of bullfrog quality.The best concentrations of sodium D-isoascorbic acid,Nisin preservative,and chitosan for bullfrog storage and preservation were 0.3%,0.05%,and 1.0%,respectively,based on the comprehensive physicochemical and microbiological indexes and sensory evaluation.The results of response surface optimization design showed that the interaction between sodium D-isoascorbate,Nisin and chitosan preservatives was more significant,and the best formulation of the composite preservative was 0.36%sodium D-isoascorbate,0.055%Nisin and 1.11%chitosan.The TVB-N value of bullfrogs in the group treated with the composite preservative was 9.45±0.17 mg/100 g at the 8th day of storage,and the relative error with the predicted value was only 0.79%.The fitted model was accurate and reliable.（4）Taking bullfrog as the test object,the effects of compound fresh-keeping agent and fresh-keeping technology combined with air conditioning package on the quality of bullfrog during storage were studied under the given soaking conditions.The results showed that both composite preservatives and composite preservatives combined with gas-conditioning packaging could inhibit the increase of bullfrog colony count,TVB-N value,TBARS value and juice loss rate,and maintain the sensory quality of bullfrog for a longer period of time.The combination of composite preservatives with gas-conditioning packaging enhanced the antibacterial and antioxidant effects on bullfrogs,which had a synergistic effect on delaying the spoilage of bullfrogs.The composite preservative and the composite preservative combined with gas-conditioning packaging also inhibited the oxidation of bullfrog myofibrillar protein,which was manifested by delaying the decrease of myofibrillar protein sulfhydryl content and solubility and the increase of carbonyl content and surface hydrophobicity.The shelf life of bullfrogs in Control group was only 2 d.The shelf life of bullfrogs in Control group could be extended to 8 d by the composite preservative and to 10 d by the composite preservative combined with gas-conditioning packaging,which has a certain potential for the storage and preservation of bullfrogs.