Effects Of Thermal Processing On Edible Quality And Protein Digestion Characteristics Of Channel Catfish

Channel catfish is a kind of delicious freshwater fish without inter-muscular spines,thus convenient to eat.In China,catering industry and family consumption are responsible for the growing sales volume of this fish which has been commonly served after cooking by roasting,steaming,boiling methods or frozen prepared products in recent years.Now,domestic and foreign researches on the thermal processing of channel catfish were still mainly focused on the impact on the sensory quality,lacking in-depth research on the impact on nutritional quality.In the context of general health,with consumers’ attention to food nutrition and health,the research on the impact of thermal processing on the nutritional quality of catfish has increasingly become the demand of the times.Therefore,this study will introduce an in vitro simulated digestion model to systematically analyze the effects of different thermal processing methods and degrees on the sensory and nutritional qualities of channel catfish,and explore the main reasons that affect the degree of protein digestion in order to provide more suitable application parameters for further thermal processing.1.The effects of different thermal processing methods on the edible quality of channel catfish were studied.Fresh channel catfish was used as raw material,and processed by three common thermal processing methods of boiling,steaming and roasting until the center temperature reached 40 ± 0.2,50 ± 0.2,60 ± 0.2,70 ± 0.2,80 ± 0.2,90 ± 0.2°C.The changes in the eating quality of fish were studied by measuring the texture,color,taste,flavor and sensory evaluation of it.The results showed that: with the continuous increase of the central temperature,the texture of catfish was gradually destroyed,and steaming had the least negative effect on the texture of catfish among the three commonly used thermal processing methods.Foods need to be cooked to ensure the eating safety,when heat to the center temperature of 70℃,the sensory quality of fish meat was better,and the sensory evaluation score was higher,too.After heat treatment,the number of flavor compounds in the fish meat increased from only 28 in raw meat to more than 38.After refrigerated storage,the texture of reheated fish deteriorated more severely,and the content of TVB-N increased significantly with the prolongation of storage time,indicated that its freshness continued to decline.2.The effects of different thermal processing methods on the nutritional quality of catfish were studied.Different heat processing methods and processing degrees were set and the simulated in vitro digestion model was introduced to measure the changes in basic nutrients,amino acid analysis,protein digestibility,protein particle size,degree of hydrolysis,SDS-PAGE gel chromatography analysis and digestion product antioxidant ability to study the changes in the nutritional quality of channel catfish.The results showed that the weight loss rate,water loss and fat loss of the roasted fish were significantly higher than those of the other two thermal processing methods.The proportion of free water in the roasted group was the least.With the increase of the center temperature,the total protein content showed a gradually increasing trend,the main reason might be that the protein concentrated because of the heat treatment.The degradation and aggregation of water-soluble and salt-soluble proteins could be visually observed from the SDS-PAGE gel pattern.The nutritional value of catfish meat was evaluated by the calculation of amino acid score,chemical score and essential amino acid index.It was found that the amino acid ratio of catfish protein met with the WHO/FAO recommended standards.When steaming to a central temperature of 70°C,the essential amino acid index of the fish meat was the highest,which was 97.66,indicated the highest nutritional value.The simulated in vitro digestion model was used to evaluate the protein digestion characteristics of channel catfish,it was found that the fish meat after thermal processing was better digested than raw meat,and the digested protein particles were obviously smaller.The steamed fish meat was best digested among the three methods.The antioxidant ability of the steamed group was the strongest,mainly because the digested product had the most components with a molecular weight of less than 500 Da,and the peptide content and the proportion of antioxidant amino acids were also the highest.The nutritional value of reheated fish meat was further reduced after refrigeration,mainly reflected in the continuous loss of water,fat and protein digestibility with the prolongation of storage time.Reheat by steaming could better maintain the nutritional value of fish.3.The reasons affecting the degree of in vitro digestion of catfish were studied.Focused on the protein oxidation,protein structure changes and fat oxidation,and used carbonyl content,sulfhydryl content,disulfide bond content,protein secondary and tertiary structural changes and schiffer base to study the reasons that affected the in vitro protein digestion of catfish.The results showed that changes in protein structure could affect the binding sites of digestive enzymes,which in turn affected the degree of digestion.Compared with raw meat,the content of α-helix in the secondary structure of fish protein decreased after heat treatment,but the content of β-turn increased significantly,and β-sheet content in the roasted group increased by 104.69%,which would lead to protein aggregation and affect the binding sites of digestive enzymes.The protein structural changes of the steamed group were the smallest,but the protein oxidation degree was stronger than that of the boiled group,with the highest protein digestibility.The protein structure change and protein oxidation degree of the roasted group were both highest,but the protein digestibility was the lowest.Proper protein oxidation would lead to the expansion of the protein structure and make it better contacts with the digestive enzymes in the body,promote its digestion and degradation.But when the protein oxidation reached a certain level,aggregation would occur,and its Digestibility might be reduced instead.