| Nanjing dry-salted duck is one of the most famous dry-salted meats of china with a long production history of over500years; It has a very strong flavor characteristic and is enjoyed by consumers. However, the laggard processing technology adopted in small-scale workshops has limited the sustainable development of Nanjing dry-salted. The main reason is the deficiency in basic research during traditional processing of Nanjing dry-salted duck, even some processing parameters are still not concrete. This research is a part of the items entitled "Producing technology and products exploitation for Chinese traditional&character food and animal products "(2002AA248031) under the high technology and development program of P. R. The aim of current project was to investigate the rule of muscle protein degradation, the changes of muscle cathepsin activities and their influencing factors, and analyzing the flavor substances of Nanjing dry-salted duck during traditional processing. This could be beneficial to provide a basic heory in improving the processing technology and realizing the industrial production of Nanjing dry-salted duck. The contents and results are as follows.1. Study on properties of Cathepsin B, L and H in duck muscleCathepsin B, L and H were extracted from muscle of cherry valley duck by biochemical techniques. The influence of temperature, pH, salt content, activators and inhibitors on the activities of Cathepsin B, L and H was studied. The activities of Cathepsin B, L and H with optimum pH were5.0-5.5,4.5-5.5, and6.5-7.0respectively. The optimum temperatures were35℃-40℃,40℃-45℃and45℃-50℃respectively. The activities of Cathepsin B, L and H were significantly influenced by salt concentration, which decreased along with increase of salt concentration. L-cysteine and DTT could increase the activity of cathepsin. On the contrary, the activities of Cathepsin B, L and H were completely inhibited by E-64. Cathepsin B and L were not inhibited by EDTA and PMSF.The effects of main influencing factors (temperature, salt content and pH) on the activities of cathepsin B, L and H were firstly studied using response surface methodology. The actual activities of cathepsin B, L and H were estimated with the gained response surface regression equations. The results showed that the activities of three cathepsins were significantly influenced by temperature and salt content (P<0.0001). The activities of cathepsin B, L and H were interaction effects between temperature and salt content (P<0.05), The activities of cathepsin L were interaction effects between temperature and pH value(P<0.05).2. Studies on changes of the activities of cathepsin B, L and H during processing of Nanjing dry-salted duckNanjing dry-salted duck were processed according to typical traditional processing technology with100cherry valley ducks. Muscle samples were taken from six ducks, raw ducks, cured, salted, piled, and then drying for5,10and15days stage. Leg muscle Samples were used to analyze the changes of the activities of cathepsin B, L, H and relative indices during the processing of Nanjing dry-salted duck. Results showed that the activity of cathepsin B was the highest; activity of cathepsin H was the lowest. While cathepsin B, L and H displayed similarly potential activities which all all dropped gradually during processing. The potential activity of three cathepsin significantly decreased in dry-cured finished (p<0.05), which was not different in drying stage. The activity of cathepsin B of leg muscle decreased from4.87U/g at raw duck stage to1.02U/g at ripened duck (drying15days). The residue activity in ripened duck was about20.84%.The activity of cathepsin L from3.72U/g to1.11U/g. The residue activity was about29.76%. The activity of cathepsin H was changed from1.75U/g to0.40U/g, and the residue activity was about22.61%. Moisture contents of leg muscle decreased from78.67%in raw ducks to60.97%in ripened duck. Salt contents of leg muscle increased from0.15%in raw duck to14.42%. The changes of pH were insignificant after salted stage.The response surface regression equations estimated that the actual activities of cathepsin B, L and H were lower than their corresponding potential activities except for cured stage during processing of Nanjing dry-salted duck.3. Studies on changes of muscle protein degradation during the processing of Nanjing dry-salted duckMuscle samples indices was studied by analyzing moisture content, salt content, total nitrogen, non-nitrogen, peptides and free amino acids during processing of Nanjing dry-salted duck.The results indicated that the moisture content decreased while the salt content increased in breast and leg muscles. The total nitrogen content decreased, and the non-protein nitrogen content increased by dry-curing and salting. The ratio of final product of traditional Nanjing dry-salted duck was63.83%.The moisture content and salt content in breast and leg muscle of ripened duck were60.12%and60.97,14.62%and14.42%respectively. The proteolytic index (P.I.%) of Nanjing dry-salted duck was6.62-8.41.Peptides of muscle were studied by RP-HPLC during the processing of Nanjing dry-salted duck. Six remarkable elution peaks were discovered in14min. Total elution area increased significantly by curing (P<0.05). The amount of peptides were positively correlated with FAA contents (R2=0.82)All kinds of FAA increased in concentration along with the progressing of Nanjing dry-salted duck. The glutamic acid content, which is the main palatable compound of the Nanjing dry-salted duck, increased2.19folds. followed by Lys、Pro、Gly and Ala. FAA concentration of the Arg, Glu, Thr and His were high in the leg muscle of Nanjing dry-salted duck.The experiment studied the changes of the myofibrillar and sarcoplasmic protein of breast muscles during the processing of Nanjing dry-salted duck. Electrophortic studies of the muscle myofibrilar and sarcoplasmic proteins showed the disappearance of protein chain. Protein of Nanjing dry-salted duck was degradated in different levels.4. Studies on the changes of flavor compounds in Nanjing dry-salted duck by typical traditional processingChanges in class, absolute and comparative amount of volatile flavor compounds in Nanjing dry-salted duck during typical traditional processing were extracted and identified by solid phase microextraction (SPME) and Gas Chromatography-Mass Spectrometry (GC/MS) technology using the samples mentioned above. The results showed that total90kinds of volatile compounds were identified during Nanjing dry-salted duck processing. These compounds including26kinds of hydrocarbons,18kinds of aldehydes,13kinds of alcohols,10kinds of ketones,8kinds of carboxylic acids,5kinds of esters,2kins sulfur compounds,1kind of amines, lkind of aethers,2kinds nitrogen,4kinds of furans.34kinds of volatile compounds were identified in leg muscle of raw duck. Ripened Nanjing dry-salted duck (drying15days) were53kinds of volatile compounds. During different processing stages of Nanjing dry-salted duck, the amount of volatile flavor compounds, total Peak area and proportion were constantly changing in leg muscle. Dry-curing accelerated the oxidation, kinds and total Peak area of volatiles compounds increased rapidly. However, total Peak area of volatiles compounds decreased in salted stages, which may be part of flavor substances into the salted liquid. From salted to drying15days,total peak area of volatiles compounds increased gradually with the increase of flavor substances. The contents of alcohols, aldehydes, carboxylic acids and ydrocarbons in leg muscle were higher than others volatile compounds in raw duck. The contents of aldehydes, alcohols, hydrocarbons and ketones in leg muscle were38.00%,26.72%,18.61and11.61%in ripened Nanjing dry-salted duck, respectively. The change trends of aldehydes and ketones compounds were very similar. Contents of aldehydes and ketones increased rapidly during drying stage. While hexanal content was23.91%in drying15days. |
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