Studing On The Changes Of Lipid And Flavor Compounds During Processing Of Nanjin Dry-Cured Duck

Nanjing dry-cured duck is a peculiarly traditional meat product of china, and also one of the most famous dry-cured meat products in the world. Nanjing dry-cured duck, with a long production history and a unique flavor, is enjoying a high reputation in china or southeast Asia. Up to now, the laggard traditional processing technology adopted in small-scale workshops has influenced the sustainable development of dry-cured duck seriously. The main reason is the deficiency in theory research, and the poor understanding on the scientific meaning of the traditional processing technology of dry-cured duck. This lead no theory can be used in optimizing the duration and parameters in practice.The aim of this research is to investigate the forming mechanism and pathways of flavorsubstances originated from lipid by studying the changes of lipid as a main precursor of flavors, the characteristics of endogenesis lipase and analyzing the flavor substances of Nanjing dry-cured duck during the processing; elucidate the role of lipid in formation of flavors; provide a theory basis for studying and accommodating the flavor, improving the processing technology, optimizing the parameters and realizing the industrial production of Nanjing dry-cured duck. The contents and results are as follows:1. Studies on the changes during the processing of Nanjing dry-cured duckExperimental dry-cured duck were processed by traditional processing technology . A tatle of seven points, which were raw duck, end of dry- cured, end of marinated, end of piled, 5 days of air-drying, 10 days of air-drying, 15 days of air-drying (finished product), were selected during this progress. Biceps femoris and subcutaneous fat were taken from 6 ducks of each point randomly as samples. Then they were used to analyze the changes of contents of lipid, triacylglycerol, phospholipids, free fatty acids and relative indices. Results showed that: triacylglycerol contents in lipid of biceps femoris and subcutaneous fat did not change significantly during processing. While the phospholipids contents in lipid of biceps femoris and subcutaneous fat changed significantly during processing. About 41.27% and 53.39% of the total phospholipids in biceps femoris and subcutaneous fat were hydrolyzed during processing of dry-cured duck. PC and PE were major component of intra-muscle phospholipids of dry-cured duck. The relativity analyze showed that R² of phospholipids and triacylglycerol contents with free fatty acids contents were 0.82/0.42,0.87/0.81,respectively. Results analysis showed that, the main source of free fatty acids was phospholipids. Phospholipids were the main precursor of flavor substances. Analysis of free fatty acids showed that, the proportion of saturated and monounsaturated fatty acids in biceps femoris and subcutaneous fat increased during processing, among which the proportion of monounsaturated fatty acids increased more strongly; the proportion of polyunsaturated fatty acids decreased during processing. These indicated that the polyunsaturated fatty acids were the key fatty acids in lipid oxidation during processing of dry-cured duck. Changes of arachidonic acid contents were gently, which indicate arachidonic acid played an important role in flavor formation of dry-cured duck.2.Research on types of molecular species of phospholipids in raw duck of Nanjing dry-cured duckA method based on high performance liquid chromatography-mass spectrometry (LC-MS) for analysis of the main molecular species of phosphatidylcholine(PC) and phosphatidylethanolamine (PE) in Nanjing salted duck muscle was established. The intramuscular phospholipids was fractionated using solid phase extraction column(NH₂), then PC and PE was separated from the phospholipid extraction on a normal phase semi-preparative silica gel column(WatersμPorosil 300mm×7.8mmi.d,10μm) by high performance liquid chromatography(HPLC).The compositions of their molecular species were identified by reverse phase liquid chromatography- electrospray ionization mass spectrometry in positive ion mode using solvent A(methanol/ chloroform, 17.5/1,V/V) and solvent B(acetonitrile/water,1/1,v/v)as mobile phase to linear gradient elution on Symmetry C₁₈ column (250 mm×4.6 mm i.d,5μm).The results showed that there were five main molecular species in PC from duck muscle, they were:C_20:4/C_16:0 PC,C_18:1/C_16:0 PC, C_18:2/C_18:0 PC,C_18:2/C_16:0 PC and C_18:1/C_18:0 PC.There were also five main molecular species in PE from duck muscle, which were C_14:0/C_22:4PE.3.Study on changes of lipases and phospholipase during processing of Nanjing dry-cured duckThe changes of acid and neutral lipases activity, phospholipase activity in muscle of dry-cured duck during processing were studied. The results showed that the activities of these three enzymes were increased during the process of curing, but decreased during piled and drying. The co-relationship analysis showed that the contents of salt and water were significantly correlated to the activities of these three enzymes during processing of Nanjing dry-cured duck. 4. Study on lipid oxidation of Nanjing dry-cured duck during processingThe lipid oxidation of muscle of breast and leg in dry-cured duck during processing was studied by determination of the peroxide value, TBARS value, carbonyl compounds and conjugated dienes in this paper. The results showed that the levels of lipid oxidation of dry-cured duck became more serious characterized by the increases in the peroxide value, TBARS value, carbonyl compounds and conjugated dienes are increased during processing. The process of salting affected the TBARS value, carbonyl compounds and conjugated dienes significantly and the peroxide value changed evidently during the process of air-drying.5.Study on Time related components and changes in flavor compounds of Nanjing dry-cured duck during processingNanjing dry-cured ducks were processed based on a traditional processing technology. Samples of Biceps femoris were taken from 6 ducks randomly at each of the key stages of production, i. e., raw duck, end of dry- cured, end of marinated, end of piled, 5 days of air-drying, 10 days of air-drying, 15 days of air-drying. Analysis of flavor compounds showed that, there were 33 compounds at green duck, 68 at the end of dry-salting,15 at end of wet-salting,31 at end of arraying, 43 at 5 days of air-drying, 57 at 10 days of air-drying and 54 at 15 days of air-drying. The flavor compounds were clustered in the following chemical families: sulphur compounds, amine compounds, alcohols, carboxylic acids, hydrocarbon compounds, ketones, esters, aldehydes, aethers, nitrogenous compounds and furans. In the flavor substances of the final product of Nanjing dry-cured duck, the contents of aldehydes, alcohols, hydrocarbon compounds and ketones were 37.78 %, 26.93 %, 18.39 % and 11.72 % of all, respectively. The first principal components were consisting of 2 aldehydes, 1 carboxylic acids and 1 alcohols, the second were consisting of 2 aldehydes, 1 ketones and 1 hydrocarbon compounds and some other flavor compounds. The two principal components explained 85.0 % of t he total variance of the flavors in Nanjing dry-cured duck.