Studies On The Flavor Compounds Of Duck Meat & Its Changes During The Processing

Flavor is one of the most important sensory property of meat and meat product. However, little reports on the volatile compounds or taste compounds of duck meat were presented, especially no reports was on the flavor of traditional Chinese duck meat product—Nanjing water-boiled salted duck. 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. China and "Modern process, preservative technique and standardization of traditional Chinese feature meat product"(BE2001400) under the science and technology tackle key problem program of Jiangsu province. The aim is to investigate the duck flavor and effects of process on the duck meat flavor and provide a theory basis in improving the processing technology and realizing the industrial production of duck meat product. The contents and results are as follows.1. Studies on the analysis method of duck volatile flavor compoundsDuck aroma compounds were analyzed using different extraction techniques. The solid phase microextraction (SPME) analysis conditions were optimized, which the extraction time, temperature, sample quality and fibers were optimize by using water-boiled salted duck breast meat. The optimized conditions of spme for analysis of duck meat were that a 75μm carboxen-polydimethylsiloxane (CAR-PDMS) SPME fiber was exposed to the vapor phase above 7 g sample for 40 min at 45℃. The simultaneous distillation extraction (SDE) method was comparatively studied by using three different extraction solvents. Result indicated that ethyl ester was the preferable extraction solvent. Three extraction techniques including SPME, purge and trap with thermal desorption (PT-TD) and SDE were comparatively used to extract the aroma compounds of duck. The extracted aroma was then analyzed by gas chromatography-mass spectrometry (GC-MS). Total 116 volatile flavor compounds were identified by comparing the mass spectrometry and Kràtz retention index (KI), which aldehydes (31 kinds), esters (4 kinds), alcohols (14 kinds), acids (7 kinds), hydrocarbons (25 kinds), kentons (15 kinds), N-containing compounds (5 kinds), S-containing compounds (8 kinds)and furans (7 kinds) were included. SPME and PT-TD method provided better extraction for volatiles having a lower molecular mass, while the results of SDE method was inverse. As for analyzing the flavor compounds of duck meat, SPME method was better than PT-TD method and SPME with SDE method may well complement each other.2. Studies on the characteristic aroma compounds of duckSimultaneous distillation extraction (SDE) by different extraction time was used to analyze the origin of indole. Result indicated that indole was detected only in the extract that the extraction time long than 2h, and the content of indole rose exponentially (R~2=0.9989) with the increasing extraction time. It is obvious that indole was not the genuine substance in the duck, but that it was formed during the extraction of the volatiles from duck meat. SDE extract was separated by pH value and then were analyzed, which found the neutural fraction and the basic fraction could be the important component of duck meat aroma. Different part of duck meat and fat including duck breast and duck caudal were analyzed using solid phase microextraction (SPME). The branch fatty acids including 2-methyl hexanoic acid, 4-methyl hexanoic acid and 4-methyl octanoic acid, were identified in the duck caudal meat and fat. These branch fatty acids were the first identified in poultry meat, which could contribute the typical flavor of duck. The SPME-GC-O and SDE-GC-O were used in the different part of duck meat and fat to identify the principal characteristic compounds of duck meat, and a total 16 compounds were identified as follows: methanethiol, 2-methyl propanal, 3-methyl butanal, 3-hydroxy-2-butanone, dimethyl disulfide, 2-methyl thiophene, 2-methyl-3-furanthiol, 3-(methylthio)-propanal, (Z)-2-heptenal, dimethyl trisulfide, 1-octen-3-ol, 4-methyl hexanoic acid, 2-acetyl-thiazole, (E,E)-2,4-nonadienal, 4-methyl octanoic acid, (E,E)-2,4-decadienal. The following sixteen compounds were contributed to the overall flavor of duck: 1-penten-3-ol, 2,3-pentanedione, hexanal, methylpyrazine, furfural, 2-furanmethanol, (E)-2-hexenal, butyrolactone, 2-octanone, 2-pentyl furan, acetic acid hexyl ester, (E)-2-octenal, nonanal, 2-formylthiophene, (E)-2-nonenal, (E)-2-decenal.3. Studies oneffect of process on duck flavor precursor and taste compounds During the processing of Nanjing water-boiled salted duck, the content of water of duck has a decrease tendency during the processing, while the protein and fat content increased continuously. The, thiamine, MUFA and PUFA content decreased during the dry-curing (p＜0.05). No significant changes of the important flavor precursor happened during the brining. During the roasting, the reducing sugar decreased significantly and the thiamine didn't change, while the free fatty acids increased. As for phospholipids fatty acid, no changed was found before the process of boiling. The important precursor compounds decreased significantly during the boiling. Before the boiling of the processing of NJWSD, all kinds of FAA and total FAA increased in different degree, especially for the processing of brining and roasting. During the boiling, all the FAA decreased (p＜0.05), most of them decreased to 50%. Most of small peptide content had a decreasing tendency, while most small peptide and the total peptide increased in brined duck than those of in dry-cured duck (p＜0.05). The flavor nucleotides 5'-IMP and 5'-GMP continuously decreased before the process of boiling, but largely increased during the boiling.The significant differences were observed in moisture and pH value of different processed duck meat product, which the NJWSD was in the lowest pH value, and roasted duck were in the highest content of moisture and lowest fat content. As for water-boiled duck, the fat content and pH value were the highest. No significant difference between three different processed duck products in reducing sugar content was observed. But the content of thiamine of three different processed duck products differed significantly, which the contend of thiamine in water-boiled duck was the lowest, followed was the roasted duck and the content in NJWSD was the highest, he main difference among different processed duck meat was on the PUFA, which the roasted duck possessed the lowest level of PUFA. As for phospholipids, the PFA of water-boiled salted was the highest among three different processed duck, while roasted duck and water-boiled duck had no difference. The content of most FAA and peptides, no difference was observed between NJWSD and roasted duck, and higher than those of in water-boiled duck. The total of FAA in NJWSD was the highest, while the roasted duck and water-boiled duck were in the same level. All the nucleotides in NJWSD were higher than that of in water-boiled duck. The content of flavor nucleotides in roasted duck was the highest among three different processed duck, followed was the NJWSD and the water-boiled salted duck.Except the brining process, else process during the processing of NJWSD had an effect on the content of flavor precursor. No difference between three process styles were observed in the effect on the flavor reaction by reducing sugar. Among three processed duck, the decomposition degree of thiamine in water-boiled salted duck was the least, while that of in roasted duck and water-boiled duck were higher. The lower pH value and suitable content of salt of water-boiled salted duck were largely contributed to the umami sensory property. The brining process in water-boiled salted duck had great effect on the taste compounds of duck meat. The higher oxidation and decomposition degree of fat, highercontent of umami amino acids asp and glu, sweet amino acid ala, small peptides and flavor nucleotides 5'-IMP and 5'-GMP were the reason of delicious for the water-boiled salted duck and roasted duck, which was one of the reasons constituting the delicious of water-boiled salted duck.4. Studies on the effect of process on the duck volatile flavor compoundsChanges in volatile flavor compounds during the processing of water-boiled salted duck were investigated using solid phase microextraction, and three processed duck meat product including water-boiled salted duck, roasted duck and water-boiled duck were compared for their aroma compounds, to study the effect of process on the duck aroma and discuss the mechanism of the formation of duck meat volatile flavor compounds. Total 92 compounds were identified during the processing water-boiled salted duck, which 57 compounds were detected firstly in duck meat. These compounds included alcohols (13 kinds), aldehydes (22kinds), hydrocarbons (25 kinds), acids (4kinds), ketones (12 kinds), esters (5 kinds), furans (3 kinds), N-containing compounds (2 compounds) and S-containing compounds (6 kinds). During the processing of water-boiled salted duck, dry-curing accelerated the oxidation and increased the formation of some characteristic aroma compounds, but the effect of brining and roasting were little. The boiling process made the duck aroma compounds level sharp and high the oxidation value. The first principal component (PC1) explained 97% of the total variance of the data, in which pentanal, hexanal, octanal, nonanal, and 2,3-octanedione were the main compounds. Three different processed duck products were not different in kinds of volatiles, but were the content, of that. Except esters, other kinds of compounds in water-boiled duck were lower than those of water-boiled salted duck and roasted duck. Compared to the water-boiled salted duck, roasted duck were higher in alcohols, aldehydes, N-and S-containing compounds, but lower in hydrocarbons, esters, ketones. Lipid oxidation and decomposition were the necessary condition for the formation characteristic aroma of duck, and Maillard reaction was decisive to the cooked duck meat aroma. Interactions between compounds produced by the reaction of intermediates of the Maillard reaction with lipid oxidation products importantly influence the overall aroma profiles of cooked duck meat.5. Studies on the effect of duck fat on the duck odourDuck fat was extracted selectively using petroleum spirit and chloroform with methanol respectively. Sensory analysis and instrument analysis showed that the duck meat sensory property were roasted and nutty when the phospholipids and triglycerides were removed, while the duck characteristic aroma score were significantly low (p＜0.01) compared to the petroleum spirit test. The volatile compounds differed greatly, which the former contained the hydrocarbons, acids and esters mainly, and no duck characteristic aroma compounds were found in the former sample. The other important active aroma compounds were also significantly lower than the later. Triglycerides had little effect on the duck characteristic and meaty aroma, but phospholipid was.Duck fat extracted by chloroform with methanol was washed by water to remove the water soluble compounds in fat. No S- and N- containing compounds were identified in washed duck fat, and the characteristic aroma compounds were significantly lower (p＜0.01) than the control duck fat. The water soluble compounds in duck fat contributed the duck meaty aroma but not the characteristic aroma of duck.Non-oxidative duck fat with a higher corn score, had a significantly lower score in duck odour than that of oxidative duck fat (p＜0.01). The oxidative duck fat had a higher score in oxidation and rancid than that of non-oxidative duck fat (p＜0.01). Little aldehydes and ketones was identified in non-oxidative duck fat, and no 2-pentyl furan was found also. But dimethyl sulfide was higher(P＜0.05) in non-oxidative duck fat than that of oxidative duck fat. Fat oxidation significantly contributed to the duck odour. The duck characteristic aroma compounds and other active aroma compounds in oxidative duck fat were higher(P＜0.05) than those of in non-oxidative duck fat.