| Water-boiled salted duck is one of the traditional meat products in China, which is known as its white skin, tender, delicious. Old brine is one of the most important factors influencing its quality and flavour. However, there has a little of research on old brine, especially to its effect on flavour and quality of water-boiled salted duck. This research is a part of the items entitled"Modern agricultural industrial technology system" of MOA (2009 NYCYTX-42).Through the research on physicochemical indexes, safety indexes of old brine of Nanjing Guihua Duck, Xiangfufu brine and new brine of Guihua Duck, the basic information on old brine and mass transfer during curing can be obtained, finally providing theories to standardization and modernization of the production of water-boiled salted duck. The contents and results were as follows:1. The research on changes of quality and microstructure of duck meat under different curing methodsFirstly, from sensory evaluation results, mix-curing showed the highest overall sensory score, verifing its practicity in actual production. As to 13 quality indexes—pH, moisture, salt, water holding capacity, hardness, shear force, meat color (L*, a*, b*), NMR, muscle fiber diameter, sarcomere length and intercellular space, most of these indexes values by mix-curing almost lied in those of dry-curing and wet-curing. The 13 indexes had some correlation with curing time from Pearson correlation analysis, and only salt content, hardness and NMR change were positively with curing time. For the principal component analysis, different curing methods had different impact on meat quality. Therefore, principal component analysis is a good way to distinguish the three kinds of curing methods. From the results of light microscopy and electron microscopy, after mix-curing the values of muscle fiber diameter, sarcomere length and muscle fiber space were just in between those of dry-curing and wet-curing. Therefore, the microstructural observation can also be used to distinguish the three kinds of curing methods.Mix-curing had a better effect on the quality of duck meat, which can not only avoid the phenomenon of water exdution from internal food during wet-curing, but avoid dehydration on the surface of meat during dry-curing. Therefore, mix-curing is a practical curing method in the actual production, Moreover, it will have a great significance on standardization and modernization of production of water-boiled salted duck if the researches on old brine was carried out.2.Analysis on the physicochemical indexes and safety indexes in brines of water-boiled salted duckAt the determination of physicochemical indexes on new brine of Guihua Duck, Xiangfufu brine, and old brine of Guihua Duck from different cooking cycle, it was found that new brine had a relatively lower pH, fat content, crude protein content, ash content value than those of the other two kinds of brines, which showed a gradual increase with the using year increasing. The so-called cooking cycle refers to cooking one time after continuous curing 3 times with brine in the practical production of water-boiled salted duck, which constituted a cycle. pH can be used as one of the key indicators to standardize old brine. As to safety indexes, the values of microbes, nitrite, TBA, POV and heavy metals concentration in the three brines were well below the corresponding provisions of the lowest concentration in the national food hygiene standards, so they were very safety under conventional factory production. In addition, the repeateded curing process hadnot much impact on these indexes of brines.3. Analysis on flavor materials in brines of water-boiled salted duckOn comparison of the flavour materials in the three kinds of brines, the concentration of sugar—one of the flavor precursors, were very low, roughly between 0.08%～0.20%, and there had no major change followed with curing and cooking. In respect of fatty acids, the highest concentrations of fatty acids in brines were saturated fatty acids. The content of total fatty acid in Xiangfufu brine was the highest than those of two brines in the whole process of curing and cooking. Moreover, glutamic acid, alanine were the main amino acids in the three kinds of brines. Contrary to new brine, the content of free amino acids after cooking in the two old brines were higher than those of curing treatment. The levels of flavor nucleotides—5’-IMP and 5’-GMP were higher in the three kinds of brines, which appeared as the increase with the curing time increasing, especially for new brine. However, after cooking the content of six kinds of nucleotides in three brines showed a decreasing trend. There had mainly 11 kinds of small peptides in three kinds of brines, showing a trend of increase with the curing time increasing. In each treatment, the content of the small peptides in new brine were significantly lower than those of Xiangfufu brine and old brine. The main volatile compounds in the three kinds of brines were fat oxidation products, such as aldehydes, ketones, alcohols, hydrocarbons and other compounds. After cooking, the aldehydes, ketones, acids, esters and alkyl substances in new brine decreased. For Xiangfufu brine and old brine, they also showed a similar trend which werenot so obvious.4.Standardization on brines of water-boiled salted duckThrough the regression analysis on eight indexes in new brine, the regression equation between cooking times and eight indexes was obtained:N= 23.257-2.567a-0.165c +0.276 e-0.106f +1.297 g+4.672 h, R2= 1.0Moreover, the regression equation between curing times and eight indexes was obtained:N=-7.825-0.016b-0.046c+0.015 f+0.053 g+4.545 h, R2= 1.0Where N-cooking(curing) times, a-pH value, b-saturated fatty acid content, c-monounsaturated fatty acid, d-polyunsaturated fatty acid content, e-Glu content, f-free amino acids, g-style nuclear glycosides acid content, h-Hexanal content.The values of eight corresponding indexes in old brine were put into the two equations, getting roughly more than 40 cooking cycles (100 curing times repeated), the new brine could be called as old brine. Therefore, old brine refers to a supersaturated salt solution, which is rich with flavor materials, a relatively fixed basic ingredients, higher food safety, higher curing times. Usually after about 40 cooking cycles new brine can be looked as old brine.5. Study on kinetics of mass transfer in water-boiled salted duck during wet-curingOld brine and the other three concentration brines (5%,15%and 25%NaCl (w/w))were used in accordance with the actual processes of water-boiled salted duck. Results showed that as brine concentration increased, the weight gain and water content decreased and the NaCl content of meat samples increased. Moreover, hardness and springiness increased with increasing brine concentration coupled with a drop in WHC. Higher salt concentration brines showed a higher proteolysis than those of lower salt concentration brines. De values decreased with salt concentration increasing. The predicted model and diffusional equation could be fitted to study the mass transfer on duck.15% NaCl (w/w) brine, the moderate salt concentration, maybe very suitable for wet-curing just because of its higher process yields, De value and lower salt content. However, its effect on the flavor of duck meat needs to be further identified.6.Effects of ambient temperature on mass transfer in water-boiled salted duck during wet-curingIn order to obtain the first-hand information of water-boiled salted duck to better guide the actual production, the kinetics of mass transfer in duck breast fillets during wet-curing were compared under three ambient temperatures (4℃,20℃and 30℃). Results indicated that curing temperature significantly affected the moisture, salt and overall weight changes of the duck breast fillets. As curing temperature increased, the weight gain and salt content in muscles increased whilst moisture content decreased. However, muscle hardness increased with brine concentration concomitant with a decline in water holding capacity. In addition, the diffusion coefficient (De) values increased with curing temperature. The De value at 30℃was the highest,2.63×10-9. According to the Arrhenius equation the curing activation energy (Ea) value was 47.45 kJ/mol. From the perspective of mass transfer kinetics, the higher ambient temperature was more suitable for curing water-boiled salted duck.In addition, if the changes of other various components during curing were thought about, the equation of mass transfer can be expressed as follows:AM= 28.701X-9.028Y+3.183 Z-3.301U+6.407 V-l 15.814, R2= 0.854Where AM-duck weight change, X-protein content, Y-amino acids, Z-fat content, U-ash content, V-salt concentration. |
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