| A variety of biochemical reactions that participate the flavor formation during cured meat products processing are usually catalyzed by various endogenous enzymes, such as lipases, lipoxygenase (LOX), proteases etc. Process temperature and salt content are colsely related to the endogenous enzyme activities. It is possible to find out the optimal temperature and salt content that influencing the enzyme activity under the premise of ensuring the flavor quality of cured meat products, so as to achieve the objectives of enhancing enzymatic activities, promoting lipid oxidation to form flavor compounds and shortening the process time. In this study, in order to explore the regulation mechanism of lipid oxidation, improve the production process and product quality during dry-cured duck processing, the characteristics of lipoxygenase and interactions of processing factors on its activity were studied by extracting and purifying lipoxygenase from the raw material of Cherry Valley duck and investigating the correlations of lipid oxidation with physic-chemical indices and LOX activity. The detailed contents and results of this study were as follows.1. Lipoxygenase activity in Cherry Valley duck muscle:Purification and paritial characterizationLipoxygenase was extracted from Cherry Valley duck muscle and purified through ammonium sulfate saturation and DEAE-Sepharose column. It was showed that lipoxygenase in raw material of Cherry Valley duck could be effectively purified through20-40%ammonium sulfate saturation. And, during separation using the DEAE-Sepharose column chromatography, when the NaCl elution concentration in the0.3-0.35mM, LOX was compeletly isolated with other proteins. The purified enzyme had a purification factor of54.3and a molecular weight of96KDa. LOX is more sensitive to heat, and the characteristics of crude and purified enzyme was slightly different. At room temperature (20℃), the optimal substrate concentrations, reaction temperature, pH value and maximum velocity (Vmax) for curde and purified enzyme catalyzing linoleic acid were6.4mM and10.0mM,40℃and30℃, pH5.0and pH5.5,0.06983U/min and0.04756U/min, respectively. The affinity of purified enzyme to linoleic acid substrate was higher than that of cured enzyme.2. The regulation mechanism of lipoxygenaseThe interactive effects of processing factors on lipoxygenase activity were studied through extraction and purification lipoxygenase from the raw material of Cherry Valley duck. The orthogonal test result showed that the order of all factors affecting LOX activity was temperature> NaC1concentration> pH>substrate concentration The result of variance analysis showed that under the conditions of reaction temperature30℃, pH5.5, NaCl concentration3%, and substrate concentration12mM, LOX had the highest activity.There were significant interactions with temperature、pH and NaC1(P<0.01). The critical value of temperature decreased gradually with an increase in NaC1content and the NaC1content critical value for LOX activity also decreased with thetemperature increasing. The temperature critical value increased with pH increasing, while NaCl content critical value decrease with pH increasing. Reducing the NaCl content and improving the temperature could promote LOX activity and accelerate lipid oxidation in initial processing stage. With the infiltration of salt and temperature increasing, the LOX activity was inhibited at a later stage. These indicated that LOX activity and enzyme-catalyzed lipid oxidation in dry-cured duck products could be regulated by controlling process factors during processing.3. Study on the regulation mechanism of lipid oxidation and the optimization of intensifying high-temperature during dry-cured duck processingIn this section, the Cherry Valley duck was processed to dry-cured duck through intensifying high-temperature ripening process.The parameters of intensifying high-temperature ripening of dry-cured duck was optimized by the response surface methodology (RSM). In order to provide theoretical guidance for dry-cured duck processing, the correlations between lipid oxidation with process factors, LOX activity and other physic-chemical indices were studied by comparing the intensifying high-temperature ripening process and the traditional process. The results showed that:both the intensifying high-temperature and salt content had a critical value on the TBARs value of final dry-cured duck products. Intensifying high-temperature had significant interactions with salt content. The critical value of intensifying high-temperature decreased gradually with an increase in salt content and the salt content critical value for TBARs value also decreased with the intensifying high-temperature increasing. The optimal processing of dry-cured duck were intensifying high-temperature45.41℃, air-drying temperature14.44℃, salt content10.07%. When the end of the process, LOX activity in intensifying high-temperature group was increased by36.98%than LOX activity in control group, and the enzyme activity of intensifying high-temperature group was always greater than the control group during the processing. It was effective to improve the LOX activity during dry-cured processing, reduce lipid peroxidation indicators of the product and promote the formation and accumulation of the flavor substances by applying intensifying high-temperature. |
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