Inactivation Of High Hydrostatic Pressure On Microorganism In Nanjing Water-Boiled Salted Duck

Nanjing water-boiled salted duck is a traditional Chinese specialty meat product, which is famous around the world. High ydrostatic pressure, as a cold sterilization technology, can inactivate most vegetative micro-organisms while maintaining the original flavor, texture and nutritional value. High hydrostatic pressure treatment may affect the composition of the surviving flora in meat products, and then the pressure-resistant bacteria became the dominant flora, which will affect the storage characteristics and the shelf-life of products. We use both culture-dependent methods and PCR-DGGE techniques to detect the effect of high hydrostatic pressure on bacterial diversity of Nanjing water-boiled salted duck during the storage, and determine the superiority bacteria. What’s more, the spoilage potential of the pressure-resistant bacteria was analyzed. Meanwhile, thermal treatment combined with high pressure was used to inhibit the pressure-resistant bacteria. The main studies and results were as follows:1. Impact of high hydrostatic pressure on microbial communities in Nanjing water-boiled salted duck during the storageThe ducks were treated by 200 MPa/40℃/10 min and 400 MPa/20℃/10 min, and the objective was to study the impact of high hydrostatic pressure on shelf-life of the ducks. We used culture-dependent methods and PCR-DGGE techniques to evaluate the effect of high hydrostatic pressure on microbial communities in the ducks during the storage. The results showed that high hydrostatic pressure could inactivate microorganisms and extend the shelf-life. The treatment of 400 MPa/20℃/10 min was better to inactivate the bacteria than 200 MPa/40℃/10 min. High hydrostatic pressure can reduce the microbial diversity in water-boiled salted duck and the microbial community becomes simple after pressure treatment during storage. Bacillus cereus and Staphylococcus warneri became the dominant bacteria of the products treated by 400 MPa/20℃/10 min.2. The spoilage capability of the pressure-resistant bacteriaThe spoilage potential, the tolerance to NaCl and the growth activity at different temperatures of the pressure-resistant bacteria were determined. And these bacteria were inoculated in the breast of the duck and the effect of the two bacteria on the storage characteristic of the breast was analyzed. The object was to reveal the spoilage capability of the pressure-resistant bacteria. The results showed that Staphylococcus warneri had a bility to produce acid and ammonia and the ability to produce casein protease and lipase was weak, but no production of gas and H2S. Bacillus cereus had a bility to produce sticky, acid and ammonia, H2S and the ability to produce casein protease and lipase was strong, but no gas. And, these pressure-resistant bacteria couldn’t grow well at 4℃ and 55℃, the optimun growth temperature was 35℃. The bacteria showed good growth activity when the concentration of NaCl was less than 7%, and Staphylococcus warneri has better salt-resistant capability. The spoilage capability of the pressure-resistant bacteria was weak, and when the total number of colonies were at acceptable range (< 4.9 lg CFU·g-1), the two strains did not significantly affect the value of pH and TVB-N, and the odor of the products. The spoilage capability of the pressure-resistant bacteria was weak.3. Effect of thermal treatment combined with high hydrostatic pressure on the inactivation of the pressure-resistant bacteria.The effect of pressure, temperature and holding time on the inactivation of pressure-resistant bacteria were investigated by single factor method. Orthogonal experiments were employed to the optimum level, evaluating the thermal treatment combined with high pressure on inactivation of the bactrtia and determining the primary and secondary order of the factors. And validation experiments were tested in real-food matrix. The results showed that the factors that affect inactivation of Staphylococcus warneri were sorted by extents as:pressure> interaction> temperature, while the factors that affected inactivation of Bacillus cereus are sorted by extents as:interaction> temperature> pressure.Treament of 300 MPa/60℃、400 MPa/60℃ and 500 MPa/50℃ could inactivate more than 107 CFU·mL-1 of Bacillus cereus in buffer, and more than 105 CFU·g-1 in the ducks. The validation experiment confirmed that the optimum conditions didn’t affect the color, texture characteristic and moisture content of the products.