Research On Preservation Technology Of Tilapia Fillet With Controlled Freezing-point And Modified Atmosphere Packaging And Shelf Life Predictive Model

Fish fillet is a representative way of production in the fresh fish consumption. Fish fillet is a main product in the export trade of Tilapia. However, it is not yet popular in our contry. With the improvement of people's living standard, fresh aquatic products in small packages gradually become an important sale mode such as the Tilapia fillets. Therefore, studies an appropriate preservation method has important and practical significance to the Tilapia fillet in the industrial production development. The thesis is focused on the quality of fresh Tilapia fillet to study the optimal modified atmosphere packaging technology through a series of physical and chemical indexs (sensory evaluation, aerobic bacterial counts, TVB-N, pH and drip loss, etc.). Analyse and identify the specific spoilage organisms (SSO) of the Tilapia fillet with controlled freezing—point and modified atmosphere packaging and then research on the spoilage microbial effect on the quality changes of Tilapia fillet by using the inoculation technology. Finally, use the growth of SSO data to establish a mathematical model to predict shelf life. The contents and results are as follows:1. The freezing—point of Tilapia fillet is determined to be -0.7℃through traditional freezing method, thus the freezing temperature is from -0.7℃to 0℃.2. Optimal modified atmosphere packaging technology. Tilapia fillets packaged with different inflatable volume (V)—fish weight (W) ratios (2/1,3/1 and 4/1) were stored at freezing—point. Tilapia fillet packaged in controlled atmosphere with 100%CO2, 70%CO2/30%N2,50%CO2/50%N2 and 30%CO2/70%N2 are stored respectively at freezing—point. The shelf life of Tilapia fillets were evaluated by the sensory evalution as well as key indexes such as drip loss and chromatic aberration, microbial changes and TVB-N. Results showed that the V/W=3:1 was one of the optimal modified atmosphere packaging technology factor. Due to the high concentration of CO2 would accelerate the drip loss, the modified atmosphere at the level of 70%CO2/30%N2 was the other optimal modified atmosphere packaging technology factor which can extend the shelf life of Tilapia fillet effectively as well as keep their good appearance.3. Comparison with conventional preservation methods. Tilapia fillets packaged with controlled freezing—point (CF) and modified atmosphere packaging (MAP,70% CO2/30%N2) compared with different conventional preservation methods.Changes in odor, elasticity, chromatic aberration, aerobic bacterial counts, psychrotrophic counts, pH, TVB-N and drip loss of Tilapia fillets were monitored during storage of 24 days. Results showed that CF and MAP exhibited a significant fresh keeping effect on Tilapia fillets. Compared to individual CF or MAP preservation method, the shelf life was extended by 3 days and 6 days, respectively. Moreover, the combinatorial CF and MAP preservation method can provide 4-fold shelf life for Tilapia fillet treated with ordinary preservation methods.4. Microflora analysis and specific spoilage organisms (SSO) identification. The microflora and SSO identification were investigated by PCR (polymerase chain reaction)—DGGE (denaturing gradient gel electrophoresis) analysis. The total bacteria DNA of three different batches fillets purchased in the same enterprise were extracted to amplify the V6—V8 regions of the bacterial 16S rDNA gene. PCR products were used to DGGE. Six types of bacteria were identified by DNA sequence and analyzed in the main band on the DGGE profiles in three different samples. The main microbes represented by 8 main bands in DGGE gel were closely related to Carnobacterium, Lactobacillus, Lactococcus, Streptococcus, Clostridium and Pseudomonas. Lactic acid bacteria (LAB) which composed mainly by Carnobacterium.sp was the dominant microorganisms in Tilapia fillet with controlled freezing—point and modified atmosphere packaging.5. Physiological characteristics of the dominant microorganism, spoilage microbial effect on the quality changes of fillets, microbial growth kinetics model of specifics spoilage oranisms (SSO) and shelf life prediction for Tilapia fillets with controlled freezing—point and modified atmosphere packaging. The dominant microorganism that isolated from Tilapia fillets were inoculated to the no contamination fillets in order to study the microbial effect on the Tilapia fillets quality. Results showed that NaCl concentration range of 0-5%(inhibited at 8%), pH 8.0 (inhibited at 4.0) and 28℃were the optimal growth conditions of SSO and the spoilage effect on sensory evaluation include color, odor and elasticity, where the color is the most affected index, followed by odor and finally the elasticity. Use the growth data of SSO in D—MRS agar medium to develop and validate the growth kinetics model of SSO for predicting the shelf life of Tilapia fillet. Results showed that the kinetics model of SSO could be well described by a Gompertz type model. Validation of built kinetic models was preferred by comparing with experimental development of SSO grown on Tilapia fillets with different inoculation concentration (OD6oo). Absolute error and relative error between predicted shelf life and real shelf life to the fresh fillets were±3 days and -15.79-20% respectively, it suggested microbial growth models established in our research were valuable for predicting the shelf life of Tilapia fillet with controlled freezing—point and modified atmosphere packaging.