| In order to study the effect of microwave sterilization on liquid food containing fruit particles, this study processed orange particles in sugar solution by microwave heating, tested the effect of power, solids content on the changes of numbers of microorganisms and temperature in the sample. The effectiveness of microwave sterilization is verified by comparing with conventional pasteurization and inoculating a certain amount of pathogens. In order to describe microbial death kinetics under microwave treatment, the sugar solution with orange particles was used as the research material, As the sample, Escherichia coli and Listeria monocytogenes was suspended in sugar solution with orange particles Two sample was exposed to microwave at 800W for 40s, the bactericidal effect was evaluated. Linear, Weibull, log-logistic and Dose-Response functions were used to model survival curves of two pathogens in sample.In order to evaluate the effect of microwave sterilization, the quality changes were study under different condition in procedure of microwave and pasteurized treatment, on the point of thermodynamics, the difference of two kinds of sterilization pattern was investigated. In addition, to study the effect of microwave power, the concentration of particles, and sterilization time on the number of microorganism, browning index, the content of vitamine C, the Box-Behnken mum compositi on of the fermentati on medium for maxi mum collagen yield, the best sterilization condition was analysed on the basis of a three level three factor Box-Behnken design. To study sterilization methods on the impact of the quality and shelf life of sample, a kinetic study of post processing quality loss was conducted after microwave processing (500W,55s) or thermal pasteurisation (85℃, 3min) of liquid food containing fruit particles. Selection of processing conditions was only based on microorganism inactivation. Total plate count, Vitamine C, and colour changes were measured at intervals during storage under different isothermal conditions (5, 10, 15, 25℃).The results were as follows: with the increase of microwave power, the heating-up rates of samples are faster, the death rates of bacteria are faster, but germicidal effect is better at the lower power. With the increase of the fruit particles content, the heating-up rates of samples are much faster, but higher temperatures and longer time were better for sterilization. Microwave sterilization had more obvious advantages compared with conventional pasteurization. When the same sterilization effect was reached, the time by microwave was one-fifth that of the time by conventional pasteurization. With aerobic bacterial count, Listeria monocytogenes and E. coli are as aobjects, the relationship expression between death characteristic values of microorganisms and temperature were obtained, at the same temperature, the D value of aerobic bacterial counted by conventional pasteurization was 7.8–2.4 times than microwave sterilization. L. monocytogenes was harder to kill and needed to take more longer time and higher temperature to inactivate by microwave heating. The results in this paper provid the technical basis for the application of microwave technology in processing and storage of food.The results showed that a treatment of 800W for 40s reduced the counts of Escherichia coli by 2.5 log10 and Listeria monocytogenes by 3.5 log10. Since the thermal sensitivity of two foodborne pathogens is different, the death rates of two foodborne pathogens was different in sterilization process. Both the survival curves showed a slow initial drop in bacterial counts followed by rapid drop when the treatment temperature to 60℃, the last tailing tail was caused by a diminishing inactivation rate. First order kinetic model can not be used to describe the law of microbial death under microwave sterilization. Weibull model, Log-Logistics Model and Dose-Response model all can better fit the inactivation of microorganisms. However, Five model parameter was used for evaluating the stand or fall of model, the comparison of these indices indicates that Dose-Response model consistently produced better fits to inactivation data of Listeria monocytogenes than others. And Log-Logistic model had better fitting degree for Escherichia coli than others.In the process of microwave sterilization, with the increase of microwave power, the terminal tempreture of sample was higher, and the rate of quality loss was higher. the results of pasteurization was the same to microwave. When based on the terminal tempreture of sample, it is propitious to hold the quality under microwave treatment. On the premise of microorganism inactivation, it is better to use lower power to sterilize. The sample have the higher activation energy , enthalpy and entropy under microwave treatmenResponse surface methodology (RSM) was employed and a second order quadratic equation for microwave sterilization was built in the present work .due to the R2 of the model equation is high, the model equation for predicting the optimum response values was effective. analyzing the response surface plots and their corresponding contour plots as well as solving the quadratic equation ,the optimum process parameters for sterilization were obtained as : microwave power 500W, the sterilization time, 55s, the concentration of paticles 60g/100mL.The results indicated that the rates of increasing of total plate count, lossing of Vitamine C, the browing rate have increased with the increasing of storage time and temperature for both types of processing. Based on the remainder of total plate count 17.5-13.5 days shelf life for microwave treated tomato juice was longer than the respective values of thermally pasteurised juice at low storage temperatures (5-10℃) used, based on the remaning of vitamine C the difference is 5.4-3.1 days ,and based on the degree of browing the difference is 8.5-7.3 days. however, the difference between pasteurize and microwave sterilization method was not significant at 15-25℃. Microwave processing resulted in better retention of the quality of untreated juice and superior sensory characteristics compared to thermal pasteurisation. The remaining shelf-life of microwave treated sample could be predicted at the storage temperature at 5-25℃based on Vitamine C and the degree of browning. It showed from the reliability assessment between predicted and observed shelf-life that accurate rate was within 80% calculated by the prediction model for the shelf-life of sugar solution with orange particles . It was better to take the remaning of vitamine C of sugar solution with orange particles as the ultimate standard for shelf life. At low storage temperatures (5℃) 9-14 days were proposed as the shelf life of microwave treated sugar solution with orange particles, and 6-9 days were proposed as the shelf life of thermally pasteurized sugar solution with orange particles. The results in this paper provid the technical basis for the application of microwave technology in processing and storage of fruit juice. |
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