| The potential hazard in food and outbreaks of diseases caused by food-borne pathogens have attracted increasing attentions from the food processing industry.When using conventional heating methods,heat transfer is slow,especially for the prepackaged solid or semi solid foods.Traditional thermal processing may involve long time heating at high temperatures,promoting chemical and physical reactions in foods,and causing quality degradation.For example,color changes,development of off-odors or off-taste,and loss of freshness or nutrients occur frequently.Radio frequency(RF)energy has been identified as one of the most promising physical heat treatment for pasteurization due to volumetric,fast,and selective heating,and keeping the performance and the quality of products better,compared with the conventional convection,conduction,and radiation heating.However,it lacks in-depth systematic research on the inactivation mechanism of pathogens induced by RF heating,including kinetics studies on microbial thermal death and food quality during the RF heating,and the thermal and non-thermal effects on inactivating pathogens by RF heating.Experimental methods under isothermal conditions are widely used in the laboratory for kinetic studies,mainly including hermetically sealed test cells filled with samples heated by water or oil bath,but which cannot control the heating rate of the sample.Thermal kinetic parameters applied in industry have complex heating profiles,and are much different from the experimental isothermal conditions.Also heating rate has been shown to have a significant effect on thermal inactivation of microorganisms and the food quality,and holds the potential to investigate the non-thermal effect of RF wave.Therefore,a heating block system(HBS)was developed to regulate the heating rates,target temperature and holding time in foods using a temperature controller.The influence of heating rate on the destruction kinetics of Escherichia coli ATCC 25922 in mashed potato and the associated changes in product color were evaluated using the HBS system.Finally,the result of inactivation comparison experiments using RF and the HBS systems under the same heating conditions suggested that non-thermal effects do not exist in RF systems at 27.12 MHz for microbial inactivation.The main contents and results were as follows.(1)The HBS system consisted of a heating unit and a control unit.The heating unit included top and bottom blocks,heating pads,and 6 pull-push boxes with 6 cells.The cell provides a sample space of 1.0 ml.The target temperature(up to 140℃),heating rate(0.1-13.5℃)and holding time can be controlled by using the proportion-integration-differentiation(PID)temperature controller in the heating block system for the microbial and food quality kinetic studies,with the the difference of temperature between predicted and experimental data was no more than 0.5℃.(2)A computer simulation model was developed to analyze the behavior of heating process in HBS.The the maximum difference heating rate between the simulation and the theoretical value was less than 0.8%.The value increased with the increase of the heating rate.Compared with higher heating rates,the lower heating rates achieved better heating uniformity in the TDT cells,and the maximum uniformity index value was 0.015.The real temperature profiles using RF energy could be approximately achieved in HBS by setting a number of linear regression curves with the maximum difference temperature of 0.85℃.(3)Distilled water,apple juice,milk,mashed potato,egg white,rice paste,rice,almond powder,peanut powder,peanut butter and ground beef were selected as representative liquid,semi solid and solid sample to evaluate the HBS’s performance by experiment.The results showed that the maximum difference of temperature between predicted and experimental data was 0.97℃ during the whole heating process.The synchronization and uniformity of temperature in peanut samples were evaluated by an infrared(IR)thermal imaging camera.The sample temperature data in six cells agreed well with each other since the maximum and average temperature differences were 0.46℃ and 0.32℃,and the maximum uniformity index value was 0.024,suggesting that the HBS could provide a sufficiently uniform heating environment over the six cells.(4)The influence of heating rate on the destruction kinetics of Escherichia coli ATCC25922 in mashed potato was evaluated using the TDT heating block system.Five heating rates(0.1,0.5,1.0,5.0 and 10.0℃/min),three final temperatures(55,57 and 60℃)and five holding times(0,5,10,30 and 60 min)were precisely controlled by the unique heating block system.Results showed that the slower heating rates(<1.0℃/min)gave significantly higher D-values presumably contributed by higher thermo-tolerance of E.coli ATCC25922 in mashed potato.At the holding temperature of 55℃,the D value was 15.44 min with the heating rate of0.1℃/min,while the D value was 2.50 min when using the heating rate of 10℃/min.The heating block system could precisely characterize the heat resistance of bacteria in foods to simulate the real sterilization and pasteurization treatments using radio frequency energy.(5)Kinetic models were also developed for color degradation in mashed potato during the same heating process with the TDT heating block system.Results showed that the first order reaction kinetic model gave a better fit to the color data.High correlation between microbial lethality and L~*&a~*values suggested that color change during thermal process could be used as an indicator not only for food quality change but also for the microbial destruction(food safety).The results confirmed that the D-and z-values derived from isothermal studies could be accurately estimated.The good thermal process could be achieved when the heating rates were≥1.0℃/min at least,and≥5.0℃/min at best in RF processing.(6)Escherichia coli and Staphylococcus aureus in apple juice and mashed potato were exposed to both RF and TDT heating block system to compare their inactivation populations.Results showed that a similar and uniform temperature distribution in tested samples was achieved in both heating systems.The survival patterns of two target microorganisms in two food samples were similar both for RF and heating block treatments since their absolute difference of survival populations was no more than 1 log CFU/ml.The solid temperature and microbial inactivation data demonstrated that only thermal effect of RF energy at 27.12 MHz was observed on inactivating microorganisms in foods. |
PDF Full Text Request