Thermal Treatment Kinetics And Microwave Pasteurization Technique Of Cold Smoked Salmon

Listeria monocytogenes,ubiquitous in nature and salt,and resistant in low temperature,is a main pathogenic bacterium in refrigerated products threatening human health.Cold smoked salmon has been largely comsummed because of its delicate meat and aromatic flavor.But it is facing high risks due to being contaminated by Listeria monocytogenes and lacking the heat pasteurization process.Microwave-assisted pasteurization,a simple and fast heating,is widely studied and applied as a heat treatment pasteurization technique,which has broad application prospects,especially in solid and semi-solid food products.Many reactions,such as sterilization,enzyme inactivation and quality deterioration,exist during heat treatments,so it's important to consider both pasteurization effectiveness and product quality.Studying on thermal inactivation and quality changing kinetics in cold smoked salmon and building the corresponding models are of great importance for technical parameter selections.Effects of microwave heat treatment are closely related to food dielectric properties and at the same time may result in the uneven heating.Due to individual differences and representativeness of real samples,developing model food product with similar dielectric properties of cold smoked salmon products and analyzing microwave pasteurization with the help of computer simulation are useful to quickly optimize the technical parameters of microwave pasteurization and establish the treatment protocols.This dissertation,focusing on microwave pasteurization of Listeria monocytogenes in cold smoked salmon,has a systematic study of heat treatment pasteurization from four aspects:thermal inactivation of Listeria spp.,quality changes kinetics,dielectric properties,model food development,and microwave-assisted pasteurization.The main content and results are as follows:?To observe its thermal inactivation curve and thermo kinetic parameters,cold smoked salmon filets inoculated with 3 strains of Listeria monocytogenes(ATCC19114,ATCC7644,ATCC19113)and 1 strain of innocuous Listeria innocua(ATCC51742)were subjected to heating with temperatures ranging from 58?to 66?in an glass capillary tube with 1.8 mm inside diameter and the 3 mm outside diameter.Results showed that all 4 strains(ATCC19114,ATCC7644,ATCC19113 and ATCC51742)exhibited the typical first-order inactivation features.The D value of both Listeria monocytogenes and Listeria innocua dropped dramatically when temperature rose from 58?to 66?.The z value of target bacteria was 5.23~6.45?.Both thermal-death-time(TDT)curve and k-T curve could reliably predict the activation energy of target microorganism with the difference less than3.0%.When at 60~64?,Listeria innocua ATCC51742 could be used as the experimental verification bacterium of heat inactivation of cold smoked salmon.?Sealed aluminum heating unit(D35 mm×H6 mm)was used to observe quality index(cook loss,CIE,and shear force)change of cold smoked salmon(D30 mm×H6 mm),which was heated at different temperature(50~70?)with different time(40~120min)so as to fix the kinetic model of various indexes and the thermal kinetic parameters.Combined with heat inactivation kinetics of Listeria monocytogenes in cold smoked salmon,the present study tried to define the product's heat pasteurization and quality keeping equilibrium region and select the best heat treatment parameter.Results showed that cook loss,a~*,b~*and shear force followed first-order reaction while L~*followed zero-order reaction;activation energy a~*>b~*>L~*>shear force>cook loss;CIE a~*was most sensitive to temperature and closely related to thermal inactivation of Listeria monocytogenes.The best region for heat treatment(temperature-time combinations)was the overlapping area below CIE a~*quality deterioration curve and above Listeria monocytogenes(ATCC19113)death curve,which could meet the needs of pasteurization and at the same time maintain high quality of the product.?Dielectric properties like dielectric constant and dielectric loss factor of cold smoked salmon were measured between 300 MHz and 3000 MHz using an open-ended coaxial-line probe technique at temperatures between 20?and 80?.Correlation between conductivity and dielectric loss factor was studied,and also the penetration depth of cold smoked salmon was estimated under different frequencies.Results showed that dielectric constant negatively correlated with frequency.When frequency was smaller than 1000MHz,dielectric constant of the sample dropped significantly with increasing frequency,but increased with increasing temperature.When frequency was between 1000~3000MHz,dielectric constant dropped mildly and had a linear negative correlation with frequency and temperature.While dielectric loss factor had a negative correlation with frequency,it had a positive one with temperature.When frequency was less than 2000MHz,dielectric loss factor dropped significantly with incrasing frequency;under higher frequency(>2000MHz),the dielectric loss factor of cold smoked salmon changed mildly with frequency.The calculating value of conductivity for dielectric loss factor in lower frequency range approached approximately the measured value and the dielectric loss factor could be calculated by measuring conductivity of the sample.Penetration depth had a negative correlation with both frequency and temperature,and was relatively small and ranged between 2.22~9.45mm under different frequencies because of the high salt contents in cold smoked salmon and its high dielectric loss factor.?Gellan gum was used to get simulated foods by using different ingredients to match with the similar dielectric properties of cold smoked salmon.Using semi-consecutive microwave heating system with single mode output under 915MHz,30kW,and with computer simulation and microwave-assisted pasteurization,the temperature distributions were mapped and analyzed and the cold/hot spots were determined after microwave heat treatment.Results indicated that simulated food with 0.5%gellan gum,0.5%polyacrylate,5%olive oil,1.5%D-ribose,1%lysine,2.5%salt,0.1%calcium chloride,and 88.4%distilled water matched well with the dielectric properties of cold smoked salmon and could be used as simulated food in microwave pasteurization.Comparing with the microwave pasteurization process of simulated food at 64?and computer simulated heat patterns suggested that computer simulation results were in great accordance with the actually measured data.So computer simulation could be a preferable way of optimizing microwave heat treatment technique.The study finally got the cold spot of microwave heat treatment.?Designed with 6 logs reduction of populations of Listeria monocytogenes in cold smoked salmon with pasteurization,experiments of microwave pasteurization and hot water pasteurizationwereconducted,showingthatwhenheated3.1m,preheat temperature/microwave heated temperature/heat preservation temperature were 30?/64?/6?,respectively,with microwave power of 7.0 kW/6.2 kW/2.6 kW/2.5 kW respectively.Preheated 10 min and cooled 3 min,sample size of 128 mm×86 mm×10 mm,under microwave treatment condition with sample mass of 113g,belt speed of 162.56cm/min met the pasteurization requirement.Microwave-assisted pasteurization claimed shorter time for sterilizing and at the same time maintained product quality,so the technique of microwave-assisted heat sterilization could be used to instantly sterilize the product.