| Almonds have high economical and nutritional values.With people’s increasing health concerns,the consumer market for almonds and other nut products has gradually increased.Almonds may be contaminated by food-borne pathogens,such as Salmonella from the soil,water or air during harvesting,drying,transportation processes.Before consuming,raw almond kernels may be stored for 12 months or longer.Since the moisture content of almonds during storage is around 8% w.b.,it is generally considered safe.However,many cases of Salmonella infection are related to raw almonds during storage.In addition,the U.S.Department of Agriculture stipulates that the food-borne pathogens in the raw almonds must meet the pasteurization requirement of at least 4-log reductions to achieve the safe consumption level.Therefore,to explore the factors that affect the survival and heat resistance of the target bacteria in almonds is necessary for the nut industry to develop an effective pasteurization process.In this paper,almond kernels are used as the research object.The research focuses on the kinetics of heat inactivation of the target bacteria,the verification of the pasteurization effect,and the exploration of the inactivation mechanism.The specific research contents are as follows:(1)Using the controlled atmosphere(CA)heating block system to change the gas concentration during the heating process,study the thermal inactivation kinetics of the target bacteria under different heating rates,target temperatures and holding times,and quickly obtain modified atmosphere-thermal inactivation kinetic parameters of the target pathogen.(2)Using the microbial heating block system,study the influence of pretreatment of different packaging methods,gas concentrations,storage time,and storage temperature on the heat resistance of the target bacteria,and simultaneously detect the change trends of gas concentration and water activity during storage.(3)Optimize the radio frequency(RF)pasteurization process parameters through the obtained thermal inactivation kinetic parameters of bacteria under modified atmosphere pretreatment.Use the free-oscillating RF system to verify the pasteurization effect of the modified atmosphere packaging(MAP)pretreatment-assisted RF pasteurization method.(4)Use the obtained modified atmosphere-thermal inactivation kinetic parameters to optimize the simultaneous atmosphere-assisted RF pasteurization process during the heating process.A 50 Ω RF system was used to verify the pasteurization effect,and finally the main quality indicators of almond kernels after treatments were evaluated.(5)Modified atmosphere-heat treatments were performed on the inoculated almond powder.Scanning electron microscopy was used to observe changes in the surface morphology of Escherichia coli,and through transcriptome sequencing technology to analyze the changes in different genes of the target bacteria under the combined action of modified atmosphere and heat,and to explore the action mechanism of modified atmosphere-heat from the cellular level to the molecular level.By analyzing the test data,the following main conclusions were obtained:(1)The air-tightness of the CA heating block system was good.The relative leakage did not exceed 2.2%,the O2 and CO2 concentrations were relatively stable,the average gas concentration deviation was within 0.2%.The accuracy of the system temperature stability could be controlled within ± 0.5 ℃.It indicated that the system could be used as a reliable device for evaluating the kinetics of E.coli ATCC 25922.The D-values and z-value of E.coli under regular atmosphere(21% O2,0% CO2)treatment were greater(p < 0.05)than those of modified atmosphere(2% O2,20% CO2)treatment.For modified atmosphere treatment to reach the 5-log reduction requirement,it needed to be held at 75 ℃ for 43 min.When the heating rate was greater than or equal to 1 ℃/min,the D-values had no significant difference(p > 0.05).When the heating rate was less than 1 ℃/min,the D-values of E.coli under regular atmosphere(RA)treatment increased with the decreased heating rates,while the D-values of E.coli under modified atmosphere treatment decreased with the decreased heating rates.(2)During the storage period,the sample moisture content,water activity,and gas concentration in the packaging bag were relatively stable.The surviving curves and thermal inactivation of E.coli ATCC 25922 fitted well to Weibull and first-order-kinetic models,respectively.As storage temperature increased,the effect of storage atmosphere reduced bacterial populations and D75-values of E.coli ATCC 25922 became more remarkable.Stored with the MAP at 24 ℃ for 12 months,the surviving populations of E.coli ATCC 25922 in 6.0 %of wet base decreased by 2.55 log/CFU,and the pasteurization of 4-log reductions can be achieved by holding the temperature at 75 ℃ for 50.4 min.Compared with RA packaging,the D-values at 75 ℃ with CA decreased by 31.2%.(3)The storage conditions(such as gas concentrations,moisture content of samples,and water activities)were relatively stable during pre-storage,and the temperature variations of the RF treated circular inoculation area were within 2 °C.E.coli ATCC 25922 was less resistant after 12-week storage at 24 °C under MAP conditions,and the 4-log reduction time needed for MAP was 18 ± 1 min holding after RF heating to 75 ± 2 °C,reducing 40% pasteurization time as compared with RAP storage.Fatty acid(FA)and peroxide value(PV)increased significantly with the increase of storage time(p < 0.05),but the quality of treated samples remained within acceptable range of nut industry standard(FA < 0.6%,PV < 1.0 meq/kg).(4)The air tightness and gas concentration of the CA-RF system were relatively stable.The water content and water activity of the samples decreased after treatments,but the small decrease range did not have a major impact on the survival populations of E.coli.In the heating stage,the RF power level was 900 W and the interval gap was 11.5 cm to quickly heat up.Based on different processing conditions,the RF input power was reduced from 900 W to the range of 25-225 W in the holding stage.The minimum pasteurization time for achieving 4-log reductions was 21 ± 1 min by holding at 75 ± 3 °C under CA conditions,reducing 38 %pasteurization time as compared with RA treatment.The major quality attributes of all samples remained within the accepted level(PV < 1.0 meq/kg,FA < 0.6%,and L* > 40)of nut industry standard during RA/CA assisted RF heating and storage periods.(5)Scanning electron microscope observation revealed that when the samples of the CAheat treatment group reached the target temperature,the surface of E.coli showed a small amount of shrinkage and deformation.The deformation of the conventional gas group and the changed concentration gas group had no much difference.In the CA-heat samples held at target temperature for 10 min,the surface morphology of E.coli had obvious shrinkage and deformation.The deformation degree of the changed concentration gas group was slightly greater than that of the conventional gas group.Transcriptome data analysis showed that RA treated samples had 23 differential genes compared with CA treated ones(18 up-regulated genes and 5 down-regulated genes),and RAT had 381 differential genes(185 up-regulated genes and 196 down-regulated genes)compared with CAT treated ones,respectively.These genes were enriched in metabolism,cell parts,catalytic activity,binding and other functions.Compared with the control group O,the number of genes expressed in cell growth,metabolic processes,reproduction,and translation regulator activity was down-regulated in the treatment group,and the number of genes expressed in functions,such as nitrogen metabolism and sulfur utilization,was up-regulated.Compared with CA and CAT,RA and RAT had up-regulated the number of expressed genes for stress response,reproduction process,and macromolecular complexes,as well as the expressed genes for signal transduction activities and carbon utilization.The enrichment of KEGG signaling pathway showed that compared with the RA and RAT in the conventional air-modified group,the differential genes were mainly enriched in the microbial metabolism in diverse environments,metabolic pathway,two-component system and ribosome pathways. |
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