Tolerance Of Salmonella Typhimurium In Red Pepper Powder Under Radio Frequency Heating Stress

Recently,many outbreaks of Salmonella associated with low-moisture foods(LMFs)have attracted people's attention.Previous studies showed radio frequency(RF)heating had good pasteurization effect on LMFs.Thus,Salmonella Typhimurium as target bacteria and red pepper powder as typical LMFs were selected for the investigation of sublethal,inactivation and stress resistance mechanism of Salmonella in red pepper powder under RF heating stress.This study mainly explored the thermal inactivation and sublethal effect caused by RF heating from the aspects of water activity(a_w),pasteurization temperatures,heating rates and food matric,and investigated the potential direct and cross protection effects of repaired cells under different environment stresses,then elucidated the tolerance mechanism of sublethal injured cells(SICs)for better understanding the cause of cell death,and finally volatile components and antioxidant activity of red pepper powder after RF treatments were investigated for providing theoretical guidance for the development of RF pasteurization technology of low moisture powdery foods.1.Sublethal injury of Salmonella under RF heating stress.According to the previous study,sample initial a_w had influence on RF pasteurization effect,on this basis,the effect of a_w on sublethal injury was studied.To further confirm the results,the influence of SICs repair conditions(repair temperature,repair time and selective mediums),RF pasteurization temperature(60-100°C)and heating rate(1.9-17.6°C/min)were investigated by conducting a series of single factor experiments.Results demonstrated that there were SICs in red pepper powder with relatively high initial a_w(?0.53),and we also confirmed that there were no SICs in the powder with low initial a_w(0.44)by excluding all potential influence factors such as repair conditions,RF pasteurizing temperature and heating rate.2.Direct protection and cross-protection effects of repaired SICs.The response of Salmonella repair cells with RF heating pretreatment under different environment stresses(15%ethanol,10%Na Cl,p H 3.0 acid buffer solution)were studied and was verified by the study of cell membrane fatty acid composition and cell morphology.Results indicated that SICs can be completely repaired within 5 h,and there were no direct protection and cross-protection effects for repaired Salmonella.Besides,the UFA/SFA ratio in cell membrane fatty acid profile and cell morphology indirectly confirmed the results.3.The effect of food components on thermal resistance under RF heating stress and related gene expression regulation.The study explored the effect of food components on thermal tolerance of Salmonella,and investigated the gene expression levels of RF treated and repaired cells from sigma fators(rpo S,rpo E,rpo H),fatty acid biosynthesis-associated genes(cfa,fab A,fab D)and cytochrome bo oxidase genes(cyo A,cyo B,cyo C).Results showed that when the protein content was 8%,or the fat content was 6%and 12%for single component study,there was significant difference on inactivation compared with control group,and increased protein and fat contents in red pepper powder were helpful to protect Salmonella from heat injury,which resulting in the decreased thermal inactivation,but the trend was contrary to starch.Also,the relative gene expressions of Salmonella Typhimurium were all up-regulation under RF heating stress,while the genes of repaired cells had down-regulated trend except rpo S after repair treatment,indicating that the membrane structure of cells was damaged under RF heating,and the stress response of cells was alleviated by the up-regulation of genes in order to reduce the damage degree.4.Study on the quality of red pepper powder based on its volatility and antioxidant activity after RF heating.With the increase of temperature,the contents of esters,alcohols and alkenes decreased due to thermal decomposition,while the contents of aldehydes and ketones increased significantly in red pepper powder after RF heating.RF heating had no significant effect on the DPPH·radical scavenging ability of red pepper powder,but it will reduce the ABTS radical scavenging ability and increase the reduction ability of iron ion and copper ion,while in the temperature range of experimental study,the effect of different holding time on the antioxidant activity was relatively small.