Study On The Inhibition Mechanism Of High Pressure Thermal Sterilization Treatment On Pressure-resistant Bacteria From The Cooked Ham

At present,the low-temperature meat products gradually come into our life.It’s urgent to solve the problem of short shelf life due to microbial contamination.It has been confirmed that Weissella viridescens and Leuconostoc mesenteroides are two typical spoilage bacteria in low-temperature meat products under ultra-high pressure processing.They still survived after ultra-high pressure processing,limiting the shelf life of low-temperature meat products.Using high pressure thermal sterilization treatment(lower than temperature of low temperature meat products processing:68～72℃),will achieve better effect of sterilization for pressure-resistant bacteria.Besides,it can reduce the nutrient loss of products but not damage the sensory quality.In the previous studies,the dynamic model of Weissella viridescens and Leuconostoc mesenteroides under high pressure thermal sterilization treatment was studied.The best processing conditions of ultra-high pressure suppression of the best smoked ham were:pressure 350 MPa,temperature 50℃,and the holding time 10 min.In this experiment,Weissella viridescens and Leuconostoc mesenteroides were used as tested strains.To investigate the inhibition mechanism of high pressure thermal sterilization treatment on these two pressure-resistant bacteria,the thesis includes two sections as following:1 Study on the effect of high pressure thermal sterilization treatment and the microstructure of Weissella viridescens and Leuconostoc mesenteroides.The section studied the effect of high pressure thermal sterilization treatment and bacterial microstructure.In this experiment,the thin agar layer was used to count after treatment.Scanning electron microscope(SEM)and transmission electron microscope(TEM)ware used to observe cell morphological changes and the ultrastructure of bacteria.The results showed that Weissella viridescens and Leuconostoc mesenteroides was in 19h and 14h respectively into the stationary phases.After high pressure thermal sterilization,the survival rate of Weissella viridescens and Leuconostoc mesenteroides was-4.84 and-6.58,respectively.Weissella viridescens were more heat-and pressure-resistant.High pressure thermal sterilization did not completely kill all bacteria,but caused cells to be in sublethal physiological condition,reduced the survival rate and delayed the growth.High pressure thermal sterilization induced obvious morphologic changes of cells.The morphologic changes and structure changes revealed by SEM and TEM showed that the in tracellular cytoplasm aggregated and a large electron transmission area appeared.2 Study on the biological changes of Weissella viridescens and Leuconostoc mesenteroides under high pressure thermal sterilization.The section studied the influence of cell membrane permeability and denaturalization of protein and nucleic acid.In this experiment,the growth curses of treated and untreated Weissella viridescens and Leuconostoc mesenteroides were made,and then the number of treated ones was evaluated by the thin agar layer(TAL).We measured flow cytometry analysis to study physiological status.The influence of cell membrane and wall permeability was evaluated by nucleic acid and protein’s leak and the activity of alkaline phosphatase.The denaturation of protein and nucleic acid was analyzed by using laser confocal micro-Raman.The results showed that high pressure thermal sterilization delayed the growth and caused cells to be in sublethal physiological condition,but damaged bacteria could be repaired in optimal conditions with adequate nutrition.High pressure thermal sterilization destroyed cell membrane and raised extracellular ultraviolet absorption content(P<0.0001).High pressure thermal sterilization affected the structure of the protein,nucleic acid,and lipid conformation of cell membrane,which destroyed the cell membrane permeability.On molecular level,high pressure thermal sterilization changed polysaccharide and protein’s structure on the surface membranes and lipid conformation of membranes of Weissella viridescens.The cell membrane permeability’s change of Leuconostoc mesenteroides was mainly due to protein denaturation.Besides,the activity of alkaline phosphatase was inhibited,which affected the normal metabolism of the cell.It indicates that the change of cell morphology and cell membrane permeability,the leak and degeneration of protein and nucleic acid could inhibit bacteria.The most important factor was cell membrane permeability’s increasing.In conclusion,High pressure thermal sterilization can not completely kill all bacteria,but cause cells to be in sublethal physiological condition.