Establishment Of Growth Prediction Model Of Staphylococcus Aureus In Salmon Storage Process And Study Of Photodynamic Sterilization Technology

Staphylococcus aureus is a kind of common foodborne pathogenic bacteria,which does not require high nutritional conditions for growth,and widely exists in fish,meat,eggs,milk and their products.When the mount of S.aureus reaches a certain level in food,heat-resistant enterotoxin may be produced under certain conditions,which will bring food safety hazards to consumers.In recent years,poisoning incidents of S.aureus have often occurred in aquatic products such as fish and shrimp shellfish.Among them,salmon,as a common raw fish,is rich in nutrients and lack of cooking treatment,which is more prone to S.aureus contamination.Therefore,through the investigation of the growth law of S.aureus during salmon cold-storage,a predictive model of S.aureus growth at different temperatures was established.And in the presence of the photosensitizer curcumin,the bacterial effect and related mechanism of photodynamic sterilization technology on S.aureus were explored,which provided theoretical reference and guidance for the monitoring and control of S.aureus in food.1.To investigate the growth regularity of S.aureus during salmon storage and build a growth prediction model.S.aureus was inoculated into fresh salmon,and primary models such as the modified Gompertz model,the modified Logistic model and the Baranyi model were used to fit the growth data of S.aureus at different temperatures.After comparison,it was found that the modified Gompertz model has the best fitting effect,and the average values of R~2,RMSE,AIC and BIC were 0.9961,0.0935,-63.2034 and-62.0122,respectively.On the basis of the first order model,the square root model of temperature-maximum specific growth rate(μ_max)√μ_max=0.02523T-0.08002and the polynomial model of temperature-lag period（λ）λ=17.52-0.9362×T+0.01333×T~2were further established,the R~2,deviation factor B_f and accuracy factor A_f values were 0.994、0.998、1.068 and 0.974、1.051、1.111,respectively,which effectively described the influence of temperature on the growth parameters of S.aureus.2.Using the natural pigment curcumin as photosensitizer,the sterilization test of photodynamic sterilization technology on S.aureus was carried out.The results showed that the concentration of photosensitizer and the illumination time had significant influence on photodynamic sterilization effect.With the increase of curcumin concentration and the extension of illumination time,the number of S.aureus decreased rapidly.The combination of 10μM curcumin concentration and 15 min illumination time was optimal,which could inactivate 6.9 log CFU/m L S.aureus.Furthermore,the inactivation kinetics of S.aureus was studied with the modified Gompertz model at five different initial bacterial concentrations,including 1×10~8,1×10~7,1×10~6,1×10~5 and1×10~4,and it was found that lower bacterial density was less resistant to photodynamic sterilization.In addition,photodynamic technology can effectively play a bactericidal role in cold fresh salmon,and the killing of S.aureus can reach 0.96 lg CFU/g,which has the prospect to be applied in food.3.The mechanism of photodynamic inactivation of S.aureus was analyzed.Scanning electron microscopy was used to observe the morphological changes of bacteria before and after sterilization,and it was found that the cell surface of S.aureus collapsed accompanied by the outflow of intracellular dissolved substances;In the determination of cell membrane function,it was found that the leakage of intracellular proteins and nucleic acid increased with the increase of sterilization time,indicating that the permeability of the cell membrane was gradually destroyed;By combining 5-CFDA and PI dyes with flow cytometry,it was found that the integrity of the cell membrane and the activity of esterase were changed after photodynamic sterilization;In addition,photodynamic photodynamic sterilization led to an increase in intracellular ROS content,further showing that ROS may be the reason why photodynamic reaction played a bactericidal role.To sum up,this study took S.aureus as the research object,and the established growth model effectively predicted the growth changes of S.aureus during the storage process of fresh salmon,and provided theoretical basis and guidance for risk assessment of microorganisms in the production and transportation of salmon.In addition,the photodynamic sterilization technology with curcumin as photosensitizer efficiently killed S.aureus,which had the characteristics of safety and low energy consumption,and had great application potential in food.