Study Of Survival Of Salmonella In Skim Milk Powder And Effect Of Radio Frequency Sterilization

Low water activity foods, such as peanut butter, chocolate, dried milk, nuts and so on, usually do not support growth of pathogens because of low water activity. However, in recent years, a number of outbreaks associated with low water activity foods occurred, making people have a new understanding of microorganisms about low water activity foods and resulting in the safety of these foods becoming a major concern. Traditional thermal sterilization methods are difficult to be applied in low water activity foods because of its poor heat transfer and strong heat resistance of microorganisms. At present, chemical fumigation and irradiation are the mainly sterilization methods used in low water activity foods. However, chemical fumigation sterilization easily leads to chemical residue and irradiation sterilization has not been widely accepted by people. Hence, there is an urgent need for new sterilization technique to control microorganisms in low water activity foods. Radio frequency(RF) is considered as one of the most potential technologies in food industry and has an excellent prospect on content of the advantages of rapid and uniform heating.In this study, Salmonella was chosen as the object. The relationship between water activity and water mobility, the factors on the survival and heat resistance of Salmonella in skim milk powder, and sublethal laws of heating of Salmonella were investigated. In addition, factors influencing sterilization effect of radio frequency on Salmonella in skim milk powder were discussed. At last, Logistic model, Weibull model and Gompertz model were used to fit the survival curve and inactivation kinetics model of Salmonella in skim milk powder.Firstly, the effects of water activity, water mobility, storage temperature and particle size on the survival of Salmonella in skim milk powder were studied. Results showed that water activity and storage temperature significantly influenced the survival of Salmonella. And water mobility effected the survival at aw 0.81. However, size of particle had no effect on its survival. Logistic model, Weibull model and Gompertz model were used to fit the survival curve of Salmonella in skim milk powder. Storage temperature had a significant effect on fitting of models. At 37℃, the best fitting model was Weibull model. At 4℃, these three models were inappropriate to predict the survival of Salmonella.Secondly, the inactivation of Salmonella in skim milk powder treated with heating was studied. And heating inactivation curve of Salmonella was fitted with selected models. Inactivation lows of Salmonella and fitting models were significant different at different heating temperature. Particle size of skim milk powder did not influence the inactivation of Salmonella at 60℃, 70℃ and 80℃ with the exception of 90℃.Method of selective medium was used to study the sublethal cells treated with heating in skim milk powder. Results indicated that sublethal laws of Salmonella differentiated with different heating temperature: the heating temperature of 60℃ mainly caused the damage of cell membrane; heat treatment of 70℃ leaded to the death of cell by the damage of cell membrane and outer membrane; heat treatment of 80℃ didn’t cause the damage of cell membrane and outer membrane when killed Salmonella cell; the inactivation of Salmonella was mainly due to the outer membrane damage at 90℃.Effect of sterilation of radio frequency on Salmonella in skim milk powder was studied. The result showed that radio frequency could kill Salmonella effectively without causing damage to the cell membrane or plasma membrane. Distance between electrode plates significantly influenced the effectiveness of sterilization and the smaller of the electrode plates distance the better of the sterilization. Logistic, Weibull and Gompertz models were used to fit radio frequency bactericidal kinetics of Salmonella. Results showed that Weibull model was the best.