The Study On The Growth, Enterotoxin Producing And There Inhibitor Methods Of Staphylococcus Aureus In Whipping Cream

The growth and production of enterotoxin by Staphylococcus aureus in whipping cream and the inbibitory effects of aw、p H and Nisin on it were studied in this thesis. The Staphylococcus aureus which producing SEA was isolated from kinds of fresh food from farmers’ market and identified by PCR.Two concentration levels of Staphylococcus aureus strains which produce SEA enterotoxin were inoculated into whipping cream stored at different temperatures, the low initial inoculating level was controlled in 2 ~ 3 lg(CFU/g) and the high initial inoculating level was controlled over 4 lg(CFU/g), the plates counts of staphylococcus aureus and staphylococcal enterotoxin were tested regularly, and the S. aureus Enterotoxin was detected by ELISA kit.Afterward, S.aureus strain which produce SEA was inoculated into whipping cream and stored at 36℃±1℃ for 36 h under various aw、p H and Nisin content conditions. The modified Gompertz model was used todescribe the S.aureus growth in whipping cream under various conditions. The inbibtory effects were analysised in this study. The conclusions of this paper were as follows:Firstly, 10 strains of staphylococcus aureus was isolated from fresh food. 6 strains among them were enterotoxin nengetive and 4 were enterotoxin positive tested by ELISA kit test. In positive strains, 2 strains of enterotoxin positive were identified to produce SEA by PCR.Secondly, growth rate of S aureus present a certain positive correlation with temperature. Growth rate decreased and the Enterotoxigenic time delayed with the decreasing of the temperature; In high inoculation level, staphylococcus enterotoxin were detected in 36℃after 12 h, in 25℃after 24 h and in 15℃after 66 h. In low temperature(5℃), S aureus did not grow and there was no staphylococcus enterotoxin could be detected.Different initial inoculation level also had effets on the growth and Enterotoxigenic time of staphylococcus aureus.the growth rate in high inoculation was faster than in the low inoculation.In low inoculation level, staphylococcus aureus enterotoxin was detected only in 36℃ after 27 h,During the moderate temperature(25 ℃ 、 15 ℃) there were no staphylococcus enterotoxin detected.However, in high inoculation level,the staphylococcus enteroxoin could be detected in 36℃,25℃and15℃,in low temperature(5℃), Saureus did not grow and there was no enterotoxin could bedetected. Enterotoxigenic time of the high initial inoculation level was early than that of the low initial inoculation level; The enterotoxin could be detected in the middle or late term of the late lgarithmic phase where the plates counts of S aureus reached 6lg(CFU/g).Thirdly, the varying of water activity(aw), p H and Nisin content in whipping cream were selected to test the inhibitory effects, the result from aw group showed that the aw of whipping cream decreased with the increased content of sorbitol,the results from the modified Gompertz model presented that the growth rate decreased and the enterotoxingenic time was delayed with the water activity decreased.In p H groups, the growth rate decreased and the enterotoxingenic time was delayed with the p H decreased. The S. aureus growth in the cream could not be inhibited completely with various nisin content, however the enterotoxin time could be delayed.Thirdly, the water activity was changed by adding different concentration sorbitol, the result showed that the growth rate decreased and the enterotoxingenic time was lengthend with the water activity decreased through the fitting Gompertz model. The lag time also were lengthened and the population in stationary phase reduced.In p H experiment groups, the growth rate decreased and the enterotoxingenic time was lengthend with the p H decreased through thefitting Gompertz model. the S. aureus growth in the cream could not be inhibited completely, but the enterotoxin time was delayed.Forthly, 7% sorbitol was the minimum lever in aw group, however, the SEA from the cream containing 7% sorbitol was much late to be detected than that from the cream of p H5.5 or containing 0.5g/Kg Nisin, the latter two were both the maximum lever in their group. Thus we can conclude that the aw of whipping cream reduced by adding sorbitol has better inhibit effect on S.aureus growth than other methods, and the results also indicated that the inhibit effect would more strong with more high content of the sorbitol. Considering the cost and effect, 7%~14% sorbitol content is probably appropriate for the whipping cream formulation.In summery, the growth and production of enterotoxin by S aureus in whipping cream and the inhibitory effects of water activity,p H and Nisin were tested in this thesis. This study could provide a theoretical reference for the risk assessment of whipping and may be beneficial for cream man-ufactures to improve the safety of the products.