Sublethal And Inactivation Law Of Staphylococcus Aureus During Frozen Stress

The study target of this paper were two strains of Staphylococcus aureusATCC6538and ATCC8095, with the purpose of understanding their sublethal andinactivation law during frozen stress, the total viable, non-injured cell number andsublethal rate under different frozen storage temperatures, freeze-thaw circle and asimulative cold chain were determined separately. On the basic of above study, developedpredictive inactivation kinetic models for pure cultures of Staphylococcus aureus atdifferent frozen storage temperatures initially, and explored the critical control point ofStaphylococcus aureus during cold chain circulation of quick frozen food. The mainconclusions of this study were as follows:1、The viable cell number of Staphylococcus aureus gradually reduced with the timeprolonged at different frozen storage temperatures, along with an increase in sublethalrate, however. In different frozen storage temperatures, the effect of frozen stress onStaphylococcus aureus were least at-25℃,-18℃，-11℃and-3℃were in the middle, at4℃, the cell were seriously hurt, with the most rapid reduction of viable cell number anda relative higher sublethal rate. The developed Weibull model at different frozen storagetemperatures indicated that when the frozen storage temperature were higher(4℃), thedecrease of viable cell number were first fast and then slower, that is to say, with thestorage time increased, Staphylococcus aureus began to adapt the disadvantageous lowtemperatures and developed some kind of capability of anti-frozen stress. On thecontrary,in the middle frozen storage temperatures, the decrease of viable cell numberwere first slow and then faster, suggested that these low temperatures had anaccumulative hurt on Staphylococcus aureus, cells were more easy to die with the storagetime increased. At the lower frozen temperature(-25℃), the inactivation mechanism ofthe two strains were different, Staphylococcus aureus ATCC6538showed theaccumulative hurt and ATCC8095developed the capability of anti-frozen.2、Comparatively, in the condition of successive freeze-thaw cycles, both total viablecells non-injured cells were increase with the frozen storage time prolonged. At the sametime, the inactivation curves of total viable and non-injured cell were coincided, and thesublethal rate were decreased with the time. All this showed that successive freeze-thaw cycles had a certain repair effect on sublethal cells. Compared with stored at constant-18℃, theviable cells of Staphylococcus aureus subjected to freeze-thaw treatment were higher, and1hour freeze-thaw treatment higher than0.5hour freeze-thaw treatment. Oppositely, thesublethal rate of Staphylococcus aureus stored at constant-18℃were obviously higherthan both freeze-thaw treatments. This also demonstrated that sublethal cells were theintermediate transition state of Staphylococcus aureus during inactivation, andfreeze-thaw cycles could promote sublethal cells recover to normal cells.3、Under the condition of temperature fluctuation simulating food cold chain,although the total viable cell number decrease during the circulation, the non-injured cellnumber kept a coincident fluctuating curve with the changes of temperature. WhenStaphylococcus aureus were at25℃before determination, both the total viable numberand non-injured cell number would increased. But stored at-18℃followed thetemperature fluctuation would lead to an accumulative hurt of cells until they were killed.Meantime, the sublethal rate showed an increased trend with the time. From the result ofsimulating food cold chain, temperature fluctuation during cold chain circulation mighthave a closely relationship with the frequently detection of S. aureus in quick frozen food.Considering the sublethal and inactivation law in all three frozen stress above, initiallydetermined the critical control point of Staphylococcus aureus during cold chaincirculation were the process of loading and unloading products and consumer purchasing.