Abstract:The technology of air cooling, vacuum cooling with different evacuation rate, final pressure value and repressuring time utilized to lower the temperature of iceberg lettuce were investigated in this work. All of samples were stored at the temperature of 5±2 ℃ after cooling. the indice including the total and internal Escherichia coli O157:H7 cell counts were detected to evaluate the effects of different evacuation rates, final pressure values and repressuring times on the iceberg lettuce. Effects of different evacuation rates, final pressure values and repressuring times on the variation of the total and internal Escherichia coli cell counts during cold storage were also evacuated. The results showed that all vacuum cooling methods had a more internal cell conts of Escherichia coli of 4.5×103 cfu/g and less total cell counts of Escherichia coli of 1×106 cfu/g compared to the internal and total cell conts of Escherichia coli of 1.9×103 cfu/g and 2.3×106 cfu/g subjected to air cooling. It was found that higher evacuation rates, lower final pressure value and quicker repressuring time led to a more infiltration of Escherichia coli into the inner of iceberg lettuce. Compared to air cooling, vacuum cooling was more effective to decrease the cell counts of total Escherichia coli. Compared to the reduction rate of 69.57% of the total cell conts of Escherichia coli after 3 d cold storage, all vacuum cooling methods had a more reduction rate of 90%. All vacuum cooling methods had a higher internal Escherichia coli cell counts of 1.5×102 cfu/g after 3 d cold storage than 1.1×102 cfu/g subjected to air cooling. It was reasonable to assumed that controlling the cell counts of Escherichia coli before vacuum cooling and adjusting properly the evacuation rate of 0.229 min-1, final pressure value of 1500 Pa and repressuring time of 9min might be a effective method to assure the quality and safety of leaf vegetable. These above-mentioned conclusions could provide a reference for the mechanism of filtration of Escherichia coli into the inner of iceberg lettuce.
