Use of hydrostatic pressure for inactivation of microbial contaminants in cheese

The objective of this study was to determine the effect of high pressure (H P) on the inactivation of microbial contaminants in Cheddar cheese (Escheri chia coli K-12, Staphylococcus aureus ATCC 6538, and Penicillium roqueforti IMI 297987), Initially, cheese slurries inoculated with E, coli, S. aureus , and P, roqueforti were used as a convenient means to define the effects o f a range of pressures and temperatures on the viability of these microorga nisms. Cheese slurries were subjected to pressures of 50 to 800 MPa for 20 min at temperatures of 10, 20, and 30 degreesC, At 400 MPa, the viability o f P. roqueforti in cheese slurry decreased by >2-log-unit cycles at 10 degr eesC and by 6-log-unit cycles at temperatures of 20 and 30 degreesC, S. aur eus and E. coli were not detected after HP treatments in cheese slurry of > 600 MPa at 20 degreesC and >400 MPa at 30 degreesC, respectively. In additi on to cell death, the presence of sublethally injured cells in NP-treated s lurries was demonstrated by differential plating using nonselective agar in corporating salt or glucose. Kinetic experiments of BP inactivation demonst rated that increasing the pressure from 300 to 400 MPa resulted in a higher degree of inactivation than increasing the pressurization time from 0 to 6 0 min, indicating a greater antimicrobial impact of pressure. Selected cond itions were subsequently tested on Cheddar cheese by adding the isolates to cheese milk and pressure treating the resultant cheeses at 100 to 500 MPa for 20 min at 20 degreesC. The relative sensitivities of the isolates to HP in Cheddar cheese were similar to those observed in the cheese slurry, i,e ,, P. roqueforti was more sensitive than E. coli, which was more sensitive than S. aureus, The organisms were more sensitive to pressure in cheese tha n slurry, especially with E. coli, On comparison of the sensitivities of th e microorganisms in a pH 5.3 phosphate buffer, cheese slurry, and Cheddar c heese, greatest sensitivity to HP was shown in the pH 5.3 phosphate buffer by S. aureus and P. roqueforti while greatest sensitivity to HP by E. coli was exhibited in Cheddar cheese. Therefore, the medium in which the microor ganisms are treated is an important determinant of the level of inactivatio n observed.