Hydrostatic pressure may affect the intracellular pH of microorganisms by (i) enhancing the dissociation of weak organic acids and (ii) increasing the permeability of the cytoplasmic membrane and inactivation of enzymes required for pH homeostasis. The internal pHs of Lactococcus lactis and Lactobacillus plantarum during and after pressure treatment at 200 and 300 MPa and at pH values ranging from 4.0 to 6.5 were determined. Pressure treatment at 200 MPa for up to 20 min did not reduce the viability of either strain at pH 6.5. Pressure treatment at pH 6.5 and 300 MPa reduced viable cell counts of Lactococcus lactis and Lactobacillus plantarum by 5 log after 20 and 120 min, respectively. Pressure inactivation was faster at pH 5 or 4. At ambient pressure, both strains maintained a transmembrane pH gradient of 1 pH unit at neutral pH and about 2 pH units at pH 4.0. During pressure treatment at 200 and 300 MPa, the internal pH of L. lactis was decreased to the value of the extracellular pH during compression. The same result was observed during treatment of Lactobacillus plantarum at 300 MPa. Lactobacillus plantarum was unable to restore the internal pH after a compressiondecompression cycle at 300 MPa and pH 6.5. Lactococcus lactis lost the ability to restore its internal pH after 20 and 4 min of pressure treatment at 200 and 300 MPa, respectively. As a consequence, pressure-mediated stress reactions and cell death may be considered secondary effects promoted by pH and other environmental conditions.

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