Bacillus sporothermodurans is a milk spoilage bacterium producing resistant endospores that survive ultra-high temperature treatment. The inactivation of B. sporothermodurans LTIS27 spores by combined hydrostatic pressure and heat treatments was studied. A central composite experimental design was used to evaluate the effect of pressure (300-500 MPa), temperature (30-50 degrees C), and pressure-holding time (10-30 min) on the inactivation of spores in distilled water and skim milk. The inactivation observed was shown to fit well with the values predicted by the quadratic equation, since R-adj(2) were 0.970 and 0.977 in distilled water and milk, respectively. By analyzing the response surface plots, the inactivation was shown to be higher in distilled water than in milk under all the conditions tested. This was probably due to a protective effect of milk against inactivation by pressure. The optimum process parameter values for a 5-log cycle reduction of spores were calculated as 477 MPa/48 degrees C for 26 min and 495 MPa/49 degrees C for 30 min in water and in milk, respectively. This study shows the efficiency of hydrostatic pressure in combination with moderate temperature to inactivate B. sporothermodurans spores. Such treatments could be applied by the dairy industry to ensure the commercial sterility of UHT milk. (C) 2012 Elsevier Ltd. All rights reserved.