FACTORS AFFECTING THE RESISTANCE OF LISTERIA-MONOCYTOGENES TO HIGH HYDROSTATIC-PRESSURE

Several factors relevant to food preservation had large effects on the resistance of L, monocytogenes to high hydrostatic pressure. Cells in the stationary phase of growth were much more resistant to pressures above 200MPa than those in the exponential phase. Following treatment for 10 min at 400 MPa viable numbers of stationary phase cells were reduced by only 1.3 log(10) units compared with more than 7 log(10) units for exponentially growing cells. Stationary phase cells were sensitised to pressure by butylated hydroxyanisole, potassium sorbate and by acid conditions. The most effective sensitiser was BHA which, at 1.55 mM, caused a 10(4)-fold enhancement of killing by treatment at 300 MPa for 10 min. Propyl hydroxybenzoate, butylated hydroxytoluene and sodium ascorbate were without effect. Xylitol (15%) protected log phase cells against inactivation by pressure: after treatment at 350 MPa for 10 min viable numbers were reduced by only 0.24 log(10) units compared with 3.4 log(10) units in the absence of xylitol. The physicochemical environment can substantially increase or decrease resistance to pressure and these effects will require quantification if the microbiological effects of pressure are to be predicted.