High‐pressure inactivation of dried microorganisms

Abstract

Dried microorganisms are particularly resistant to high hydrostatic pressure effects. In this study, the survival of Saccharomyces cerevisiae was studied under pressure applied in different ways. Original processes and devices were purposely developed in our laboratory for long-term pressurization. Dried and wet yeast powders were submitted to high-pressure treatments (100-150 MPa for 24-144 h at 25 degrees C) through liquid media or inert gas. These powders were also pressurized after being vacuum-packed. In the case of wet yeasts, the pressurization procedure had little influence on the inactivation rate. In this case, inactivations were mainly due to hydrostatic pressure effects. Conversely, in the case of dried yeasts, inactivation was highly dependent on the treatment scheme. No mortality was observed when dried cells were pressurized in a non-aqueous liquid medium, but when nitrogen gas was used as the pressure-transmitting fluid, the inactivation rate was found to be between 1.5 and 2 log for the same pressure level and holding time. Several hypotheses were formulated to explain this phenomenon: the thermal effects induced by the pressure variations, the drying resulting from the gas pressure release and the sorption and desorption of the gas in cells. The highest inactivation rates were obtained with vacuum-packed dried yeasts. In this case, cell death occurred during the pressurization step and was induced by shear forces. Our results show that the mechanisms at the origin of cell death under pressure are strongly dependent on the nature of the pressure-transmitting medium and the hydration of microorganisms.

High gas pressure: An innovative method for the inactivation of dried bacterial spores

“…This effect has already been reported on yeasts, where higher inactivation has been observed at lower water activity. However, inactivation has been observed for fully hydrated yeasts using pressurized nitrogen, but this inactivation was essentially due to the compressive effect rather than a specific gas effect (Espinasse et al, 2008a, b).…”

“…The gas bags were then placed in an hydrostatic press, 1.5 L, operating up to 600 MPa (GEC Alsthom ACB, Nantes, France), to carry out the high pressure experiment. This gas pressurization protocol has been detailed in a previous study (Espinasse et al, 2008a). All the experiments were performed at room temperature.…”

“…An inactivation of over 5‐log was achieved with Saccharomyces cerevisae after treatment up to 150 MPa for 1 h. The most evident hypothesis to explain the inactivation of yeasts resides in the sorption of the gas, mostly in the lipid part of the cell, followed by a sudden drop in pressure. The gas sorbed in the different cell layers does not have enough time to diffuse through the structure, provoking a mechanical alteration of essential layers such as the cell wall or the plasma membrane (Espinasse et al, 2008a, b).…”

High gas pressure effects on yeast

“…In a previous paper (Espinasse et al, 2007), we showed for the first time that high pressures of nitrogen were efficient in inactivating dried yeast cells after a treatment of 150 MPa during 24 h whereas the same treatment in a non-aqueous liquid (octadecafluorooctane) without any gas atmosphere had no influence on cell viability. Then, pressurized nitrogen proved to exert specific effects on dried cells, distinct from compression effects observed in octadecafluorooctane.…”

Microtomographic investigation of a yeast grain porous structure

“…Although yeast is known to survive at very high pressure (Espinasse et al, 2008), little is known about its behavior in contact with high pressure mercury. Therefore X-ray microtomographic investigation of a yeast grain is realized.…”