Inactivation of vegetative cells by continuous high-pressure processing: new insights on the contribution of thermal effects and release device

Dynamic or continuous high-pressure processing of fluid foods has drawn significant interest as a microbial reduction process in the past decade, and many attempts have been made to better understand the mechanisms involved in that reduction. This study was intended to provide insight into the contributions of thermal effects and differences in pressure release components in the inactivation of 2 vegetative pathogen analogs--the Gram-positive Listeria innocua and the Gram-negative Escherichia coli. Fluids containing microbial loads of 10(8) or greater were subjected to continuous high-pressure processing at 200 to 210 MPa. Without active cooling of the release components, all fluids experienced a temperature rise in excess of 70 °C, thus occluding any pressure-related effects for all release components. Active cooling of the valve bodies of the 2 valve-style release components (a conical disruption valve and a micrometering valve) allowed the temperature rise to be abated enough to isolate the effects unique to a given valve. In Tryptic soy broth trials, the mean inactivation levels of E. coli between valves were similar--5.16 log and 5.33 log for the micrometering and conical disruption valves, respectively. When repeated with L. innocua, a similar inactivation level was observed in the conical disruption valve (5.1 log) but not the micrometering valve (3.02). Listeria innocua trials were also repeated using fluid whole milk, which showed a lower levels of inactivation--2.04 log for the micrometering valve and 2.51 log for the conical valve.

This paper compares some of the most common pressure release components used in continuous high-pressure processing and attempts to isolate the contributions of thermal effects from those of pressure and shear. This information is important to those seeking to compare and evaluate the effectiveness of a proposed set of process parameters for microbial inactivation. Further, the ability to reduce the extreme nature of the temperature rise has the potential to expand the use of process into more temperature sensitive products.