HIGH PRESSURE–HIGH TEMPERATURE STERILIZATION: FROM KINETIC ANALYSIS TO PROCESS VERIFICATION^†

Abstract

The inactivation of Clostridium sporogenes PA 3679 spores by high pressure at high temperatures (HP–HT) in phosphate buffer was investigated in a lab‐scale temperature‐controlled HP system (QFP‐6) with an internal heater to maintain the sample temperature. Some inactivation of spores occurred during the pressurization come‐up time (CUT) and depressurization time. The inactivation of PA 3679 was found to be exponential during the adiabatic holding period of the HP cycle at constant pressures and temperatures. The inactivation rate increased with both pressure and temperature. The kinetic parameters – such as D‐values at tested temperatures and pressures that are necessary for the design of process parameters of HP sterilization process – were determined. Within the pressure range of 600–800 MPa, the calculated D‐values ranged from 270.3 to 357.4 and 49.0 to 67.6 s at 91 and 108C, respectively. These studies provided basic data on the effects of pressure and temperature on the inactivation of PA 3679 spores under conditions applicable to the development of preservation specifications for commercial HP–HT processing of low acid foods. The spore strips of C. sporogenes were used as indicators for microbiological verification of delivered lethality of HP–HT sterilization process at different processing conditions in a pilot scale HP vessel.

Synergistic Inactivation of Spores of ProteolyticClostridium botulinumStrains by High Pressure and Heat Is Strain and Product Dependent

“…For mashed carrot, a 12-min process at 600 MPa and 80°C was required to inactivate a proteolytic type A C. botulinum strain by 5 log 10 , whereas a proteolytic type B C. botulinum strain was inactivated by Ͻ3 log 10 by a process of 600 MPa at 80°C for 60 min (18). Meat and carrot broths inoculated with 5 log 10 spores/ml Clostridium sporogenes PA 3679, the nonpathogenic surrogate for C. botulinum for thermal processing studies, were sterilized only with pressure treatments of Ͼ800 MPa for 5 min at initial temperatures of 80 to 90°C (16).Most studies comparing the heat-only and high-pressureplus-heat resistances of bacterial spores have concluded that, in most cases, pressure and heat do act synergistically to deliver lethality (1,15,17,27,28,30,33). Predominantly, the approaches used by others to demonstrate synergy assume loglinear inactivation kinetics during the pressure hold phase of the high-pressure thermal (HPT) process, ignore inactivation during the pressure come-up time (CUT) and decompression, derive decimal reduction times (D T values; the time required at a constant temperature [T] to achieve a decimal reduction in the number of surviving spores) under constant pressure conditions, and compare these with D T values determined at T and ambient pressure.…”

Strong and Consistently Synergistic Inactivation of Spores of Spoilage-AssociatedBacillusandGeobacillusspp. by High Pressure and Heat Compared with Inactivation by Heat Alone

“…In this study, we elected to assume that thermal-only z T values do not change under HPT conditions. Generally, z T values under HPT conditions have been found, given hold time kinetics only, to be greater than those determined under thermal-only conditions (14,21,23,25,29), indicating that spores are less sensitive to temperature changes during HPT than during thermal-only processing. Therefore, by utilizing thermal-only z T values to derive F T z values under HPT conditions, our results may underestimate resistance.…”

“…While we have noted some synergy between heat and high pressure for the inactivation of spores of proteolytic C. botulinum, this synergy appears to be dependent on both the strain and the product (7). Contrary to the case for proteolytic C. botulinum, there are a number of published studies suggesting that the most heat-resistant spoilage bacterium of concern for LASSF, Geobacillus stearothermophilus, is not nearly as resistant to thermal processing under high pressure as its heat resistance would predict (2,14,17,21,24). In comparison, strains of the aerobic mesophilic species Bacillus amyloliquefaciens, which produces spores with intermediate heat resistance, have been shown to produce highly HPTresistant spores (1,17,18,23) that under some conditions appear to be stabilized by high pressure against thermal inactivation (16).…”

“…B. amyloliquefaciens is closely related to Bacillus subtilis and Bacillus licheniformis, but to date, the spores of these related bacteria have been found to be relatively sensitive to HPT processing (17,18,26). A number of B. amyloliquefaciens strains have been proposed as relevant target spoilage organisms for 18) and even as potential surrogates to assess the efficacy of HPT processes with respect to proteolytic C. botulinum (23).In assessing the resistance of spores to inactivation by HPT processing compared with that achieved under thermal-only conditions, the primary approach of others (1,14,21,23,24) has been to compare inactivation as a function of the hold time, mostly assuming log-linear inactivation kinetics. For HPT inactivation, this approach ignores lethality accrued during the preheating, pressure come-up (CUT), and depressurization phases.…”

“…In assessing the resistance of spores to inactivation by HPT processing compared with that achieved under thermal-only conditions, the primary approach of others (1,14,21,23,24) has been to compare inactivation as a function of the hold time, mostly assuming log-linear inactivation kinetics. For HPT inactivation, this approach ignores lethality accrued during the preheating, pressure come-up (CUT), and depressurization phases.…”