Purpose: Study and compare thermal and pressure-assisted thermal resistances of the most resistant proteolytic C. botulinum strains and C. sporogenes PA 3679 spores suspended in ACES buffer (pH 7.0).
Methods: Spores of Giorgio-A and 69-A strains of C. botulinum and C. sporogenes PA 3679 were prepared using biphasic media and diluted in ACES buffer (0.05M, pH 7) to 105-106CFU/ml. Diluted samples were placed into a modified sterile transfer pipette, heat-sealed and subjected to combinations of temperature (93-108°C) and high pressure (600-750 MPa) in a laboratory scale high pressure system. Another set of diluted spores were placed in NMR tubes, both ends heat-sealed, and subjected to various temperatures (93-108°C) and times in a Fluke 7321 High Precision Bath with Duratherm S as heat transfer fluid. Survivors were determined by a 5-tube MPN method using TPGY broth after incubation for 3 months.
Results: Thermal and pressure-assisted D-values (min) of Giorgio-A, 69-A, and PA3679 decreased as process temperature increased. Highest log reduction (> 4.5) of spores occurred at 108°C and the highest temperature and pressure combination (108°C and 750 MPa). Thermal D-values were higher for Giorgio-A, 69-A, and PA 3679 at any temperature compared those processed at the same temperature combined with pressure. For example, thermal D-values at 108°C for Giorgio-A, 69-A, and PA3679 were 3.5, 2.9, and 12.5 minutes, respectively. These times are higher than those obtained at the same temperature combined with 600 MPa (e.g. 2.4 for Giorgio-A, 2.1 for 69-A, and 5.7 for PA3679). Thermal and pressure-assisted D-values for PA3679 were higher than those for C. botulinum under the same conditions.
Significance: C. sporogenes PA3679 appears more resistant to thermal and pressure-assisted HPP than C. botulinum strains based on the thermal and pressure-assisted D-values.