The combined high pressure and heat resistances of spores of five proteolytic Clostridium botulinum strains and of the nonpathogenic surrogate strain Clostridium sporogenes PA3679 were compared with their heat-only resistances on the basis of equivalent accumulated thermal lethality, expressed as equivalent minutes at a reference temperature of 105°C (F 105°C ). Comparisons were made with three model (i.e., diluted) products, namely, 30% (wt/wt) Bolognese sauce, 50% (wt/wt) cream sauce, and rice water agar. Pressure was determined to act synergistically with heat during high-pressure thermal (HPT) processing for C. botulinum FRRB 2802 (NCTC 7273) and C. botulinum FRRB 2804 (NCTC 3805 and 62A) in the Bolognese and cream sauces and for C. botulinum FRRB 2807 (213B) in the Bolognese sauce only. No synergy was observed for C. botulinum FRRB 2803 (NCTC 2916) or FRRB 2806 (62A) or C. sporogenes FRRB 2790 (NCTC 8594 and PA3679) in any of the model products. No significant protective effect of pressure against spore inactivation was determined for any Clostridium strain in any product. Because synergy was not consistently observed among strains of C. botulinum or among products, the prediction of inactivation of C. botulinum spores by HPT sterilization (HPTS) for the present must assume a complete lack of synergy. Therefore, any HPTS process for low-acid shelf-stable foods must be at least thermally equivalent to an F 0 process of 2.8 min, in line with current good manufacturing practices. The results of this study suggest that the use of C. sporogenes PA3679 as a surrogate organism may risk overestimating inactivation of C. botulinum by HPT processing.Low-acid shelf-stable products that are microbiologically safe and stable are not obtainable by high-pressure processing (HPP) at near-ambient temperatures, as bacterial spores can survive pressures above 1,500 MPa (3,6,16,35), which far exceed the pressure capabilities of current commercial HPP equipment. Extensive inactivation of bacterial spores by high pressure is likely only to be realized in combination with initial process temperatures that exceed 60°C (16,20,23,24,31,37,39; C. M. Roberts and D. G. Hoover, presented at the Institute of Food Technologists 1996 annual meeting). Of particular interest (and concern) for low-acid shelf-stable foods is the ability of a combined high pressure and heat process to synergistically inactivate spores of the major bacterial spore-forming pathogens of concern, which are proteolytic strains of the neurotoxigenic species Clostridium botulinum.The combined high pressure and heat resistances of spores of a number of proteolytic C. botulinum strains in a number of different matrices have been investigated. Proteolytic C. botulinum type A BS-A and 62A spores were inactivated by 2 and 3 log 10 , respectively, in phosphate buffer (0.067 M, pH 7.0) after 20 min at 827 MPa at 75°C and by 3.2 log 10 and 2.7 log 10 , respectively, in a crabmeat blend (pH 7.2 to 7.4) after 15 min (29). For mashed carrot, a 12-min process at 600 MPa and 80°C ...

Page 2318, column 1, line 43: "B. coagulans was sporulated on Campden sporulation medium (6)" should read "B. coagulans and B. amyloliquefaciens were sporulated on Campden sporulation medium (6) and nutrient agar, respectively."

The inactivation of spores of four low-acid food spoilage organisms by high pressure thermal (HPT) and thermal-only processing was compared on the basis of equivalent thermal lethality calculated at a reference temperature of 121.1°C (F z 121.1°C, 0.1 MPa or 600 MPa ) and characterized as synergistic, not different or protective. In addition, the relative resistances of spores of the different spoilage microorganisms to HPT processing were compared. Processing was performed and inactivation was compared in both laboratory and pilot scale systems and in model (diluted) and actual food products. Where statistical comparisons could be made, at least 4 times and up to around 190 times more inactivation (log 10 reduction/minute at F T z 121.1°C ) of spores of Bacillus amyloliquefaciens, Bacillus sporothermodurans, and Geobacillus stearothermophilus was achieved using HPT, indicating a strong synergistic effect of high pressure and heat. Bacillus coagulans spores were also synergistically inactivated in diluted and undiluted Bolognese sauce but were protected by pressure against thermal inactivation in undiluted cream sauce. Irrespective of the response characterization, B. coagulans and B. sporothermodurans were identified as the most HPT-resistant isolates in the pilot scale and laboratory scale studies, respectively, and G. stearothermophilus as the least in both studies and all products. This is the first study to comprehensively quantitatively characterize the responses of a range of spores of spoilage microorganisms as synergistic (or otherwise) using an integrated thermal-lethality approach (F T z ). The use of the F T z approach is ultimately important for the translation of commercial minimum microbiologically safe and stable thermal processes to HPT processes.High-pressure thermal (HPT) processing has been identified as a potential alternative to conventional thermal processing to deliver low-acid shelf-stable foods (LASSF) with improved sensory and nutritional qualities (5,11,15,28). Previously, we have shown that, in laboratory scale experiments, the inactivation of spores of proteolytic Clostridium botulinum by HPT processing is similar to that achieved under thermal-only processing conditions compared on the basis of accumulated thermal lethality (F T z ) (7). 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 condi...