High pressure destruction kinetics of bacterial spores in low acid food at elevated temperatures

High pressure (HP) sterilization of low acid foods is a process involving HP at elevated temperatures. Data available on HP destruction kinetics of pressure resistant pathogenic, spoilage-causing and surrogate bacterial spores are limited, but essential for establishing such a process. While compression heating could be employed to provide the elevated temperature, accurate data gathering under such conditions is difficult due to rapid heat loss during the treatment. The objectives of this research were to first standardize the set up carrying out HP tests at elevated and stabilized temperatures, and subsequently to evaluate HP destruction kinetics of selected bacterial spores under HP elevated temperature processing conditions.The set-up consisted of a thick insulated chamber for holding samples during the test. A relationship was established for the adiabatic temperature rise milk which was used as the main low acid food medium: DeltaTP = -0.306 + 0.0224Ti + 0.0423P + 4.49x10-4T i2 + 1.31x10-4TiP -- 1.24x10 -5P2 (R2 =0.999, n = 50, SE = 0.20°C, p<0.05). Initial temperatures at 83.5, 91.7, 95.8°C and 80.7, 88.9, 93.0°C provided operating process temperature 121, 130, 135°C, at 800, 900MPa, respectively.Destruction kinetics tests were carried with two strains of Clostridium sporogenes (11437, 7955) and Geobacillus stearothermophilus 10149 spores suspended in milk at 700-900 MPa and 70-100°C. These strains were selected for their relatively high pressue and thermal resistance. The survival counts were well fitted by first order linear models. The D values C. sporogenes 11437 varied from 0.73 min at 900 MPa 100°C to 17.0 min at 700 MPa 80°C HP treatments while they ranged from 6.0 to 833 min at 80-100°C under thermal processing conditions. The D values associated with of C. sporogenes 7955 spores were higher and varied from 1.3 min at 900 MPa 100°C to 38.2 min at 700 MPa 80°C HP treatments, and from 12.1 to 156 min at 80-100°C during thermal treatments. The D values of Geobacillus stearothermophilus 10149 spores varied from 0.6 min at 900 MPa 90C to 20.9 min at 500 MPa 70°C HP treatments with 6.3 to 49.4 min for thermal treatments at 110-120°C. Hence C. sporogenes 7955 spores were the most resistant among those studied. The HP destruction kinetics of C. sporogenes 7955 spores were also studied in salmon and were lower than in milk.Pressure resistance screening of selected group I C. botulinum spores was carried out at 800-900 MPa at 90-100°C. Nominal D values of each strain were evaluated which demonstrated that PA9508B, H09504A and CK2-A were the more resistant spores (PA9508B > HO9504A > CK2-A). HP destruction kinetics of C. botulinum PA9508B spores in milk were evaluated in detail at 700-900 MPa at 90-100°C and parallel thermal treatment at 90-100°C. The survival counts were again described by first order linear models (R2 >0.86). The D values varied from 0.35 min at 900 MPa 110°C to 38.9 min at 700 MPa 90°C for HP treatments and 14.4 to 273 min at 90-100°C for thermal treatments. These demonstrated that HP processing combined with elevated temperatures will accelerate the spore destruction rate. However, the associated D values of C. botulinum PA9508B spores in milk were more resistant than those of C. sporogenes 7955 at 90, 100°C, which indicated that the surrogate may not be very effective for using as a target for verification HP at elevated temperature processing conditions.Overall, this work has demonstrated several findings. The non-pathogenic C. sporogenes 7955 spore was the most resistant surrogate but the pathogenic C. botulinum PA9508B spore was even more resistant. D values associated with HP at elevated temperatures were higher than under conventional thermal treatments, and hence provide accelerated destruction kinetics at least for the non-pathogenic spores and hence better spoilage control. However, from safety point of view the conventional thermal sterility requirements would still persist even under HP processing conditions. Milk as a low acid food medium provided more resistance for HP destruction than fish