Inactivation of Bacillus subtilis spores by high pressure CO2 with high temperature

• The spore clumping induced a protective effect on Bacillus subtilis spores.
• The inactivation kinetics of Bacillus subtilis spores showed nonlinear behavior.
• High pressure CO2 inactivated bacterial spores by damaging the inner membrane.

The objective of this study was to investigate the inactivation of the Bacillus subtilis spores by high pressure CO2 combined with high temperature (HPCD + HT) and to analyze the clumping effect of the spores on their HPCD + HT resistance. The spores of B. subtilis were subjected to heat at 0.1 MPa and HPCD at 6.5–25 MPa, and 82 °C, 86 °C, and 91 °C for 0–120 min. The spores were effectively inactivated by HPCD + HT, but a protective effect on the spores was also found, which was closely correlated to the pressure, temperature and time. The spores treated by HPCD + HT at 6.5 and 10 MPa exhibited a two-stage inactivation curve of shoulder and log-linear regions whereas the spores at 15–25 MPa exhibited a three-stage inactivation curve of shoulder, log-linear and tailing regions, and these curves were well fitted to the Geeraerd model. Approximately 90% of pyridine-2,6-dicarboxylic acid (DPA) was released after HPCD + HT and the 90% DPA release time depend on the pressure and temperature. Moreover, the spore clumping in suspensions was examined by dynamic light scattering. The particle size of the spore suspensions increased with the increase of pressure, temperature and time, indicating the spore clumping. 0.1% Tween 80 as a surfactant inhibited the spore clumping and increased the inactivation ratio of the spores by HPCD + HT. These results indicated that the spore clumping enhanced the spores' resistance to HPCD + HT and induced a protective effect.