The Effect of Water Activity, Solutes and Temperature on the Viability and Heat Resistance of Freeze-dried Bacterial Spores

Abstract

Freeze-dried spores of Bacillus megaterium, B. stearothermophilus, Clostridium bifermentans and C. botulinum type-E suffered little or no loss in viability after storage at 25[degree] at water activity (aw) values between 0.2 and 0.8. When stored over P2O5 (0.00 aw) the spores of all four species showed a marked loss in viability. The above results were similar for spores whether stored in air or in vacuum. With spores stored over distilled water (1.00 aw) the Bacillus spores underwent a large loss of viability in vacuum but not in air; for spores of the clostridia the reverse was true. The addition of DL-glyceraldehyde, diacetyl or ribose (0.05 M) to the spore suspensions before drying caused increased death during storage at 0.50 aw and to a lesser extent at 0.20 aw. Death was greater at 30[degree] than at 10[degree]. The addition of sucrose, glutamate or semicarbazide did not decrease the viability. When the dried spores were resuspended in dilute phosphate buffer after storage for 2-6 years, their resistance to heating was greatest after storage at aw values of 0.4, 0.6 and 0.8.