In recent years reports have been published on non-sterility problems in UHT milk caused by the survival of very heat-resistant
spores, which have been identiÞed as belonging to the species Bacillus sporothermodurans. In order to solve the problems in dairy
practice more information is needed on the thermal death kinetics of these spores. The heat resistance of spores of three Bacillus
sporothermodurans strains isolated from non-sterile UHT milk was determined in the temperature range of 110Ð145¡C and was
compared with the heat resistance of Bacillus stearothermophilus spores. For the low temperatures (110Ð125¡C) the heating was
carried out in tubes. For the higher temperatures (130Ð145¡C) a direct UHT sterilizer was used. D
140
values of 3.4Ð7.9 s
(B. stearothermophilus D
140
"0.9 s) indicate an exceptionally high heat resistance of spores of B. sporothermodurans under UHT
conditions. Thermal death time (TDT) curves show di¤erent slopes for B. sporothermodurans and B. stearothermophilus, with
z"13.1Ð14.2¡C and 9.1¡C, respectively. To our knowledge there are no observations of other spores in the literature for which such
high D
140
values and z values have been demonstrated. ( 1999 Published by Elsevier Science Ltd. All rights reserved