Factors Affecting the Development of Clostridium botulinum in Whole Milk

Abstract

This study was undertaken to determine the effect of the sterilization process and the age of the medium at the time of inoculation on the development of Clostridium botulinum type 62A. Whole milk was autoclaved at 121 C for 18 or 30 min and inoculated with C. botulinum so as to contain 2,000 to 5,000 spores per milliliter. No effort was made to remove dissolved oxygen or to reduce the oxidation-reduction (O/R) potential of the medium by adding sodium thioglycolate. A 3- and a 5-day-old medium were used to study the influence of aging. Eh determinations were made periodically on inoculated and uninoculated samples. Culture development was followed by use of an oval tube counting procedure. The technique used to sterilize the milk influenced the initial O/R potential as well as the autoreductive capacity of the medium. The initial Eh of whole milk sterilized in 500-ml volumes for 18 min was 234 mv. In milk sterilized for 30 min it was 192 mv. The lag phase was 7 days in the former and 5 days in the latter. The exponential growth phases were similar. Autoreduction occurred in sterilized milk. The Eh of uninoculated milk sterilized for 18 min decreased 45 mv in 6 days. In milk sterilized for 30 min the decrease was 63 mv. In milk inoculated 3 or 5 days after sterilization the lag phase was shorter than when the medium was inoculated within 2 hr after autoclaving, regardless of the sterilization procedure employed. The autoreductive property of sterilized whole milk plays a major role in the development of C. botulinum type 62A.