Microbial heat resistance determinations by the multipoint system with the thermoresistometer TR-SC Improvement of this methodology

Abstract

The thermoresistometer TR-SC for microbial heat resistance determinations by the multipoint system was improved. In manual mode (for the determination of high D t values) the need of pipetting was eliminated: Selected volumes of heated menstruum were collected directly into plating medium by means of an automatic sampling valve operated by a solenoid activated by a timer. In the automatic mode, used for the determinations of very low D t values, the inoculation of menstruum, equiliberated at the required temperature and the collection of samples during the experiment, were made fully automatic. The injection of the inoculum was made with a solenoid operated syringe, and samples were collected, with a specially adapted sample collector, directly into tubes containing melted plating medium. The maximum number of samples per unit of time was increased to 52 samples/s. The minimum detectable D t value was reduced from 0.009 to 0.0009 min without decreasing precision (SD = 0.00017). This improvement eliminated the chance of handling errors, and reduced time and effort required for heat resistance determinations. This modification makes this method especially suitable for the determination, by the multipoint system, of very low D t values as needed in experiments at ultrahigh temperatures.