Inactivation of Pseudomonas fluorescens and Streptococcus thermophilus in Trypticase (R) Soy Broth and total bacteria in milk by continuous-flow ultrasonic treatment and conventional heating

A continuous-flow ultrasonic treatment apparatus has been set up in order t o apply high-intensity ultrasound for the inactivation of bacteria importan t for the dairy industry. A comparative study has been carried out using a conventional tubular heat exchanger under similar process conditions. Pseud omonas fluorescens and Streptococcus thermophilus, inoculated in Trypticase (R) Soy Broth (TSB), were inactivated by ultrasound. the former (Gram-negat ive) being less resistant than the latter (Gram-positive). Although the eff ectiveness of ultrasound can be decreased by temperature increase during th e treatment, an additive effect was observed between heat and ultrasound. T his result was indicated by computer simulations comparing ultrasound with conventional heating. Preliminary results obtained in milk demonstrated tha t continuous-flow ultrasonic treatment could be a promising technique for m ilk processing. This method has several advantages such as homogenisation o f milk and less energy consumption than batch systems. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.