Inactivation of Pseudomonas fluorescens and Streptococcus thermophilus in Trypticase® 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 to apply high-intensity ultrasound for the inactivation of bacteria important for the dairy industry. A comparative study has been carried out using a conventional tubular heat exchanger under similar process conditions. Pseudomonas fluorescens and Streptococcus thermophilus, inoculated in Trypticase® Soy Broth (TSB), were inactivated by ultrasound, the former (Gram-negative) being less resistant than the latter (Gram-positive). Although the effectiveness of ultrasound can be decreased by temperature increase during the treatment, an additive effect was observed between heat and ultrasound. This result was indicated by computer simulations comparing ultrasound with conventional heating. Preliminary results obtained in milk demonstrated that continuous-flow ultrasonic treatment could be a promising technique for milk processing. This method has several advantages such as homogenisation of milk and less energy consumption than batch systems.

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