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

Abstract

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.

Introduction

During the last years, ultrasonic treatment and other alternative food technologies have been studied due to consumers’ continuing demand for products with improved quality and safety.

High-intensity ultrasound (10–1000 W/cm² and ⩽0.1 MHz) Povey & McClements, 1988, McClements, 1995 has been used for some dairy process applications such as cleaning (Kivelä, 1996), inactivation of bacteria Ordóñez et al., 1987, Garcı́a et al., 1989, Wrigley & Llorca, 1992 and enzymes (Vercet, López & Burgos, 1997), extraction of chymosin Kim & Zayas, 1991a, Kim & Zayas, 1991b and β-galactosidase (Sakakibara, Wang, Ikeda & Suzuki, 1994) and homogenisation of milk (Gaffney, 1997), even increasing the cheese yield (Müller, 1992). Although the bactericidal effect of ultrasound is known, especially when combined with heat Ordóñez et al., 1984, Ordóñez et al., 1987, Garcı́a et al., 1989 and static pressure (Raso, Pagán, Condón & Sala, 1998), the dairy industry does not currently use this technique for preservation purposes. This could be due to the fact that detailed studies, including energy consumption aspects, about continuous-flow ultrasonic milk processing are scarce. It is evident that continuous processes are easier to scale-up than batch treatments.

During ultrasonic treatment the main active force is mechanical in nature, resulting in the formation and implosion of bubbles in a liquid (cavitation) (Shukla, 1992). However, the action of cavitation is a heat-generating process, since input of mechanical energy causes molecular motion, raising the temperature (Berliner, 1984). Moreover, part of the energy can be absorbed for the heating of the sample (Floros & Liang, 1994). As the heating is within the sample, the equipment wall temperature is lower than the liquid temperature. This could offer some advantages over conventional heating systems in terms of better temperature distribution and less fouling formation, and thus the product quality might be improved Jong de et al., 1992, Jong de, 1996.

On the other hand, Pseudomonas fluorescens (Gram-negative) is one of the most important psychrotrophic bacteria responsible for undesirable flavours in milk and dairy products. The heat treatment applied during conventional pasteurisation (72–78°C, 10–20 s) is sufficient to destroy large numbers of vegetative cells of this microorganism (Nickerson & Sinskey, 1972). Streptococcus thermophilus (Gram-positive) is a thermoduric bacterium which may appear in large numbers in pasteurised milk, giving the false impression that the milk has not been properly pasteurised (Frobisher, 1968).

The effectiveness of ultrasound in killing Gram-negative and Gram-positive microorganisms is still unclear. Ahmed and Russell (1975) stated that Gram-negative cells are less resistant to ultrasound than Gram-positive. However, Scherba, Weigel and O’Brien (1991) did not find any difference in the killing rate of the two kinds of microorganisms by ultrasonic treatment.

The aim of this research was to evaluate the effects of continuous-flow ultrasonic treatment using the heating produced by ultrasound on P. fluorescens and S. thermophilus in Trypticase® Soy Broth (TSB), and on total bacterial counts of milk. A comparative study has been made using a conventional heating system (heat exchanger) under similar process conditions. In addition, the shelf life of milk subjected to a mild continuous-flow ultrasonic and conventional treatment has been evaluated by microbiological and proteolytic indices.