The Effect of High Intensity Ultrasound Treatment on the Amount of Staphylococcus aureus and Escherichia coli in Milk

Zoran Herceg¹*, Anet Režek Jambrak¹, Vesna Lelas¹ and Selma Mededovic Thagard<sup>2

1</sup>Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia
²Clarkson University, Department of Chemical and Biomolecular Engineering, 8 Clarkson Avenue, Potsdam, NY 13699-5705, USA

Article history:
Received February 2, 2011
Accepted October 24, 2011

Key words:
milk, high intensity ultrasound, Staphylococcus aureus, Escherichia coli, response surface methodology

Summary:
Inactivation of Staphylococcus aureus and Escherichia coli in milk containing 4 % milk fat was carried out using a 20 kHz power ultrasound. The experiments were planned and performed according to the statistical experimental design. Specifically, central composite design was used to optimize and design three experimental parameters: temperature (20, 40 and 60 °C), amplitude (60, 90 and 120 μm) and treatment time (6, 9 and 12 min). It was found that Gram-negative bacteria (Escherichia coli; D_{120 μm}=2.78 min at 60 °C) are more susceptible to the ultrasonic treatment than Gram-positive bacteria (Staphylococcus aureus; D_{120 μm}=4.80 at 60 °C). Nevertheless, all three parameters studied seem to substantially affect the inactivation of both Staphylococcus aureus and Escherichia coli in milk using ultrasonic treatment. The results also indicate increased inactivation of microorganisms under longer period of treatments, particularly in combination with higher temperature and/or amplitude.

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