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Low frequency ultrasonication of Saccharomyces uvarum: effect of temperature and wave amplitude

INTRODUCTION The application of ultrasound to biological systems offers a range of possibilities in food technology and biotechnology. The biological parameters which influence the intensity of the ultrasound effects are: microbial strain tested, the suspending medium, the size of the cell. The objective of this research was to study the possible lethality of the combined effect of ultrasound and mild heat treatment on brewer’s yeast Saccharomyces uvarum viable cells suspended in wort. The effect of the amplitude of the sonication waves was also evaluated. METHODOLOGY Ultrasound at 20 kHz and 108 m (25 %), 173 m (40 %) and 260 m (60 %) of wave amplitude was applied to the brewer’s yeast S. uvarum cells wort suspension (pH 5.6 ; temperature 45 and 55 oC) with an immersed 13-mm diameter probe (1-10 min). Decimal reduction time (D) value was calculated at a given temperature and sonic wave amplitude. Determinations of cell size and percent of dead cells (methylene blue staining) were measured by light microscopy with digital camera and recorded onto a PC. RESULTS AND DISCUSSION The resistance of S. uvarum cells suspended in wort at pH 5.6 to the action of ultrasound (20 kHz, wave amplitude in the range 108-260 m) was analyzed at 45 and 55 oC. The inactivation rate where a first-order kinetic was observed exhibited D values between 1 and 25 min. At 45 oC, D values were decreased by the simultaneous effect of ultrasound from 19.6 to 14.5, but at 55 oC no advantages were obtained by adding sonication. Physiological studies of cell size and percent of dead cells proved that the ultrasonic waves do not destroy the brewer’s yeast cells ; they damage them, thus increasing their activity to heat. However, when ultrasound was applied at 55 oC, no structural differences were appreciated between sonicated cells and only heat-treated cells. CONCLUSIONS 1. Studies of survival of S. uvarum suspended in wort at 45 and 55 oC at different ultrasonic wave amplitude show that application of ultrasound at non-lethal temperature (45 oC) did not display a deactivation action while synergy between ultrasound and heat was confirmed at the 55 oC. 2. Determinations of cell size and percent of dead cells proved that the ultrasonic waves do not destroy the brewer’s yeast cells ; they damage them, thus increasing their activity to heat.

ultrasound; S. uvarum; temperature; wave amplitude

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