Impact of the frequency of high-power ultrasound on the microbial load and physical properties of carrots (Daucus carota)

Abstract

Ultrasound is a promising non-thermal clean technology in the fresh-cut industry of vegetables due to the inactivation of microorganism caused by cavitation [1]. Depending on the intensity of the ultrasound, time of application and temperature, reduction levels up to 1.3 Log UFC/g have been achieved [2]. However, there are fewer studies that evaluate the effect of the frequency of the ultrasonic wave and treatment time on the reduction of microorganism. ¿e main aim of this work was to evaluate the effect of bath sonication treatments of 25 and 42 kHz during 10 and 20 minutes on slices of carrots. Experiments were conducted at 24.0 ± 1.0°C. ¿e temperature of the water bath and carrot slices was monitored during treatments. Mesophilic aerobic microorganisms, yeast and moulds were evaluated in accordance with ISO 4833:2003 and ISO 7954:1987 protocols. Additionally, instrumental colour and texture of carrot slices were also evaluated. ¿e higher the frequency–time applied the higher core temperature obtained. A¿er 20 minutes at 42 kHz, the core temperature raised 7.3 ± 0.2°C. However, at 25 kHz half of the increase was found, 3.3 ± 0.2°C. ¿e effect of ultrasound frequency and time was evaluated through two way-Anova analysis (¿ < 0.05). From our results there were no significant interaction between the studied factors (¿ > 0.05). ¿e reduction of mesophilic bacteria was higher at 25 kHz but there were a significant effect of the time of application. ¿e maximum reduction achieved was 1 Log10 at 10 minutes. ¿ere were no significant differences on the reduction of moulds and yeast and 1 Log10 reduction was achieved. Alegria et al. [3] obtained lesser microbial reduction in sonicated fresh-cut carrots (45 kHz / 1 min). ¿e authors reported a maximum of 0.5 Log10 reductions for both mesophilic flora and yeast and moulds group. ¿e instrumental firmness and colour of the all the sonicate carrot slices were no significantly different to the raw carrot (¿ > 0.05). From our results, ultrasonic treatment can be an alternative to reduce the microbial load in fresh-cut products with no effect on texture and colour. However, higher microorganism reductions may be needed by the fresh-cut industry. Further studies on the possible combination of clean treatments may be the solution to reduce the chlorine concentration used on these food products.