Inactivating Salmonella Enteritidis on shell eggs by using ozone microbubble water

The major pathogen associated with eggs is Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) and chlorine washing is the most widely used for sanitization. Microbubble, a novel technique and able to operate in large quantity, has been presented to be an alternative method. Thus, microbubble water combining with ozone (OMB) was applied to disinfect S. Enteritidis spiked on shells at 10⁷ cells per egg. OMB was generated by injecting ozone into a Nikuni microbubble system, then delivered into 10 L of water. After 5, 10, or 20 min of activation time, the eggs were placed into OMB and washed for 30 or 60 s. The controls involved unwashed, water washing, ozone only, and microbubble only (MB). The highest reduction, 5.19 log CFU/egg, was achieved by the combination of 20-min activation and 60-s washing, which was used for following tests of large water quantities. Comparing with the unwashed control, 4.32, 3.73 and 3.07 log CFU/egg reductions were achieved in 25, 80, and 100 L of water, respectively. The other system, Calpeda, with higher motor power was tested in 100 L and obtained a reduction of 4.15 log CFU/egg. The average diameter of bubbles generated by Nikuni and Calpeda pump systems were 29.05 and 36.50 μm, respectively, which both were within the microbubble definition of ISO. Much lower reductions, around 1-2 log₁₀ CFU/egg, were shown with the treatments of ozone only and MB by the same operative parameters. After 15-day storage at ambient temperature, the OMB-treated eggs showed similar sensory quality with the unwashed ones. This is the first study demonstrating that OMB effectively inactivates S. Enteritidis on shell eggs in large quantity of water and does not diminished the sensory characteristics of eggs. Furthermore, bacterial population was under the detection limit in the OMB-treated water.

Mechanism of OH radical production from ozone bubbles in water after stopping cavitation

It has been experimentally reported that OH radicals are produced from ozone microbubbles even after stopping cavitation. Microbubbles are mostly produced using acoustic or hydrodynamic cavitation. In the present paper, numerical simulations of chemical reactions inside an ozone bubble and an oxygen bubble are performed during and after acoustic cavitation in order to study the mechanism of OH radical production. The results have indicated that less than one molecule of OH radicals is produced from a dissolving ozone bubble after stopping cavitation when local pH near the bubble wall is <8. On the other hand, more than 10⁸ or 10⁶ molecules of H₂O₂ are produced inside an ozone or oxygen microbubble, respectively, in water during acoustic cavitation per violent collapse. It suggests that OH radicals are possibly produced by the chemical reaction of H₂O₂ with O₃ in the liquid phase even after stopping cavitation. Furthermore, in strongly acidic conditions (pH < 5), the reaction of H₂O₂ with O₃ in the liquid phase is relatively slow, and OH production considerably continues after stopping cavitation. This may be the reason for the enhanced OH signals in strongly acidic conditions observed experimentally after stopping cavitation.