A case study on the use of ultrasound for the inhibition of Escherichia coli O157:H7 and Listeria monocytogenes in almond milk

The interest towards not-dairy beverages is increasing and almond milk is widespread diffused. The main topic of this paper was a focus on Ultrasound (US) to inhibit Escherichia coli O157:H7 and Listeria monocytogenes. The variables of the treatment (power, duration, and pulse) were combined through a randomized design; the use of DoE theory (Design of Experiments) and its outputs (Pareto chart, 3D plots, desirability profiles) pointed out that the effect of the treatment relies upon the total energy distributed into the system on E. coli O157:H7, as suggested by the significance of interactions of power, pulse and time, while power was the most important factor for L. monocytogenes. A final challenge test was done by using two combinations (H-80% of power, 8 min and pulse at 6 s- for E. coli and F-80%; 2 min; 6 s- for L. monocytogenes) and storing the samples at 4 °C for 2 weeks. This experiment suggests that the treatment could exert a sub-lethal injury on the pathogens, which, combined with the storage under refrigeration, could contribute to increase the shelf life.