Synergistic effects of shear stress, moderate electric field, and nisin for the inactivation of Escherichia coli K12 and Listeria innocua in clear apple juice

Abstract

Rotational shear stress (SS), moderate electric field (MEF), and nisin (NS) treatments were applied to study their effect on inactivation of Gram-negative (Escherichia coli K12) and Gram-positive (Listeria innocua) bacteria in clear apple juice. We evaluated the combined effects of shear rate (up to 2879 s−1) and MEF with a 60 Hz square-waveform and a duty cycle of 0.50–0.99, at 27 °C; followed by treatment with NS (0–100 IU/ml). The mechanisms of bacterial inactivation were investigated using plate counts and flow cytometry with SYTO-9/propidium iodide (PI) staining. Selected quality attributes (pH, color, and antioxidant activity) of juice samples were also evaluated.It was possible to achieve 5-log cycle reductions of both E. coli K12 and L. innocua, in samples treated for 5 min with SS at 2879 s−1 and MEF under a duty cycle of 0.99 in combination with NS treatment at 100 IU/ml without significant degradation in quality attributes when compared to the control. Flow cytometry results showed that the mechanism of cellular damage differed for E. coli K12 and L. innocua. In conclusion, the synergistic effects of the combination of SS+MEF with NS could represent a useful alternative to inactivate target pathogens in foods at temperatures below 30 °C.