Effects of combination shear stress, moderate electric field (MEF), and nisin on kinetics and mechanisms of inactivation of Escherichia coli K12 and Listeria innocua in fresh apple-kale blend juice

This work explored the possible synergistic effects of simultaneous application of shear stress-moderate electric field (SS+MEF) in combination with nisin (NS) at ambient temperature on microbial inactivation and quality changes of apple-kale blend juice. In microbiological studies, Escherichia coli K12 and Listeria innocua were exposed to a) SS+MEF; b) NS; and c) their combinations either simultaneously (SS+MEF+NS) or sequentially (SS+MEF→NS). Quality parameters, including pH, color, antioxidant capacity, and chlorophyll contents of uninoculated juice were also evaluated. Under individual SS+MEF treatment for 10 min, maximum inactivation levels of E. coli K12 and L. innocua were 5.4 and 5.2 log CFU/mL respectively. NS treatment alone inactivated E coli K12 and L. innocua by 1.0 and 2.6 log CFU/mL, respectively. Nevertheless, subpopulations of live cells observed by flow cytometry and by plate counts were significantly reduced after both simultaneous and sequential combination treatments, in comparison to individual treatments. SS+MEF+NS treatment produced inactivation levels up to 5-log reductions for both microorganism species within 5 min. The modified Gompertz model best-fitted the inactivation kinetics. SS+MEF+NS treatment for 5 min preserved pH, color, antioxidant capacity and chlorophyll contents of the product, suggesting its potential as a non-thermal preservation technique for fruit and vegetable juices.