Nisin-based antimicrobial combination with cold plasma treatment inactivate Listeria monocytogenes on granny smith apples

Authors: Ukuku, Dike; Niemira, Brendan; Uknalis, Joseph

Submitted to: LWT - Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2019
Publication Date: 1/30/2019
Citation: Ukuku, D.O., Niemira, B.A., Uknalis, J. 2019. Nisin-based antimicrobial combination with cold plasma treatment inactivate Listeria monocytogenes on granny smith apples. LWT - Food Science and Technology. 104:120-127. https://doi.org/10.1016/j.lwt.2018.12.049.
DOI: https://doi.org/10.1016/j.lwt.2018.12.049

Interpretive Summary: Farm and supermarket apples were inoculated with 7 log CFU/ml of Listeria monocytogenes bacteria and then treated to kill the pathogenic bacteria with cold plasma followed by nisin-based antimicrobial exposure. Cold plasma treatments alone caused minimal reduction of bacteria while the nisin based antimicrobial solution alone killed more bacteria. When the treatments were combined, the bacterial populations killed was higher compared to individual treatments suggesting that this treatment combination process is the best approach to reducing bacteria on apples.

Technical Abstract: Many studies on the development of new and/ or value added processing intervention techniques to improve food safety for US consumers have been reported. However, information on the effect of treatment parameters on microbial inactivation on some of these technologies is limited. In this study, we investigated the effect of integrated treatments of nisin-based antimicrobial and cold plasma treatments in reducing Listeria monocytogenes inoculated on apple surfaces. Fresh apples were purchased from a New Jersey farm and a supermarket in Philadelphia area. All apples were inoculated with L. monocytogenes at 5.8±0.24 log CFU/ farm apples and 4.6±0.12 log CFU/supermarket apples and they were treated with antimicrobial solution for 30 s, 40 s, 3 min (180s) and 1h (3,600 s), cold plasma treatments for 30 and 40s, and a combination of antimicrobial and cold plasma treatments. Efficacy of the treatments on microbial reduction were investigated by enumerating surviving colony forming units on selective and non-selective agar plates. Effect of treatments on surface structure of apples and bacterial populations were examined using scanning electron microscopy (SEM). Cold plasma treatments, alone at 30 and 40 s yielded 0.3 log reduction of L. monocytogenes. Similarly, antimicrobials alone led to slightly higher bacterial reduction at 30 and 40 s. Cold plasma treatment at 40s, followed immediately with antimicrobial treatments at 180s and 3,600 s led to 2.5 and 4.6 log CFU/g inactivation of L. monocytogenes, respectively. SEM observation showed changes on apple surface structures but not on bacterial cell structure. This results suggests that a pre-treatment with cold plasma at 40s reduces sanitizer treatment time from 1 h to 3 min to achieve approximately 3.5 to 4 log inactivation of L. monocytogenes on apples.