Cold plasma inactivation of salmonella in pre-packaged mixed salads is influenced by cross-contamination sequence

Authors: Hertrich, Sarah; Boyd, Glenn; Sites, Joseph; Niemira, Brendan

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2017
Publication Date: 11/22/2017
Citation: Hertrich, S.M., Boyd, G., Sites, J.E., Niemira, B.A. 2017. Cold plasma inactivation of salmonella in pre-packaged mixed salads is influenced by cross-contamination sequence. Journal of Food Protection. Volume 80, No. 12, Pages 2132-2136. 10.4315/0362-028X.JFP-17-242.
DOI: https://doi.org/10.4315/0362-028X.JFP-17-242

Interpretive Summary: Novel processing techniques, such as cold plasma (a form of electrically energized gas), are currently being investigated to enhance the safety and shelf-life of pre-packed foods. The purpose of this study was to determine the effects of cold plasma corona discharge on the inactivation of Salmonella on pre-packaged, tomato-and-lettuce mixed salads. Two different inoculation methods were evaluated to address cross-contamination of Salmonella from cherry tomatoes to lettuce and vice versa. In separate studies, either cherry tomatoes or romaine lettuce were inoculated with a Salmonella cocktail and placed into a commercial PET plastic container and thoroughly mixed together with its non-inoculated counterpart. Samples for each inoculation method (lettuce-onto-tomato or tomato-onto-lettuce) were treated with high-voltage cold plasma corona discharge inside plastic containers for 3 minutes. When lettuce was the inoculated counterpart, the kill level of Salmonella was significantly greater on tomatoes (83% reduction) compared to lettuce (55% reduction). Salmonella was significantly reduced on mixed salad only when lettuce was the inoculated counterpart (49% reduction). These results show that the nature of the contamination can have a significant impact on the way cold plasma reduces pathogen loads on packaged mixed salads. Cold plasma can kill Salmonella in a pre-packaged, mixed salad, with efficacy dependent on the nature of contamination, direction of transfer, and on the surface topography of the contaminated commodity.

Technical Abstract: The demand by consumers for convenient food products has led to an increased production of pre-packaged and ready-to-eat food products. Most of these products rely mainly on surface disinfection and other traditional approaches to ensure shelf-life and safety. Novel processing techniques, such as cold plasma, are currently being investigated to enhance the safety and shelf-life of pre-packed foods. The purpose of this study was to determine the effects of cold plasma corona discharge on the inactivation of Salmonella on pre-packaged, tomato-and-lettuce mixed salads. Two different inoculation methods were evaluated to address cross-contamination of Salmonella from cherry tomatoes to lettuce and vice versa. In separate studies, either cherry tomatoes (55g) or romaine lettuce (10g) were inoculated with a Salmonella cocktail and placed into a commercial PET plastic container and thoroughly mixed together with its non-inoculated counterpart. Mixed salads were allowed to dry in a biosafety cabinet for 1 h. Samples were treated with 35 kV cold plasma corona discharge inside plastic containers for 3 min. Samples were stomached and serially diluted in BPW then plated onto APC Petrifilm and incubated for 18 h at 37°C. When lettuce was the inoculated counterpart, log kill of Salmonella was significantly greater on tomatoes (0.75 log CFU/g) compared to lettuce (0.34 log CFU/g) (p=0.0001). Salmonella was reduced on mixed salad only when lettuce was the inoculated counterpart (0.29 log CFU/g) (p=0.002). Cold plasma can kill Salmonella in a in a pre-packaged, mixed salad, with efficacy dependent on the nature of contamination, direction of transfer, and on the surface topography of the contaminated commodity.