2018, Fröhling, A., Ehlbeck, J., Schlüter, O.

Cold plasma is described as a promising technique for the treatment of fresh food. In particular, the application of plasma-treated water gained interest in fresh-cut produce processing. This study aimed to evaluate the effectiveness of plasma-treated water (PTW) to decontaminate lettuce during washing on a pilot-scale level with special interest in the dynamics of the culturable microbial community in a first approach. PTW was used in pilot-scale washing at different processing steps, and the total viable count (TVC) of endive lettuce was determined after treatment and after storage (seven days, 2 °C). Microflora representatives were identified using MALDI-ToF MS. The highest reduction of TVC (1.8 log units) was achieved using PTW for washing whole lettuce before cutting. The microbial community structure showed high variations in the composition along the processing chain and during storage with a decrease in diversity after washing with PTW. PTW reduced the microbial load of endive lettuce; however, this was not clearly detectable at the end of storage, similar to other sanitizers used in comparable studies. To assure the safety of fresh products, detailed knowledge about the microbial load and the composition of the microbial community close to the end of shelf life is of high interest for optimized process design.

2018, Fröhling, A., Rademacher, A., Rumpold, B., Klocke, M., Schlüter, O.

In this study, the composition of the microbial community on endive lettuce (Cichorium endivia) was evaluated during different postharvest processing steps. Microbial community structure was characterized by culture-dependent and culture-independent methods. Endive lettuce was sampled exemplarily at four different stages of processing (raw material, cut endive lettuce, washed endive lettuce, and spin-dried (ready to pack) endive lettuce) and analysed by plate count analysis using non-selective and selective agar plates with subsequent identification of bacteria colonies by matrix-assisted laser desorption/ionization time-of light mass spectrometry (MALDI-TOF MS). Additionally, terminal-restriction fragment length polymorphism (TRFLP) analysis and 16S rRNA gene nucleotide sequence analysis were conducted. The results revealed structural differences in the lettuce microbiomes during the different processing steps. The most predominant bacteria on endive lettuce were detected by almost all methods. Bacterial species belonging to the families Pseudomonadaceae, Enterobacteriaceae, Xanthomonadaceae, and Moraxellaceae were detected in most of the examined samples including some unexpected potentially human pathogenic bacteria, especially those with the potential to build resistance to antibiotics (e.g., Stenotrophomonas maltophilia (0.9 % in cut sample, 0.4 % in spin-dried sample), Acinetobacter sp. (0.6 % in raw material, 0.9 % in cut sample, 0.9 % in washed sample, 0.4 % in spin-dried sample), Morganella morganii (0.2 % in cut sample, 3 % in washed sample)) revealing the potential health risk for consumers. However, more seldom occurring bacterial species were detected in varying range by the different methods. In conclusion, the applied methods allow the determination of the microbiome's structure and its dynamic changes during postharvest processing in detail. Such a combined approach enables the implementation of tailored control strategies including hygienic design, innovative decontamination techniques, and appropriate storage conditions for improved product safety.

2018, Schottroff, F., Fröhling, A., Zunabovic-Pichler, M., Krottenthaler, A., Schlüter, O., Jäger, H.

The viable but non-culturable (VBNC) state, as well as sublethal injury of microorganisms pose a distinct threat to food safety, as the use of traditional, culture-based microbiological analyses might lead to an underestimation or a misinterpretation of the product's microbial status and recovery phenomena of microorganisms may occur. For thermal treatments, a large amount of data and experience is available and processes are designed accordingly. In case of innovative inactivation treatments, however, there are still several open points with relevance for the investigation of inactivation mechanisms as well as for the application and validation of the preservation processes. Thus, this paper presents a comprehensive compilation of non-thermal preservation technologies, i.e., high hydrostatic pressure (HHP), pulsed electric fields (PEFs), pulsed light (PL), and ultraviolet (UV) radiation, as well as cold plasma (CP) treatments. The basic technological principles and the cellular and molecular mechanisms of action are described. Based on this, appropriate analytical methods are outlined, i.e., direct viable count, staining, and molecular biological methods, in order to enable the differentiation between viable and dead cells, as well as the possible occurrence of an intermediate state. Finally, further research needs are outlined.

2018, Durek, J., Schlüter, O., Roscher, A., Durek, P., Fröhling, A.

Ochratoxin A (OTA) is one of the most abundant food-contaminating mycotoxins. Besides their high toxicity, mycotoxins are highly stable to physical, chemical or biological detoxification. Therefore, the treatment with cold atmospheric plasma could be one approach to reduce the amount of mycotoxins in different products. The aim of this study was to determine the influence of cold atmospheric plasma on the inactivation of Aspergillus niger and Penicillium verrucosum inoculated on barley and their production of OTA. Inoculated barley was treated with plasma generated by dry air, CO2 or CO2 + O2 for 1 or 3 min and stored for up to two weeks at 9, 25, or 37°C. Three minutes of air plasma treatment effectively significantly reduced the total mold count of both microorganisms by 2.5–3 log cycles. The production of OTA from A. niger was only low, therefore the treatment effect was indistinguishable. The treatment of P. verrucosum on barley after an incubation of five days using a CO2 + O2 plasma resulted in a reduction of the OTA content from 49.0 (untreated) to 27.5 (1 min) and 23.8 ng/g (3 min), respectively. In contrast, CO2 plasma caused an increase of the OTA amount from 49.0 (untreated) to 55.8 (1 min) and 72.9 ng/g (3 min). Finally, the use of air plasma resulted likewise in a decrease of the OTA concentration from 56.9 (untreated) to 25.7 (1 min) and 20.2 ng/g (3 min), respectively. Reducing the incubation time before the treatment to 24 h caused in contrast an increase of the OTA content from 3.1 (untreated) to 29.1 (1 min) and 20.7 ng/g (3 min). Due to the high standard deviation, these changes were not significant, but the tendencies were clearly visible, showing the strong impact of the plasma gas on the OTA production. The results show, that even if the total mold count was reduced, under certain conditions the OTA amount was yet enhanced, probably due to a stress reaction of the mold. Concluding, the plasma gas and incubation conditions have to be considered to allow a successful inactivation of molds and in particular their toxic metabolites.