## 3 Results and Discussion
### Microbial Diversity on Endive Lettuce along the Washing Process Chain
The application of PTW during washing of endive lettuce led to a pH value below 2 in the wash water as a consequence of the presence of nitrite and nitrate and their reaction with water to form nitric acid and nitrous acid [19]. Hence, it is conceivable that changes in the ribosomal proteins occur in response to the stressful conditions during washing with PTW, and these changes may hamper the identification of the mass spectra. During storage, the microorganisms might regenerate, leading to a decreased amount of not identified spectra. However, this is only a hypothesis, and changes in MALDI-ToF mass spectra of bacteria after the application of certain decontamination processes need to be evaluated and verified in further studies. Nevertheless, it is obvious that the culturable microbial community structure is diverse and varies greatly between the batches. This can be attributed to the natural microbial diversity on lettuce but it may also be influenced by the different environmental conditions during the experiments, since there was a temperature increase from 19 \({}^{\circ}\)C to 31 \({}^{\circ}\)C and an increase in relative humidity from 69% to 95% in the working environment during the experimental trials [19]. Therefore, it has to be taken into account that it is not possible to determine the microbial community structure of endive lettuce during an experiment and transfer the results to other experiments and that it is, therefore necessary to evaluate the dynamics in the microbial community structure of products during different processing steps singularly. Nevertheless, the results indicate that the culturable microbial community is influenced by the application of PTW during washing, indicating that some microorganisms are more sensitive to PTW than others. Changes in the microbial diversity after certain processing steps were also found in other studies. The microbial diversity of soybean sprouts changed due to washing with bacteriocins [31], and the microbial diversity of lamb's lettuce changed after washing with essential oils and subsequent storage [10]. In our study, members of the family Staphylococcus-caceae were predominately found after washing with PTW. In comparison to the washing without sanitizers, a decreased culturable microbial diversity was found in all samples after washing with addition of PTW at different processing steps. During storage, changes in the culturable microbial diversity were also observed, which might be attributed, on the one hand, to the application of sanitizers and, on the other hand, to the refrigeration temperatures. The storage temperature of minimally processed produce should be between 1 \({}^{\circ}\)C and 4 \({}^{\circ}\)C to extend the shelf life of the products [32]; however, psychrotrophic bacteria such as Pseudomonas-Pseudomonasaceae are able to proliferate at refrigerator temperatures, where even mesophilic bacteria can grow albeit at lower growth rates [33]. Changes of the composition of the microbial community during storage were also found for ready-to-eat baby spinach and mixed salad with Pseudomonas-cae as predominant microorganisms [34]. Even though the TVC of the final product was reduced after application of PTW to some extent at the end of the storage (seven days, 2 \({}^{\circ}\)C), no significant difference of TVC on the final product could be observed between the different treatments applied. This implies that the shelf life of endive lettuce could not be enhanced in comparison to the endive lettuce washed without sanitizers, which is in accordance with other studies using, e.g., chlorine as a sanitizer. However, the TVC was only tested at the end of storage period of seven days and, in future studies, the TVC should be obtained following different lengths of storage to evaluate the development of TVC during storage, and if any differences between the application of PTW during washing and washing with water only occur.