## 4 Discussion
### Inactivation of E. coli and Selection of Equivalent Processing Conditions for Each Technology  
_E. coli_ O157:H7 is a pathogenic bacterium commonly present in apple and apple processing environments. It is known to have the ability to tolerate acidic conditions and thrive in refrigerated conditions [31]. The chosen pasteurization methods were able to effectively inactivate _E. coli_ by at least 5 log units, which is the minimal level of inactivation necessary to attain pasteurization status. Although apple products are typically pasteurized at higher temperatures and for longer durations to eliminate thermophilic bacteria like _Alicyclobacillus_ spp., the flash pasteurization method employed in this study successfully inactivated _E. coli_ by more than 5 log units. This could allow us to reduce the processing temperature and time of apple products, if they are kept under refrigeration conditions, as some thermoresistant bacteria and spores could develop [32]. Concerning the use of ultrasound (US) for pasteurization purposes, specifically in relation to targeting _E. coli_, there are reports indicating that this technique is capable of inactivating at least 5 log units. However, it is important to note that this effectiveness is achieved when accompanied by moderate temperatures. [PERSON] and colleagues demonstrated a significant decrease in _E. coli_ K12 loads in apple cider treated with ultrasound (20 kHz and 0.46 W/mL of acoustic energy density). The reduction was at least 5 logs and occurred after 5, 10, and 15 min of processing at temperatures of 60, 55, and 50 \({}^{\circ}\)C, respectively [33]. In a separate investigation, [PERSON] and colleagues [34] examined the viability of utilizing ultrasound (US) with specific parameters (5 s on, 30 s off, 24 kHz, at 60 \({}^{\circ}\)C and 45% amplitude) for the pasteurization of apple juice and pulp. The findings indicated that this method also met the requirement of achieving a 5-log reduction, which was not attainable through conventional pasteurization at 60 \({}^{\circ}\)C. This study showcases the feasibility of integrating US (ultrasound) technology with moderate temperatures (60 \({}^{\circ}\)C) for the purpose of pasteurizing food. This combination enables shorter processing times and reduces the amount of heat exposure, resulting in pasteurized foods that possess enhanced nutritional and sensory qualities. At selected processing conditions, PEF was the processing technology used in this study with the lowest impact on _E. coli_ loads (regardless of achieving the 5-log threshold), and the electric field strength selected was in the mid-range of the ones tested (i.e., the electric field strength varied between 7 and 12.5 kV, and the highest inactivation level was observed for a field strength of 10 kV). The reader may find this result rather curious, as usually more intense processing conditions yield higher inactivation rates, but this did not happen in the present study. It is possible that a synergistic effect between the applied electric field strength and the processing time led to an overall temperature rise of about 31 \({}^{\circ}\)C (from an initial temperature 26 to a final temperature 57 \({}^{\circ}\)C after processing) and greater inactivation effects of PEF, as observed by [PERSON] and colleagues (2021), who reported that the inactivation of _E. coli_ was proportional to the increase in the set processing temperature, i.e., higher inactivation levels were observed at higher temperatures when compared to conventional PEF at room temperature [35]. The inactivation of _E. coli_ O157:H7 on Granny Smith apple peels subjected to microwave for 35 s at 652 W that achieved an average peak temperature of 75.3 \({}^{\circ}\)C was examined. These operational parameters are akin to the time-temperature profiles utilized in the CTP process.