## 4 Current Studies Focused on PPO Activity
### Fruit and Vegetables Characterizationand Processing  
In order to understand PPO behavior in fresh-cut produce, some characterization in whole fruits and vegetables is needed. As is known, PPO and phenol content in fruits are important in order to explain browning. Many authors have claimed that these contents might vary depending on the cultivar and fruit development or ripening [58; 59]. That is the reason why some studies have focused on pre-harvest behavior or ripening evolution (when the sample is collected) of some fruits and vegetables. For instance, the characterization of membrane-bound polyphenol oxidase from Granny Smith apples was carried out, which provided interesting information to understand the behavior of PPO during processing in order to improve the quality of minimally processed apples [60]. On the other hand, PPO content was measured during ripening for medlar, finding rapid enzymatic browning as the fruit softened and observing an increase in PPO activity and a decrease in polyphenol content during ripening [58]. Another study on rambutan was carried out so as to characterize it when it comes to PPO activity, showing high polyphenol contents and PPO activity (especially in spinterns, compared to peel), which could explain (along with dehydration) browning during storage [57]. Furthermore, a study about the suitability of five potato cultivars for fresh-cut processing was published, paying special attention to browning and PPO activity. In this case, \"Marabel\" cultivar showed the lowest phenol content and PPO activity and the longest shelf-life, due to the maintenance of global quality (including the subsequent decrease in browning evolution) [36]. Lettuces cultivated in different salinity conditions were evaluated in order to assess its suitability for fresh-cut processing, paying special attention to PPO activity and browning. Thus, low salinity conditions (2.8 dS m\({}^{-1}\)) were not suitable due to the high PPO activity, being reduced when salinity was increased [31]. Another similar study showed that the use of high salinity solutions (4.8 mS cm\({}^{-1}\)) could slightly reduce PPO activity, especially at the end of storage [61]. In addition, six genotypes of artichoke were studied for minimal processing, focusing on PPO activity and browning, with \"Romolo\" cultivar being the most suitable one [14]. Minimally processed sweet peepers were studied according to their PPO activity evolution during storage. Thus, yellow peppers were less suitable for minimal processing, showing the green ones lower PPO activities [42]. Finally, some authors have studied how irrigation affects fresh-cut lettuce, finding that high irrigation regimes conveyed high PPO activity during storage [32]. But not only pre-harvest conditions are vital to understand PPO activity. In that sense, processing (including cutting, heat treatments, etc.) can affect phenol content, due to PPO activity, among other factors [62]. As pointed out in the literature, processing, even when it comes to cutting styles, can accelerate PPO activity, as it happened in the case of fresh-cut potatoes, where shredding had a negative effect [63] or lettuce, where cutting induced an increase in PPO activity in minimally processed lettuce [64].