## 5 Current Treatments to Avoid or Reduce PPO Activity in Fresh-Cut Produce
### Modified Atmosphere Packaging and Controlled Atmosphere  
#### 5.2.1 Traditional MAP or CA  
Traditional modified atmosphere packaging can extend fresh-cut produce's shelf-life because of its low O\({}_{2}\) and/or high CO\({}_{2}\) composition. Thus, by reducing oxygen (avoiding anaerobic respiration) cut browning effects might be reduced, among other effects in fruit and vegetables [89; 80]. Following this conservative technique, many authors have studied the effect of different MAP treatments on fruits and vegetables. For instance, the effect of different materials used for MAP was assessed for fresh-cut cambola, finding the polyolefin bags as the most suitable film because of PPO activity reduction (probably due to firmness conservation, which avoided the contact of enzymes and substrates) [47]. Different MAP treatments (combined with antioxidant and calcium washing treatments) were carried out for pitted jackfruit fruits, in order to assess browning index (which was considerably reduced), extending the shelf life of the samples for 35 days at 6 \({}^{\circ}\)C [90]. Even controlled atmospheres (1 kPa CO\({}_{2}\) + 1 kPa O\({}_{2}\) and 3 kPa CO\({}_{2}\) + 3 kPa O\({}_{2}\)) has been used as a pre-treatment so as to avoid browning in fresh-cut apple, and changes in PPO activity depending on CA were negligible [19]. In addition, the use of high CO\({}_{2}\) concentrations in fresh-cut pear kept quality parameters such as firmness or color, inhibiting browning [91; 92]. However, the effect of these techniques might be short. Thus, when the sample is put in contact with oxygen (for example, by opening the package) browning might restart [33]. Recently, new materials have been tested to be introduced in MAP, such as the use of ZnO nanoparticles into poly-lactic matrix, showing interesting results and keeping the properties of fresh-cut apples for 14 days at 4 \({}^{\circ}\)C, decreasing PPO activity. In addition, other studies have focused on anoxia treatments, where the sample is exposed to pure N\({}_{2}\). In that sense, the browning inhibition of this technique was proved in fresh-cut Chinese water chestnut, including changes in PPO activity [93]. In addition, the use of sustainable materials seems to play an important role in active packaging, as in the case of onion solid wastes, which were used to produce active packages which reduced browning in fresh-cutproduce [94]. However, some studies did not find MAP effective enough to prevent cut browning. Therefore, some complementary treatments are needed (for example cysteine washing treatments) [95]. Indeed, many authors have used MAP combined with other washing treatments such as citric and ascorbic acid, calcium chloride, calcium ascorbate, chitosan, etc. [69; 96; 97; 98; 99]. Finally, as explained previously, pre-harvest or pre-processing treatments can influence on the performance of fresh-cut produce, as in the case of the use of modified atmospheres in whole fruits before minimal processing. Thus, an interesting study showed the effect of 1-MCP during postharvest on fresh-cut planquilla pear. Its antioxidant potential was found responsible for the better tissue integrity, hindering PPO coming in contact with phenols once minimal processing was carried out [100].