## 5 Current Treatments to Avoid or Reduce PPO Activity in Fresh-Cut Produce
### Edible Coatings  
Coatings of edible materials applied as a thin layer in order to improve quality of fresh-cut fruits and vegetables act as a barrier that reduces respiration and water loss. Plus, some additives can be placed in edible coatings, acting more effectively in browning inhibition or texture maintenance, reducing PPO activity in many cases [81; 117; 118]. Thus, some authors have used chitosan coatings in order to increase quality in fresh-cut produces. For example, it was applied on Chinese water chestnut, reducing PPO and phenylalanine ammonia lase (PAL) activity and therefore increasing its shelf-life [119]. The use of alginate in combination with calcium chloride was studied for apples, retarding browning probably due to PPO inhibition by calcium chloride (the latter interacting with cooper) and O\({}_{2}\) reduction on account of the subsequent edible coating's barrier action [67]. Other edible coating used in apple was a trehalose (0.8%), NaCl (0.1%) and sucrose (1.0%) solution for 5 min. This innovative coating was able to reduce browning due to the ability to conform with polyphenol compounds, avoiding weight loss and maintaining color during storage [23]. Additionally, in minimally processed Red Delicious apples, the use of a coating consisting oftocopherol/mucilage nanoemulsion was tested, observing 65% decrease in PPO activity after 21 days of storage at 4 \({}^{\circ}\)C [45]. Other chitosan coatings (combined with AA and calcium chloride) were also used, being its effect on PPO activity proved in fresh-cut Fuji apples [120]. Regarding other minimally processed fruits, as in the case of fresh-cut pineapples, the combination of alginate treatment and MAP did not show PPO activity during storage nor browning evolution [121]. For fresh-cut banana, dipping the produce into chemical solution (0.5% ascorbic acid, 2% calcium chloride and 0.75%cysteine) combined with carrageenanan coating plus storage under controlled atmosphere (3% O\({}_{2}\) and 10% CO\({}_{2}\)) showed PPO activity inhibition and therefore this method could be suitable to preserve them for up to 5 days [18]. For peers, chitosan (1%) and its derivative carboxichitosan (1%) was also proved in combination with sodium chlorite (600 mg/L), being the latter more effective with over 50% PPO inhibition [55]. Finally, konjac glucomanan coating incorporated with pineapple fruit extract was used for fresh-cut rose apple fruit, being the PPO and POD activities lower than in control samples and therefore retarding browning [122]. In addition, the effect of chitosan coating and MAP on PPO activity was positively proved in fresh-cut lotus root [99]. Similar results were obtained with another edible coating based on cysteine and citric acid, applied to fresh-cut lotus root slices, inhibiting browning and microbial growth [123]. Thus, as in many other treatments covered in this review, the combination of treatments could be a suitable way to strengthen their effectiveness, showing synergistic effects. In addition, the use of natural products (which are increasingly interesting for customers) seems to become important to produce edible coatings. This way, some authors have tested the effectiveness of aloe vera gel and honey-based edible coatings to keep quality parameters in fresh-cut papaya. As a result, its combination with other chemical dips (such as ascorbic acid or vanillin) reduced PPO activity and kept global quality parameters (such as weight loss or sensory quality) during a storage period of 12 days at 4 \({}^{\circ}\)C [51].