## 5 Current Treatments to Avoid or Reduce PPO Activity in Fresh-Cut Produce
### Heat Treatments  
Thermal methods have been extensively used for food preservation. In order to avoid loss of some food properties such as vitamins, texture, fresh appearance, etc., it is applied in short times. Thus, temperature affects microbial and enzymatic activity whereas time affects quality loss [8,85]. Regarding PPO, its activity is extremely affected. As commented earlier, temperatures from 50 to 80 \({}^{\circ}\)C seem to be effective for inhibiting PPO activity during washing of many products such as lettuce, celery or peaches, among others, retaining quality parameters such as firmness in the case of minimally processed peaches [52]. Another aspect to take into account is the time of application. Thus, blenching treatments have been carried out in order to inactivate undesirable enzymes in fruit and vegetable tissues. However, softening might be found in blanching. Some authors have studied the effect of washing treatment temperature on PPO activity for apples, pointing  
Figure 8: Main trends for treatments in order to keep quality of fresh-cut produce. Search words: Heat treatments, modified atmosphere packaging (MAP), edible coatings, washing treatments, essential oils and extracts and antioxidants. Source: Scopus [7].  
out that high temperatures (from 55 to 65 \({}^{\circ}\)C) for short periods (30 s) were more effective, combined with a treatment of 0.6 % calcium chloride [86]. The use of blanching (by steam or microwaves) and osmotic dehydration was also carried out in minimally processed strawberries (where the effect of PPO does not imply browning, but loss of red color due to anthocyanin deterioration), showing no color changes possibly due to enzyme deactivation [13]. Similar findings were found for mild heat pre-treatments on pears, with an optimal temperature-time range of 35-45 \({}^{\circ}\)C for 40-150 min [66]. For fresh-cut iceberg lettuce, different calcium lactate solutions were found to be more effective in inhibiting PPO at 50 \({}^{\circ}\)C for 1 min, pointing out the suitability of the combined use of different treatments in order to keep global quality in minimally processed fruits and vegetables [15]. In parallel, in the case of fresh-cut apple slices, the use of hot water (at 60 or 70 \({}^{\circ}\)C, depending on the variety) combined with ascorbic acid or citric acid solution (1 % in both cases) showed the best results to control browning development [87]. However, some side effects can be observed during heat treatments, such as vitamin deterioration, which points out the need of further studies to optimize temperatures and time treatments in order to avoid these inconvenient effects. For instance, blanching at 100 \({}^{\circ}\)C was effective at reducing PPO activity in lettuce, but other antioxidants such as carotenes and ascorbic acid were reduced due to the heat treatment [56]. Indeed, further studies were carried out in order to optimize blanching time, concluding that the longer the treatment was, the less PPO activity was found (especially for vascular tissues), needing an intermediate solution in order to avoid extreme carotenoids or ascorbic acid deterioration in fresh-cut lettuce [17]. For peaches, a 50 \({}^{\circ}\)C heat pre-treatment for 10 min was effective in avoiding browning when the fruit was minimally processed. However, no PPO activity differences were found between treated and control samples [52]. Regarding the effect of heat treatments on quality parameters of fresh-cut sugarcane, a treatment at 45 \({}^{\circ}\)C for 3 h was tested, showing positive results in PPO activity (reducing its activity compared to control samples) [88].