Natural elicitors as preharvest treatments to improve postharvest quality of Butterhead lettuce

Lettuce (Lactuca sativa) is one of the most popular leafy vegetables in the world, characterized by a diverse composition of phytochemical compounds but at low levels. However, their content may be increased with abiotic stresses. Accordingly, phytochemical enhancement and related microbiological and organoleptic quality and peroxidase (POD) and polyphenol oxidase (PPO) activities of Butterhead lettuce elicited during preharvest with natural compounds (chitosan 10 g L-1 and tea tree essential oil 2.7 mL L-1) were studied. Quality indices were evaluated at harvest and during 21 days of refrigerated storage (0-2 degrees C). Treatments with chitosan and tea tree increased the total phenolic content of freshly lettuces by 30.5 and 21.1%, respectively, and the total flavonoids concentration by 43.3 and 36.4%, respectively, compared with control samples. The antioxidant capacity at harvest of the elicited plants, measured with DPPH and TEAC assays, was also higher. Notably, these improvements were maintained during refrigerated storage. Conversely, although a higher concentration of ascorbic acid was initially detected in treated plants with chitosan and tea tree, these differences were not observed at later storage times. No differences were detected in the organoleptic quality of elicited and control plants, meanwhile the microbiological quality and enzymatic activity were affected by the preharvest treatments. In particular, the application of chitosan exerted a fungistatic effect reducing yeast and molds population counts by 1.6 log throughout the storage, compared with control samples. Furthermore, chitosan also reduced the activities of PPO and POD, enzymes related with browning processes. Preharvest treatments with chitosan and tea tree enhanced the content of health-promoting phytochemicals in lettuce, without affecting its organoleptic quality. Moreover, chitosan treatment appears as a promising method to improve the safety and reduce the enzymatic activity of lettuce.