Effect of free and microencapsulated thyme essential oil on quality attributes of minimally processed lettuce

In the present study, thyme essential oil (TO) was used as a natural antioxidant and antimicrobial agent to enhance the quality and shelf-life of minimally processed lettuce during refrigerated storage. For this purpose, the effect of dipping treatments of free TO (0.5, 1.0, and 1.5 g L⁻¹) and microencapsulated TO in β-cyclodextrin (TO:β-CD, 15.8, 31.6, and 47.5 g L⁻¹) on quality aspects of minimally processed Romaine lettuce was compared. Quality parameters such as weight loss, color, total phenolics and flavonoids content, antioxidant activity (DPPH), microbial populations, and organoleptic quality were evaluated during 12 d of refrigerated storage. Weight loss of lettuce samples was not affected by treatments with TO. Immediately after application of free TO treatments, the total phenolic and flavonoids content and the antioxidant activity of lettuce increased significantly. However, these improvements were not maintained during storage. Conversely, TO:β-CD treatments did not affect the phytochemical content and antioxidant activity of the samples immediately after application but a significant increase was observed during storage. Moreover, these quality parameters increased as the concentration of the microcapsules in the dipping solution increased. Treatments with free TO were not effective in reducing the microbial load of lettuce compare to control samples during the entire assayed period. Instead, treatments with TO:β-CD (31.6, and 47.5 g L⁻¹) exerted a bacteriostatic effect over mesophilic and psychrotrophic bacteria and reduced Enterobacteriaceae and yeast and molds counts throughout the storage period. Additionally, samples treated with microencapsulated TO presented better organoleptic quality scores than control and free TO treated lettuce. From this study, it can be concluded that TO:β-CD microcapsules can be used to control decay and increase the antioxidant health benefits of minimally processed lettuce due to an enhancement of the antimicrobial and antioxidant properties of the essential oil.