Postharvest Evaluation of Essential Oil on Lettuce and Other Leafy Greens

As alternatives to chlorinated water, some plant-derived essential oils (EOs) have been evaluated at the microbial level as potential washing sanitizers on fresh produce. Although lettuce is frequently studied for the efficacy of EOs as antimicrobials, effects on postharvest quality are rarely reported. In this experiment, the effects of EOs on quality of lettuce and other leafy greens were evaluated. Treatments included semi-water soluble whey protein emulsified thyme, cinnamon, and clove bud oils at 0.05%, 0.1%, and 0.5% with whey protein emulsion without oil and water with 200 ppm free chlorine as controls. Locally purchased romaine lettuce leaves were dipped for one minute in solutions, then stored at 4 °C, (95% to 100% relative humidity). Two leaf systems (whole leaf versus fresh cut leaf) were investigated; small sections of leaf (2.5 by 5.5 cm in size) were found to effectively emulate EO events. A 0–5 rating scale was developed to follow severity of leaf damage, and ratings were done at 48 hours after treatment when symptoms were worst. Significantly higher damage was found on leaves treated with any of the three EOs at 0.5%, compared to those with EOs applied at lower rates or with controls. Damage at the 0.5% rate was generally classified as necrosis and browning, with thyme oil resulting in the highest damage rating among the three EOs. At lower rates (0.1% and 0.05%), thyme oil caused minor damage described as glassy spots, while cinnamon and clove bud oil caused no damage. Electrolyte leakage from leaves was significantly higher (>50%) for all three EOs at the 0.5% rate, indicating leaf membrane was damaged and a higher percentage of electrolyte was leaked out of the cells. Electrolyte leakage was lower (<20%) from leaves treated with 0.05% and 0.1% oils and was not statistically different from the controls. In addition to this experiment, other leafy greens (cabbage, kale, and baby spinach) also showed damage when treated with 0.5% of all three EOs. Using a vapor system (7 uL/L) in place of dipping, romaine lettuce showed damage after seven days; thyme oil vapor caused severe pink spots on both ribs and photosynthetic tissue, while clove bud and cinnamon oils caused moderate browning on only photosynthetic tissue. Our results indicate that leafy green quality is highly dependent on the type and amount of EOs used and application systems that will deliver efficacious antimicrobial activity without postharvest damage need to be developed.