Effects of D-Limonene Nanoemulsion Coating on Post-Harvest Quality and Physiology of Papaya

Papaya (Carica papaya L.) is a climacteric fruit, and its quality will rapidly decrease after ripening. Hence, the storage life of its fruit is short. D-limonene is a terpene compound in citrus essential oil and has antibacterial and antioxidant properties. The addition of D-limonene in edible coating can delay volatilization, prevent microorganism and pathogen invasion, decrease water loss, inhibit softening, decrease gas exchange, and extend the storage life of fruits. In this study, 0.25%, 0.5%, and 1% D-limonene nanoemulsion coatings were used for post-harvest immersion treatment of "Tainung No. 2" papayas and its effects on appearance, chlorophyll content, respiration rate, ethylene production, pectin methylesterase, polygalacturonase activity, decay loss, firmness, total soluble solid, titratable acidity, ascorbic acid, and total plate count were investigated. After the papayas were treated with 0.5% D-limonene nanoemulsion coating, polygalacturonase and pectin methylesterase activities decreased, fruit firmness was maintained, and ascorbic acid content was high. On the last day of storage, polygalacturonase and pectin methylesterase activities were 0.01 and 0.02 U/kg FW lower than the control group, respectively; firmness was higher than the control group by 1.75 N, and ascorbic acid content was higher than the control group by 31.97 mg/100 g FW. On Day 2, the treatment showed delay in total soluble solid accumulation and chlorophyll degradation, consequently delaying the color change in fruits. The coating decreased decay loss by 40% on Day 2, decreased respiration rate by 97.0 mg CO2 kg-1 h-1, and ethylene production by 5.7 & mu;L kg-1 h-1 on Day 2. Simultaneously, the coating decreased the total plate count and resulted in a good appearance. Fruits coated with a 1% D-limonene nanoemulsion coating showed defects in color change. In summary, 0.5% D-limonene nanoemulsion coating delayed "Tainung No. 2" papaya ripening and decreased microbial infection, consequently extending its storage life.