Effect of An Alkaline Surface Treatment On Edible Coating Performance as Pertains to Gas and Water Vapor Exchange of Carambola and Mango Fruit

Freshly harvested ‘Keitt’ mangoes and ‘Arkin’ carambolas were washed with or without alkaline solution and then coated with carnauba wax or water. After treatments, fruit were stored at 15°C and relative humidity 70% room for 2 days to allow drying of the coating and equilibration of the internal gas in the fruit. Stabilized fruit, average weight 522 g for mango and 153 g for carambola, were individually sealed in 3.6 L glass jars initially flushed with dry air and incubated for up to 8 hours. RH in the jars was measured at 1 min. intervals to determine water loss. For CO₂ and O₂ transmission determination, internal gas components were measured two and four days after treatment. The results showed that the alkaline wash slightly increased water loss in mango fruit, but decreased water loss in carambola fruit while coating the fruit remarkably reduced water loss. In addition, coating application on the alkaline washed fruit had a lower water loss rate than coating alone. Similar results were observed for CO₂ and O₂ gas exchange. Both the alkaline wash and coating reduced the gas transmission rate, and coating application on the alkaline washed carambola had the lowest gas permeability. In conclusion, 1) the alkaline wash  reconditioned the fruit surface, smoothing out cuticular microcracking, thus explaining the reduced transmission of CO₂ O₂ and water vapor in carambola fruit; 2) possible removal of cuticle by the alkaline wash may cause increased water loss, as seen on mango; 3) the alkaline washed fruit surface allowed better coating adhesion to the fruit surface, thus, application of coating after an alkaline wash always had lower gas transmission rate in both mango and carambola fruit compared to coating alone or untreated controls. Another study showed that an alkaline wash significantly improved fruit sanitation by removing of surface microbial organisms and spores. However, care must be paid to avoid phytotoxicity due to the high pH treatment.