Hot Water Treatment and Coatings to Modify Internal Atmosphere of Navel Orange As Determined By Respiration Rate, Gas Exchange, and Sensory Quality

Abstract-Respiratory activity is important for plant metabolic processes and increases as plants respond to stress. Upregulation of the plant antioxidant system in response to stress increases resistance to subsequent stress exposure. When submerged in heated water, fruit respiration can increase substantially with minimal gas exchange possible, potentially creating extreme internal modified atmospheres (MA) that may also be perceived by the tissues as a stress. Coatings applied to citrus fruits may also create internal MA during storage. The purpose of this research was to select the optimum hot water treatment (HWT) temperature and duration plus fruit coatings in order to create beneficial internal MA for maintaining postharvest quality of fruit stored at 25ºC. Naval orange (Citrus sinensis) fruit harvested at commercial maturity in January 2016 were dipped for 30 seconds in 1,000 ppm thiabendazole. Respiration rates were measured at from 25ºC to 60ºC in 5ºC increments. A data logger recorded the time and temperature when peel and center fruit temperatures were equilibrated and GC was then used to measure respiration rate and internal atmosphere. The maximum respiration rate (200.31 ml CO₂/kg-h) occurred at 55ºC with lower rates at lower and higher tempertures. Different HWTs were evaluated with temperatures from 25ºC to 55ºC in 5ºC increments and durations from 10 to 80 min. Those treatments resulted in internal O₂ from 1% to 20% and CO₂ from 1% to 30%. Immersion in 45ºC water for 30 min resulted in the most extreme internal atmosphere (~3% O₂ and ~18% CO₂) without causing any detrimental effect on external peel appearance. Polyethylene-, carnauba-, or shellac-based coatings were applied to hot-water-treated fruit using a centrifugal applicator on a semi-commercial citrus packingline in order to create possibly beneficial MA within fruit during storage at 25ºC with 85% RH. Considering the flavor and overall quality of the fruit, consumers ranked fruit with carnauba-based coatings higher than the other treatments, while shellac-coated fruit received the lowest ratings. This was likely due to the internal MA in the fruit, with carnauba resulting in an apparently beneficial atmosphere of ~10% O₂ and ~8% CO₂ while shellac resulted in more extreme ~5% O₂, ~18%CO_2; shellac also resulted in elevated ethylene (2-3 ppm), which suggests that injury occurred. Therefore, it can be concluded that HWT followed by application of semipermeable coatings can be used to create and maintain beneficial internal MA in citrus fruits to better maintain fruit quality during storage at ambient temperature.