Lenticel discolouration on 'B74' mango fruit and under-skin browning on 'Honey Gold' mango fruit

Abstract

Lenticel discolouration (LD) on mango fruit is evident as red, brown or black ‘halos’ surrounding the lenticels. It is a common skin disorder on ‘B74’ (CalypsoTM) mangoes. LD is exacerbated by exposure to γ-irradiation, a disinfestation treatment. Postharvest treatments of ‘B74’ fruit with chemicals (viz., anti-browning agents: ascorbic acid, citric acid and calcium ascorbate), wax (viz., carnauba coatings), and bags (viz., types and atmospheres) prior to γ-irradiation were investigated with a view to reduce LD induced by γ-irradiation. Different fruit ripeness stages (viz., hard, rubbery and sprung) prior to γ-irradiation were also investigated. With a view to better understand the browning biochemistry, polyphenol oxidase (PPO) and peroxidase (POD) activities and total phenolics concentration were quantified. Anti-browning agents did not reduce LD. Coating with three layers of 75% carnauba wax reduced LD, but the fruit failed to ripen. Maintaining fruit inside macro-perforated bags and paper bags did not reduce LD. Holding fruit inside closed polyethylene bags reduced LD, but only while fruit remained in the bags. Moreover, maintaining fruit in polyethylene bags impaired subsequent ripening. Irradiating partially ripe sprung stage fruit increased LD less that developed at eating ripe as compared to fruit treated when they were hard green. Thus, irradiating fruit at more advanced stage of ripeness is a promising approach to reduce LD associated with irradiation. Total fruit skin phenolics concentration was not correlated to LD. PPO activity after γ-irradiation in hard fruit skin increased more than that in sprung fruit. Also, LD in irradiated hard fruit skin increased more than in sprung fruit after γ-irradiation. Therefore, LD induced by irradiation was evidently related to PPO activity. Similarly, POD activity may also be involved in LD as it was also higher in irradiated fruit. Polyethylene bagging was associated with reduced PPO and POD activities when fruit were in bags and for a short time after their removal. Therefore, the transient lessening of LD after bagging is potentially associated with limited oxygen concentrations within the bags. Under-skin browning (USB) is manifested as sub-epidermal discolouration. This disorder predominantly affects ‘Honey Gold’ mango fruit. Towards understanding the causes of USB and reducing economic losses to industry, the influences of fruit growing region (viz., Northern Territory, North Queensland and Southeast Queensland), physical stress (viz., abrasion and vibration [0, 3, 9 and 18 h at 12 Hz], and reduced storage temperature (viz., 6 – 20°C) were investigated. Also, transporting fruit in soft polystyrene liner tray inserts was compared to commercial polyethylene liner tray inserts as a potential means to reduce USB. In addition, mango sap influences for harvest time (viz., morning, afternoon) and type (viz., spurt and ooze) were evaluated along with holding temperature (viz., 12 and 20°C) and mechanical damage type (viz., abraded, cut and peeled). USB incidence and severity, PPO and POD activities and total phenolic concentrations were measured. USB incidence was strongly influenced by the fruit growing region. Fruit grown in the Northern Territory were more susceptible to developing USB than fruit from North Queensland. Fruit produced in Southeast Queensland had no USB. The test measure of abrading the fruit skin elevated the incidence of USB on fruit grown in the Northern Territory and North Queensland. Moreover, the USB area (severity) surrounding the abrasion was generally larger than was USB expression away from the abrasion site. Simulated road transport vibration of 12 Hz for 18 h induced USB. Compared to at 20°C, vibration at low temperature increased the incidence of USB on fruit vibrated for 3, 9 and 18 h. Therefore, USB is not simply a chilling injury response. Physical stress is most likely to directly induce USB, and low temperature exacerbates the disorder. However, shipment of fruit in a polystyrene liner did not consistently lessen USB incidence and severity as compared to the polyethylene liner. As for the vibrated fruit, a higher incidence (%) of USB was found on the ‘shoulder’ position than on the ‘cheek’ position. Total phenolics concentration, and PPO and POD activities were less possible to be closely associated with USB incidence and severity. Anatomically, USB occurred in sub-epidermal cells surrounding resin ducts and extending away from resin ducts. Sub-epidermal browning similar to USB could be induced by injecting mango spurt sap underneath the skin. Exposure of fruit to a low temperature of 12°C resulted in a higher incidence of browning than at 20°C. Overall, observations suggested that physical stress possibly results in the leakage of sap from resin ducts into surrounding cells to cause USB, and low temperature intensifies USB.