Reported herein is a high throughput method to quantify in a single analysis the key volatiles that contribute to the aroma of commercially significant mango cultivars grown in Australia. The method constitutes stable isotope dilution analysis (SIDA) in conjunction with headspace (HS) solid-phase microextraction (SPME) coupled with gas-chromatography mass spectrometry (GCMS). Deuterium labelled analogues of the target analytes were either purchased commercially or synthesised for use as internal standards. Seven volatiles, hexanal, 3-carene, α-terpinene, p-cymene, limonene, α-terpinolene and ethyl octanoate, were targeted. The resulting calibration functions had determination coefficients (R) ranging from 0.93775 to 0.99741. High recovery efficiencies for spiked mango samples were also achieved. The method was applied to identify the key aroma volatile compounds produced by 'Kensington Pride' and 'B74' mango fruit and by 'Honey Gold' mango sap. This method represents a marked improvement over current methods for detecting and measuring concentrations of mango fruit and sap volatiles.

Under-skin browning (USB) is an unsightly physiological disorder that afflicts ‘Honey Gold’ mango fruit. Under-skin browning symptoms develop after harvest upon the interaction of physical abrasion and physiological chilling stresses. Less understood preharvest and/or harvest factors may also influence fruit susceptibility to USB. In this study, we examined the impact of harvest time during the diurnal cycle and fruit sap components on USB development. Fruits were harvested at 4- to 6-h intervals, lightly abraded with sandpaper to simulate vibration damage during refrigerated road transport, held at 12 ± 1°C for 6 days, transported to the research facilities and ripened before USB assessment. Spurt and ooze sap from the fruit were collected at each harvest time. The samples were separated and analysed by gas chromatography–mass spectrometry. Fruit harvested at 10:00, 14:00 and 18:00 h had 3- to 5-fold higher incidence of USB than did those picked at 22:00, 2:00 and 6:00 h. Sap concentrations of the key aroma volatile compounds 2-carene, 3-carene, α-terpinene, p-cymene, limonene and α-terpinolene were higher for fruit harvested at 14:00 h compared to those picked at other times. In the fruits harvested in the afternoon, abraded skin treated with spurt sap sampled at 14:00 h had 14.3- and 29.0-fold higher incidence and severity, respectively, of induced browning than did those treated with sap collected at 6:00 h. The results showed that fruit harvested in the afternoon were more susceptible to USB than those picked at night or in early morning. The diurnal variation in fruit sensitivity was evidently associated with specific compositional differences in sap phytotoxicity. Topical application to the fruit skin of pure terpinolene and limonene resulted in induced USB damage, whereas pure carene and distilled water did not. Microscopy examination showed that while skin damage caused by pure terpinolene and limonene was not identical to USB per se, similarities suggested that sap components cause USB under inductive commercial conditions. Considered collectively, these findings suggest that night and early morning harvesting will reduce USB and thus improve the postharvest quality of Honey Gold mango fruit.

BACKGROUND: Under-Skin Browning (USB) is a physiological skin disorder that significantly reduces quality of 'Honey Gold' mango (HG) fruit. Relationships between potential causative factors (vibration, holding temperature, sap) and expression factors (enzymes activities, phenolic concentration, anatomy) were investigated. RESULTS: USB incidence was 2.6-3.6-fold higher in ripe HG fruit vibrated for 3-18 h at 12°C to simulate transport damage and held then at 12°C for 8 days compared to control fruit held under the same conditions. USB severity of fruit lightly abraded with sand paper to simulate physical damage and artificially induce USB was higher in fruit held at 10°C than at 6-8°C or 12-13°C for 6-8 days. Compared to non-affected skin, USB-affected tissue had a 7.4% increase in total phenolics concentration. However, polyphenol oxidase (PPO) and peroxidase (POD) activities decreased by 19%. Anatomical similarities were observed between USB symptoms and sapburn caused by spurt sap or terpinolene (a major sap component) to abraded skin areas. Incidence of sapburn was higher in abraded fruit held at 12°C than at 20°C. CONCLUSION: Holding HG mango fruit at 10°C can intensify USB. Activities of PPO and POD appear not to be regulatory factors in USB expression in HG. Sap components may be involved in USB expression under conducive postharvest conditions.

Lenticel discolouration (LD), under-skin browning (USB), and resin canal disorder (RCD) are three fruit skin disorders of concern to the Australian mango industry. LD is confined to lenticels on the fruit skin. It was suspected that irradiation may differentially induce LD on cvs. 'B74', 'Honey Gold', 'Kensington Pride', and 'R2E2' fruit. Irradiation at 0.5 kGy significantly (P≤0.05) increased LD and delayed the loss of green skin colour of all four cultivars. There was generally no significant differences (P>0.05) between 0.5 ('commercial' dose) and 1.0 kGy ('upper limit' dose).The least pronounced effect was on cv. 'Honey Gold'. Irradiation also diminished aroma volatiles production by cvs. 'Kensington Pride' and 'R2E2'. Examination of the morphology of LD showed it in tissue sections as browning of sub-lenticellular cells around lenticel cavities. No such browning was evident in sub-lenticellular cells around non-coloured lenticel cavities. Coloured polymerised phenolic compounds appeared accumulate in the cell wall and in the cytoplasm of sub-lenticellular cells around the cavities of discoloured lenticels.USB is a physiological disorder evident as spreading grey -brown sub-surface lesions in cv. 'Honey Gold' mango fruit. In contrast, RCD in cv. 'Kensington Pride' mango fruit is evident as ramifying dark brown sub-surface resin canals. These two largely cv. specific browning disorders were compared and contrasted at the cellular level. USB was characterised by dark brown surrounding parenchyma cells around the epithelial cells that line resin ducts. This disorder involves starch retention as well as deposition of phenolic compounds in parenchyma cells surrounding the epithelial cells of resin ducts. In contrast, RCD was distinguished by localised browning inside resin duct lumens. The browning process evidently involved accumulation of polymerised phenolics.Polyphenol oxidase and peroxidase enzymes were determined by tissue printing to be associated with both USB and RCD browning.The effect of harvest time over the diurnal cycle was investigated with regard to the propensity of cv. 'Honey Gold' fruit to develop USB. USB expressed in postharvest is related to the cumulative interaction of physical (viz., vibration) and physiological (viz., chilling) stresses. Fruit harvested during the day were relatively more susceptible to developing USB than were those picked at night.The higher USB incidence in the afternoon harvest was related to temporally variable sap phytoxicity. An important contributory factor to greater afternoon sap phytoxicity was concomit ant increases in the concentrations of volatiles in the sap and, especially, in the non-aqueous sap phase.Changing from day to night and early morning harvesting afforded reduced incidence and severity of USB on cv. 'Honey Gold' fruit. v Li, G.Q., Gupta, M ., San, A.T., Joyce, D.C., Hofman, P.J., M acnish, A.J., M arques, J., 2016.Postharvest treatment effects on 'B74' mango fruit lenticel discolouration after irradiation. Acta Horticulturae (ISHS) 1111...