John P. Bower

Anti-oxidant levels in various tissues during the maturation of ‘Hass’ avocado (Persea americana Mill.)

Summary Avocado is a fruit high in polyunsaturated fatty acids, with a high mineral content. However, no reports are available on how valuable avocado fruit could be as a source of anti-oxidants, depending upon their stage of maturity. Such a feature might become an important marketing tool in future. To evaluate the presence of anti-oxidant systems throughout avocado fruit development, various tissues were analysed for both their total and specific anti-oxidant compositions.Total anti-oxidant levels were found to be higher in the exocarp and in seed tissues than in the mesocarp, a possible reason for the susceptibility of avocado to post-harvest mesocarp disorders. While seed tissues contained predominantly ascorbic acid (AsA) and total phenolics (TP), the anti-oxidant composition of the mesocarp was characterised by the presence of the C7 sugar, D-mannoheptulose. Among the anti-oxidant enzymes assayed, peroxidase (POX) and catalase (CAT) were present in higher concentrations than superoxide dismutase (SOD) in mesocarp tissue. The low concentration of anti-oxidants in the mesocarp could be a major reason for the susceptibility of avocado fruit to physiological disorders.

Ripening physiology and quality of ‘Hass’ avocado (Persea americana Mill.) after cold storage at 1ºC

Summary The export of avocado (Persea americana Mill.) fruit into foreign markets has become a high priority for the South African industry due to increased competition in the European market. Some of these countries require phytosanitary treatment before access is granted. Cold disinfestation is the only suitable phytosanitary treatment available for avocado, but the fruit are generally considered to be sensitive to temperatures below approx. 5ºC. Phytosanitary disinfestation of South African avocados of acceptable fruit quality has been achieved by maintaining the pulp temperature below 2ºC for 28 d, but the effect on ripening physiology has not been determined. The effect of cold storage at 1ºC (air delivery temperature) on ‘Hass’ avocados with or without modified humidity packaging (MHP) for 28 d was examined. Compared to a conventional storage temperature of 5.5ºC for 28 d, fruit stored at 1ºC had significantly reduced rates of respiration and ethylene production, less softening, water loss, fresh weight (FW) loss, and mannoheptulose consumption measured immediately after cold storage. The use of MHP resulted in similar reductions at the conventional temperature and, when MHP was used in combination with 1ºC storage, these reductions were enhanced. Cold storage at either temperature resulted in increased cellulase and polygalacturonase activities during ripening when compared to non-stored fruit (kept at 21ºC until ripe), but no significant differences were recorded between the four cold storage treatments (i.e., 1ºC and 5.5ºC, with or without MHP) in the activities of cellulase, polygalacturonase, and pectin methylesterase during ripening. The number of days to ripen increased slightly after storage at 1ºC, but the percentage of marketable fruit was not significantly reduced. Significantly reduced FW and water losses from fruit stored under MHP resulted in significant reductions in the severity of skin spotting. The ripening physiology and quality of South African ‘Hass’ avocados were not negatively affected when fruit were stored at 1ºC for 28 d. When considered with previous results, this treatment appears to be suitable for future use.

Some initial changes in ‘Hass’ avocado (Persea Americana Mill.) physiology due to ethephon

Ethylene is used commercially to hasten ripening of avocados, but the presence of ethylene in the storage atmosphere can increase the risk of physiological disorders. To investigate the short term effect of ethylene on avocado physiology, fruit were dipped in a low concentration ethephon solution. The ethephon treatment significantly increased the ethylene production rate of avocados after 6 h, compared to untreated fruit. After 6 h, the respiration rate of treated fruit increased significantly during the investigation while that of the untreated fruit did not show a significant increase from the initial measurement. Lipid peroxidation increased after dipping, reaching a peak after 4 h, and then declined to a level similar to untreated fruit. Mannoheptulose declined significantly during the 6 h investigation in both the untreated and treated fruit. It is concluded that avocado fruit can undergo significant physiological changes, and quality loss, in a short amount of time, which is exacerbated by exposure to ethylene.

Effect of production site, storage duration, and hot water and molybdenum dips on bioactive compounds with antioxidant properties in lemon flavedo during cold storage

Certain bioactive compounds, such as vitamin E (α-tocopherol), vitamin C (ascorbic acid), β-carotene, polyphenols (e.g. flavonoids and flavonones, viz. hesperidin and naringin) have antioxidant properties and their presence in the citrus flavedo plays a significant role in mitigating oxidative stress, which could possibly result in chilling injury during cold storage. The purpose of this investigation was, therefore, to alter the concentration of these bioactive compounds in lemon flavedo using hot water and molybdenum (Mo) treatments prior to cold storage to allow fruit to withstand such storage, and avoid the occurrence of chilling injury. Fruit from three sources (Ukulinga Farm, Sun Valley and Eston Estates) were treated for 2 min with hot water dips (HWD) (at 47 °C or 53 °C) with or without a subsequent soaking in 1 or 10 µM Na2MoO4 for 30 min. Fruit were then stored at -0.5 °C for up to 7, 14, 21 or 28 d and thereafter moved to room temperature (±21 °C and ±68% RH) for 7 d. Lemon fruit from Sun Valley Estates were chilling-susceptible and had the lowest concentration of bioactive compounds, significantly lower than chilling-resistant fruit from Ukulinga Farm. Application of HWD at 53 °C, as well as 1 µM Mo in combination with HWD at 53 °C, enhanced the flavedo concentration of bioactive compounds. Furthermore, storage duration seemed to induce an increase or decrease in flavedo bioactive compounds concentration. In general, HWD and Mo dips enhance the rind concentration of phenolics and ascorbic acid at specific storage times.