Ripening physiology and quality of 'Hass' avocado (Persea americana Mill.) after cold storage at 1°C

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. 5 degrees C. Phytosanitary disinfestation of South African avocados of acceptable fruit quality has been achieved by maintaining the pulp temperature below 2 degrees C for 28 d, but the effect on ripening physiology has not been determined. The effect of cold storage at 1 degrees C (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 5.5 degrees C for 28 d, fruit stored at 1 degrees C 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 1 degrees C 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 21 degrees C until ripe), but no significant differences were recorded between the four cold storage treatments (i.e., 1 degrees C and 5.5 degrees C, 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 1 degrees C, 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 1 degrees C for 28 d. When considered with previous results, this treatment appears to be suitable for future use.