Manipulation of ripening during storage of Sun Supreme plums to improve storage quality

In 2004, Sun Supreme plums were harvested at a flesh firmness of 7, 9 and 5 kg. Quality was evaluated after a cold storage period of 35 days as well as after a simulated shelf life period of 5 days at 10°C. Dual-temperature storage employing a 9 days warming at 7.5°C during cold storage at – 0.5°C gave the best results in terms of internal quality. In the fruit harvested at 9.0 kg, taste was rated as poor and was only marginally acceptable in the fruit harvested more mature. However, excessive fruit drop was reported when the fruit matured to a flesh firmness of 7 and 5 kg on the tree and the quality of this fruit after storage was inconsistent. It was speculated that the unique fruit size and the fact that the fruit ripens from the inside could be reasons for the premature fruit drop in this cultivar.

In 2007, Sun Supreme was harvested at a flesh firmness of 10.8 kg. The TSS was 12.0% and the malic acid was relatively high at 2.63%. It was confirmed that fruit stored according to the dual- temperature regime described above did not develop internal disorders. Similar results in terms of internal disorders were obtained by substituting the –0.5°C temperature used during the initial cooling and accumulation period of 10 days, with 4.5°C. However, fruit from both these regimes had an acidic taste. This fruit ripened to a deep yellow colour and the taste improved to acceptable levels by ripening the fruit at ambient (± 20°C) for 10 days.

In 2008, Sun Supreme plums were harvested at a flesh firmness of 9.4 kg and 10.2 kg and a respective colour rating of 6.2 and 3.6 on DFB colour plate PL19. The fruit were stored for 35 days using different dual-temperature storage regimes and evaluated after cold storage and subsequent shelf-life. It was confirmed that internal quality was good, using similar regimes as decribed above, with a modification in that the 9 day warming period at 7.5°c was substituted by a 10 day warming period. However, the taste of this fruit was poor, both before and after shelf life, as it tended to be sour. By ripening the fruit at 20°C after cold storage, to a flesh firmness of 7 and 6 kg, the fruit coloured up (Fig 2) and the taste improved to acceptable levels. Fruit that was pre-ripened to a flesh firmness of 7 and 6 kg at the onset of storage developed internal breakdown.