Effect of forced-air cooling rate on the quality of plums (Laetitia)

Laetitia plums from two producers in Grabouw were sampled from Kromco, and a third fruit population was sampled from Stellenbosch. Fruit used in this trial were sampled very late in the picking window of Laetitia in response to commercial problems experienced with gel breakdown. As a result, fruit sampled from Grabouw was softer (3.0 kg) than the minimum flesh firmness (4.5 kg) for exportable Laetitia plums, according to the PPECB production guides. The fruit from the three producers were cooled at different forced-air cooling (FAC) rates, namely rapid, intermediate and slow cooling. After cooling, the fruit were stored for 49 days using the commercially used dual-temperature storage regime. Internal quality evaluations were conducted on the fruit after 7 and 14 days in cold storage, with a full evaluation conducted after the storage protocol period. Results showed that rapid cooling, within 12 h, increased the risk of GB and IB in fruit from the three producers. This result was evident as early as after 7 days in storage. The high internal disorders in fruit cooled within 12 h were again observed after 14 days cold storage and also after the full protocol period. However, after 49 days cold storage, fruit sampled from Grabouw, which were soft at harvest, and cooled slowly over 51 and 72 h, developed overripeness. No overripeness developed in fruit sampled from Stellenbosch, which were harvested at a firmer flesh firmness. Shrivel and decay did not show definite trends associated with FAC rate. However, there was a tendency for these disorders to be higher in fruit cooled over 72 h compared to fruit cooled faster. Generally, fruit cooled over 72 h were softer than fruit cooled faster. This trial also demonstrated the influence of fruit inherent storage potential on fruit quality after storage. Laetitia plums sampled from Grabouw, which for some unknown reason had a poor inherent storage potential in the 2005 season, and which was cooled rapidly, developed up to 28% GB within 7 days of storage, whereas fruit from Stellenbosch, and also cooled rapidly, developed only 7.5% GB after this time period. In previous cooling trials conducted on Laetitia, only 6% internal disorders developed in fruit cooled rapidly and stored for 49 days. This illustrates that even though rapid cooling is detrimental to fruit quality, the levels of internal disorders vary depending on the inherent storage potential of the fruit. Fruit with poor inherent storage potential develop internal disorders earlier in storage and to a much higher level than fruit with a good inherent storage potential. The reason for poor inherent storage potential could not be established in this trial. Cooling trials, on Laetitia should be continued to establish the effect of cooling tempo, and not solely the time to reach -0.5 °C, on development of internal disorders. Investigation of the use of cooling rate in a prediction model should also be investigated. In conclusion, the current cooling recommendation on Laetitia should be upheld, i.e. Laetitia plums should be cooled to temperature between 24 and 48 h.