Revision of temperature tolerance at loading for plums

The effect of temperature during the accumulation phase, prior to loading of plums onto ships for export was tested. Temperatures selected were -0.5 °C, because this is the recommended pulp temperature at loading, 2 °C which is 0.5 °C higher than the current maximum temperature of 1.5 °C; and 3 °C. A temperature of 4.0 °C was also tested in 2015. Accumulation periods of 3, 7 and 10 days were used. The objective was to determine if maximum temperature at loading of plums can be adjusted upwards without a negative effect on post storage quality.

Various plum cultivars not cooled before packing were obtained from a commercial pack house. Each cultivar was divided into 9 treatments. The variables were accumulation periods of 3, 7 and 10 days and temperatures of -0.5 °C, 2 °C and 3 °C. After the accumulation periods, the plums were cold stored according to the recommended cold storage protocols. The plums were evaluated for flesh firmness, decay, shrivel and internal disorders after cold storage, and after 5 days shelf life at 10 °C. This was followed up by semi commercial trials.

Cultivars differed in the extent to which fruit quality was affected by higher temperatures during accumulation before loading onto ships for export to overseas destinations. Quality of plums accumulated at -0.5° C and 2.0° C was similar. In most cases, accumulation at 3.0 °C had no negative effect on fruit quality compared to -0.5 °C and 2.0 °C. Exceptions were Sunkiss in 2016 that were softer when accumulated at 3.0 °C compared to -0.5 °C and 2.0 °C, and Songold in 2017, which developed more GB before shelf life if accumulated at 3.0 °C. In a semi commercial trial in 2018 on Ruby Star plums fruit was marginally softer but shrivel was reduced. However, the slightly softer fruit was not regarded as a problem.

The recommendation is that the temperature tolerance at loading for plums can be adjusted from 2.0 °C (maximum temperature 1.5 °C) to 3.5 °C (Maximum loading temperature 3.0 °C). There is a warning that a maximum temperature tolerance for Songold plums with flesh firmness below 6 Kg (Non SmartFresh treated) should not be exceeded.