The effect of elevated storage temperatures on plums

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DURATION: Two years
Lead institution: Stellenbosch University (Department of Horticultural Science)
BENEFICIARY: The deciduous fruit sector

Some plum varieties are cold sensitive and prone to develop chilling injury during prolonged periods of cold storage at -0.5°C. Intermittent warming, during which the plums are kept at -0.5°C for a maximum of ten days, warmed to 7.5°C for a specific period and then cooled again to -0.5°C, counteracts the risk. Although effective, the dual temperature regime also causes so-called hot spots to develop in the shipping containers. It means that some of the fruit will arrive overripe in export markets, to the detriment of the producer. The effect of elevated storage temperatures on cold sensitive plum cultivars was evaluated in a project, financed by the Post-Harvest Innovation Programme.

Cold sensitive plums selected for testing

Mariana Jooste led the project at the Department of Horticultural Science of Stellenbosch University. The objective of the study was to determine the effect of elevated storage temperatures on plum quality, which was established by cold storage trials, additional cell membrane studies and antioxidant tests over two seasons (2009 and 2010).

Two cold sensitive plum cultivars, ‘Sapphire’ and ‘Laetitia’, were selected for testing. The trial was conducted in collaboration with Arrie de Kock of ExperiCo (Fruit Technology Solutions).

Cold storage trials over two seasons

In 2009, ‘Sapphire’ and ‘Laetitia’ plums of optimum maturity were sourced from a pack house in Simondium. In 2010, the results of the best performing regimes were tested on optimum maturity fruit sourced from two growing areas, namely Simondium and Robertson. The fruit was kept at different single and dual temperature storage regimes, using six replicates per treatment. ‘Sapphire’ fruit was cold stored for 35 days and ‘Laetitia’ for 49 days, with each temperature regime being concluded with an additional shelf life period of seven days at 10°C to simulate export conditions. The traditional intermittent regime (-0.5°C, alternated with 7.5°C and -0.5°C) was used alongside the test treatments as control.

  • Four different single storage regimes: -0.5°C, 2.0°C, 5.0°C and 7.5°C were tested for ‘Sapphire’ and ‘Laetitia’.
  • Three dual storage regimes of 2.0°C and -0.5°C, 5.0°C and -0.5°C, as well as 7.5°C and -0.5°C were used. For both cultivars, the first temperature of each regime was kept for 28 days to simulate the shipping period. These regimes were concluded at -0.5°C, during which ‘Sapphire’ was kept for seven days and ‘Laetitia’ for 21 days to simulate overseas stock-rolling periods.

Test results for ‘Sapphire’ plums indicated high levels of internal browning (chilling injury) and shrivel in the majority of treatments, with low flesh firmness after shelf life. The dual regime of 5.0°C and -0.5°C compared best to the control, which exhibited the lowest level of internal disorders. In ‘Laetitia’, all the treatments caused chilling injury after shelf life. The single temperature treatments of 5.0°C and 7.5°C compared favourably to the control treatment.

Outcome of study and benefits to industry

  • ‘Sapphire’ and ‘Laetitia’ plums developed chilling injury at an elevated storage temperature of 2.0°C. The reason for chilling injury is that the treatment caused low antioxidant levels (ascorbic acid and glutathione) and low levels of the highly unsaturated phospholipid fatty acid, linolenic acid.
  • Elevated temperatures are therefore not recommended for ‘Sapphire’ plums – chilling injury (dependent on production area) or over ripeness (dependent on harvest maturity) may develop.
  • ‘Sapphire’ plums from Robertson developed less chilling injury than fruit from Simondium, due to higher levels of water-soluble antioxidant activity (HAA), reduced ascorbic acid (L-AA) and the phospholipid fatty acid, linolenic acid.
  • Single treatments at 5.0°C and 7.5°C compared well to the traditional dual regime for ‘Laetitia’, since high ascorbic acid levels were retained. However, these regimes should only be considered if the fruit is harvested towards the upper end of the maturity window, to prevent decay (at 5.0°C) and over ripeness (at 7.5°C).
  • ‘Laetitia’ plums developed no internal defects when treated with 1-MCP (Smartfresh™) at -0.5°C, irrespective of harvest maturity or area – the treatment can be safely recommended for ‘Laetitia’.
  • This study informs producers of the correct plum temperature regime for different cultivars – fewer overseas claims increases profitability.