The plum-transport dilemma

Can plums tolerate higher CO₂ levels during shipping? By Kyra-Kay Rensburg.

The current reefer container-vent setting of 15 m³ per hour for stone fruit aims to prevent CO₂ build-up but exacerbates moisture loss. Closing the vents could increase relative humidity and protect the fruit against shrinkage and shrivel, but some plum cultivars suffer damage and internal disorders when exposed to high CO₂ levels.

Starting in 2019, ExperiCo Agri-Research Solutions has been testing various plum cultivars for CO₂ sensitivity to see whether they are candidates for shipping with lower air exchanges.

“The main thing is that you can’t control what’s happening during the voyage,” says project leader Handré Viljoen, stone-fruit portfolio manager at ExperiCo. “We don’t know what the CO₂ build-up is in a container.”

Plum cultivars that are shown to tolerate higher CO₂ levels will be used for semi-commercial shipping trials.

Modified-atmosphere trials

The initial steps are to assess how plums respond to modified atmospheres. Four single-temperature stored cultivars are selected annually for testing based on export volumes.

Plums sourced from commercial pack houses are split into four treatments with five single-carton replicates per treatment. The fruit is placed in sealed bags, and the atmosphere inside the bags is modified, as shown in Table 1.

Table 1: Treatments applied during 20 days of modified-atmosphere exposure.

Fruit is stored at -0.5 °C in regular atmosphere for seven days, followed by 20 days in the modified-atmosphere treatments, followed by another 15 days in regular atmosphere. The quality of half the fruit is assessed immediately after cold storage, while the other half is evaluated after another five days shelf life at 10 °C.

How cultivars reacted

In 2019, when ExperiCo started these trials, they identified two single-temperature plum cultivars — Fortune and September Yummy — that did not develop internal disorders or reduced firmness in CO₂ levels up to 6%, but the effect on shrivel was inconclusive. Laetitia suffered internal disorders and negative effects on flesh firmness at CO₂ levels above 2%.

The cultivars currently being trialled are African Delight, Angeleno, Ruby Star, and Ruby Sun.

According to Viljoen, Ruby Sun reacted well to 4% and 6% CO₂ last season, but had slight internal browning. There were no internal disorders when it was retested this year, but further testing is required.

Ruby Star performed poorly last season, displaying shrivelling, gel breakdown, aerated tissue, and skin cracks — it will be retested. “Ruby Star plums were too mature last season,” notes Viljoen. “That may be why they had internal issues.”

Other than greater aerated tissue levels in Angeleno, it showed no sensitivity to elevated CO₂ levels. Fruit quality in African Delight was not negatively affected by higher CO₂.

“So far, the African Delight and Angeleno plums handled the elevated CO₂ well, so that’s a good thing. This year Ruby Sun also handled it well,” comments Viljoen.

He adds that seasonal variation in fruit maturity and quality is why the trials need to be repeated.

“The practical part of it is, if we can identify the right levels of CO₂, then we can actually give exporters the option of closing the vents if needed,” says Viljoen.

Further research is required once it is proven that closing vents will reduce shrivelling. A semi-commercial trial will be conducted that involves closing container vents during export and monitoring CO₂ build-up and fruit quality.

What are people saying about this research?

“Since deregulation, the export of plums in integral containers has grown significantly and the default protocol, irrespective of variety, has always been to ventilate all plum containers. This introduction of fresh air is supposed to prevent build-up of CO₂ within the container. However, this can have a negative effect on the delivery air temperature as well as the relative humidity within the container, which could contribute to an increase in moisture loss and shrivelling of the fruit.”

Karin van Rensburg. Technical adviser, In2Stone.