Scanning plum cultivars for CO2 sensitivity and the effect of closed vents in shipping containers on relative humidity and CO2 levels.

Moisture loss and shrivel are of concern in the plum industry. High relative humidity (RH) in the supply chain reduces the fruit vapour pressure deficit (VPD) and thus protects fruit against moisture loss and shrivel. Closing vents on shipping containers may increase RH and consequently limit shrivel, but there is a risk that resultant higher CO₂ levels may cause damage to the fruit. This project was to establish CO₂ tolerance of plum cultivars during cold storage, since this may enable increasing RH in shipping containers and so reduce shrivel with export fruit, with possible energy saving benefit.

Modified atmosphere (MA) treatments (UTC, 2% CO_2, 4% CO₂ and 6% CO₂) were applied to plum cultivars to establish the C0₂ upper threshold before fruit quality is compromised. Plum cultivars tested were Ruby Sun, Ruby Star, African Delight and Angeleno. Fruit was subjected to single-temperature storage for 42 days followed by 5 days shelf life. The fruit was packed in D05D cartons (5 Kg). Shrivel sheets with 48 x 6 mm perforations were used on Ruby Sun, Angeleno and Ruby Star, while bags with 54 x 2 mm perforations were used for African Delight.

Ruby Sun

Following evaluations over three seasons, Ruby Sun plums seem suitable for export at higher CO₂ levels (< 6%) with no negative effects on fruit quality and the possibility of reduced shrivel. Flesh firmness retention was evident in the 2022 season with fruit treated with 4% and 6% CO₂, with no significant differences occurring in the 2023 and 2024 seasons.

Ruby Star

Over all three seasons, no significant differences in flesh firmness occurred between treatments for both the 11 mm and 8 mm plungers after the cold storage period, but with a tendency for the 4% and 6% CO₂ treated fruit to exhibit higher flesh firmness when compared to the untreated control (UTC) fruit. The shrivel levels over the three seasons did not show a clear tendency with regards to different treatments. Gel breakdown (GB) incidence was low after the cold storage period, but increased to unacceptable levels over shelf-life, across MA treatments. Hence, Ruby Star is not suitable for export in shipping containers with closed vents if there is a possibility of CO₂ build-up.

African Delight

Although significant differences occurred in two seasons after either cold storage or shelf life, there were no clear trends indicating that higher CO₂  improves flesh firmness retention. Shrivel levels across seasons and treatments exhibited no clear trends. In the 2022 season, a tendency occurred for lower GB, internal browning (IB) and aerated tissue (AT) with increased CO₂. This tendency was not evident in the 2023 and 2024 seasons, which suggests that CO₂ at these levels would not exacerbate these disorders. Overall, African Delight plums seem suitable for export at higher CO₂ levels (<6%), with no negative fruit quality effects.

Angeleno

Flesh firmness didn’t reveal trends across treatments and seasons. Shrivel and decay levels were low to non-existent. AT levels varied from low in one season, to high in others, especially after shelf life. Angeleno seems suitable for export at higher CO₂ (<6%) with no negative fruit quality effects observed.

The findings of three seasons research indicated that Ruby Sun, African Delight and Angeleno can withstand CO₂ of 6% with 15% O₂ balance, without negative fruit quality effects and with the possibility of fruit quality improvement in some cases. Ruby Star consistently exhibited sensitivity to elevated CO₂ levels and does not seem suitable for export in shipping containers with closed vents.

This research over three years indicated that Ruby Sun, Angeleno and African Delight show promise for export in shipping containers with closed vents if CO₂ does not exceed 6%, while Ruby Star is not suitable. Previous research indicated that Fortune and September Yummy may also be suitable for export under MA, but Laetitia was likely to be CO₂ sensitive. Commercial research using MEARSK AV+ system can be considered.