Moisture loss studies in Japanese plums (Prunus salicina Lindl.)

Excessive moisture loss can lead to postharvest shrivelling of plums which is not accepted by our clients, causing financial losses. The objectives of this study were to determine:

• if there are openings/cracks in the fruit peel that contribute to moisture loss,
• the effect of fruit developmental stage on peel permeability to moisture,
• the contribution of the pre-storage and storage vapour pressure deficit (VPD) between the fruit and its environment to post-storage shrivel manifestation,
• and if preharvest silicate applications could reduce the development of concentric rings in ‘African Delight^TM’ plums.

• Peel cracks and lenticels were determined in ‘Sapphire’, ‘Songold’, ‘Laetitia’ and ‘African Delight^TM’ plums by quantifying the water vapour permeance (P’H₂O; mol s^-1 m^-2 Pa^-1) of 100 fruit per cultivar. Subsequently the fruit were sorted from the lowest to highest water vapour permeability and fluorescence microscopy was employed to determine peel openings in five random peel segments per fruit.
• Changes in peel permeability was determined weekly on three plum cultivars from 4 weeks prior to their optimum harvest dates until post-optimum maturity using the steady state solution of Fick’s first law of diffusion.
• Ten commercially used handling chains for plums were simulated to determine the contribution of the VPD between the fruit and its environment on fruit moisture loss on arrival at the packhouse, after forced air cooling, after cold-storage and after shelf-life.
• To determine the effect of preharvest silicate on the development of concentric rings in ‘African Delight^TM’ plums, three treatments were evaluated (a control where no Si was applied, a foliar Si spray and a root Si application) in a randomised complete block design.

• Concentric rings at the pedicel end of ‘African Delight^TM’ plums are open hairline cracks. Peels with wider cracks had higher moisture loss. Peels of ‘African Delight^TM’, ‘Laetitia’ and ‘Sapphire’ contain open lenticels. ‘Songold’ had no peel openings which explains why it is not a shrivel susceptible cultivar.
• During fruit development on the tree fruit to fruit variation made the largest contribution (> 45%) towards differences in peel permeability between sampling dates, followed by sampling date (> 20%) and orchard differences (> 15%). Peel permeability increased two- fold as fruit became over mature.
• Exposing plums to ambient conditions for 48h to 72h after harvest induced severe moisture loss. A 2% loss in the total mass of the fruit due to moisture loss causes shrivel
• Preharvest silicate treatments did not prevent or reduce the concentric rings at the pedicel end of ‘African Delight^TM’ plums.

Fruit with excessive hairline cracks at the stem end should not be packed. Cultivars with concentric rings at the pedicel end should be excluded from the breeding programs. Overmature fruit are more susceptible to shrivel – sell it as soon as possible after harvest and prevent mixed maturities in the same carton. Field heat must be removed as soon as possible after harvest to reduce moisture loss. Plums may be stored at 0 °C or just above the packhouse dew point up to 72 h before it is packed and force air cooled, but will be softer after storage compared to fruit packed and cooled immediately after harvest. Plums must never be left at ambient for extended periods after harvest