Why do some plum cultivars shrivel more than others? By Grethe Bestbier.
Shrivel, especially in plums, is a major problem in the South African fruit industry. This post-harvest condition manifests as shrinkage, shrivelling and reduced mass, and is related to moisture loss. In South Africa, moisture loss in plums is an especially big problem, since plums are stored for extended periods and mostly exported by sea.
Moisture loss is influenced by various factors including temperature, humidity, and fruit type. The extent of moisture loss in plums is also strongly impacted by the specific cultivar. A recent project by the Department of Horticultural Science at Stellenbosch University explored how shrivel was affected by differences between plum cultivars. The research was conducted by Dr Imke Kritzinger for her PhD, and supervised by Dr Elmi Lötze and Prof. Karen Theron.
Comparing three plum cultivars
“Even when handled carefully from harvest up to packaging, moisture loss in plums will still occur,” says Kritzinger. “There are also very big differences between cultivars in terms of how prone they are to shrivel.” Kritzinger looked at three plum cultivars with different shrivelling tendencies: Laetitia, Songold and African Delight. Of these, Songold tends to show the fewest symptoms of shrivel, followed by Laetitia and African Delight.
Kritzinger studied the lenticels of unharmed fruit with no visible cracks or holes in the skin, from three weeks prior to harvest up to harvest. Lenticels are pores in the peel that allow for gas exchange between the fruit and the atmosphere. Plum cultivars have different numbers of lenticels with varying degrees of openness. Kritzinger expected that a greater number of open lenticels would increase the peel’s permeability and that cultivars with many open lenticels will lose more moisture. However, she found that in some cultivars, the correlation between permeability and the number of open lenticels was poor. This indicated that lenticel number is not the main factor that determines fruit peel permeability.
Next, Kritzinger considered the composition of the cuticles. A fruit’s cuticle consists of wax and cutin. Looking at the alcohols within the waxes first, she picked up a significant difference in primary alcohol content between the different cultivars. Kritzinger found that the more primary alcohols a cultivar has, the lower the shrivel incidence seems to be. In addition, higher concentrations of secondary alcohols in the waxes at harvest coincided with lower shrivel incidence in plums.
Looking for clues in the cutin
The peel’s cutin forms a framework which holds the waxes and increases its rigidity. Large amounts of phenols in the cutin will strengthen the cuticle even more, so that it cannot deform easily. The study found that the amount of phenols present in the cutin at harvest affects shrivel development. Songold, for example, has a large amount present at harvest, while Laetitia starts out with very low phenol levels and increases its phenol production after harvest. According to Kritzinger, phenol production in Laetitia is “too-little-too-late,” and the cultivar shows more wrinkles when losing moisture.
The study also looked at fatty acid content, which increases the flexibility of the cutin. A flexible cutin is prone to collapse when losing moisture. Laetitia has more of these acids than Songold, further explaining its likeliness to show symptoms of shrivel. In contrast, Songold’s cuticle is more rigid due to its many phenols and low concentrations of fatty acids, reducing the appearance of shrivel.
Lastly, Kritzinger took a morphological approach by studying the epidermal and hypodermal cells of the peel. The cells were stained and examined under a light microscope. In Songold these cells are packed more tightly together than in Laetitia and African Delight. The densely packed cells reduce the space available for the peel to collapse and wrinkle, further explaining why Songold tends to shrivel less than the other cultivars.
“Everything together formed a picture,” says Kritzinger. “One thing on its own did not necessarily give us the answers we wanted.”
Further research and possibilities for shrivel management
Kritzinger’s study also involved a packaging test, which yielded very useful results. To reduce moisture loss, plums are sometimes packed in high-density polyethylene (HDPE) bags. In this study a range of perforated HDPE bags, currently used in the South African fruit industry, were compared to low-density polyethylene (LDPE) bags with different numbers of microperforations.
The study showed that LDPE microperforated bags reduced shrivel incidence in Laetitia with plus-minus 50% and in African Delight with about 93%. Under ideal conditions, the use of LDPE-92 and LDPE-72 microperforated bags may be a viable option to reduce moisture loss, while still maintaining fruit quality. However, this will have to be tested on a commercial level to verify the results.
“The microperforated bags look promising, so I think more research is needed there,” says Kritzinger. “However, you will never be able to completely solve the issue. It is just the way that the fruit is.”
According to Kritzinger, the study paves the way for other research possibilities such as looking into edible coatings. Edible coatings are wax-like substances sometimes used in citrus to limit moisture loss. Successful use of edible coatings requires an understanding of natural cuticle composition, for which this study is a great start.
While this project compared three different cultivars, another study by Kritzinger currently underway compares plums of the same cultivar that shrivelled with those that did not. The hope is to identify certain wax properties within a cultivar that reduces shrivel susceptibility, and potentially find a way to produce more plums with such properties.
The factors found by Kritzinger to contribute to shrivel might also provide plum breeders with markers to select against. Trees that may produce fruit with a propensity to shrivel could be culled from breeding programmes at an early stage.
Image supplied by Imke Kritzinger | Stellenbosch University.