New research finds that UV-C irradiation is comparable to SO2 for maintaining the postharvest quality of blueberries.
The standard postharvest practice of SO2 fumigation is under pressure from markets due to concerns about residues and potential adverse effects in individuals sensitive to sulphites. “But SO2 is hard to beat because it’s very effective,” says Carl Schulenburg.
Schulenburg is in the final stages of completing his MSc on UV-C as an alternative to SO2. The BerriesZA-funded project is led by Prof. Lise Korsten of the Department of Plant and Soil Sciences at the University of Pretoria.
What was done?
Schulenburg obtained blueberries from a commercial farm immediately after harvest. He conducted two laboratory trials.
In the first, he disinfected the surface of the berries and inoculated them with botrytis spores. Subsets of the inoculated berries were treated with UV-C or SO2, and the development of grey mould was compared to an untreated control after storage at 4 °C for 21 days.
The second trial was similar to the first, except that Schulenburg didn’t inoculate the berries with spores, but relied on natural infections that occurred in the field before harvest.
In addition to assessing mould development, he analysed berry defence mechanisms and evaluated fruit quality (firmness, mass, pH, and total soluble solids).
The results
Both UV-C and SO2 treatments significantly reduced the occurrence and severity of grey mould in inoculated berries compared to the untreated controls. Roughly 60% of the untreated berries developed mould compared to only about 10%–15% of the treated berries. There was no significant difference between the UV-C and SO2 treatments.
Schulenburg saw similar results for the naturally infected berries. Whereas 40%–50% of the uninoculated, untreated berries developed botrytis infections, less than 10% of the berries treated with UV-C and SO2 became mouldy.
He reports that the UV-C treatment increased the antioxidants and activated the berry’s defence mechanisms.
“UV-C has a direct and indirect effect on spores,” explains Schulenburg. “The irradiation inhibits spores directly. Secondly, it stimulates berry defence mechanisms, which creates an unfavourable environment for spore germination.”
He didn’t observe adverse effects on the fruit-quality parameters he tested.
Where to next?
Schulenburg only used two cultivars in his experiments. He highlights the need to test UV-C in other cultivars, as berry characteristics such as the thickness of the wax layer could impact efficacy.
His trials were also entirely laboratory-based and need to be replicated in a pack house. “Although we didn’t upscale and implement this in a pack house, the UV-C treatment is designed for future pack-house studies,” he says.
The UV-C treatment can integrate within existing pack-house workflows. Schulenburg identified a window of time during which fruit could be exposed to an effective dose of radiation.
Irradiation with UV-C is already being used commercially to sterilise fresh produce overseas. Schulenburg’s results confirm that it’s a viable option for South African blueberry pack houses.
“There’s still a lot of research to be done,” he says, “but it’s exciting that we have a possible alternative to SO2 fumigation.”
