By Marianne Stein
Fungal contamination of cereal grains poses a substantial threat to food security and public health while causing hundreds of millions of dollars in economic losses annually. In a new study, researchers at the University of Illinois Urbana-Champaign evaluated far-ultraviolet C (far-UVC) light as a safe way to alleviate fungal contamination of corn and wheat and found this technology to be effective.
"Light-based technology is easy to use, and the cost is minimal compared to many other methods. However, conventional UVC lamps emit light at a wavelength of 254 nanometers, which can cause skin or eye damage to humans, so it's not safe to use when workers or consumers are around," said study co-author Yi-Cheng Wang, an assistant professor in the Department of Food Science and Human Nutrition, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois.
"So instead, we are using a technology called microplasma-based far-UVC light. It emits light at 222 nanometers, a wavelength other studies have found to be safe for humans, even at prolonged exposure. We wanted to see if it can also be used to mitigate fungal contamination."
Conventional 254-nanometer light disinfection works by damaging cells' DNA, whereas the shorter 222-nanometer wavelength is mainly absorbed by peptide bonds and amino acids. Wang said this means far-UVC causes cell damage to microorganisms, but cannot penetrate humans' outermost layer of dead skin cells or the tear layer of their eyes, and thus poses no threat to them.
Wang and lead author Zhenhui Jin, a recent graduate of FSHN's doctoral program, tested the efficacy of far-UVC light against two fungi, Aspergillus flavus and Fusarium graminearum. Both fungi affect grains in the field; they can lead to substantial losses in grain quality and produce mycotoxins that threaten human and animal health.
First, the researchers suspended the fungi's spores in a liquid buffer and treated them with various doses of far-UVC light. They found that, at the highest treatment doses, 99.999% of the spores of both Aspergillus and Fusarium were inactivated via changes to the cells' membranes and their mitochondria.
The next step was to test the far-UVC light treatment against the two fungi's mycelia—a network of threadlike strands that invade host plants' tissues after spores germinate. On agar plates, the growth of mycelia for both fungi was successfully inhibited. But that wasn't the end.
"For the liquid and agar, we could just put the lamp above the petri dishes containing the fungi. However, food products are three-dimensional. Therefore, we constructed a treatment system with six lamps that shine light over and around the grains," Wang said.
The researchers tested the system on corn kernels and wheat grains. The treatment reduced more than 90% of both fungi. The surface roughness of the cereal grain was likely the reason for lower treatment effects than in liquid buffer, Wang said. However, the results were comparable to, or even better than, previously published studies in which cereals were treated with conventional 254-nanometer UVC light.
The team also investigated whether the light treatment affected the quality of the grains. They found no significant effect on moisture content in either the corn or the wheat, and no significant change in the percentage of the wheat that germinated within seven days after treatment.
However, for the corn kernels treated with the highest dosage of light, there was a 71% increase in germination over the same period. This could have been because the light treatment increased the corn cells' permeability, facilitating their uptake of water, but Wang said this idea will need to be tested through future research.
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