Investigating the Use of Ultraviolet Light Emitting Diodes (UV-LEDs) for the Inactivation of Bacteria in Powdered Food Ingredients

Abstract

The addition of contaminated powdered spices and seasonings to finished products which do not undergo further processing represents a significant concern for food manufacturers. To reduce the incidence of bacterial contamination, seasoning ingredients should be subjected to a decontamination process. Ultraviolet light emitting diodes (UV-LEDs) have been suggested as an alternative to UV lamps for reducing the microbial load of foods, due to their increasing efficiency, robustness and decreasing cost. In this study, we investigated the efficacy of UV-LED devices for the inactivation of four bacteria (Listeria monocytogenes, Escherichia coli, Bacillus subtilis and Salmonella Typhimurium) on a plastic surface and in four powdered seasoning ingredients (onion powder, garlic powder, cheese and onion powder and chilli powder). Surface inactivation experiments with UV mercury lamps, UVC-LEDs and UVA-LEDs emitting at wavelengths of 254 nm, 270 nm and 365 nm, respectively, revealed that treatment with UVC-LEDs were comparable to, or better than those observed using the mercury lamp. Bacterial reductions in the seasoning powders with UVC-LEDs were less than in the surface inactivation experiments, but significant reductions of 0.75-3 log₁₀ colony forming units (CFU) were obtained following longer (40 s) UVC-LED exposure times. Inactivation kinetics were generally nonlinear, and a comparison of the predictive models highlighted that microbial inactivation was dependent on the combination of powder and microorganism. This study is the first to report on the efficacy of UV-LEDs for the inactivation of several different bacterial species in a variety of powdered ingredients, highlighting the potential of the technology as an alternative to the traditional UV lamps used in the food industry.

Keywords: LED; bacteria; foods; inactivation; powder; ultraviolet.

Figure 1

(A) Emission profiles of (1) mercury lamp at 254 nm; (2) UVC-LED at 270 nm; (3) UVA-LED at 365 nm. (1) and (2) were measured at 20 mm distance while (3) was measured at 100 mm distance. (B) Schematic diagram of the UV-LED experimental set-up. This image was created with BioRender.

Figure 2

Mean log CFU values of (A) L. monocytogenes; (B) E. coli; (C) B. subtilis; (D) S. Typhimurium following exposure to UV light at wavelengths of 254 nm (white bars) and UV-LED light at 270 nm (grey bars) and 365 nm (black bars) on a plastic Petri dish surface. Error bars represent the standard deviation of replicate experiments. Absence of a bar indicates that bacteria were reduced to below detection level (10 CFU). *** denotes p ≤ 0.001, ** denotes p ≤ 0.005, * denotes p ≤ 0.05 and – denotes no statistical significance

Figure 3

Inactivation curves of (A) L. monocytogenes; (B) E. coli; (C) B. subtilis; (D) S. Typhimurium following exposure to UVC-LED light at 270 nm in garlic powder (▲ green triangle), onion powder (█ red square), cheese and onion powder (● yellow circle) and chilli powder (♦ blue diamond). Data are represented as the mean ± standard deviation of three biological replicates.

Figure 4

Correlation between experimentally observed log reductions and predicted log reductions of L. monocytogenes (▲ green triangle), E. coli (█ red square), B. subtilis (● yellow circle) and S. Typhimurium (♦ blue diamond) as predicted by (A) the biphasic model, in cheese and onion powder; (B) the Geeraerd shoulder–tail model, in garlic powder; (C) the Geeraerd-tail model, in onion powder and (D) the biphasic model, in chilli powder.