Tribo-sanitizer decontaminates food storage materials
According to the World Health Organization (WHO), 600 million people fall ill and 420,00 die each year because of unsafe food. A single strain of foodborne bacteria, Escherichia coli, costs the US economy more than $300 million per year, and the costs of Listeria monocytogenes outbreaks are very similar. The burden of foodborne diseases is substantial, which has led to the development of a large number of decontamination methods.
Those that use light – specifically, ultraviolet-C (UVC) irradiation – are growing in popularity due to their relative low cost and effectiveness. These systems disinfect surfaces by causing damage to the nucleic acid in bacterial cells. However, to date, they’ve relied on access to electricity to operate, which limits their use in low-resource environments. A group of researchers from the University of Illinois have developed a UVC sanitizer that can be self-powered via a triboelectric nanogenerator (TENG). Writing in Nano Energy [DOI: 10.1016/j.nanoen.2023.108675], they say this is the first time a TENG-based device has been used to successfully disinfect food.
TENGs generate electricity from mechanical energy via contact electrification and electrostatic induction. They’re particularly effective at low frequencies typical of ‘waste’ mechanical energy produced by the human body, or as vibrations in industrial systems. The Illinois team, led by Assistant Professor Yi-Cheng Wang, designed a free-standing rotational TENG that consisted of two parts; a stator and a rotator. The stator was made from a thin piece of fibreglass that had two radial tin/lead electrodes – each with four sectors – printed onto it. A nylon film was attached on top of the electrodes before the assembly was mounted onto a vertical stand. The rotator was made from an acrylic disc with four trenches cut into it at 90° from each other. Into each trench, they added a piece of polyvinyl chloride (PVC) film that had been trimmed and shaped to curve upwards at one end. The rotator was then loaded onto a rotary shaft that could be driven by a motor or by a hand crank. This TENG was used to power a low-pressure amalgam UVC lamp (λ = 254 nm), initially via a motor but also later, via the hand crank.
They chose to test their tribo-sanitizer’s decontamination performance against two foodborne pathogens known to cause illness outbreaks – Escherichia coli O157:H7 (E. coli O157:H7), which is gram negative, and Listeria monocytogenes (L. monocytogenes), gram positive. These bacterial suspensions were inoculated onto the surfaces of apple skin, romaine lettuce, and polyethylene terephthalate (PET); a polymer widely used in drinks bottles and food packaging.
By analysing the electrical output of TENG, they found that there was an optimal effective contact area between the PVC and the nylon – it occurred when that area was very similar to the area of one electrode sector. In addition, when frequency increased from 3 to 12 Hz, the TENG’s voltage output increased as well: from 1.55 kV to 4.07 kV. The TENG maintained 95% of its voltage output after eight hours of continuous operation, suggesting that it has good stability. The researchers also took inspiration from lightning’s ability to electrostatically break-down the air to further increase the output of the TENG when connected to the UVC lamp. By adding an airgap into the circuit, they significantly increased the brightness of the lamp. They write, “… when the system was switched into discharge-connection mode (i.e., the mode incorporating the air-gap design) from direct-connection mode, voltage across the lamp increased from around ± 500 V to ± 1500 V, and current through it from 50 to around 120 μA.” The discharge-connection mode was used for all disinfection experiments.
Testing the tribo-sanitizer against liquid samples of the bacterial suspensions found a greater than 5-log reduction in E.coli, and a 4.36-log reduction in L. monocytogenes¸ both after a 90-min exposure. The reduction in E.coli is in-line with that required by the FDA for beverages. The tribo-sanitizer’s disinfection efficacy for PET surfaces was particularly noteworthy. E. coli O157:H7 inoculated on PET dropped from an initial concentration of 6.13 log CFU/cm2 to below the limit of detection (0.18 log CFU/cm2) after 60 min of treatment. The concentration of L. monocytogenes on the same type of surface also reduced significantly, from 5.57 log CFU/ cm2 to 1.76 log CFU/cm2 in the same timeframe.
On the real-food samples, the system was slightly less effective. On apple skin, E. coli O157:H7 was reduced by 3.66 log, and L. monocytogenes by 2.96 log. On lettuce, the reductions were 2.68 and 1.67 log, respectively, again after 60 minutes of treatment. The authors attributed this reduced performance to differences in the three surfaces’ morphology and topography, with PET having the lowest overall roughness, giving bacteria nowhere to hide.
The authors say that their tribo-sanitizer “…has strong potential for real-world use,” and that it could “…potentially be applied to enhance food safety in homes, restaurants, and other food-preparation environments by decontaminating surfaces, food, and drinking/washing water.”
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Zhenhui Jin, Fujunzhu Zhao, Longwen Li, Yi-Cheng Wang. “Tribo-sanitizer: A portable and self-powered UV device for enhancing food safety,” Nano Energy 115 (2023) 108675. DOI: 10.1016/j.nanoen.2023.108675