Ice, widely used in the food industry, is a potential cause of food poisoning resulting from microbial contamination. Direct microbial inactivation of ice is necessary because microorganisms may have been present in the source water used to make it and/or may have been introduced due to poor hygiene during production or handling of the ice. Nonthermal and nondestructive microbial inactivation technologies are needed to control microorganisms in ice. We evaluated the applicability of a UVC light-emitting diode (UVC-LED) for microbial inactivation in ice. The effects of UV intensity and UV dose of the UVC-LED on Escherichia coli ATCC 25922 and a comparison of UVC-LED with a conventional UV lamp for effective bacterial inactivation in distilled water and ice cubes were investigated to evaluate the performance of the UVC-LED. Finally, we assessed the effects of the UVC-LED on pathogens such as E. coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes in ice cubes. The results indicated that UVC-LED effectiveness depended on the UV dose at all UV intensity conditions (0.084, 0.025, 0.013, 0.007, and 0.005 mW/cm2) in ice and that UVC-LED could more efficiently inactivate E. coli ATCC 25922 in distilled water and ice than the UV lamp. At a UV dose of 2.64 mJ/cm2, E. coli in distilled water was decreased by 0.90 log CFU/mL (UV lamp) and by more than 7.0 log CFU/mL (UVC-LED). At 15.2 mJ/cm2, E. coli in ice was decreased by 3.18 log CFU/mL (UV lamp) and by 4.45 CFU/mL (UVC-LED). Furthermore, UVC-LED irradiation reduced the viable number of pathogens by 6 to 7 log cycles at 160 mJ/cm2, although the bactericidal effect was somewhat dependent on the type of bacteria. L. monocytogenes in ice was relatively more sensitive to UVC irradiation than were E. coli O157:H7 and Salmonella Typhimurium. These results demonstrate that UVC-LED irradiation could contribute to the safety of ice in the food industry.

Ice is widely used to cool foods, such as drinking water, seafood, and fresh produce. Ice is mainly made by home freezers or by ice-making machines in restaurants, cafes, and ice-making companies. Ice must be microbiologically and chemically safe because consumers eat ice directly or eat foods that come in direct contact with ice. However, ice can be microbially contaminated (58, 10, 15, 16, 20) and, thus, can cause food poisoning (5, 12). Gerokomou et al. (10) investigated bacterial contamination in 100 ice samples that were collected at 10 different retail points in the Epirus region in Greece. They found Escherichia coli and Salmonella in 15 and 4% of ice samples, respectively, ranging from 102 to 103 CFU/mL. Lateef et al. (15) evaluated the microbiological safety of commercial ice using ice collected from four ice-manufacturing factories in Ogbomoso, Nigeria. They found that all the samples were microbially contaminated, with the microbial load ranging from 1.88 × 104 to 3.20 × 104 CFU/mL.

Microorganisms can contaminate ice owing to the poor quality of source water used and/or poor hygiene during production or handling (16). Moreover, Burnett et al. (3) indicated that ice-making machines can be contaminated due to seeding from the main supply, faulty plumbing that allows backflow from the drains, and irregular cleaning of ice machines. Once ice-making machines are contaminated, contaminated ice will be produced until the machines are cleaned.

To prevent food poisoning by ice, sufficiently hygienic water should be used, and ice-making machines should be cleaned regularly. However, both of these processes require additional work and utilize chemicals that can be problematic. Additionally, it is particularly difficult to evaluate the safety of water and the cleanliness of ice-making machines, or automatic vending machines, which may be individually owned. Therefore, development of an inactivation technology for microorganisms in ice may be the most effective approach for preventing ice contamination. However, because such a technology must be nonthermal and nondestructive, no studies have yet reported the direct microbial inactivation of ice.

UV irradiation is a common nonthermal microbial inactivation technology. UV light covers a wavelength spectrum from 100 to 380 nm and is subdivided into three regions by wavelength: UVA (315 to 400 nm), UVB (280 to 315 nm), and UVC (<280 nm) (19). UV radiation is believed to inactivate microorganisms by acting directly on the DNA in the cell, blocking cellular reproduction. In general, UVC radiation is believed to have high inactivation effects because DNA mainly absorbs UV radiation from 200 to 300 nm, with an absorbance peak around 260 nm. However, the main UV sources are mercury lamps, which are fragile and contain toxic mercury. In contrast, UV light-emitting diodes (UV-LEDs) have recently emerged as a new source for UV irradiation. UV-LED has several advantages such as compactness, robustness, faster start-up time, less energy consumption, longer lifetime, ability to turn on and off with high frequency, and lack of mercury (21). However, few studies have evaluated the effectiveness of UV-LED in microbial inactivation. In a review of the published literature, Song et al. (19) looked at the effects of UV-LED on various microorganisms and concluded that, in many cases, the results were inconsistent because of a lack of uniformity in research materials and methods. Different UV-LEDs have various radiation patterns, such as emission spectra, viewing angles, and radiation distributions. Similarly, there have been few studies comparing the effects of UV-LEDs with those of conventional UV lamps. Bowker et al. (2) compared the inactivation effect of a 254-nm UV lamp with that of 275- and 255-nm UVC-LEDs in which the UV intensities were 0.34, 0.094 to 0.11, and 0.049 to 0.060 mW/cm2, respectively. The log reduction for the same UV dose increased with the increase in UV intensity. Bowker et al. (2) assumed that lower UV intensity might cause the lower inactivation rates, despite using the same UV dose. Thus, fundamental knowledge of the effects of UVC-LEDs on microbes in ice is necessary to evaluate its applicability.

Accordingly, in the present study, we evaluated the performance and the effectiveness of UVC-LED for bacterial inactivation in ice.

Bacterial strains

E. coli (ATCC 25922, American Type Culture Collection, Manassas, VA), four strains of E. coli O157:H7 (RIMD 0509939, RIMD 05091896, RIMD 05091897 [Research Institute for Microbial Diseases, Osaka University, Japan], and HIPH 12361 [High Institute of Public Health, Alexandria University, Egypt]), five strains of Salmonella Typhimurium (RIMD 1985007, RIMD 1985009, ATCC 29057, ATCC 29629, and ATCC 29630), and six strains of Listeria monocytogenes (ATCC 13932, ATCC 15313, ATCC 19111, ATCC 19117, ATCC 19118, and ATCC 35152) were used in this study. These strains were maintained at −80°C in tryptic soy broth (TSB) containing 10% glycerol. A platinum loop was used to transfer the frozen bacterial cultures by scratching the surface of the frozen culture into tryptic soy agar (TSA) plates. The inoculated plates were incubated at 37°C for 24 h, and an isolated colony of each bacterium was then transferred to fresh TSB (5 mL) in a sterile plastic tube. The cultures were transferred using loop inocula at two successive 24-h intervals to obtain a more homogeneous and stable cell population. Grown cells were collected by centrifugation (3,000 × g, 10 min), and the resulting pellet was washed in sterile 0.1% peptone water three times and then was resuspended in 10 mL of sterile 0.1% peptone water, corresponding to approximately 108 to 109 CFU/mL. Strains of three pathogenic species, except E. coli ATCC 25922, were combined to prepare culture cocktails for use in experiments. The suspensions were stored at 5°C until use.

Sample preparation

For inactivation experiments in distilled water, a 1-mL aliquot of E. coli ATCC 25922 suspension (108 to 109 CFU/mL) was added to 50 mL of distilled water in a glass beaker such that the initial concentration of the inoculum was approximately 107 to 108 CFU/mL. For inactivation experiments in ice, a 1-mL aliquot of E. coli ATCC 25922 suspension and the cocktail suspensions of each pathogen were added to 25 mL of distilled water in a glass beaker. The inoculated distilled water was placed into an ice cube tray (30 by 30 by 30 mm) and then was frozen at −80°C for 5 to 6 h. The initial concentrations of E. coli and other pathogens in the ice were approximately 106 to 107 CFU/mL, because viable bacterial numbers were reduced by 1 log during the freezing process.

Experimental apparatus

One UVC-LED module (5.5 by 0.2 mm; UVC-EC910ZA, Panasonic Photo lighting, Co. Ltd., Osaka, Japan) with a wavelength of 270 to 280 nm was connected on a direct-current power supply (GW Instek, Taipei, Taiwan). The electric current and voltage were 120 mA and 9 to 10 V, respectively. The UVC-LED and UV lamp were placed in the vertical direction relative to the sample. The intensity of UVC was adjusted by varying the distance between the samples and the LED. The intensity of the UV was determined with a spectrometer (UV-37SD, Custom, Tokyo, Japan). The UV intensity was held constant for 30 min. A low-pressure UV lamp (336 by 82.3 by 65 mm; Handy UV lamp SUV-16, AS ONE, Osaka, Japan) with a peak wavelength of 254 nm was used to compare the effects of the UVC-LED. The UV lamp was housed in a UV collimated beam apparatus (1).

Inactivation in distilled water by UVC

Water samples containing E. coli ATCC 25922 in a glass beaker were irradiated by UVC-LED at various UV intensities (0.084, 0.025, 0.013, 0.007, and 0.005 mW/cm2) or by UV lamp at a UV intensity of 0.025 mW/cm2 at room temperature (20 to 25°C). A 1-mL aliquot was collected by pipetting at each sampling time during UVC irradiation treatment.

Inactivation in ice by UVC

In this experiment, all ice samples (30 by 30 by 28 mm) placed on the petri dishes were irradiated at −30°C in a deep freezer. Ice samples containing E. coli ATCC 25922 were irradiated by a UVC-LED of various UV intensities (0.084, 0.025, 0.013, 0.007, and 0.005 mW/cm2) or by a UV lamp at a UV intensity of 0.025 mW/cm2 at room temperature (20 to 25°C). Ice samples containing pathogens, such as E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes, were irradiated by a UVC-LED at 0.084 mW/cm2; this was the highest UV intensity used in the present study and was appropriate to evaluate the reduction of bacterial cell numbers in a short irradiation period. After UVC treatments, irradiated ice samples were taken from the deep freezer and completely melted at room temperature. A 1-mL aliquot of sample was collected from melted ice. Samples were irradiated with UVC for a maximum of 30 min.

Microbiological analysis

After UV irradiation treatment, to determine the number of surviving bacteria at each UV dosage, appropriate serial dilutions (1:10) were performed with sterile 0.1% peptone water and were plated in duplicate on TSA plates. The plates were then incubated at 37°C for 24 to 48 h.

Survival kinetics analysis

The sensitivity of microorganisms to UV radiation can be evaluated by the following formula (11):

\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicodeTimes]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\begin{equation}\tag{1}{\rm{log}}{{N(t)} \over {{N_0}}} = - k \times {\rm{\ UV\ dose}}\end{equation}

where N(t) and N0 are the momentary and initial counts, respectively. The initial counts (N0) were viable cell counts at time zero. The parameter k (cm2/mJ) is the inactivation rate from the linear portion of the relationship between log inactivation and the applied UV dose.

Statistical analysis

All experiments were repeated three times, and each data point was determined based on duplicate plate counts. Data from three samples subjected to each treatment in each of three independent replicate experiments were analyzed. To determine statistical significance (P < 0.05) between the UV treatment conditions and type of pathogenic bacteria, we used Student's paired t tests for comparisons with two groups, such as UVC-LED versus UV lamp, and we used the Tukey-Kramer method for multiple comparison tests, such as for UV intensities and type of pathogenic bacteria. All statistical analyses were conducted using the R statistical environment (version 3.2.4 for Macintosh operating system [http://www.R-project.org]).

Performance of the UVC-LED

Changes in the irradiation intensity of the UVC-LED that depend on the distance were examined. As shown in Figure 1, the UV intensity decreased as the irradiation distance became longer. Whereas the UV intensity was 0.084 mW/cm2 at a distance of 2 cm, it decreased by 0.005 mW/cm2 at a distance of 10 cm. We also evaluated durability as another aspect of UVC-LED performance. Changes in the UV intensity of the UVC-LED for 30 min at room temperature and −30°C are shown in Figure 2. The UV intensity was almost constant during the 30-min irradiation, and temperature did not affect the intensity of the UVC-LED.

FIGURE 1.

Changes in the intensity of UVC-LED depending on the distance between UVC-LED and the objective.

FIGURE 1.

Changes in the intensity of UVC-LED depending on the distance between UVC-LED and the objective.

Close modal
FIGURE 2.

The intensity of UVC-LED at different distances and temperatures: 2 (○), 6 (△), and 10 (□) cm at room temperature, and 2 (•), 6 (▴), and 10 (▪) cm at −30°C.

FIGURE 2.

The intensity of UVC-LED at different distances and temperatures: 2 (○), 6 (△), and 10 (□) cm at room temperature, and 2 (•), 6 (▴), and 10 (▪) cm at −30°C.

Close modal

Inactivation of E. coli and pathogens in distilled water by UVC irradiation

Numbers of E. coli ATCC 25922 in distilled water decreased as the UV dose of the UVC-LED increased (Fig. 3). The reduction of E. coli depended on the UV dose at all UV intensities, except for 0.084 mW/cm2. E. coli ATCC 25922 was inactivated completely at 4 mJ/cm2 when the UV intensity was 0.084 mW/cm2. In contrast, E. coli was completely inactivated at 1 to 2 mJ/cm2 when other UV intensities were applied. The k value of the UVC-LED at 0.084 mW/cm2 UV intensity was significantly lower (P < 0.05) than that at other UV intensities (Table 1).

FIGURE 3.

Changes in the survival ratio of E. coli ATCC 25922 in distilled water at room temperature irradiated by UVC-LED at UV intensities of 0.085 (○), 0.025 (△), 0.013 (□), 0.007 (⋄), and 0.005 (×) mW/cm2, and irradiated by a UV lamp (•) at a UV intensity of 0.025 mW/cm2.

FIGURE 3.

Changes in the survival ratio of E. coli ATCC 25922 in distilled water at room temperature irradiated by UVC-LED at UV intensities of 0.085 (○), 0.025 (△), 0.013 (□), 0.007 (⋄), and 0.005 (×) mW/cm2, and irradiated by a UV lamp (•) at a UV intensity of 0.025 mW/cm2.

Close modal
TABLE 1.

UV sensitivity of E. coli ATCC 25922 in distilled water for UVC-LED and low-pressure UV lampa

UV sensitivity of E. coli ATCC 25922 in distilled water for UVC-LED and low-pressure UV lampa
UV sensitivity of E. coli ATCC 25922 in distilled water for UVC-LED and low-pressure UV lampa

Furthermore, we compared the performance of the UVC-LED with that of a conventional UV lamp at a UV intensity of 0.025 mW/cm2 (Fig. 3). The UVC-LED could inactivate E. coli ATCC 25922 in distilled water more efficiently than the UVC lamp at the same UV dose (Fig. 3). The k values of UVC-LED and that of the UV lamp at 0.025 mW/cm2 were significantly different (4.27 ± 0.49 versus 0.50 ± 0.02, respectively; P < 0.05; Table 1).

Inactivation of E. coli and other pathogens in ice by UVC irradiation

We next investigated the inactivation effects of the UVC-LED on bacterial pathogens in ice cubes. As shown in Figure 4, the reduction of E. coli ATCC 25922 in ice depended on the UV dose at all UV intensity conditions, unlike the case in distilled water (Fig. 3). E. coli ATCC 25922 required a higher UV dose to be inactivated in ice (160 mJ/cm2) than in distilled water (1 to 4 mJ/cm2). However, the UVC-LED could inactivate E. coli ATCC 25922 in ice more efficiently than the UVC lamp (Fig. 5), similar to the results observed in distilled water. There were significant differences (P < 0.05) between the survival ratios of pathogens with the UVC-LED and the UV lamp at UV doses of 1.52, 6.08, and 45.8 mJ/cm2, but there were no significant differences at 15.2 mJ/cm2.

FIGURE 4.

The reduction of E. coli ATCC 25922 in ice at UV intensities of 0.085 (○), 0.025 (△), 0.013 (□), 0.007 (⋄), and 0.005 (×) mW/cm2 at −30°C.

FIGURE 4.

The reduction of E. coli ATCC 25922 in ice at UV intensities of 0.085 (○), 0.025 (△), 0.013 (□), 0.007 (⋄), and 0.005 (×) mW/cm2 at −30°C.

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FIGURE 5.

Comparison of the effectiveness of UVC-LED (○) and a conventional UV lamp at a UV intensity of 0.025 mW/cm2 (△) at −30°C on inactivation of E. coli ATCC 25922 in ice.

FIGURE 5.

Comparison of the effectiveness of UVC-LED (○) and a conventional UV lamp at a UV intensity of 0.025 mW/cm2 (△) at −30°C on inactivation of E. coli ATCC 25922 in ice.

Close modal

Furthermore, a contamination level of 106 to 107 CFU/mL of E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes in the ice was efficiently inactivated by UVC-LED irradiation (Fig. 6). Although the reduction trends of pathogens in ice by the UVC-LED appeared to differ slightly, there were no significant differences among pathogens at each UV dose.

FIGURE 6.

Comparison of the inactivation of E. coli O157:H7 (○), Salmonella Typhimurium (△), and L. monocytogenes (□) in ice by UVC-LED at a UV intensity of 0.085 mW/cm2 at −30°C.

FIGURE 6.

Comparison of the inactivation of E. coli O157:H7 (○), Salmonella Typhimurium (△), and L. monocytogenes (□) in ice by UVC-LED at a UV intensity of 0.085 mW/cm2 at −30°C.

Close modal

In this study, we evaluated the ability of a UVC-LED to inactivate pathogens in distilled water and ice. The results indicated that the microbial inactivation effect of the UVC-LED depended on the UV dose at all UV intensity conditions in distilled water and ice and that the UVC-LED could more efficiently inactivate E. coli ATCC 25922 in distilled water and ice than the UV lamp.

In the present study, E. coli ATCC 25922 in distilled water was not inactivated more efficiently at higher intensities (such as 0.084 mW/cm2) than at other UV intensities. Notably, microbial inactivation by UV irradiation appeared to depend on UV dose. However, Sommer et al. (18) reported that, at the same UV dose, a higher UV intensity and shorter exposure time were more effective for E. coli inactivation in water than a lower UV intensity and longer exposure time, which is inconsistent with our current results. This discrepancy may be related to the inability of the sample to absorb UV light at 0.084 mW/cm2, due to the linearity of the UVC-LED. Kim et al. (13) reported that light from UV-LEDs converges at one point vertically, whereas UV lamps scatter light over a large area. Song et al. (19) reported that a UV-LED is a point source and that uniform irradiance is not expected on the water sample surface. Distilled water could not be uniformly irradiated by the UVC-LED when the distance between the distilled water and the UVC-LED was short because the UVC-LED light exhibited straightness. Thus, the UVC-LED may not be able to inactivate microorganisms in a liquid efficiently when the distance between the UVC-LED and the irradiated material is less than 2 cm. Notably, distance did not affect the inactivation of E. coli ATCC 25922 in ice, potentially owing to scattering of light. UV light may have been dispersed throughout the ice by reflection off of molecular crystals in the ice, whereas UV light passed straight into the water. Thus, our current findings indicate that UVC-LED efficiency differed depending on the material being irradiated when the UVC-LED and sample were close in proximity.

In the present study, the UVC-LED could more efficiently inactivate E. coli ATCC 25922 in distilled water and ice than the UVC lamp, consistent with the findings of Chatterley and Linden (4) and Kim et al. (13). Chatterley and Linden (4) compared the log reduction of E. coli K-12 by irradiation from low-pressure lamps (254 nm) and LEDs (265 nm) and concluded that the UV-LED could slightly improve inactivation compared with the UV lamp, although there was no significant difference at a 95% confidence level. Kim et al. (13) studied the reduction levels of E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes spread on selective culture media after treatment with a 254-nm UV lamp or a 266-nm UV-LED. They concluded that UV-LED treatment at a UV dose of 0.7 mJ/cm2 inactivated nearly all inoculated pathogens, whereas the UV lamp resulted in 3.06-, 1.42-, and 0.34-log reductions of E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes, respectively, which were significantly less (P < 0.05) than the UV-LED inactivation levels at the same dose. However, Song et al. (19) compared the average of k values for E. coli inactivation using a UV lamp at 254 nm with the k values of UV-LEDs at different wavelengths (255, 265, 272, 275, and 280 nm) from previous studies and found a higher average k value (0.506 cm2/mL) for the UV lamp than for UV-LEDs (0.170 to 0.422 cm2/mL). Thus, the results of comparisons of inactivation by UV-LEDs and UV lamps have been inconsistent, potentially because of a lack of uniformity in research materials and methods, including individual differences in UV-LEDs. However, our results showed the potential of UVC-LEDs to inactivate bacterial cells efficiently compared with conventional UV lamps.

In this study, the reduction of pathogens in ice by UVC-LED depended on the type of bacteria. Although L. monocytogenes was completely inactivated at 40 mJ/cm2, 102 CFU/mL E. coli O157:H7 and Salmonella Typhimurium survived, even at 100 mJ/cm2. In general, gram-positive bacteria, such as L. monocytogenes, are more resistant to UV light than gram-negative bacteria, such as E. coli O157:H7 and Salmonella Typhimurium, providing an explanation for these findings. However, Koutchma (14) assumed that the UV sensitivities of L. monocytogenes and Salmonella in liquid foods were similar to the UV sensitivity of E. coli. Kim et al. (13) studied the reductions in viable counts of E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes spread on selective media after treatment with a 254-nm UV lamp or a 266-nm UV-LED. They found that the inactivation level was the lowest for L. monocytogenes, similar to our study.

One unit of the UVC-LED used in this study inactivated 106 to 107 CFU/mL of the pathogens, although the effect depended on the type of bacterial strain. These strains are highly pathogenic and can cause food poisoning when present at only 101 to 102 CFU/mL. Notably, a UV dose of more than 160 mJ/cm2 was needed to prevent food poisoning if the initial pathogen concentrations were more than 107 CFU/mL, a level that would rarely be encountered in commercial ice (10, 17).

Finally, we found that bacterial pathogens in ice were not always inactivated completely at a UV dose of 160 mJ/cm2. We assumed that the cloudiness of the ice, produced by air bubbles, could explain this result. In general, the penetration of light is lower when the liquid has greater color or turbidity. The presence of dissolved organic solutes and compounds in liquid foods leads to strong UV attenuation effects (9). Moreover, the cloudiness of ice could prevent the penetration of UV light into the ice. The ice used in the present study was not clear because we intended to obtain fundamental data for evaluating the microbial inactivation effect of UVC-LED in commercial ice.

In the future, we will need to investigate the effects of cloudiness of ice, the number of UVC-LED modules used for treatment, and changes in the angle of the UVC-LED on the results of bacterial inactivation. These efforts will be realized by investigation of ice-making procedures and development of a small-scale apparatus for optimal ice inactivation using UVC-LED.

In summary, our results demonstrate that the use of UVC-LEDs, rather than UV lamps, may contribute to efficient microbial control in ice and to ensuring the safety of ice in the food industry.

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