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Introduction
Agriculture is an important activity owing to its response to the nutritional demands of millions of people, preserves natural environments, and stimulates the process to generate a better quality of life (Organización de las Naciones Unidas para la Alimentación y la Agricultura [FAO], Organización Panamericana de la Salud [OPS], Programa Mundial de Alimentos [WFP] & Fondo de las Naciones Unidas para la Infancia [UNICEF], 2019). Fresh produce is continuously exposed to microbial contamination at every stage of production (growing, transport, packaging, storage and final sale) and accounts for a high percentage of total food expenditures for consumers worldwide. The fresh products that have a great impact worldwide are the tomato, garlic, and serrano peppers.
The demand for quality fresh products (fruits and vegetables) is crucial for a healthy diet from a microbiological point of view. However, when consumed without any disinfection process, fresh products become potentially dangerous for humans. Contamination from pathogen causes substantial losses of food (55 % of fruits and vegetables, 25 % for cereals, 35 % for fish and shellfish, and 20 % for meat and dairy products) during storage, transport, and marketing (FAO, OPS, WFP, & UNICEF, 2019; Nüesch-Inderbinen & Stephan, 2016).
Harmlessness and food-safety are considered the backbone that sustains the food industry and are closely linked with the community’s demand for food in good condition and risk-free. More than 200 infections (from small diarrhea to cancer) are referred to due to a lack of safety (World Health Organization [WHO], 2019).
Numerous processing approaches have been established to stop food spoilage and increase safety. Pasteurization (by heat), canning, freezing, refrigeration, and chemical conservatives are considered traditional methods (Ravindran & Jaiswal, 2019). But there are changes in texture, flavor, and modification in the sensory and nutritional qualities of fresh products. Another technology that can be included in the list is irradiation. Irradiation is a food preservation technique used to extend and improve the shelf life of fresh or processed foods. Food irradiation is an energy-efficient method (quantity of particles or rays) that contributes to reducing the enormous losses generated due to spoilage or contamination by harmful bacteria and other parasites (Ashraf et al., 2019). Different countries have recognized the potential of the food irradiation method, because it helps to reduce losses in the post-harvest stage, conforms with quarantine requirements, extends exports, and ensures the hygienic quality of food. Irradiation also offers the opportunity to maintain nutrients in foods and replace chemical preservatives, which can be considered risky or harmful to health (Cardello et al., 2007; Frewer et al., 2011; Nayga et al., 2005)
The use of ultraviolet (UV) light in food irradiation is presented as an alternative method to conventional thermal techniques for disinfection. An important issue generated by the thermal methods is the nutritional and organoleptic properties are modified (Fan et al., 2017). Mercury, xenon, and UV-LEDs lamps in recent years have been integrated into these processes. At the beginning of the 20th century, UV irradiation was used in the disinfection of water (Kowalski, 2009), and different foods, like tomato and serrano peppers. Several authors reported that in cherry tomato, eggs, chicken drumsticks, frankfurters, bratwurst, tomato and jalapeño peppers exposed to ultraviolet light C (UV-C at 254 nm) with doses of 0.4 J∙cm-2 and 2 KJ∙m-2 showed a germicidal effect over 90 % (Sommers et al., 2010; Choi et al., 2015).
UV light technology has been proved to be a suitable option for disinfection, although it has several disadvantages such as not penetrating food, that is, decontamination is carried out superficially (Koutchma, 2009). When food goes through a decontamination process, there are shaded areas; that is, irradiation does not contact all flanks of the product, which is considered a challenge for the design and development of new disinfection systems.
A proposal by Stoops et al. (2003) was a UV disinfection system for granulated or powdered foods (Brésilienne nuts, meringue Chunks, cacao powder, small ground Hazelnuts, large ground hazelnuts, and a hazelnut-with-waffle-mixture), irradiation was established with UV at 254 nm (2.88 mW∙cm-2) during 1 h and showing results over 90 % in two of the six irradiated foods. The authors did not indicate the irradiation area, and the height of the lamps. Hosseini et al. (2019) developed a rotational UV system for the decontamination of pistachio using mercury lamps. The dose is emitted at 2.1 and 4.5 KJ∙m-2 for 7 and 15 min while the product receives the rays in all flanks, obtaining germicidal effects superior to 99.9 %.
The current UV-LEDs developments have advantages over conventional mercury lamps such as longer life, better temperature control, greater energy efficiency, better drive voltage, flexibility in design, no risk of mercury release, and versatility to generate different wavelengths (Muramoto et al., 2014; Chen et al., 2017). Green et al. (2018) used the germicidal effect of the UV-LEDs (A, B, and C) at 259, 268, 275, 289 and 370 nm versus a mercury lamp at 253.7 nm. The obtained results showed that UV-LEDs at 259 y 268 nm were the best.
Additionally, the authors combined the UV-LEDs at 259 and 289 nm with dose to 7 mJ∙cm-2 and they were potent to Escherichia coli and Listeria.
Yagi et al. (2007) used UV-A light in sterilization for the inactivation of E. coli with a dose of 54 J∙cm-2 and Vibrio parahaemolyticus with 27 J∙cm-2, achieving a germicidal effect of 100 and 85 % for 30 and 10 min, correspondingly. The bacteria reduction was determined according to those that appeared on the agar. Malik et al. (2017) applied 57.6 J∙cm-2 of UV-A to E. coli ATCC 11229 and had a germicidal effect of 99.9 %. Though the germicidal effect is high, the experiment was carried out on agar. Food in real conditions is totally contaminated and not on agar, therefore the information from these investigations is only a guide.
According to the literature, the use of UV light emitted diodes has shown germicidal power in food, higher energy efficiency, a longer lifetime, constant light intensity, management in heat up, and temperature, including the application of light in specific bands (λ) of the UV spectrum (210 to 400 nm). Therefore, the objective of this study was to improve knowledge and the use of UV-A LED technology in an automatic routine through a semi-industrial mechatronic system for the agro-industrial sector in the disinfection of fresh products. The integrated system allows us to act accurately in the irradiation dose (controlling the exposure time), because one of the problems of this technology is the non-irradiated area or part of the products, which can be solved by applying some strategies associated with the mechatronic system (steps, vibrations, stop/start techniques, among others).
Materials and methods
Based UV-A LED food disinfection mechatronic system
The design and manufacture of the Based UV-A LEDs food disinfection mechatronic system were developed by the Artificial Lighting Laboratory (LIA) at Instituto Tecnológico de Pabellón de Arteaga in Aguascalientes, México (Figure 1). The mechatronic system for food disinfection has a conveyor belt where the fresh product is moved to the different stages of the process. In the first stage there is a hopper where the product is selected (garlic, serrano peppers, and tomato). The second stage, the fresh product is moved to the irradiation area where there are three UV-A of 374 nm (Figure 2a) of 25 watts each. The lamps are mounted on a platform of variable height. Controlling the height of the platform allows us to have better control over the irradiation dose to which the fresh product is exposed.
Figure 1. General scheme of UV-A LEDs Mechatronic system: a) design, implementation and validation, and b) sequenceof the disinfection process.
Figure 2. Irradiation of UV-A LEDs: a) maximum irradiation emitted by the lamps and b) the irradiated area by UV-A LEDs.
The irradiation zone is shown in Figure 2b and has an effective irradiation area of 800 cm2 (approximately 50 x 16 cm). The conveyor belt is controlled by an external system that allows to program the speed of advance of the product and thus control the exposure time. Immediately after the product leaves the irradiation zone, a thermal camera takes the image for temperature monitoring. In the third stage, a bacterial count is performed to analyze the germicidal effect. All material in the system is sanitary grade.
Characterization of the irradiation zone
We used UV-LEDs type A installed in an aluminum lamp with fans to dissipate the heat. The UV-A LEDs reached a maximum power of 0.544 mW∙cm-2 at 374 nm wavelength. We applied a configuration with three lamps on the system. The irradiated zone and characterization were determined by an ILT950 spectroradiometer (ILT950, International Light Technologies, USA) (Figure 2).
Doses experimental setup
The dose programming includes the values of the time and the height of the lamps. The doses are determined by the Equation (1):
where D refers to the doses applied (J∙cm-2), I is the irradiance (mW∙cm-2), and t is the time (s). Four doses were established: 7.9, 23.7, 47.4, and 71.1 J∙cm-2.
Experimental design
Three types of fresh products (garlic, serrano peppers, and tomato) were used to validate the mechatronic system. The fresh products were from the same harvest and were purchased from a local retailer or farmers market. From the fresh products was determined the initial number of bacteria before the irradiation process. After the irradiation process, 10 g of garlic, serrano peppers, and tomato were selected to evaluate the germicidal effect. Triplicate experiments for all conditions were carried out with all doses.
Organism and counting approach
The microorganisms analyzed for the experimentation were aerobic mesophilic (AM) to verify the sanitary quality of food, the handling conditions, and hygienic conditions (Gould, 1988). The same procedure was used for the three fresh products. 10 g of product were aseptically transferred to a sterile bottle with 90 mL of Peptone and the samples were homogenized for 2 min. Subsequently, 15 mL of agar for standard methods were poured into petri dishes and 1 mL of the dilution was inoculated in duplicate. The samples were left 48 h in an oven at 35 °C. Colony-forming unit (CFU) were counted per plate.
Temperature monitoring of fresh products
An infrared camera (One Pro LT, FLIR®, USA) was used to monitor the temperature of the fresh product. The camera was mounted on the system and measurements were taken at the same point of the product every 10 min for 90 min. Weight and color were measured before and after each treatment to determine product change.
Statistical analysis
The experiments were based on a completely randomized design, and results are expressed as mean ± standard deviation or ± standard error. The experiments were performed twice in triplicate. A Shapiro-Wilks test was performed to determine the normality of the data. Subsequently, for the validation of the germicidal effect, one analysis of variance was performed (P () 0.05) to determine if there are significant germicidal differences between doses or techniques. Statistical tests were performed in R software.
Results and discussion
In LIA, the disinfection process with UV-A LED irradiation was carried out. The system was configured and programmed to irradiate every one of the products under the same conditions (7.9, 23.7, 47.4 and 71.1 J∙cm-2 for 10, 30, 60 and 90 min), correspondingly calculated in the methodology section. About 95 % of the germicidal effect was achieved during the irradiation process. Also, our system takes advantages such as it can be irradiated with different products, variables like the range of treatments, doses, on/ off time, the conveyor belt speed, the height of the lamps are controlled through an automatic routine. Can be incorporated into a production line; besides shall be scalable; that is, higher production capacity, irradiation area, among others (Figure 1).
Stoops et al. (2013) proposed a rotating system, but with several disadvantages, such as the inability to adjust height, irradiation zone and dose. Therefore, this system is not suitable for integration into a production line. Hosseini et al. (2019) also designed a system for equivalent products and no adjustment of height, irradiation zone or rotational speed.
Temperature monitoring of fresh products
Figure 3 shows the results obtained in the temperature monitoring during 90 min of treatment for fresh products (tomato, serrano peppers, and garlic). Figure 3a displays the thermograms obtained with the thermal camera, assuming the same reference point for all. The initial temperatures of tomato (Figure 3a1), serrano peppers (Figure 3a3), and garlic (Figure 3a5) were 19.9, 21.9 and 22.0 °C, respectively. Figures 3a2, 3a4, and 3a6 show the final temperatures 38.3, 41.9 and 40 °C correspondingly to each product (tomato, serrano peppers, and garlic, respectively). According to the data obtained, the irradiation time and the heat generated by the UV-A LED system are factors of the temperature increase in tomato, serrano peppers, and garlic. The weight and color of each product were measured, obtaining as results that there are no significant changes.
Figure 3. a) Thermographic images at the beginning and end of the experiment: tomato (a1 and a2), serrano peppers (a3 and a4) and garlic (a5 and a6), and b) temperature behavior during fresh product treatments.
The research reported have emphasized the product temperature when it is submitted to a disinfection process hence an excessive increase would cause changes in the physical and chemical properties. For this reason, it is essential to monitor during the treatment. Disinfection systems developed with mercury lamps in comparison with those with UV-LED light can cause dehydration if exposure is prolonged, this being one of the main challenges for the development of new strategies for disinfection of fresh products (Fan et al., 2017; Mandal et al., 2020).
Germicidal efficiency
Table 1 shows the number of CFU in each product (garlic, serrano peppers and tomato) at the beginning of the experiment, and garlic had the highest number of CFU.
The germicidal effect by treatment and dose is shown in Table 2. At a dose of 7.9 and 23.7 J∙cm-2, tomato had the highest germicidal effect (41.0 and 71.4 %), followed by serrano peppers (35.6 and 67.2 %) and garlic (19. 4 and 67.6 %), while at 47.4 J∙cm-2, the germicidal efficacy was highest in garlic (92.7 %), and was similar for serrano peppers (85.4 %) and tomato (83.2 %). The maximum cumulative germicidal effects (90 min) were 94.4, 90.6 and 93.8 % for garlic, serrano chili and tomato, respectively, at a concentration of 71.1 J∙cm-2.
Table 2. The germicidal effect of UV-A LEDs irradiation.
| Product / Producto |
Dose (J·cm-2) / Dosis (J·cm-2) |
Germicidal effect (%) / Efecto germicida (%) |
Log (N/N0) / Logaritmo (N/N0) |
|---|---|---|---|
| Garlic / Ajo | 7.9 | 19.4 ± 5.6 | 0.09 ± 0.03 |
| 23.7 | 67.6 ± 1.9 | 0.49 ± 0.03 | |
| 47.4 | 92.7 ± 0.4 | 1.13 ± 0.04 | |
| 71.1 | 94.4 ± 0.7 | 1.25 ± 0.05 | |
| Serrano peppers / Chile serrano | 7.9 | 35.6 ±13.8 | 0.20 ± 0.09 |
| 23.7 | 67.2 ± 4.08 | 0.47 ± 0.09 | |
| 47.4 | 5.4 ± 1.4 | 0.79 ± 0.10 | |
| 71.1 | 90.6 ± 2.9 | 1.05 ± 0.16 | |
| Tomato / Jitomate | 7.9 | 41.0 ± 9.9 | 0.23 ± 0.08 |
| 23.7 | 71.4 ± 12.4 | 0.58 ± 0.18 | |
| 47.4 | 83.2 ± 5.2 | 0.79 ± 0.14 | |
| 71.1 | 93.8 ± 2.0 | 1.24 ± 0.18 |
The development of the mechatronic disinfection system using UV LED light allowed the study of different wavelengths with UV-C light, while the proposals with UV-A mentioned in the literature were carried out on agar inoculated with bacteria.
In comparison with the experiment reported by Yagi et al. (2007) and Malik et al. (2017), where they found germicidal rates of 100 and 99.9 %, were obtained respectively. The highest germicidal effect achieved in this research was 94.4 % in garlic. Different factors can influence the germicidal effect, the two most important are bacteria and the contamination origin of the samples. AM represent a wide range of microorganisms with different resistance to UV light, as reported by Yagi et al. (2007), who with the same doses achieved different germicidal effects on Salmonella (20 %) and Vibrio parahaemolyticus (100 %). Birmpa et al. (2013) reached reductions of 1.75 and 1.27 Log in E. coli and L. innocua, respectively using UV-C light. In this case, the products were irradiated for up to 60 min, and no significant changes in organoleptic properties were found. Compared to our proposal, we achieved a similar germicidal effect but with a higher UV-A dose. Choi et al. (2015) applied doses of 10 kJ∙m-2 to cherry tomatoes inoculated with S. typhimurium. The results showed reductions of 1.28 Log.
Germicidal effect performance
Germicidal Effect Performance is defined as the difference in germicidal efficacy between periods, this means, the difference in the percentages of germicidal effect in treatments. Figure 4 represents the effective germicidal effects though the time of each treatment, the behavior of the germicidal effect in each fresh product was indicated, specifically showing what percentage was obtained by period (from 0 to 90 min). The first column in Figure 4 shows the percentages obtained in each period of the treatments.
Figure 4. The figures on the left side show the zone of maximum germicidal effect in the treatments, and the figures on the right side show the performance of the germicidal effect over time.
The shaded area reflects the maximum germicidal efficacy at the intervals for each product, i.e., the interval in which irradiation with LED UV-A light was most potent. The figures on the right-hand side show the percentage values for each period, and it can be seen whether the germicidal effect increases or decreases over time. In the 10 to 30 min interval, the highest germicidal percentage was produced in garlic, with 48.1 %, while at 60 min it decreased 25.1 % and at 90 min it was only 1.7 %. With serrano peppers, the highest germicidal efficacy was obtained in the first 10 min (35.6 %); subsequently, the value began to decrease until reaching 5.6 % at 90 min (Figure 4).
The tomato achieved 41.0 % in the first 10 min of treatment, subsequently decreased in 30 min to 30.4 %, 11.8 % to 60 min, and in 90 min to 10.7 %. The maximum germicidal effect between intervals was obtained in 30 min on garlic, 10 min on serrano peppers, and 10 min on tomato; inactivating a large number of AM. The germicidal effect represents an important parameter when establishing new strategies for food disinfection, because it is important to know the time in which there is synergy with the different wavelengths to obtain the most effective irradiation treatments to reduce AM and energy consumption. A large number of jobs reported with UV-C, but there are very few UV-A for disinfection.
Compared to food disinfection work with UV-C emitted with mercury lamps, the work presented shows a lower germicidal effect, however, for those products that require a decrease in their microbial load it may be a viable option. Also, UV-A LEDs have a greater coverage area, an advantage that can lower the cost of disinfection systems. No changes in weight were observed with the treatments, nor color changes with the naked eye, although it would be interesting to perform an analysis of the chemical compounds to characterize tomato properties.
Conclusions
The mechatronic system using UV-A LED light designed to disinfect food proved to be suitable for fresh food, because significant bacterial reductions were observed. The system was designed and developed for the agroindustrial sector considering that it can be inserted as a process within the production line. Ensuring the food safety is possible to control the doses and irradiation time of each product. According to needs, the irradiation area will be extended along a conveyor belt establishing more arrays of UV-A LEDs. For this experiment did not cause secondary effects but is a variable that should be examined as a part of the application when used this kind of technology. For the food disinfection such as garlic, serrano peppers, and tomato, UV-A LEDs irradiation proved to be a versatile instrument, also could be configured for different foods. Germicidal efficacy can be influenced by bacterial resistance due to the great variety of mesophilic aerobic bacteria and food infection process (production line, packaging, touching the surface, among others). These are real conditions of the products and must be taken into account. Finally, few studies have assessed the efficacy of UV-A LEDs directly in food. The presented technique opens a gap in the microbiological processes in preservation and decontamination, intending to provide nutritional and safe food.
