Comprehensive Study of Light-Emitting Diodes (LEDs) and Ultraviolet-LED Lights Application in Food Quality and Safety

Light-Emitting Diodes (LEDs) and Ultraviolet Light-Emitting Diodes (UV LEDs) consist in a semiconductor of light, that are emerging in the market, due to their singular characteristics, as being a solid-state cold source of light, which has potential application in food preservation. For this reason, this study lens to provide a review of the effects of LED and UV LED application in fresh fruits and vegetables, under refrigeration storage. Analyzing the LED role, in extending the shelf-life of postharvest food, these present the capability of improving the quality physicochemical and microbiological of fruits and vegetables, such as: color (chlorophyll), weight loss, total phenolic and flavonoid content, phenylalanine ammonia-lyase activity and total soluble solids. In addition, it's able to stop chemical reactions and increasing the activity of fruits and vegetable defenses. UV LED light, on the other hand, operates in an effective and straightway in the inactivation the food pathogens, such as Escherichia coli, pseudomonas fluorescens and Salmonella spp, for example. Therefore, UV LED light can be applied to delay the senescence of foods, however, the wavelength must match the target organism, depending on the food.

stimulation, color, weight loss reduction, water activity and pH). UV lights, on the other hand, are responsible for the surface microbial counts reduction, by avoiding yeast, mold and bacteria proliferation 26 , responsible for accelerate food decay.
Therefore, LED lights has the capacity of efficiently attend the food industry, according to their specifications and needs, being an increasingly inexpensive approach for food safety and preservation 11 . This review study highlights the effects of LED and UV lights in fruits and vegetables postharvest preservation. The main characteristics of LED and UV lights, and its mechanisms of action in food, to extend shelflife and enhance properties, are presented. The industrial application of LED in food preservation are also presented.

Principles of led mechanism in food
The LED system works by the principle of passing an electrical current through the device in one direction and blocking the current flow that comes from the opposite, being capable of emitting light with narrow emission wavelength bandwidths, high photoelectric efficiency and photon flux or irradiance 9 . In addition, the LED has non-breakable glass envelopes, low heat irradiation, higher efficacy, and can be used in postharvest preservation 11 .
Among the LED lights advantages, are the capability to control the spectral output, light intensity and the possibility to select several wavelengths that match the absorbance of plant photoreceptors 62 (Fig. 1), which can be used to improve the physicochemical and microbiological components and consequently the shelf-life and the food quality during postharvest stage.

Visible LED
LED is a semiconductor that can produce light by using a safer approach because is a cold lightning and does not have glass envelopes or mercury in their composition 12 . In addition, is environmentally friendly and energetically efficient 13 , one of the characteristics of the LED is that it can provide different colors, depending on their composition. Red, green, yellow and orange lights are made of indium, gallium, aluminum and phosphide, while blue lights of gallium, nitride and silicon carbide 9 .
LED lights are a solid-state lighting that provides a non-conventional source. Due to the capability of monitor their spectral and temporal properties, color and wavelength, which possess several applications, as well as automobiles, communication, agriculture, medicine and food preservation 14 .
The cost benefit of LED light has become very attractive in the food industry due to its price and efficiency 9 . Moreover, the directed light provided by LED, allows the use of LED at its highest lighting efficiency, with a large amount of light and color emitted, yet still presenting energy savings 15 . Each one of the LEDs several colors is used in the food industry for cultivation and postharvest preservation. Analyzing this last factor, LED can provide an increase in the physicochemical properties and bacterial inactivation, responsible for food degradation, therefore extending the shelf-life of fresh foods.
Blue light (emission spectrum: 455-465 nm) may be responsible for regulating the biomass production, leaf expansion and stomatal opening of plants 16 , also being able to protect the food against harmful pathogens, such as Salmonella spp. on fresh-cut pineapple slices 17 , for example. Also, vegetables such as cabbage, when exposed to blue LED present higher levels of vitamin C, total polyphenolic contents 18 and chlorophyll content 19 .
Red LEDs act in the wavelength of 660 nm, which is similar to the absorption of plants. Therefore, is responsible for assisting the photosynthetic apparatus of fresh food 16 , increasing de plant growth, besides, this light color can increase the phenolic compound in vegetables, such as broccoli, for example 20 . On the other hand, the green light has a positive effect on the chlorophyll content 18 , mainly present in green vegetables, such as lettuce and cabbage, for example.

UV LED
UV LEDs are a light source based on the conversion of electricity to photons 21 . Those photons are absorbed by the food genetic material and form dimers, inhibiting the transcription and replication of the cell 22 . This light can possess wavelengths of 100 -400 nm and is typically made of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) 23 , the wavelengths are divided into UV A, UV B and UV C light.
UV LEDs have been receiving attention in the last decade, those can be used instead of conventional low-pressure mercury lamps 21 , and also due to their many advantages compared with the traditional UV lamps 24 . Besides been a green source of light, UV LEDs are also compact, have a fast start-up and less energy consumption, are a cold source of light and possess a long lifetime of 100,000 hours and 75% wall-plug efficiency, due to new improvements in this technology 25 .
Therefore, UV LEDs may be applied in the food industry 26 , mainly due to their potential of inactivating pathogens, on the surface food, without producing undesirable by-products 24 , which can be an effective mechanism for food safety, preserving the food in postharvest stages 9 .  On the other hand, UV-C is capable of inducing the formation of DNA photoproducts, such as cyclobutane pyrimidine dimers (CPDs) and pyrimidine 6-4 pyrimidone (6-4PP), capable of inhibiting transcription and replication 63 , as well as DNA and RNA polymerases. The inhibition of both replication and gene expression, can cause cell death 23 . In addition, UV is ultimately limited by the shading of microbes in protective sites leading to tailing in inactivation curves 66 .
In the matter of food preservation, the electromagnetic spectrum possesses several radiation forms, each one has different penetration power, frequency and wavelength, and, for the food industry, the most interesting and passive applications are gamma and ultraviolet radiation 28 . Over the last decade, UV radiation has been used both for water disinfection and microbial decontamination of surfaces for fresh food preservation 29 .
The consumer demand for fresh minimal processing food is growing 28 . For this reason, UV LED shows efficiency on slowing the senescence of fresh food and delaying the nutritional loss 22 . Therefore, to ensure the UV LEDs efficiency against food pathogens, the wavelengths should match the target organisms 26 , this way, it presents effective results (Table 1).
The principle of the UV LED mechanism to bacterial inactivation works directly on the bacteria DNA, the UV-C light, which provides the wavelength 100 to 280 nm, is considered to have the highest efficiency, due to the bacteria DNA absorbance of 260 nm 34 . The microorganism is hit by the UV light, changing the DNA, and stopping the reproduction, which leads to the bacteria death 35 , as presented in Fig. 2. Similar to UV-C, the UV-B spectrum has the ability to partially inactivate bacteria by damaging DNA as well as other cellular structures 23 .
In addition, LED UV-A is capable of causing damage to cell structures by forming reactive oxygen species (ROS), causing oxidative damage to lipids, proteins, and DNA 23 (Fig. 2). However, the UV-A gamma is hardly absorbed by native DNA, not inducing severe damage by dimer formation, and can still produce photoproducts or modified DNA by indirect photosensitization reactions 63 . As UV LEDs can possess wavelengths of 250 nm to 365 nm, allows selecting the most effective wavelength to the specific target 36 , wavelengths lower than 250 nm have poor penetration power.

LED in food preservation
Over the last years, LED lights have been studied for food preservation, due to their positive effects over the physicochemical and microbiological aspects of food in the postharvest stage, such as the reduction of weight loss and

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water activity, maintenance of the vitamin C index, color, chlorophyll and pH, as well as inactivation of bacteria responsible for food degradation. Also, for being economical and energetically efficient, which contribute to reduce the fresh food senescence, prolonging food shelf-life 37 .

Enhance in food properties
The possibility to select several wavelengths is an advantage to the antibacterial effect 10 and to improve properties, such as vitamin C, total phenolics, color, chlorophyll content, weight losses, total titratable acid and total soluble solids content. Table 2 present some successful applications of LED lights in food preservation. The LED highlight comes from the food waste concern on the postharvest stages, and the LED light application can supply these losses, due to their capability of improving the food shelf-life 38 .

Food safety
The concept of food safety has increased due to the importance of microorganism control in food 28 . New policies and regulations, related to food safety, are been constantly developed and updated 49 . In the matter of food industries, the use of LED lights became an attractive source to ensure and keep the safety of food during the cultivation and storage stages 38 .
Therefore, the development in food safety is related to environmental protection 50 , noticing that the food market needs to ensure the food safety and quality, those may be available from traditional and nontraditional technologies, including the nonthermal lights (LEDs) 28 .
UV LED lights, in this scenario, might have an important role in food safety, due to their antimicrobial power to treat food during the storage stage, acting on the food surface and in the air, inhibiting the bacteria and pathogens, the next step for the food industry 23 . For this reason, LEDs and UV LEDs have an industrial and commercial potential if applied in the refrigerator, because of their positive effect over postharvest food 38 .

Industrial applications
Since the first household refrigerator development, innovations in food preservation assisted by cold storage became more frequent and necessary. Analyzing the LED light role in the food preservation under refrigeration, domestic refrigerators with LED technology have already been developed and are available in the market. The LED light " Vitamin Power " technology applied in refrigerators uses green, blue and white LED lights to simulate sunlight and potentialize food vitamins in a process similar to photosynthesis 52 . Vitamin Power works with the principle of pulsed LED lights, attached on the bottom shelves of the refrigerator, the fruits and vegetables, located under the light, have their properties, such as vitamin C and D enhance. Also, with the "Antibacteria Technology" this refrigerator provides an antipathogenic effect. The technology operates with a blue LED light inside the refrigerator to avoid/reduce microorganism proliferation, capable of eliminating 99,99% of bacteria 53 , conserving the food for a longer storage time.
The "Smart Side by Side" refrigerator has an air purification based on the Higiene Fresh+ technology, using air purified (odorless and bacteria reduction) by carbon filter and UV LED photocatalyst, with an automatic fan. The Higiene Fresh+ technology is capable of eliminating 99.99% of bacteria, and maintain 88% of fruits and 95% of vegetables moister, keeping the food, inside the refrigerator drawer, fresh for a longer period 54 . Carbon filter has a well-known catalytic property and can be applied to remove gas molecules produced during the food deterioration process 55 . UV light in the bactericidal range (200 to 280 nm) effectively inactivates bacterial microorganisms in the air, as well as in water and surfaces (as food surfaces) 56 .
The Nasa-Inspired Air Purification System is an air purification system, based on UV-C light with TiO 2 , to reduce bacteria and ethylene gas from the air, maintaining the food freshness. Some studies have shown that TiO 2 nanoparticles have antimicrobial properties [57][58][59] , which is improved in UV light presence 55 . Also, ethylene scavenger activity was already verified in TiO2 60 .

CONCluSiONS
The promising LED and UV LED technologies have several characteristics, capable to enhance the food properties and keep fruits and vegetables fresh for a longer period. Due to the lack of mercury in their composition, LED light became an emerging technic in the matter of food preservation, in an account of being a cold source of light, this device could be attached inside refrigerators as a mechanism for extending food shelf-life and preventing the food waste. The effectiveness of LED light over the physicochemical and microbiological content of food depends on the right combination of color, wavelength and type of food, studies proved that the use of LED lights and UV LED lights can enhance the food properties and inactivates food pathogens. Those factors classify the LED light technology as an efficient method to extend the shelf-life of fresh food, which is already being applied in the refrigerator industry and available in the market.

FUnDInG
The study is funded from Coordination of Superior Level Staff Improvement -CAPES (finance code 001) and Foundation for Research and Innovation of the State of Santa Catarina -FAPESC (TO 2018TR342).

DATA AVAILAbILITy
All datasets generated or analyzed during this study are included in the manuscript

ethiCS StAteMeNt
Not applicable.

1.
Food and Agriculture Organization of the United Nations. Food loss and waste and climate change: An interdependent relation. SAVE FOOD FOR A BETTER CLIMATE -Converting the food loss and waste challenge into climate action. 2017:3-7. http://www. fao.org/3/a-i8000e.pdf. Accessed April 13, 2020.