LEDs Impact on Post-Harvest Fruit Quality

• LEDs for the postharvest treatment of fruits are new, and the industrial application looks promising.
• LEDs influence the development of quality parameters like color, firmness, taste, nutritional value, control of ripening, microbial spoilage and senescence, and extending shelf-life.
• LEDs can be useful for fruit preservation for long-duration transportation and storage and at retailers.

Fruits are among the most highly perishable food items and suffer significant yield and quality loss in the postharvest stages due to physiological processes. The food industry has used Light-emitting diode (LED) during the preharvest stage as standard procedure, but postharvest applications are still not widespread. However, new research suggests LEDs can be safe, chemical-free, and physically preserve fruit quality. Find out what is known so far about potential postharvest applications of LEDs for fruit quality enhancement.

Current Postharvest Problems

Fruits can lose quality and senescence since their physiological processes, like respiration, ripening, and senescence, continue after picking. Fruits lose moisture and weight, flavor, texture, and nutritional value. They are also vulnerable to microbial spoilage. Postharvest conditions are designed to reduce respiration and ripening rates and postpone senescence.

Traditional chemical treatment methods are no longer popular due to awareness of their health risks.

Research findings on controlling fruit quality and senescence in the postharvest phase with Light-emitting diode (LED) have been promising and suggest LED can also enhance various quality traits.

LEDs

LED is a solid-state technology that produces light with narrow wavelengths, varying intensity, high photoelectric efficiency, and low thermal properties. LEDs come in wavelengths from infrared to ultraviolet and are applied at different fluences. Light fluence is the radiation incident per unit surface area per unit time.

Many physiological processes in fruits are controlled by light radiation, so LEDs of appropriate wavelength can optimize the ripening process, nutritional value, secondary metabolites production, delay senescence, and realize fruit quality and shelf-life improvement, see Figure 1. The research findings on these potential effects of LEDs on postharvest fruit quality are discussed here.

Figure 1: Graphical representation of LED effect on postharvest fruits, Nassarawa et al. 2021. (Image credits: https://doi.org/10.1007/s11947-020-02534-6)

Influencing Ripening

Ripening at the targeted time is advantageous, as it produces a quality change in appearance, flavor, texture, and nutrition consumers want. However, early ripening during storage will lead to senescence.

LEDs will hasten ripening by increasing respiration and ethylene production. LED light also triggers gene expression controlling the production of pigments like carotenoids and flavonoids that change color. Thus, the overall effect of LEDs is to increase ripening.

However, the genotype is significant, and blue accelerates ripening in bananas, followed by red and green light.

The effect of LED color on ripening depends on the genotype. Blue LED quickens ripening in peaches, bananas, satsuma, strawberries, and sweet oranges, by increasing ethylene synthesis. In bananas, blue, red, and green are suitable for ripening.

Blue light hinders the ripening process and softening of tomatoes, while red LEDs promote the ripening process. So blue can extend shelf life and red speeds up ripening.

Therefore, the choice of LED spectrum can depend on whether ripening needs to be limited or hastened.

Development of Fruit Quality Parameters

LED treatments can improve color, firmness, taste, and nutritional quality.

Color Development

Color is one of the first quality parameters consumers evaluate to choose a fruit. Depending on the species and cultivar, various pigments like anthocyanins, lycopene, chlorophyll, or flavonoids are involved in color development. Usually, temperature and fruit physiology, like ripening, determine color changes. However, light can also affect the accumulation of these pigments.

Blue light prevents green tomatoes from turning red, but red light can turn tomatoes red. However, temperatures matter, and lower temperatures of 2^oC compared with 21°C reduce color development. So, red light at any fluence produces color development at 21^oC in blueberries.

Red light is the best for color development, followed by blue and green.

Total Soluble Solids

Total soluble solids (TSS) measured in Brix determine maturity in climacteric and non-climacteric fruits. An increase in TSS accompanies ripening, and any sugar decrease delays ripening but extends shelf life. It should be possible to control sugar production by choosing suitable light colors and influencing shelf-life.

The light colors that enhance or reduce TSS production are species-specific but are moderated by temperature and storage time.

• Green is good for prolonging shelf life at even high temperatures of 21°C for over 15 days in blueberries compared to other colors and control. Red light increased TSS in blueberries at 21°C, followed by blue light. After 15 days of storage, low to medium fluence (20 to 40 W m−2) intensity blue light can increase TSS even at low temperatures of 2°C.
• Blue LEDs increase TSS in strawberries and peaches. In peaches, ethylene production was delayed from six to 9 days compared to untreated fruits. But after nine days, ethylene production increased, leading to color and TSS development.
• The red: far-red light in tomatoes increases TSS and titrable acidity in storage temperatures of 23°C.

Figure 2: Peppers exposed to blue and red light were firmer, but differences exist between cultivars, Liu et al., 2022. (Image credits: https://doi.org/10.3390/

foods11172712)

Firmness

Firmness is another essential sensory quality parameter, and as fruits ripen, they grow softer. The effect of LEDs on fruit firmness depends on species and cultivars. For example, blue and red LEDs kept pepper fruits firmer to extend shelf life, though the light color varies depending on cultivars. For instance, P1622 peppers were the firmest in blue light, Hangjiao-2 peppers were treated with red light, while Xinxiang-2 performed best in darkness, see Figure 2. If people want softer fruits, choosing other colors, such as white light in peppers would be necessary.

Physiological characteristics

Various physiological processes, like respiration, transpiration, and senescence, continue in harvested fruits, causing quality problems.

Weight Loss

Blue light causes stomata to open, increasing respiration and transpiration or loss of water. The result is a loss in fruit weight, leading to quality deterioration. For instance, strawberries stored at 5°C and treated with blue LEDs began to show a higher respiration rate after four days.

Kumato cherry tomatoes stored for 13 days at 5°C lost as much as ∼70% weight compared to darkness. Even though LEDs emit less heat than conventional light, they raise tray temperature by ∼2°C. Continuous light could also trigger oxidative stress that causes weight loss. Red and far-red light caused the most weight loss; blue light had the least weight loss of 25%, as shown in Figure 3. The weight loss increased over time, even in refrigerated conditions.

Figure 3: “Weight losses (%) of Kumato® cherry tomatoes stored during 13 days at 5°C under several illumination treatments,” Martínez-Zamora et al., 2023. (Image credits: https://doi.org/10.1016/j.lwt.2022.114420)

Senescence

Senescence or aging causes fruit quality deterioration due to loss of nutrients and biochemical and structural changes. Oxidative stress produces reactive oxygen species (ROS) that degrade proteins, lipids, and membranes, eventually leading to cell death, so senescence reduces shelf life.

LEDs of suitable intensity and wavelength can delay senescence. Blue and white LEDs are ideal for lowering pigment change, and they have less effect on respiration and transpiration.

Low fluence rate light below the light compensation point (when there is enough light for photosynthesis) can be better for storage than darkness. LEDs can reduce yellowing in mandarin and kiwifruit.

Biochemical Production

Light wavelength and intensity determine the biosynthesis of secondary metabolites. LED treatment helps the accumulation of secondary metabolites, such as vitamins, phenolic compounds, chlorophyll, total soluble solids, anthocyanin, phenols, and carotenoids.

Fruits react to light with other triggers like temperature, humidity, duration, and carbon dioxide. Blue and red LEDs increase secondary metabolites. The blue light stimulates enzymes phenyl-alanine ammonia-lyase (PAL), which are involved in synthesizing and accumulating secondary metabolites. Red light increases the accumulation of tocopherols and terpene.

Phenolic Compounds: Plants produce phenolic compounds responding to abiotic and biotic stresses. UV-B light acts as stress and triggers the production of phenols, like flavonoids, flavonols, and tannins. In blueberries, medium fluence (40 W m−2) blue light increases total phenolic content after treatment.

Anthocyanin Content: Various fluences and wavelengths affect the anthocyanin content of fruits.

• Blue, red, and green and fluence of 20 to 60 W m−2 increased anthocyanin in blueberries.
• UV-B, blue, white, and green LED increase anthocyanin in sweet cherries, see Figure 4.

Antioxidants: Having more antioxidants and antioxidant enzymes can help fruits neutralize ROS produced by stress, which leads to senescence and improve shelf life. Antioxidants can be vitamin E, polyphenols, carotenoids, and reduced glutathione; enzymes are glutathione oxidase, ascorbate oxidase, catalase, glutathione oxidase, and superoxide dismutase. Once again, the LED effect is species-specific.

• High fluence blue LEDs increased antioxidants in strawberries and blueberries stored in refrigerated conditions.
• A combination of red and blue light decreased carotenoids in citrus fruit.

Figure 4: “Postharvest irradiation with blue light increases anthocyanin content in sweet cherries,” Kokalj et al. 2019. (Image credits:https://doi.org/10.1016/j.postharvbio.2018.11.011)

Nutritional Attributes

Several secondary metabolites like sugars, vitamins, anthocyanin, total phenolic compounds, and antioxidants are good for fruits and people. When LEDs increase secondary metabolites, they improve the nutritional quality of fruits.

Microbial Spoilage of Fruits

LED exposure can reduce microbial spoilage and delay. The effect depends on genotype and cultivar.

• Blue LEDs at a fluence rate of 40 W m−2 inhibit fungal pathogenic microorganisms through photodynamic inactivation of the microbes. They reduce the mycelial growth of Penicillium digitatum, italicum, and Phomopsis citri and rotten areas. For example, blue, green, and red LEDs inhibit the three fungi in sweet orange, tomatoes, and tangerine fruits.
• Blue, green, and red LEDs had antibacterial action against Escherichia coli, Salmonella typhimurium, and Staphylococcus aureus.
• UV has bactericide effects and limits microbial growth.

Shelf-life Improvement

By delaying ripening, texture, color, and TSS development, promoting antioxidants to control senescence, and reducing microbial spoilage, fruits can last long and maintain quality without chemical treatment. The LED treatment is often effective even at room temperatures around 20^oC, possibly reducing the need for expensive and energy-intensive cold storage.

At the end of the storage period, chosen light spectra can aid in hastening ripening and quality parameters’ development to satisfy consumer demands.

More Information

The research in the postharvest application of LEDs, for improving fruit shelf life and quality, shows that LED spectra and fluence requirements are specific to species and cultivars. To be able to use this novel technique, more studies, and trials will be necessary. Time series experiments with repeated observations of the same set of fruits will need non-destructive and precise tools. Felix Instruments Applied Food Science has a series of fruit Quality meters for the purpose:

• F-750 Produce Quality Meter
• F-751 Avocado Quality Meter
• F-751 Kiwifruit Quality Meter
• F-751 Mango Quality Meter
• F-751 Melon Quality Meter

These tools collect near-infrared spectroscopy (NIR) based data and analyze them in real-time to give results in a few seconds. These are industry standards that scientists and the supply chain rely on for monitoring and control of fruit quality.

Sources

Aphalo, P. (2014). Re: What is the definition and difference between light dose and fluence rate? Retrieved from: https://www.researchgate.net/post/What_is_the_definition_and_difference_between_light_dose_and_light_fluence_rate/52ee55b6d039b1093d8b464c/citation/download.

Huang, J.-Y., Xu, F., & Zhou, W. (2018). Effect of LED irradiation on the ripening and nutritional quality of postharvest banana fruit. Journal of the Science of Food and Agriculture, 98(14), 5486–5493. https://doi.org/10.1002/jsfa.9093

Kokalj, D., Zlatić, E., Cigić, B., & Vidrih, R. (2019). Postharvest light-emitting diode irradiation of sweet cherries (Prunus avium L.) promotes accumulation of anthocyanins. Postharvest Biology and Technology, 148, 192–199. https://doi.org/10.1016/j.postharvbio.2018.11.011

Liu, C., Wan, H., Yang, Y., et al. (2022). Postharvest LED Light Irradiation Affects Firmness, Bioactive Substances, and Amino Acid Compositions in Chili Pepper (Capsicum annum L.). Foods, 11, 2712. https://doi.org/10.3390/foods11172712

Martínez-Zamora, L., Castillejo, N., & Artés–Hernández, F. (2023). Effect of postharvest visible spectrum LED lighting on quality and bioactive compounds of tomatoes during shelf life. LWT, 174, 114420. https://doi.org/10.1016/j.lwt.2022.114420

Nassarawa, S. S., Abdelshafy, A. M., Xu, Y., Li, L., & Luo, Z. (2020). Effect of light-emitting diodes (leds) on the quality of fruits and vegetables during postharvest period: A Review. Food and Bioprocess Technology, 14(3), 388–414. https://doi.org/10.1007/s11947-020-02534-6

Poonia, A., Pandey, S. & Vasundhara. (2022). Application of light emitting diodes (LEDs) for food preservation, postharvest losses and production of bioactive compounds: a review. Food Prod Process and Nutr 4, 8. https://doi.org/10.1186/s43014-022-00086-0