Nutritional and Physiological Effects of Postharvest UV Radiation on Vegetables: A Review

Effective ultraviolet (UV) irradiation has been used as a postharvest technology to reduce decay, delay ripening, and delay senescence in crop products. In this review, the effects of UV radiation of different wavelengths and doses on physiological and phytochemical parameters in postharvest vegetables are discussed in summary, including appearance (color and texture), microbial load, respiration rate, enzymatic antioxidant system, and various bioactive compounds (phenolic compounds, carotenoids, chlorophylls, vitamins, glucosinolates, betalains, and antioxidant activities). In particular, postharvest UV radiation affects oxidative metabolism and increases the antioxidant activity of plant products, which could help delay yellowing and senescence of vegetables, trigger defense responses, and reduce decay and diseases. In some cases, irradiation stimulates the synthesis of bioactive secondary metabolites that may improve the nutritional value of vegetables. The findings presented in this review are very useful and valuable for the preservation and improvement of the nutritional quality of vegetables after harvest. It will also provide scientific support for industrial and commercial applications of UV radiation in postharvest.


■ INTRODUCTION
Vegetables are rich in phytochemicals, which are considered essential components of a healthy human diet and are beneficial to the human body. Studies have shown that there is a link between a high intake of vegetables and fruits as part of the daily diet and a lower risk of many diseases such as heart disease and stroke. 1−3 In recent decades, consumer awareness of food quality has been growing and the demand for fresh, nutritious, and minimally processed vegetables rich in healthpromoting ingredients has progressively increased. However, freshly harvested vegetables are highly susceptible to qualitative and quantitative deterioration in sensory and nutritional properties and loss due to microbial or enzymatic spoilage. 4 Preservation technologies and postharvest quality modification have become the focus of research. 5 Ultraviolet radiation (UV; 100−400 nm, including the UV-C (100−280 nm), UV-B (280−315 nm), and UV-A (315−400 nm) wavebands) comprises a relatively small portion of the total solar radiation. The effects of UV-B and UV-A in plant growth and biochemistry were shown in a number of studies. In this context, there has recently been an increasing number of studies showing the beneficial effects of ultraviolet (UV) light on the postharvest preservation of vegetables. The effects of UV treatment on a particular vegetable product depend on many factors, including the type of UV wavelengths (UV-A, UV-B, or UV-C), intensity, dose, stage of maturity or development of the vegetable, plant species, variety, and uniformity of treatment. 5,6 Postharvest treatment of vegetables with relatively moderate doses of UV radiation, especially UV-B and UV-C, has been shown to be effective in delaying ripening and senescence, reducing decay, and even improving the quality of vegetables such as broccoli, 7,8 leafy vegetables, 9−11 cucumber, 12,13 tomato, 14 water bamboo, 15 bell pepper, 16−18 etc. Few studies have addressed postharvest UV-A treatment, although it also has the potential to induce phytochemicals and increase the antioxidant activity. In pigeon pea leaves, UV-A was found to have a relatively weak effect on flavonoids and stilbenes compared to UV-B and UV-C radiation. 19 Different visible wavelengths and doses of UV-A also have an effect on biologically active compounds and antioxidant activity of tomatoes. 20 Results showed that UV-A treatment at a wavelength of 365 nm had a positive effect on bioactive plant metabolites, such as lycopene and phenolic compounds, and antioxidant activity. With an irradiation duration of at least 180 min, UV-A irradiation was able to achieve a safe effect, enriching photosynthetic pigments, especially carotenoids, activating antioxidant activity, and inducing flavonoids. 20 However, our understanding of the basic effects of postharvest UV radiation on both the physiology and quality traits of vegetables is still incomplete. A better understanding of the mode of action of UV treatment on the physiological metabolism of vegetables will be helpful in developing more efficient postharvest radiation treatments. Here, we have summarized recent advances in the effect of UV on postharvest  and whiteness index greenness of control was noticeable on day 4 and turning completely green on day 8; L* value and whiteness index were maintained by the UV-C treatment, whereas the control decreased markedly during storage, and the difference was significant after day 4 until the end of storage Hue and chroma values of control markedly increased during storage, vegetable quality and recent applications of UV-A, UV-B, and UV-C in postharvest vegetable storage, including effects on color, various phytonutrients, including phenolic compounds, carotenoids, vitamins, minerals, glucosinolates, and betalanines, and key quality indicators divided into texture, respiration rate, chlorophyll metabolism, and enzymatic antioxidant system. The long-term goal is to develop irradiation protocols. The extent to which extrinsic factors, such as UV radiation itself and its environmental conditions or intrinsic factors originating from the vegetable itself, affect the quality parameters must be examined.

■ APPEARANCE AND MICROBIAL DECAY
Color. The color and texture of vegetables are decisive factors for the consumer to buy one vegetable or another. UV irradiation of vegetables should have a positive effect on color, especially for red varieties containing anthocyanins, as anthocyanin synthesis can be stimulated by UV radiation. However, at high UV irradiance levels, it is important to check that there are no bleaching effects or texture changes due to water loss caused by radiation-induced heat generation. The aim is that at least if no positive effect can be achieved, ultimately, no change in the desired color and texture will occur.
The inevitable aging of vegetables after harvest is associated with a shorter life span. We have summarized previous studies on the effects of postharvest UV irradiation on color in various vegetables (Table 1). UV-C irradiation can reduce aging symptoms such as visible yellowing in broccoli and cucumber. 12,13 Reduced postharvest senescence in broccoli by UV-C treatment was explained by reduced chlorophyll degradation at all UV doses (4.0−14.0 kJ m −2 ). However, the highest UV dose resulted in increased phaeophytin content, a degradation product of chlorophyll. 12 High phaeophytin content is responsible for a color change toward graying. Several processes are responsible for the increase in phaeophytin: for example, membrane disorganization and progressive destruction of thylakoids in the chloroplast. Costa et al. 12 argue that this is mainly caused by high doses of UV light. The study on cucumber also showed an altered surface of the irradiated side, which hardened without wilting and showed a damaged cuticle layer. 13 In a study on romaine lettuce, brightness decreased with increasing UV-C dose, in a dose−response relationship starting at the second irradiation dose (12,24,36,54 and 72 kJ m −2 ), while yellow and green colors did not decrease, although this was expected for the green color value at high UV-C doses due to chlorophyll degradation. 21 The authors argue that higher doses may cause nonenzymatic browning responses. High UV-C doses (40.8 kJ m −2 ) also cause a decrease in brightness and green color and an increase in yellowing in iceberg lettuce during seven days of storage. This is indicative of faster aging and thus loss of the desirable light green color. 22 In the same study, lower UV-C doses (2.0, 4.1, and 20.4 kJ m −2 ) had no effect on the brightness, green, or yellow color values. Similarly, no change was observed in Chinese cabbage irradiated with a low dose of 1.8 kJ m −2 . 23 UV-C treatment may also prevent the increase in green coloration of water bamboo during eight days of storage at 10°C or two days of storage at 20°C. The swollen, crisp, white stems of water bamboo, better known as Manchurian wild rice, are consumed raw or cooked after the outer green layer is removed. Color change was measured as brightness, and UV-C irradiation of 4.2 kJ m −2 caused the best inhibition of color change (1.1, 2.1, 3.2, 4.2, 5.3, and 6.4 kJ m −2 ). 15 UV-A and UV-B also have the potential to retard chlorophyll degradation. In a study by Aiamla-or et al. 7 on broccoli, UV-B doses of at least 8.8 kJ m −2 showed the best effect in preventing yellowing of flower heads. At a lower UV-B dose of 4.4 kJ m −2 or similar UV-A doses of 4.5 and 9.0 kJ m −2 , more yellowed flowers occurred during the six days of storage. Similar results were obtained when broccoli was irradiated with a similar UV-C dose and stored in the dark at 22°C for five days (10 kJ m −2 ). 24 However, lower UV-B doses may be more suitable for longer storage periods. During 21 days at 4°C, a lower UV-B dose (1.5 kJ m −2 ) delayed yellowing of broccoli flower heads best compared to the high doses (7.2 kJ m −2 ) or the control. 8 However, this could also be a cultivar effect, as the cultivar "Diplomat" was used in the study by Duarte-Sierra et al. 8 In the study by Aiamla-or et al. 7 the two cultivars were compared and the inflorescences of "Pixel" yellowed faster than those of "Sawayutaka". Apparently, the applied intensities of UV treatment have a great influence on color. Darréet al. 25 showed in broccoli that in a short storage experiment of 18 h at 20°C, no effect of UV treatment could be detected. However, when stored at 4°C for 21 days, color results were best in flower heads treated with a low dose (2.0 or 4.0 kJ m −2 ). Medium and high doses showed values comparable to the control (up to 12.0 kJ m −2 ). 25 Ripe green tomatoes also retained their green color better after UV treatment, as untreated fruits lost their green color much faster than UV-B treated fruits (10.0−80.0 kJ m −2 ). 26 Thus, UV-B treatment may help to extend the storage time. Studies on several bell pepper cultivars showed no color change, regardless of dose (between 2.2 and 7.0 kJ m −2 ) or over extended storage periods of up to 18 days. 16−18 Tomatoes irradiated with low UV-B (0.6 and 1.1 kJ m −2 ) also showed little effect on color development during 21 days of storage. 27 UV-A and UV-B radiation is generally higher in the tropics, and tropical fruits may be adapted to higher UV radiation. However, the yellow and red colors of these fruits are carotenoids. No clear trend due to UV radiation in postharvest was identified for carotenoids (see the Carotenoids section). Nevertheless, carotenoids have a protective effect in the presence of excessive light by quenching reactive oxygen species (ROS). 28,29 These results suggest that high doses of irradiation may accelerate the change to undesirable colors, which are a sign of wilting or spoilage to the consumer. Lower doses have no effect on color parameters or stabilize natural colors over time.
Adjusted UV-C doses have the potential to prevent color degradation in green vegetables.
Texture. The effect of UV radiation on texture, more specifically firmness, has been investigated in only a few studies. Low and medium UV-C doses (2.0, 4.1, and 20.4 kJ m −2 ) had no effect on the firmness of iceberg lettuce during the seven days of storage. Iceberg lettuce treated with the highest UV-C dose (40.8 kJ m −2 ) had lower levels from the first day and was thus softer. 22 Higher UV-C doses (648 kJ m -2 ) showed the same softening effect as the control in bell peppers during 21 days of storage. 16 Lower UV-C doses (64.8 and 194.4 kJ m -2 ) showed less softening after 15 days of storage. In two other studies with peppers, medium UV-C doses (6.6 and 7 kJ m −2 ) resulted in a reduction of softening after several days of storage. 17,18 Similarly, reduced softening was obtained in tomatoes with a low UV-C dose (3.7 kJ m −2 ). 30 This study also analyzed the cell wall enzymes. The authors demonstrated that UV-C can reduce the activity of cell-wall-degrading enzymes and thus delay softening. Cutting energy and thus toughness of the tissue are other aspects of texture. In white asparagus, the cutting energy of all asparagus spears was increased within four days of storage, and most significantly by UV-C treatment (1.0 kJ m −2 ). 31 The effect was rather small, possibly due to the low UV-C doses. In another experiment by the same authors, the osmotic potential in white asparagus was also found to be increased by UV-C. 31 Nevertheless, the result is a consequence of a delayed increase in cell wall components (pectin and lignin) and a greater reduction in cell wall protein in UV-C treated asparagus compared to the control. Reduced lignin formation was also observed in water bamboo after the UV-C treatment. Again, the cutting force of the control increased and remained higher than that of the UV-C treated samples during eight days of storage. 15 UV-B radiation can also reduce the formation of cellulose and lignin. This was shown in bamboo, where during 15 days of storage, the cellulose content decreased from the third day. Lignin concentration decreased on the sixth day. The strength itself increased in both treated and untreated samples. However, UV-B treatment (8 kJ m −2 ) resulted in lower firmness from day nine compared to the control. 32 High UV-B doses (20 and 40 kJ m −2 ) have the potential to increase firmness during prolonged storage periods from day 14 until the end of storage. Untreated tomatoes with a very high dose (80 kJ m −2 ) showed the lowest fruit firmness compared with lower doses. 26 UV-C and UV-B thus have two main effects on fruit softening. They can reduce postharvest hardening by reducing the formation of cellulose and lignin. On the other hand, UV-C and UV-B irradiation can cause vegetables to retain their firmness longer compared to untreated vegetables. This effect is due to a reduction in the activity of cell-wall-degrading enzymes. High doses of UV-C can cause excessive tissue damage, leading to wilting or spoilage. No studies on vegetables are known about the effect of UV-A on the tissue firmness.
Microbial Load. An important aspect of UV irradiation is to extend the shelf life by reducing the microbial load of food. UV radiation has been shown to cause DNA strand breaks and oxidative damage to lipids in microorganisms and to increase intracellular ROS levels in microorganisms. 33 Inactivation of pathogens in drinking water and wastewater is already being used commercially. 34,35 So the question is not whether it is possible to inactivate microorganisms but whether the inactivation is uniform enough for vegetables such as lettuce that have layers and spaces between them. Another consideration is the preservation of desirable quality characteristics, which should not be compromised.
UV irradiation at postharvest has been shown to play an important role in reducing microorganisms on vegetables, such as Escherichia coli, Listeria pseudomonas, Salmonella typhimurium, and Listeria monocytogenes ( Table 2). Irradiation with UV-C resulted in a reduction of the initial microbial load in almost all cases, as expected. 9,21,22,36 This reduction was observed in leaves such as spinach, 37,38 iceberg lettuce, 22,36 romaine lettuce, 21 and amaranth, 39 but also in fruits such as bell peppers 40 and tomatoes. 36 Several studies also showed that longer duration and/or higher intensity of UV irradiation had greater efficiency in reducing microbes. 21,22,37,38 In a few samples, initial irradiation had no effect on microbes: e.g., white asparagus 31 or iceberg lettuce. 36 The authors of the study on asparagus used a low dose (1.0 kJ m −2 ) and stated that the initial microbial load was already very low and therefore disinfection was less effective. 31 Huang and Chen 36 used a water-assisted UV system for lettuce disinfection (13 or 28 mW cm −2 ) and showed reduction rates comparable to those of other authors. It remains to be assumed that the disinfection effect of UV radiation could be reduced by water but is still effective. Overall, UV radiation has a good disinfection effect on vegetables regardless of whether they have different layers or interstices.
The effectiveness of UV-C irradiation in reducing microbes in various vegetables has already been studied in detail. The microbial load after UV-C irradiation during storage was quite variable. In some cases, bacterial and yeast counts were as high as in the control after a few days, e.g. in amaranth 39 or spinach. 41 Baby spinach leaves showed no difference from the control in Pseudomonas marginalis after two days and in Salmonella enterica after five days. 42 In the same study, Listeria monocytogenes contamination was reduced for 12 days. 42 Bell pepper irradiated with a low dose of UV-C (0.88 kJ m −2 ) were able to be stored much longer due to the reduction in fungal contamination. 40 Also lettuce of the cultivar Lollo rosso also showed reduced microbial contamination by day eight at higher irradiation doses (4.1 or 8.1 kJ m −2 ). 11 The authors argue that more microbes are reduced by a lethal UV-C dose at higher irradiation doses. On the other hand, high UV-C doses can also cause tissue lesions. As a result, microbes can easily invade the tissue through the lesions, leading to more rapid deterioration. Therefore, for commercial UV irradiation of vegetables, a range should be selected in which microorganisms are greatly reduced without damaging the tissue.
Few studies were found on the antimicrobial effects of UV-A and UV-B irradiation after harvest. Recently, the combination of UV-A light irradiation with various organic acid treatments has been investigated as a novel antimicrobial treatment for fresh and minimally processed crop products. 37 UV-A irradiation in combination with acetic acid or phenolic acids such as gallic acid and ferulic acid reduced the inoculation of microorganisms on fresh spinach. 37,43 UV-A irradiation is able to inactive foodborne pathogens by inducing the formation of free radicals and singlet oxygen with photosensitization by type I and II mechanisms. 44 However, UV-A cannot inactivate microbes as effectively as UV-C light 45 and therefore must be supplemented with other antimicrobial treatments to be effective. Combining UV-A irradiation with some food-grade organic acids is a very novel treatment that could have a potential synergistic antimicrobial effect.
Respiration. High respiration rates of harvested vegetables lead to faster senescence and thus a shortened shelf life. However, not many studies have investigated the relationship among UV radiation, respiration, and ethylene production.
There is no clear trend between UV-C radiation and the respiration rate. Rather, the opposite is true, as all possibilities are shown during storage: increase in lettuce, 10,11 decrease for carrot, broccoli, asparagus, or pepper, 12,18,31,46 or no effect of spinach and lettuce. 9,41 The inconsistency was also found within the lettuce species. In "Lollo rosso" and "Red oak", increasing respiration rates were observed with increasing UV-C dose (range 0.4−8.1 kJ m −2 ). 10,11 In contrast, in iceberg lettuce, UV-C irradiation showed no effect (0.1−0.5 kJ m −2 ), 9 even though the doses were low and it was water-assisted UV-C radiation. UV-C radiation had no effect on the respiration of  Salmonella radiation reduced initial bacterial colonization (d0); no difference after 12 days for Listeria, 2 days for Pseudomonas and 5 days for Salmonella spinach leaves in two different studies. In the first study, the doses were higher (4.5−11.4 kJ m −2 ) and the period studied was up to 13 days. 41 In a study by Collazo et al. 9 the doses were relatively low (0.2 and 0.3 kJ m −2 ) and the storage period was only up to eight days, yet the respiration rate was not affected. Costa et al. 12 and Vicente et al. 18 argued that increased respiration is a sign of cellular damage from UV radiation. Thus, if UV-C irradiation caused little or no cellular damage, then respiration would not be altered. Because respiration increased with increasing UV-C dose (1.2−7.1 kJ m −2 ) in a study of "Red oak" lettuce, 10 this indicates that the hypotheses of Costa et al. 12 and Vicente et al. 18 may be correct. However, Allende et al. 10 argues that UV-C light has the potential to activate several biological processes in higher plants, including stimulation of respiratory activity. Both theories explain very well why increasing effects occur. The lack of effect on iceberg lettuce can also be explained by the low dose (0.1−0.5 kJ m −2 ). 9 However, this does not explain why respiration decreases in some cases. Huyskens-Keil et al. 47 found a reduction in the physiological activity of asparagus spears. Also, Costa et al. 12 argues that lower UV-C doses have the potential to delay tissue damage, thereby reducing reparation for a period of time. Thus, the theory by Costa et al. 12 and Vicente et al. 18 may be correct.
There are only two studies investigating the effect of UV-B on respiration, and to our knowledge there are none on UV-A. The respiration rate of broccoli florets increased immediately after UV-B irradiation, but more dramatically at a high dose (1.5 and 7.2 kJ m −2 ). 8 By day seven, the two UV-B treatments and the control showed almost the same decreased respiration rate of about 180 nmol kg −1 s −1 CO 2 , which remained constant until the end of storage on day 21. Capia peppers showed no difference between the UV-B treatments (4.5 and 8.9 kJ m −2 ) and control throughout the 49 days of storage. 48 However, the respiration rate decreased during the first 21 days of storage and then increased. This increase could be a sign of aging. Again, these two studies did not reveal a clear trend but rather showed that UV radiation can cause cellular damage depending on the species.

■ EFFECTS ON BIOACTIVE COMPOUNDS
Antioxidant Activity Sum Parameters. Sum parameters can usually be analyzed much faster, and the costs for these tests are lower. Therefore, they are a good orientation when certain groups of substances are affected by a stressor. Predominantly, the total phenolic compound (TPC) and total antioxidant capacity (TAC) were analyzed after UV irradiation. In addition, in some cases, the total flavonoid content (TFC) and total carotenoid content (TCC) were analyzed. Phenolic compounds, which include flavonoids, are antioxidants. Among other things, they have a protective function against ROS within the plant cell (see Phenolic Compounds section). Consequently, oxidative stress in plants caused by UV radiation should increase TPC and TFC. TPC and TFC, together with other antioxidants, are part of the TAC, and therefore, TAC should also be increased by UV radiation.
A general positive trend was observed for TPC, TFC, and TAC upon UV irradiation, regardless of the UV wavelength. Increased TPC, TFC, and TAC levels after UV-C irradiation are found in green leafy vegetables, such as in pigeon pea leaves, 19 amaranth, 49 and red cabbage 50 (Table 3). A positive effect of UV-C on TPC, TFC, and TAC was also found in Table 2. ); UV-C treatments prolonged the shelf life by 3 days based on microbial growth; coliform bacterial growth was significantly inhibited when the highest UV-C radiation dose was applied; however, significant reductions were found at all the UV-C radiation doses from day 0 to 5, similar results were found in yeast; UV-C seemed to stimulate LAB growth and the highest increase was obtained at the highest UV-Cdoses

11
Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Review   total carotenoid content (TCC), TPC, TFC and TAC UV-C enhanced TCC content and increased continuously with storage, but no difference just after UV treatment, it showed significantly higher than control after 9 d; TFC content of UV-C treated sample was significantly higher than control at 3 and 6 days, no difference after that; TPC content seemed constant both on UV-C (6.6 kJ m −2 ) treated sample and control during 15 d storage; TAC content was similar on control and treated after UV-C treatment day 0, then it was significantly higher than control, and increased until day 6 and then remained constant for UV-C treated  multiple UV-C treatments indicated a great deal of enhancement in Vc content, which almost made this content remain at the initial level, by contrast, Vc contents in control and single UV-C treated leeks dropped rapidly to less than 0.02 mg/g.       15 total chlorophyll content UV-B treatment delayed floret yellowing in both cultivars. "Pixel" florets displayed yellowing more rapidly than "Sawayutaka" cultivar florets; Chl contents were significantly higher in "Sawayutaka" with UV-B treatment as compared to "Pixel" with UV-B treatment on day 6; Table 3. continued other vegetable forms such as tomato, 51,52 broccoli, 12,53 and water bamboo. 15 Similar positive trends were also detected in pigeon pea leaves, 19 broccoli, 25,53 kale, 54 tomatoes, 20,26 and carrots 55 after irradiation with UV-B and UV-A. The results of the studies above indicate that the vegetable form does not play a role in the effect. However, a dependence on extrinsic factors such as dose, cultivation, and UV wavelength was present. A significant difference in TFC between control and UV-C treatment was present on the fourth day for amaranth stored at 20°C, whereas it was present on the second day at 5°C . 49 Thus, lower temperatures may accelerate the increase in the number of flavonoid syntheses. In this experiment, the UV-C doses (1.7 and 3.4 kJ m −2 ) had little effect, as higher doses rarely showed a higher flavonoid concentration. In a study on pigeon pea leaves, UV-C elicited a more pronounced and earlier effect than UV-B and UV-A. 19 Also, in a study by Dyshlyuk et al. 20 longer UV-A irradiation times (180 and 360 min) at three different wavelengths (353, 365, and 400 nm) resulted in a stronger increase of TPC and TFC, while a short irradiation for 10 min had no effect. This suggests that a higher dose is required for longer wavelengths to elicit a similar effect. Tomatoes generally appear to tolerate higher UV doses. In a study by Liu et al. 26  However, this correlation was not found in other studies, such as in spinach, 41 peppers, 18 and red cabbage. 56 The UV-C dose for spinach was relatively high (4.5, 7.9, and 11.4 kJ m −2 ), so that TAC and TPC decreased even more drastically. 41 The authors argue that oxidative stress induced by high UV-C doses probably causes membrane damage and alters the cell composition. This reduced the content of antioxidant compounds. This phenomenon was also detectable in red cabbage with respect to the total anthocyanin content. There was a positive effect at lower UV-C doses (1.0 and 3.0 kJ m −2 ), but not at higher UV-C doses (5.0 kJ m −2 ) until the eighth day of storage. The oxidative stress induced by lower doses leads to a positive feedback loop with increasing levels of TPC, TFC, or TAC. Higher doses have a decreasing effect, as they are likely to lead to increased ROS levels and consequently cellular damage, thus decreasing TPC, TFC, or TAC. In addition, TAC was measured using two different FRAP and ABTS assays on red cabbage cultivar "Zi Guang" and was found to be little affected by increasing UV-C doses (1.0, 3.0, and 5.0 kJ m −2 ) during 12 days of storage. 56 Nevertheless, the two lower doses in particular were found to increase TAC levels during the first four days of storage. On day 12, both assays were affected differently, possibly a coincidence. 56 These results indicate that irradiation with intermediate and lower doses has the potential to increase the sum parameters of TPC, TFC, or TAC.
TCC showed a quite different picture, partly depending on the species or UV wavelength. Carrots irradiated with a low dose of UV-C (0.8 kJ m −2 ) initially showed a sharp decrease in TCC and then an increase in TCC during storage, just like the control samples. 46 A decrease after irradiation could be due to the antioxidant function of carotenoids, 29 as they were oxidized and no longer detectable. A delayed increase may be due to a positive feedback mechanism. In bell peppers treated with UV-C, TCC increased during storage (15−18 days), mainly due to ripening processes. 17,18 In the first study, UV-C samples (6.6 kJ m −2 ) showed increased levels the sixth day compared with the control. However, in the second study, the UV-C treated samples (7.0 kJ m −2 ) showed a smaller increase than the In red cabbage, the control initially increases, and after four days of storage, the UV-C-treated samples (1.0, 3.0, and 5.0 kJ m −2 ) show an increase in TCC. 56 Again, a dose dependence was present as the mean irradiation dose (3.0 kJ m −2 ) provided the highest values, but all values were well below the control. The authors argue that UV-C can cause photobleaching and degradation of carotenoids, decreasing the total concentration. In wild rocket, the highest TCC value was recorded with 0.2 kJ m −2 UV-B treatment for 45 s under scattered light film. 57 UV-B irradiation (1.4 kJ m −2 ) of carrot slices of different size showed no variation of TCC; however, the UV-B samples tend to be reduced at day three. 55 It is possible that the TCC value could be increased by a positive feedback mechanism. Longer doses of UV-A radiation could increase TCC in tomato. 20 A general statement on the influence of a longer wavelength is not yet possible, as further research is needed. However, results similar to those of UV-C can be expected if comparable doses are used. Vitamins. UV radiation affects various phytochemicals. However, very few studies have examined the vitamin content in vegetables after UV irradiation (Table 3). Ascorbic acid (AA), which serves as vitamin C in humans, is present in all plant cells and is the strongest contributor to the cellular redox state. 58 Therefore, it is an important quencher of ROS because its concentration exceeds that of other antioxidants. Other vitamins such as β-carotene, provitamin A, or tocopherols, vitamin E derivatives, are also potent antioxidants, both of which are fat-soluble. Carotenes protect plant cells by quenching triplet chlorophylls and ROS under excessive light energy conditions. 28 Tocopherols, on the other hand, are important for stabilizing membranes in plants. 59 In a study on leek, spinach, and cabbage, the reduction of AA was measured after single (2.46 kJ m −2 ) and multiple (5 × 2.46 kJ m −2 = 12.3 kJ m −2 ) UV-C irradiations. The reduction in AA during storage was less pronounced with multiple irradiations than with single or control irradiation. 60 In studies on cucumber (3.9, 7.7, 11.6 kJ m −2 ), water bamboo (4.2 kJ m −2 ), and bell pepper (6.6 kJ m −2 ) a correlation of AA and UV-C irradiation during storage was not present. 13,17 The author of the cucumber study explained the lack of an effect by drastic weight loss. The water loss caused by respiration leads to degradation of AA, making any possible influence difficult to detect. In a study of UV-C (10.0 kJ m −2 ) treated bell peppers AA, dehydroascorbic acid (DHA), and 2,2-diphenyl-1picrylhydrazyl (DPPH) radicals increased only on day 21 of storage. AA and DHA were not measured on day 14 of storage, but DPPH radical inhibition was already elevated. 61 The control samples showed increased wounding and AA is highly susceptible to oxidation. Nevertheless, the UV-C samples showed higher values, which can be attributed to better shape stability due to the UV-C treatment (see the Texture section) and the physiological response of the fruit. 61 A beneficial effect of UV radiation was also found in tomato in a review (2.0−8.0 kJ m −2 ). 14 Membrane damage during storage can lead to increased oxidation of AA. 14,62 UV-B irradiated parsley (42.0 kJ m −2 ) showed no difference from the control during 6 days of storage. 63 In ripe green tomatoes, UV-B irradiation had a negative effect after 37 days of storage, as the irradiated samples had 18% lower AA levels at the end of storage (10.0−80.0 kJ m −2 ). 26 In another study, only the tomato cultivar Money Maker was one of two varieties to show a positive trend for AA in flesh and skin. 64 Tomatoes received daily UV-B irradiation (6.1 kJ m −2 ) until they were fully ripe (red ripening stage). They were harvested at either the ripe green stage or the envelope stage and thus received a daily UV-B dose between 10 and 22 days. Money Maker is a common commercial cultivar, while the second variety is a mutant with high pigment content (hp-1) and produces higher amounts of lycopene. The hp-1 mutant showed no changes in skin or flesh, except for a lower AA concentration in the flesh of ripe green fruits. Nevertheless, a clear trend is not detectable for any UV species. It could be that lower values can also trigger a positive feedback mechanism, while high values cause too much cell damage, as explained by Andrade Cuvi et al. 61 However, further research is needed here, especially for UV-B and UV-A.
In a study of amaranth leaves, vitamin E was measured in the form of α-tocopherol. During storage, α-tocopherol was hardly affected by UV-C irradiation, except after 14 days at 5°C where leaves treated with UV-C (1.7 and 3.4 kJ m −2 ) had higher vitamin E content than the control group. 49 Storage conditions (20 or 5°C) had a greater effect than UV-C irradiation, as storage temperature (5°C) helped to maintain α-tocopherol content longer. 49 Plant physiological activities are reduced by low temperatures and thus also inhibit tocopherol synthesis, while high temperatures (20°C) promote synthesis. 49 The authors claim that this is an explanation not only for the difference in concentration at the beginning of storage but also for the late UV-C effect. Since UV-C radiation causes ROS production in leaves, this triggers tocopherol production only after some time. Further studies are needed, especially on the influence of UV radiation on the vitamin E content of vegetables. This study gives only a first impression. Again, further research is needed to investigate UV-A and UV-B radiation after harvest on vegetables.
Phenolic Compounds. Phenolic compounds are the largest group of secondary plant metabolites and are subdivided into several subgroups, such as phenolic acids, flavonoids, and tannins. 65 Their function in plants ranges from defense, especially against high radiation, to coloration. Therefore, phenols contribute to the overall fitness of plants. 66 They are also desirable as antioxidants in food. Thus, their induction by UV treatment would be a great benefit to the nutritional value of vegetables.
UV radiation has an enhancing effect on the phenolic compounds; especially UV-C leads to the accumulation of various phenolics (Table 3). 19,50,52,67−70 A review by Urban et al. 71 summarized that UV-C light induces phenylalanine ammonia lyase activity at the post-transcriptional level, along with various enzymes of the phenolic biosynthetic pathway, depending on the species and variety. Studies on carrots showed a simple increase in 6-methoxymellein (0.9 kJ m −2 ). 67,68 In contrast, studies analyzing several different phenolic compounds showed that accumulation is more complex. Wei et al. 19 found an increase in some phenolic compounds (naringenin, luteolin, and apigenin) with prolonged UV-C irradiation (0.9−3.6 kJ m −2 ) of pigeon pea leaves, whereas others initially increased and then decreased (orientin, apigenin-6,8-di-C-α-L-arabinopyranoside, pinostrobin, longistyline C, and cajaninstilbenic acid). One phenolic compound, pinostrobin chalcone, was not affected by UV-C irradiation. Similar results were published for a storage experiment with UV-C irradiated tomatoes (4.0 kJ m −2 ): Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Review Several phenolic compounds increase during storage (pcoumaric acid, caffeic acid, ferulic acid, chlorogenic acid, and protocatechuic acid), while some decrease (naringenin and quercetin). 51 In the same experiment, gallic acid and rutin initially increased during storage and then had levels similar to control samples after 35 days. Irradiation with UV-C (1.0, 3.0, and 5.0 kJ m −2 ) can also lead to the formation of new phenolic compounds: e.g., in a study on red cabbage control samples accumulated 11 anthocyanin compounds, while samples treated with UV-C had 15 compounds. 50 50 In another study, the storage condition also played a role, as flavonoids increased at 5°C for all doses (1.7 and 3.4 kJ m −2 ) on the second day. At the highest dose at 20°C, there was no increase until day four. 49 In the same study, all phenolic acids were increased regardless of the dose and temperature. The authors expected a higher increase at 20°C because higher temperatures increase enzymatic activities and, thus, accelerate synthesis. However, they argue that responses to UV-C, especially flavonoid accumulation, depend on the physiological stage of development as well as the species and cultivar. An increasing effect of UV radiation on phenolic compound concentrations is already more or less common sense, as described in reviews. 72,73 Nevertheless, the question remains why only some phenolic compounds are increased by UV irradiation and not all. Because of the great diversity of phenolic compounds, they have different roles in the plant, such as defense mechanisms against pathogens, parasites, and predators, reproduction and growth, and contribution to plant color. 66,74 Consequently, only those phenolic compounds that help to remedy the destructive effects of UV radiation are increased, especially those with high antioxidant activity to quench ROS. Irradiation with UV-B or UV-A showed trends similar to that of irradiation with UV-C. In one of the studies mentioned above, the effect of UV-B and UV-A on pigeon pea leaves was also investigated. 19 Some phenolic compounds increase more with UV-B irradiation (pinostrobin chalcone, pinostrobin, longistyline C, and cajaninstilbene acid). Some of the phenolics that increased with increasing UV-C dose were not affected by UV-B (0.2−0.9 kJ m −2 ) (naringenin, luteolin, and apigenin). 19 Irradiation with UV-A (0.05−0.22 kJ m −2 ) showed similar trends to UV-B, but less marked. 19 In spinach, UV-B and UV-A have different effects. After three days of irradiation with UV-B (42.0 kJ m −2 ), the levels of p-coumaric acid increased, and jaceidin was newly formed. The same treatment with UV-A (165 kJ m −2 ) showed no difference from the control samples. 63 UV-A and UV-B light has an enhancing effect on flavonoids after three days of irradiation: the content of kaempferol glycosides increased in radish shoots. In parsley, the aglycone apigenin increased 7-fold under UV-B and 2.5fold under UV-A compared to the control. Vitexin content in Indian spinach increased with UV-B irradiation on the third day and with UV-A irradiation on the sixth day. 63 This study clearly shows that flavonoids increase after UV treatment and thus play a protective role against UV-B and UV-A radiation. However, flavonoids increase differently in different species due to individual adaptation to abiotic stress. Also, in a study on broccoli, only 1-sinapoyl-2-feruloyl gentiobiose was increased, whereas other hydroxycinnamic acids remained unchanged (1,2-disinapoyl-2-feruloyl gentiobiose, 1,2,2-trisinapoyl gentiobiose, 1,2-diferuloyl gentiobiose, and 1,2-disinapoyl gentiobiose) regardless of the applied UV-B doses (1.5 and 7.2 kJ m −2 ). 8 In a study on carrots, the effect of a different pruning style before UV-B irradiation (1.4 kJ m −2 ) was analyzed. An increase in chlorogenic acid was observed in all carrots after three days of storage following UV-B irradiation. 55 The different cut types had an effect, as carrot chips responded the most and baby carrots the least. This may be due to the large surface area to volume ratio and greater wounding of the carrot chips. This suggests that pretreatment affects the sensitivity to UV radiation. UV-B irradiation was also able to stimulate the synthesis of newly formed phenols in white cabbage, a plant low in hydroxycinnamic acids (mainly sinapic acid derivatives) and lacking flavonoids. 75 After 2 or 4 days of UV-B irradiation (13.0−17.0 kJ m −2 ), newly formed hydroxycinnamic acid glycosides such as coumaroyl glycoside, feruloyl glycoside, and caffeoyl glycoside were found. De novo formation of the flavonoid quercetin triglycoside was also higher in the outer leaves than in the newly formed hydroxycinnamoyl glycosides. The authors suggest that this is due to the activation of various enzymes in the biosynthetic pathway of polyphenols under UV-B treatment after harvest. Among others, phenylalanine ammonia lyase and cinnamate 4hydroxylase are activated for the formation of early phenols and, in addition, hydroxylation and methylation reactions are promoted. 75 The reduced and altered synthesis of phenolic compounds by UV-A and UV-B radiation is expected, as these wavelengths are less energetic and therefore cause less cell damage and ROS production. Therefore, the need to synthesize protective phenolic compounds is less. UV-A and UV-B are part of natural radiation, but UV-C is blocked by the ozone layer. Therefore, plants have special adaptation mechanisms to these UV wavelengths. Nevertheless, UV-C and UV-B and UV-A seem to be effective in increasing the content of phenolic compounds in the postharvest.
Carotenoids. Carotenoids are pigments that range in color from yellow to red. The color is the result of the physical property of a polyene chain with multiple conjugated double bonds that act as a chromophore. 29 Some carotenoids, such as β-carotene, lutein, and zeaxanthin, are involved in light collection at the photosynthetic membranes of the chloroplast. In the presence of excessive light, they can protect the photosynthetic apparatus by quenching triplet chlorophylls, singlet oxygen, and ROS. 28,29 Induction of carotenoids in vegetables is beneficial for human health.
There have not been many studies analyzing individual carotenoids after UV irradiation in vegetables ( Table 3). The few studies that investigated the effect of individual carotenoids mostly used tomato. They were already reviewed with no clear trend. In the review, 14 several studies found a reduction in lycopene and β-carotene with UV-C radiation (3.7−4.2 kJ m −2 ), while other studies found no effect (4.2−6.1 kJ m −2 ).

Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Review
Two of the eleven studies in this review found an increasing effect, but mainly for lycopene (1.0−13.7 kJ m −2 ) 14 One of these studies found a positive effect for lycopene, β-carotene, and lutein concentration in the skin of a tomato variety, in addition to an increase for lycopene in the flesh. In contrast, no effect was observed in a particularly lycopene-rich mutant (Hp-1). The two cultivars studied received 6.1 kJ m −2 UV-B daily after being harvested immature until they were fully ripe. 64 This study showed that the cultivar choices may be one of the main reasons why conflicting results sometimes occur. In a more recent study, increases in lycopene, β-carotene, and lutein were found in several cases at different UV-A wavelengths. However, this was significant only when a long irradiation time such as 180 min (2.0 kJ m −2 ) or 360 min (4.0 kJ m −2 ) was used. 20 There were two studies by Gogo et al. on amaranth and African nightshade. 39,49 UV-C irradiation initially decreased the content of carotenoids, but as the storage time progressed, the content increased in amaranth and African nightshade. Higher irradiation (3.4 kJ m −2 ) resulted in higher β-carotene and lutein content on days 2 and 4, whereas lycopene content increased on day 4 and decreased thereafter. 49 These contradictory effects may be due to a variety of reasons. Several studies have shown that different tomato cultivars respond differently to abiotic stress. 76−78 In addition, some of the carotenoid synthesis steps are light-mediated, such as carotene isomerase or lycopene-β-cyclase. 14 The latter converts lycopene to β-carotene, which only reduces the lycopene content. This explains why some studies found an increasing effect but most found a decreasing effect. The ability to suppress excessive light and ROS as an antioxidant is most likely responsible for their degradation. There is little evidence on the effects of UV-B and UV-A radiation on postharvest tomatoes, except for the studies cited. Therefore, further research is needed for tomatoes and especially for UV-B and UV-A. Chlorophyll. The chlorophyll molecules are the most abundant molecules and are essential for photosynthesis, which allows plants to absorb energy from light. In photosynthetic organisms, chlorophylls a and b predominate. Decolorization would mean a significant loss of quality in vegetables.
UV-C radiation has the potential to inhibit chlorophyll loss in leaves and inflorescences. In spinach, broccoli, leeks, and cabbage, degradation is reduced by UV-C irradiation (Table  3). 12,24,41,60 Higher irradiance levels are better at preventing chlorophyll degradation (4.0−14.0 kJ m −2 ). 12,41 However, the highest dose (14.0 kJ m −2 ) delayed chlorophyll degradation while increasing the pheophytin content. This resulted in a surface color similar to that of the control samples after a few days of storage. 12 In this study, decreased chlorophyllase and MDS activity were observed but not chlorophyll peroxidase activity. The authors relate the decreased enzyme activity to UV-C irradiation, as has been shown in other studies. 12 Also in a study by Liao et al. 60 multiple irradiations (5 × 2.46 kJ m −2 = 12.3 kJ m −2 ) showed the best results in preventing chlorophyll loss in leeks, spinach, and cabbage, while single irradiation (2.46 kJ m −2 ) did not prevent degradation or did so to a lesser extent. Prolonged storage (10 days or longer) increased chlorophyll content in amaranth and African nightshade, while shorter storage times at 20°C showed no effect in African nightshade and there was an increase in chlorophyll a and b in amaranth (1.7 and 3.4 kJ m −2 ). 39 Contrasting results were seen in a study of cucumbers (4.0−14.0 kJ m −2 ), in which irradiated samples had lower chlorophyll content from the third day of storage to the end of the experiment (tenth day). 13 However, on the day of irradiation, the values of the irradiated cucumber fruit samples were higher than those of the control. The authors suggested that a high respiration rate due to high turnover may be responsible for chlorophyll degradation. 13 In conclusion, UV-C radiation has a great potential to inhibit chlorophyll degeneration. However, it may also inhibit chlorophyll biosynthesis in water bamboo. In this vegetable, the green outer layer is peeled, and it should remain white. The content of chlorophylls a and b was higher in the control samples than in the UV-C treated shoots. The author claims that this shows that UV-C treatment (4.2 kJ m −2 ) can inhibit chlorophyll biosynthesis in water bamboo. 15 There are few studies that have investigated the effect of UV-B and UV-A irradiation on chlorophyll in vegetables after harvest. There is one study with UV-A irradiation on three red ripe tomato cultivars. 20 Longer irradiation periods with different UV-A wavelengths led to an increase in total chlorophyll in all cultivars. This suggests that UV-A may also have a protective effect against chlorophyll degeneration, although only trace amounts of chlorophyll are detectable in red ripe tomatoes. 20 In tomatoes, plastids change from green chloroplasts to red chromoplasts during the ripening process. 79 This process was slowed by UV irradiation. Also, in two studies by Aiamla-or et al. 7,80 broccoli showed delayed yellowing of the flower after UV-B irradiation (8.8 to 19.0 kJ m −2 ). However, slight differences were observed. The flowers of the cultivar Pixel yellowed faster than the flowers of "Sawayutaka". This not only suggests that UV-B may have the same effect as UV-C but also shows that the choice of cultivar is important for the outcome. In a study of wild arugula, plants were grown under three different cover films with different UV-B transmittances, and fresh-cut arugula was irradiated with UV-B for 45 s after harvest. 57 For all three different films, UV-B treatment (0.2, 0.7, 1.5, and 3.0 kJ m −2 ) for 45 s resulted in higher chlorophyll a levels and was different from the other treatments of each film, also indicating a degradation of chlorophylls when a longer UV-B treatment was applied. 57 However, parsley showed no change in chlorophyll content compared with the control after UV-B and UV-A irradiation (42.0 kJ m −2 ). 63 Other phytochemicals measured, such as moisture and AA, were also not changed. This suggests that parsley may not be very susceptible to UV radiation. UV-B and UV-A need to be studied further to understand their effects on chlorophyll.
Glucosinolates. Glucosinolates (GLs) are also an important class of bioactive plant compounds. They are derived from amino acids and belong to a large group of secondary metabolites containing N and S. They have a common structure that includes a β-thioglucose group, a sulfonated oxime unit, and a variable aglycone side chain derived from the various amino acids. 81 Based on the amino acid precursors, GLs are mainly classified into three groups: aliphatic, aromatic, and indolic glucosinolates. 82 GLs are mainly abundant in the Brassicaceae family, including Brassica oleracea (i.e., broccoli, cabbage, cauliflower, kale, Brussels sprouts), Brassica rapa (i.e., turnip, Chinese cabbage, pak choi), Brassica napus (i.e., oil seedrape), Ranphanus sativus (radish), and Sinapis alba (mustard). 83 The biosynthesis of glucosinolates can be activated by biotic and abiotic stress. 84 Duarte-Sierra et al. 85 reported that the glucobrassicin content of broccoli florets increased by both 1.2 and 3.0 kJ m −2 UV-C treatments, and the concentrations remained nearly steady throughout the storage period compared to the control. At the same time, the concentrations of 4-hydroxyglucobrassicin (4-OH-GLB) in broccoli treated with 1.2 and 3.0 kJ m −2 UV-C radiation were higher by the end of the storage after 14 days compared to the control. 85 UV-B and UV-C light doses and storage times differentially tailor GLs profiles in broccoli florets, and UV-B could enhance the content of GLs after storage (Table 3). 8,25,85,86 It was reported that hormetic and higher doses of UV-B (1.5 and 7.2 kJ m −2 ) increased the total glucobrassicins by 18%, and 22%, respectively, relative to the control. 8 Aliphatic glucoraphanin showed the highest induction in response to UV-B exposure. 25 Besides, storage time has a positive effect on GL accumulation in broccoli florets. For instance, high-intensity (5.0 W m −2 ) and low-dose (2.0 kJ m −2 ) UV-B radiation in postharvest could decrease total GL content after 2 h, though a subsequent increase was observed and the highest levels of GLs were achieved after 18 h of UV-B treatment. 25 In Bimi broccoli, single UV-B and UV-B combined with UV-C radiation were applied and it was reported that all UV treatments induced higher glucoraphanin and glucobrassicin contents after storage compared to nonirradiated samples in such broccoli florets; among them 5.0 kJ m −2 of UV-B radiation induced the highest glucoraphanin/glucobrassicin in florets after 72 h regarding their respective initial levels. 53 On the contrary, another study showed that decreasing contents were observed for the glucosinolates glucobrassicin and 4-methoxyglucobrassicin in the UV-B treated (13.0−17.0 kJ m −2 ) white cabbage but there was formation of the degradation products dihydroascorbigen and dihydro-4-methoxyascorbigen, which might be related to cell shrinking as mechanical damage. 75 It can be concluded that UV radiation can increase glucosinolates.
Betalains. Betalains are an excellent substitute for thermolabile anthocyanins as food colorants. They contain two groups of water-soluble pigments: red-purple betacyanidins and yellow betaxanthins. Betacyanidins are conjugates of cyclo-dihydroxyphenylalanine and betalamic acid, and betaxanthins are conjugates of amines or amino acids and betalamic acid. 87 Betalains are also an important group of plant secondary metabolites whose accumulation can be triggered by abiotic factors or environmental stressors. To our knowledge, water and salt stress can induce the pigments, but there have been few studies on betalanines under UV stress on vegetables, none of which were conducted with UV-C (Table 3). 88,89 In the postharvest environment, a low UV-B treatment (1.23 kJ m −2 ) of the beetroots, followed by short-term storage for three and seven days, increased the ratio of betalain to vulgaxanthin in the beets without adversely affecting beet quality, whereas neither three and seven day storage nor UV-B irradiation altered the betalain content of "Monty Rz" and "Belushi Rz" beets. 87 However, after three and seven days of storage, vulgaxanthin I content in beets decreased, regardless of whether the samples were control or UV-B treated. 87 Betalain pigments may also play some role in protecting against UV stress, but further evidence is needed to support this hypothesis.
Enzymes. Enzymes act as biocatalysts and, thus, accelerate chemical reactions in living organisms. Like any other catalyst, enzymes do not alter the equilibrium and are not changed by chemical reaction. However, enzyme activity is altered by other molecules that act as either activators or inhibitors. UV radiation has the potential to inactivate enzymes by denaturing them and consequently reducing their activity. 90 UV-C treatment increased the activities of red cabbage antioxidant enzymes, especially after eight days of storage. Only at the beginning were superoxide dismutase and peroxidase increased at lower UV-C irradiances (1.0, 3.0, and 5.0 kJ m −2 ). 56 In the same study, H 2 O 2 was also measured; on the first day of storage, levels were increased at all UV-C irradiance doses compared with the control. After four days of storage, only the lowest dose showed increased H 2 O 2 levels, whereas no differences were detectable on the eighth day. This suggests that the increased activity of antioxidant enzymes reduces the levels of ROS, such as H 2 O 2 . In UV-C treated bell peppers, levels of catalase and ascorbate peroxidase increased immediately and after seven days of storage, while superoxide dismutase and guaiacol peroxidase increased only after seven days of storage. 61 In another study on bell pepper, some enzymes (catalase and superoxide dismutase) increased shortly after UV-C irradiation and remained until the twelfth or fifteenth day, while guaiacol peroxidase was increased only on the third day and ascorbic acid peroxidase was not increased at all. 17 Similarly, in water bamboo, only the enzyme catalase was increased after UV-C treatment during the eight days of the study, but significantly only from the fourth to the sixth day. Guaiacol peroxidase and ascorbate-dependent peroxidase were not affected by UV-C treatment (4.24 kJ m −2 ). 15 Again, it appears that the different species and cultivars do not respond uniformly to UV-C but have individually elevated activities of antioxidant enzymes for the detoxification of superoxide and hydrogen peroxide. Studies of UV-A and UV-B are missing in this context so far.
Phenolic biosynthesis enzymes are positively affected by UV radiation. In fresh-cut cabbage, the expression of most anthocyanin biosynthetic and regulatory genes was slightly upregulated by different UV-C doses in UV-C treated samples (1.0, 3.0, and 5.0 kJ m −2 ). 50 Most of the genes showed their maximal expression levels at day 12. There was a dose dependence, as most anthocyanin biosynthetic genes were most increased at a UV-C dose of 3.0 kJ m −2 . The gene responsible for the increased expression of anthocyanin biosynthesis was PAP1, which was immediately and dramatically upregulated by UV-C treatment in cabbage. This gene, PAP1, was reported to activate anthocyanin biosynthesis in Arabidopsis thaliana by regulating the expression of phenylalanine ammonia lyase and chalcone synthase, among others. The authors argue that it may also be responsible for the increase in cabbage. 50 In tomato, phenylalanine ammonia lyase was upregulated by UV-C irradiation (4.0 kJ m −2 ), among others such as cinnamate 4-hydroxylase or 4-coumarate CoA ligase. 51 Phenylalanine ammonia lyase was also upregulated in UV-C treated water bamboo compared with the control, but only on the second and eighth day (4.2 kJ m −2 ). 15 Again, most enzymes showed the highest concentration between the seventh and 28th days of storage. This positive influence of UV radiation on the enzymes of phenolic biosynthesis was also evident in the results of the phenolic compounds. Not all phenolics increased after UV irradiation, and some increased later than others. This shows the great variability with which plants respond to abiotic stress. One possible reason for a delayed increase is the denaturation of some enzymes by UV radiation. 90 However, this has been demonstrated for juices, so it may be less important for intact vegetables. Irradiation with UV-B (42.0 kJ m −2 ) also enhanced the phenolic biosynthesis in parsley. Phenylalanine ammonia lyase expression increased after 6 h and was maintained for 24 h. In addition, the enzymes cinnamic acid 4-hydroxylase, 4-coumarate CoA ligase, chalcone synthase, and flavone synthase were increased within the 72 h study period. As a result, cinnamic acid production increased in parsley leaves. 63 However, in two other studies, phenolic biosynthesis enzymes were not affected. 8,32 In one of the two studies, lignification of edible bamboo shoots was delayed, possibly due to decreased activities of the enzymes phenylalanine ammonia lyase, 4-coumarate CoA ligase, cinnamyl alcohol dehydrogenase, and peroxidase. 32 Phenolic compounds are precursors of lignin, and an increased lignin content in edible bamboo is undesirable. The second study, which analyzed the activity of phenolic biosynthetic enzymes in broccoli, was also unaffected by UV-B (1.5 and 7.2 kJ m −2 ). They showed a low relative gene expression in the treated florets. Most hydroxycinnamic acids (1,2-disinapoyl-2-feruloyl gentiobiose, 1,2,2-trisinapoyl gentiobiose, 1,2-diferuloyl gentiobiose, and 1,2-disinapoyl gentiobiose) were also not affected. Only the concentration of 1-sinapoyl-2-feruloyl gentiobiose increased, but for both doses. 8 However, enzyme activity was measured 6 h after irradiation, but the concentration of hydroxycinnamic acids was measured four, seven, and 14 days later. Thus, the different results can be explained by different measurement times. These two studies do not fit into the general picture of a positive influence of UV radiation on phenolic compounds (see Phenolic Compounds section). It is possible that the results of these two studies would have been different at other time points. Nevertheless, this review has shown that there are varieties and species that are less sensitive to UV radiation.
Chlorophyll-degrading enzymes in UV-C treated broccoli were reduced during six days of storage, although at different times (10.0 kJ m −2 ). 12 Chlorophyllase increased on the second day in the control samples but not in the UV-C treated samples. Peroxidase increased in both UV-C and control samples but increased more in the control on the sixth day. Mg-dechelatase increased only in the UV-C treated samples but decreased below the control value from the fourth day. Therefore, in this study by Costa et al. 12 the decreased chlorophyll degradation can be attributed to the decreased activities of chlorophyll-degrading enzymes. The authors argue that these enzymes are stimulated by ethylene and that ethylene is reduced by UV irradiation. This also allows a higher chlorophyll content to be maintained during storage. Also, in a study by Buchert et al. 24 UV-C treated broccoli (10.0 kJ m −2 ) showed higher chlorophyll content compared to the control after five days of examination. While the first chlorophyllase (BoCLH1) showed decreased expression, the second (BoCLH2) showed twice the expression compared with the control on the fifth day of the study. At the same time, the expression of pheophytinase (BoPPH) was reduced by almost half in the UV-C treated samples. This result again shows the variation of enzymes, although the same result was obtained. Irradiation of broccoli with UV-B also showed different results for chlorophyll-degrading enzymes. First, the expression of chlorophyllase genes (BoCLH1, BoCLH2, and BoCLH3) increased immediately after UV-B treatment (19.0 kJ m −2 ). On the fourth day, the expression of BoCLH1 was reduced in the UV-B treated samples, whereas the expression of the other two chlorophyllase genes was not changed compared with that of the control. Pheophytinase (BoPPH) showed a small reduction in UV-B treatment on the second and fourth day. The treated broccoli showed a two day delay in senescence. 80 These three studies confirm that UV-C and UV-B can delay chlorophyll degradation in broccoli. The studies also show that there is variability in the expression of chlorophyllase genes. Buchert et al. 24 concluded that BoPPH has the best potential to predict chlorophyll degradation. The expression of this enzyme is reduced when the chlorophyll content of treated broccoli has higher values than that of the control.
An accumulation of indole-type glucosinolates was observed in broccoli after hormetic and higher UV-B exposure (1.5 and 7.2 kJ m −2 ), while a lower accumulation of aliphatic-type glucosinolates was observed. 8 In this study, some precursor amino acids and the expression of genes related to the biosynthetic pathway of glucosinolates were also investigated. Postharvest exposure to UV-B radiation showed that several genes of the glucosinolate synthesis pathway were overexpressed. 8 Among them, the most relevant was the overexpression of tryptophan N-hydroxylase (CYP79B3), which was increased 6-fold in flowers exposed to the hormetic UV-B dose (1.5 kJ m −2 ) and 10-fold in the higher UV-B dose (7.2 kJ m −2 ). At the same time, the higher UV-B dose resulted in a 3fold overexpression of dihomethionine N-hydroxylase (CYP79F1). This result suggests that the target of UV-B is probably the branching pathway of indole glucosinolates. 8 In another paper by the same author, three genes associated with the glucosinolate pathway after UV-C irradiation on postharvest broccoli florets were studied. The relative gene expression of enzymes encoding glucosinolate biosynthesis in flowers was affected by UV-C. 85 In this study, the results showed that the genes encoding phenylalanine N-hydroxylase (CYP79A2) and tryptophan N-hydroxylase (CYP79B3) were increased 2.6-and 3.7-fold, respectively, by the high UV-C dose of 3.0 kJ m −2 compared with the control, whereas no significant overexpression of dihomo-methionine N-hydroxylase (CYP79F1) was observed. 85 However, overexpression of CYP79B3 by 3.1-fold was also observed in flowers exposed to the hormetic UV-C dose (1.2 kJ m −2 ). A similar trend was observed at days two and four after UV-C exposure, where significant overexpression of CYP79B3 was measured. The changes in CYP79B3 expression were associated with the abundance of glucosinolates. 85 These results suggest that the use of a high dose of UV-C as a postharvest treatment may not only preserve broccoli floret quality but also enhance phytonutrients during storage.
These results suggest that the use of a hormetic dose of UV-C as a postharvest treatment may not only preserve broccoli floret quality but also enhance phytonutrients during storage.

■ CONCLUSION
Ultraviolet irradiation in postharvest as a nonchemical, innovative light treatment is a promising illumination to extend the shelf life and improve the nutritional quality of vegetables. In general, there are a large number of results using UV-C. However, other wavelengths such as UV-B, UV-A, and even violet and blue light are rarely studied and used. One reason for this could be lower efficiency in reducing the microbial load. However, UV-A and UV-B wavelengths are less harmful to humans and plants, so more attention could be paid to their effect on vegetables, especially leafy vegetables. It will become easier to study the singular effects of UV-A, as special luminaires, but especially the development of narrow-band UV LEDs will make it possible to decouple UV-B. The so far small number of UV-A studies will increase significantly in the next few years. In combination with UV-C treatment, these wavelengths could be helpful for the increase in anthocyanins Journal of Agricultural and Food Chemistry pubs.acs.org/JAFC Review and other antioxidants. From the results, it appears that there is an optimal UV dose. Doses that are too high lead to loss of antioxidants, and doses that are too low have no effect. It is becoming clear that there are effects of species and cultivars, possibly due to morphological characteristics and a certain phytonutrient status before UV treatment. Rather unanswered is the question of whether longer treatments at lower intensities or shorter treatments at higher intensities are more beneficial. Ultimately, postharvest UV treatments can help reduce vegetable loss due to rot and, if tailored to the needs of the vegetable, improve the quality during storage. To date, no clear protocols can be derived and must be tested individually for each vegetable. Therefore, more studies that are systematic are needed.