Postharvest LED Treatment of Tomatoes Harvested at an Early Stage of Coloration

Abstract

The tomato plant is one of the most important vegetable crops, with a global production of around 188 million tones. The greatest losses in quantity and quality occur during storage, transport, and sale. The aim of the study was to determine the effect of irradiation on the quality and storability of the tomato ‘Tomimaru Muchoo’. Fruit harvested at the turning ripening stage were illuminated for the first two weeks at 15 °C with four visible LED light spectra, with different percentages of blue, green, and red light (BGR). The illumination times were 4 and 8 h per day (hpd). After illumination, the tomatoes were stored at 20 °C in the dark for 4 weeks. Immediately after 14 d of illumination, all tomatoes were fully ripe, although they showed varying red color intensity. In addition, all fruit retained very good quality and freshness. During further storage at 20 °C, there was a gradual decrease in tomato quality. However, LED lighting helped delay softening, reduce rotting, and thus maintain better tomato quality. Longer daily irradiation (8 h) delayed tomato senescence to a greater extent than shorter irradiation (4 hpd). Comparing the spectra, the greatest reduction in softening and rotting occurred in tomatoes illuminated with the spectrum containing the highest amount of blue light (56%). These tomatoes also maintained the lowest color index (a*/b*) throughout storage at 20 °C, which was especially evident in tomatoes that had been illuminated for 8 hpd. The light treatment influenced the maintenance of higher levels of ascorbic acid and antioxidant activity in tomatoes. However, irradiation did not increase the polyphenol content of tomatoes or reduce the lycopene levels in the fruit. Overall, the results showed that LED irradiation during storage improves storability and affects the health-promoting components of tomato fruit. It is a promising tool for reducing losses of horticultural produce.

1. Introduction

Light plays a key role in plant growth and development. It is perceived by plants through special photoreceptors [1,2] and the quality and quantity of light intake affect the expression of relevant genes, which can result in physiological and biochemical changes in the plant [3,4,5]. Recently, light-emitting diodes (LEDs) have been gaining increasing use in horticulture. LEDs are a light source that is energy-efficient, compact, durable, and has low heat and CO₂ emissions. Mostly, they are used in crop cultivation, but some research is also being conducted on their use in storing horticultural products. The objective is to improve the shelf life and health-promoting value of these products. According to D’Souza et al. [6], LED light is suitable for use in cold storage because it reduces thermal damage and the degradation of fruit and vegetables. Its use is also being considered in long-term transportation, wholesale, and retail [7,8,9,10]. According to Poonia et al. [10], LED illumination can be effective even at room temperatures (around 20 °C), reducing the need for costly and energy-intensive refrigerated storage.

Color is a key parameter of vegetable and fruit quality from the point of view of both producers and consumers. It depends on the variety, stage of ripeness, temperature, and light. Adequate light promotes the accumulation of pigments responsible for color development (chlorophyll, anthocyanins, lycopene, and flavonoids) [8,9,11]. Another key quality parameter is firmness. Fruit and vegetables soften during ripening and senescence. Nassarawa et al. [9] found that the effect of LEDs on firmness depends on the species and variety of the stored products.

In addition to morphological processes, LED of an appropriate wavelength enables the accumulation of secondary metabolites, such as vitamins, phenols, soluble solids, anthocyanins, and carotenoids [7,9,10,12]. Blue light stimulates phenylalanine-ammonia-lyase (PAL) enzymes, which are involved in the synthesis and accumulation of phenolic secondary metabolites. Red light increases the concentration of tocopherols and terpenes [13]. According to Kanazawa et al. [14] and Shi et al. [15], blue LEDs regulate the gene expression of certain steps in the phenylpropanoid pathway, thereby increasing the flavonoid and anthocyanin content. Ma et al. [16] showed red light-regulated genes related to carotenoid metabolism in mandarins.

Many studies on the application of light during storage have been conducted on green vegetables. According to Nassarawa et al. [9], LED illumination (red, blue, green, and white) improves the nutritional quality and bioactive compound content of vegetables, including romaine lettuce, spinach, cabbage, and broccoli. Lee et al. [17] confirmed that LED light irradiation was effective in increasing the concentration of chlorophyll, vitamin C, and polyphenols in stored cabbage. In a study conducted by D’Souza et al. [6] on stored cabbage, green and white LEDs stimulated chlorophyll production, while blue and white LEDs maintained higher levels of vitamin C and polyphenols compared to cabbage stored in the dark. Braidot et al. [18] found that lamb lettuce exposed to light retained better quality than that stored in the dark. Spinach leaves illuminated during storage showed higher ascorbate, folic acid, and phylloquinone content than samples stored in the dark [19]. Light inhibited browning and reduced the decrease in total phenolic content and antioxidant capacity during the storage of fresh-cut celery [20]. Ma et al. [21] found that broccoli treated with red LED (50 µmol m⁻² s⁻¹) for 4 d produced less ethylene, maintained a higher ascorbic acid content, and had less yellowing than broccoli treated with blue light or stored in the dark.

The tomato plant is a very popular vegetable around the world and deserves special attention in both agrotechnical and storage research as a rich source of pro-healthy phytochemicals (lycopene, phenolic compounds, and ascorbic acid). The growing location (open field, under roof) and growing season management (protection, fertilization, pruning, leaf removal, maintaining proper soil and air temperature and humidity, etc.) are crucial to tomato quality and storability [22,23]. Fruit color and firmness are important characteristics of tomatoes that indicate their ripeness, freshness, and attractiveness. Based on the available literature, postharvest irradiation studies of tomatoes have been conducted on fruit harvested at the green stage. Ntagkas et al. [24] concluded that ripe green tomatoes (photosynthetically active) exposed to light accumulated more phytochemicals than fruit stored in the dark, whereas ripe red fruit (photosynthetically inactive) did not respond to light.

Red LEDs promote the ripening of green tomatoes by affecting chlorophyll breakdown and accelerating lycopene synthesis. Subsequently, fruit coloration occurs more rapidly [7,12,25]. In addition, during irradiation, there is an increase in the β-carotene, phenol, and flavonoid content and a rise in antioxidant capacity. This positively influences the health-promoting properties of tomatoes [25]. In contrast, ripe green tomatoes illuminated with blue light turn from green to red more slowly than samples from dark conditions. In addition, blue light delays the softening of tomatoes, extending their shelf life [6,7].

A mixture of blue and red LEDs increases the lycopene and carotenoid content in tomatoes harvested at the green stage [4,26]. Cherry tomatoes stored at 5 °C under a mixture of blue and red LEDs for 13 d also showed an increase in lycopene, carotenoids, and flavonoids [27]. There are no comprehensive research reports on the response of tomatoes at the turning stage of coloration to post-harvest LED lighting. Therefore, research has been undertaken to improve the quality and shelf life of tomatoes harvested at the early maturity stage.

The aim of this study was to determine the effects of four visible LED light spectra on the storability and nutritional properties of tomatoes harvested at the stage of partial surface coloration (10–30%), that is, at a stage often harvested for distant markets. The effects of two exposure lengths were compared: 4 and 8 h per day (hpd). This study examined how 2-week LED irradiation affected the further storage of tomatoes at 20 °C. This research used a third physical factor (in addition to temperature and humidity) in tomato storage that can delay ripening, control senescence, improve nutritional value, and generally extend shelf life.

2. Materials and Methods

2.1. Material, Treatments, and Storage Conditions

This study was conducted on tomato cultivar Tomimaru Muchoo F₁, supplied by one of the leading horticulture farms in Poland, Mularski Group S.A. Tomimaru Muchoo is a cultivar belonging to the pink beefsteak tomato type, with a round ribbed shape, relatively few seeds, and a plump meaty texture. Although it is a pink type, the color of the mature fruit is described as raspberry or red. Tomatoes were grown in a greenhouse and harvested at an early stage of fruit coloration—grade 3 (turning, USDA color chart 1975, cited by Messina et al. [28]. The fruit showed a color change from green to tannic yellow, pink, or bright red on the surface in the range of 10–30%. The weight of the fruit was in the range of 100–140 g, and the diameter was 57–65 mm.

Immediately after harvest, the fruit was transported to the storage laboratory, where the tomatoes spent one night at about 18 °C. The next day, the fruit was washed in water at 5 °C higher than the temperature of tomato flesh. After drying, the fruit was divided into 9 groups for individual experimental objects and each fruit was weighed. Four LED spectra were used in this study. One object was a control, with no illumination for the tomatoes (stored in the dark). To set up the experiment, tomatoes were placed in Styrofoam trays (60 cm long and 40 cm wide) with 25 tomatoes in each tray. For each experimental combination, 3 trays of fruit were prepared. The experimental light spectra contained the following percentages of blue (B, 400–499 nm), green (G, 500–599 nm), and red light (R, 600–699 nm): I, 35:11:53; II, 17:12:70; III, 56:23:21; and IV, 7:3:88 (B:G:R). The photosynthetic active radiation (PAR) for each spectrum were as follows: I, 124 µmol m⁻² s⁻¹; II, 127 µmol m⁻² s⁻¹; III, 117 µmol m⁻² s⁻¹; and IV, 128 µmol m⁻² s⁻¹ (LI-189 Light Meter, LI-COR, Lincoln, NE, USA). The LED spectra were analyzed using a GL Spectrolux VIS spectrometer (GL Optic, Puszczykowo, Poland). The tomatoes were irradiated for 4 and 8 hpd at 15 °C for 2 weeks. Illumination for 8 hpd was conducted continuously from 8 AM to 4 PM every day, while illumination for 4 hpd was conducted intermittently, i.e., from 8 to 10 AM and from 2 to 4 PM.

After exposure to 15 °C, 10 tomatoes from each combination were destructively sampled for firmness tests. The remaining tomatoes were transferred to the dark at 20 °C. One tray from each treatment was stored for another 28 d. The remaining tomatoes were stored in the dark for 5 d, after which they were used for chemical analysis.

This experiment was repeated twice.

2.2. Quality Assessment

Tomato quality assessment was carried out after 14 d of exposure to 15 °C and then every 7 d during the storage period at 20 °C. Twenty-five of the same tomatoes were evaluated throughout the study period during one experiment. Fruit softening, rotting, and marketable value were determined. The evaluation was carried out organoleptically based on a 9-grade scoring scale:

-

Softening: 1—no perceptible softening; 3—light; 5—moderate; 7—strong; 9—very strong;

-

Rotting: 1—lack of visible rotting; 3—affected up to 1% of surface (1–2 small spots); 5—affected up to 15% (quite strong); 7—affected up to 50% (strong); 9—affected over 75% (very strong);

-

Marketable value: 1—no marketable value; 3—limited (eatable threshold); 5—fair (shelf life threshold); 7—good; 9—excellent.

On the same days as the quality assessment, the fruit was weighed to calculate the natural weight loss. Losses were counted as percentage differences between the initial weight of the fruit and the weight after exposure to light and a given storage period at 20 °C.

In two terms, immediately before the start of tomato irradiation and after 14 d of light exposure (pre-storage), fruit hardness was measured using a Zwick/Roel ZO10 (Ulm, Germany). The fruit was laid on its side, and the force required to obtain a 10% depression in relation to the fruit diameter was measured with a cylindrical punch tip of 20 mm diameter. Overall, 10 tomatoes were sampled for measurement on the first term and 10 from each treatment on the second term.

2.3. Color Parameter Measurements

The color parameters were measured on the day of the start of irradiation, immediately after the end of irradiation, and every 7 d during storage in the dark at 20 °C for 3 weeks. The measurement was performed in a CIE L* (brightness) a* (redness–greenness) b* (yellowness–blueness) system using a spectrophotometer (Minolta CM -700d, Konica Minolta Optics Inc., Sakai, Japan). Measurements were recorded on 10 tomatoes on the day the study began, and then, starting from day 0 at 20 °C for 3 weeks of storage, the tests were conducted on 10 of the same fruit from each combination. The result for a single fruit was the average of 3 measurements recorded at 3 different points at a distance of 1–2 cm from the top of the fruit. Data for the a* and b* parameters were converted to C* (chroma) and H° (hue angle) [29].

2.4. Chemical Analysis of Metabolites and Antiradical Activity in Tomatoes

Analyses were performed on tomatoes after 14 d of illumination at 15 °C and 5 d of storage in the dark at 20 °C. All tomatoes were stained red and suitable for consumption. For analysis, tomato samples were quartered, frozen, pulverized, and stored at −20 °C. Five grams of a disintegrated frozen tomato sample were homogenized in 6% HPO₃, then filtered and transferred to HPLC vials [30].

The L-ascorbic acid content was determined using high-performance liquid chromatography (Agilent 1200 HPLC system, equipped with a DAD detector) (Boston, MA, USA) using a Supelcosil LC-18 column (Merck KGaA, Darmstadt, Germany) (250 mm × 4.6 mm; 5 µm) with a precolumn according to IFU [31] procedures. Phosphate-buffered solution KH₂PO₄ (1%, pH 2.5) was used as the mobile phase. The column temperature was maintained at 30 °C with a flow rate of 0.8 mL min⁻¹. The detection of L-ascorbic acid was determined by absorbance at 244 nm. The results were expressed as mg kg⁻¹ f.m.

The polyphenol content was measured using a modified spectrophotometric method with Folin–Ciocalteu reagent (Sigma-Aldrich Chemie GmbH, Stainheim, Germany) [32]. To a 5-g sample of the raw material, 50 mL of 80% ethanol was added. The sample was homogenized for 2 min and centrifuged for 10 min at 15,652 relative centrifuge force (g)—20,000 rtm. The prepared solution was filtered under reduced pressure through a Büchner funnel filter. Then, 0.75 mL of the extract was transferred to a 25-mL volumetric flask, and 10 mL of distilled water, 1.25 mL Folin–Ciocalteu reagent, and 2.5 mL of 20% NaCO₃ solution were added and mixed. The volume was topped up (25 mL) with distilled water. The reaction mixture was left in the dark for 1 h at room temperature. After this time, the absorbance was measured using a UviLine 9400 spectrophotometer (Hofheim am Taunus, Germany) at a wavelength of 750 nm in relation to the blank sample. The total polyphenol content was expressed in catechin equivalents as mg kg⁻¹ f.m.

The lycopene content was determined using the method reported by Bohoyo-Gil et al. [33]. The ground sample (2 g) was homogenized in the extraction solution (hexane:acetone 6:4) with the addition of 0.1 g magnesium carbonate. The solution was filtered through a Büchner funnel under reduced pressure. The extract was transferred to a separating funnel, and 50 mL of water was added and shaken. After phase separation, the water–acetone phase was discarded. The acetone rinsing operation was repeated until the lower phase was free of acetone and the upper hexane phase containing lycopene was filtered into an evaporation flask through filter paper containing anhydrous sodium sulfate. Hexane was evaporated to dryness in a vacuum evaporator; the dry residue was quantitatively transferred to a 25-mL flask with a solution of acetonitrile:methanol:ethylacetate (55:25:20), 0.1% BHT, 1 mL TEA, and 4 mL of hexane. The flask extract was filtered with a 45 μm polytetrafluoroethylene (PTFE) filter. Separation was performed using a Kinetex C-18 column (250 mm × 4.6 mm; 5 μm) (Torrance, CA, USA) on an Agilent Technology 1200 Series high-performance liquid chromatography (HPLC) system equipped with a DAD detector. The elution conditions were as follows: 0.7 mL min⁻¹; temperature 28 °C; wavelength 472 nm; and mobile phase: acetonitrile, ethyl acetate, methanol, 1 mL TEA, and 1 g BHT in gradient flow. The calculations were performed according to the standard curve for the lycopene standard (Sigma-Aldrich, Taufkirchen, Germany). The lycopene content was expressed in mg kg⁻¹ f.m.

The antioxidant activity was determined according to Re et al. [34]. The free radical scavenging activity was determined using the 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt radical cation (ABTS•+). The tomato sample (10 g) was mixed with an ABTS•+ solution (5 mL). The reaction was then carried out for 6 min, and the mixture was kept at room temperature in a dark place. The absorbance was measured at 734 nm using a spectrophotometer of the Cary 3E UV-Visible (Varian, Markham, ON, Canada). At least 4 measurements were recorded for each sample at different concentrations to reduce the initial ABTS•+ solution absorbance from 20 to 80%. The linear regression method was applied to calculate the content of the sample, leading to a 50% decrease in the ABTS•+ solution absorbance, and recalculated to mg of Trolox equivalents. The results are given in mg kg⁻¹ f.m.

2.5. Statistical Analysis

The experiments were set up as a two-factorial design of light spectrum and exposure time. Tomatoes were subjected to four LED spectra containing the following percentage compositions of blue, green, and red light (B:G:R): I, 35:11:53; II, 17:12:70; III, 56:23:21; and IV, 7:3:88. The exposure times were 4 and 8 hpd. Analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) procedures were used for data comparison. The calculations were performed in the STATISTICA 13 package (Dell INC.).

3. Results

3.1. Storage Ability of Tomatoes

After 14 d of LED light irradiation at 15 °C, all tomatoes were fully stained red, although differences in color intensity were noted. All tomatoes showed very good freshness and a good appearance.

All fruit was hard to the touch, although the force required for 10% deflection of the fruit under the influence of the punch tip decreased from 30.5 N (immediately after harvest) to 15.4–18.2 N (after 14 d of irradiation), depending on the experimental object. The most force had to be used for control fruit, while the least force was used for fruit exposed to spectrum III for 4 hpd (Figure 1).

Immediately after irradiation, the fruit showed no signs of senescence or rotting. Both irradiated and dark-stored fruit received the highest quality rating—excellent. During the first week of shelf life at 20 °C, the tomatoes began to soften slowly. Despite slight differences, the best firmness was significantly retained by tomatoes illuminated for 4 hpd with spectrum IV, and the worst by fruit illuminated for 8 hpd with spectrum II. In general, tomatoes illuminated with spectra I, III, and IV (with scores ranging from 1.7 to 1.8) retained better firmness than tomatoes illuminated with spectrum II (score of 2.1). Additionally, tomatoes irradiated for a shorter time, i.e., 4 hpd, significantly maintained better firmness (1.6) than those irradiated for 8 hpd (2.0) and those not irradiated (1.9). During the following weeks at 20 °C, a trend was observed in which the best firmness was maintained by tomatoes irradiated with spectrum III and I, both in the group subjected to longer and shorter daily irradiation. After 3 weeks of storage, tomatoes irradiated for 8 hpd showed significantly better firmness than those irradiated for 4 hpd. This relationship persisted until the end of the storage period. After 4 weeks, the greatest fruit softening was significantly found in the control group (Figure 2A).

As the storage period increased, the symptoms of fruit rot intensified (Figure 2B). After 7 d at 20 °C, very slight signs of decay (single small spots not exceeding 1% of the fruit surface) appeared on individual fruit from objects treated with spectra IV (4 and 8 hpd) and II (4 hpd). However, these very minor defects did not significantly reduce the fruit’s marketable value (Figure 2B,C). After 2 weeks at 20 °C, the rot symptoms increased slightly but were still minor and occurred only on a few tomatoes. Nevertheless, tomatoes irradiated with spectrum III had the best quality, while the control fruit showed the worst quality (Figure 2B). For 3 weeks at 20 °C, the least rot developed on tomatoes illuminated for 8 hpd with spectrum III, showing a significant difference. In addition, better overall quality was maintained by tomatoes irradiated for 8 hpd (1.3) than for 4 hpd (1.7). After 4 weeks of storage, the rot increased. Significantly, the least rot was observed on tomatoes after irradiation with spectrum III for 8 hpd and slightly greater on tomatoes exposed to spectrum I for 8 hpd. Overall, spectrum III inhibited decay to the greatest extent, followed by spectrum I, and to the least extent by spectra II and IV. Greater benefits were obtained after longer exposure, after which the tomatoes rotted more slowly than after shorter exposure. The greatest rot developed in tomatoes that were not irradiated.

Softening and rotting affected the marketable value of stored tomatoes (Figure 2A–C). The fruit retained its high commercial value for 2 weeks after storage at 20 °C. Although the scores ranged from 8.3 to 9.0, statistically, the best tomatoes were those illuminated by spectrum III and the worst from the control. On average, tomatoes irradiated for a longer time per day retained statistically better quality than non-irradiated tomatoes, with scores of 8.8 and 8.4, respectively. After another 7 days, the market value decreased and the differences between the tested tomato groups increased (scores ranging from 7.2 to 8.8). The highest result was obtained by tomatoes after irradiation with spectrum III for 8 hd. The worst results were for fruits irradiated with spectrum IV for 4 hd and non-irradiated fruit. After 4 weeks of storage, the decrease in commercial value was significant. The value of irradiated tomatoes ranged from good to fair, depending on the treatment. The control tomatoes already had a value below fair, which is below the marketability threshold. Significantly, the best, with a rating slightly above good, were tomatoes exposed to spectrum III for 8 hpd, followed by those exposed to spectrum I for 8 hpd, with a value slightly below good. Overall, the best value was retained for tomatoes exposed to spectra I and III. Longer illumination (8 hpd) influenced the maintenance of a higher commercial value (6.5) than shorter illumination (5.5). The lowest commercial value was found for non-illuminated tomatoes, which dropped to 3.8 after 4 weeks at 20 °C (Figure 2C).

3.2. Weight Loss

The weight loss after 14 d of irradiation was low, not exceeding 2% in any combination. However, there were statistically significant differences among combinations, and the lowest was measured for control tomatoes, which was higher for fruit irradiated by an LED for 4 hpd and highest for tomato irradiated by 8 hpd. As the storage period at 20 °C was extended, weight loss increased, but for 2 weeks, it was still the lowest in the control. After this time, the trend changed, and the fastest increase in weight loss was recorded for tomatoes treated with spectrum IV for 4 hpd, followed by control tomatoes and then tomatoes treated with spectrum II for 4 hpd. However, due to the high LSD, no statistically significant differences were found among the combinations (Figure 3).

3.3. Color Parameters

Immediately after harvest, the color parameters had the following average values: L*, 50.3; C*, 12.9; and H°, 59.9. This indicates an early stage of fruit coloration. After 14 d of irradiation at 15 °C, the tomatoes turned red, but the intensity varied depending on the postharvest light treatment (Figure 4). In all of the objects, there was a decrease in the value of the L* parameter, indicating fruit darkening; the increase in the C* parameter indicated an increase in color saturation. The sharp drop in the H° value indicated greater fruit redness. The largest decrease in the H° value wa found in the control group, which was indicative of the most intense red coloration of the non-illuminated tomatoes. The least intensely colored tomatoes were those from the group illuminated with spectra I and III for 8 hpd (Figure 4).

During storage at 20 °C, the decrease in the L* parameter clearly deepened only in the second week. After this time, the lowest values of the L* parameter were recorded for control tomatoes, while the highest values were recorded for tomatoes after 8 hpd of exposure to spectrum III, followed by spectrum I. Comparing the averages for the spectra, the highest values were statistically obtained for tomatoes from spectra I and III, lower for fruit from spectra II and IV, and the lowest for non-illuminated tomatoes. Considering the exposure time, the lowest decrease in L* values occurred after 8 hpd of exposure (41.5), and it was statistically lower after 4 hpd (40.5) and lowest after no exposure (38.6). In the following week, the decline in L* values slowed, but the highest value was retained by tomatoes after 8 hpd of exposure to spectrum III, followed by spectrum I (Figure 5A).

The C* parameter also changed noticeably during the second week of storage at 20 °C. The value increased in each combination, but the lowest was recorded for tomatoes irradiated with spectra I and III and the highest for non-irradiated fruit. Tomatoes irradiated for 8 hpd showed a similar C* parameter value to those irradiated for 4 hpd. In the following week, the C* parameter values decreased slightly but still maintained the same relationship between the combinations (Figure 5B).

The value of the hue angle (H°) during tomato storage at 20 °C showed an increasing trend in all combinations. Throughout the storage period, the highest values occurred in tomatoes treated for 8 hpd with spectrum III, followed by spectrum I, and the lowest values in the control fruit. By comparing the effect of the spectra overall (averages for 4- and 8-hpd irradiation), the highest scores were obtained for tomatoes irradiated with spectra I and III, lower with spectra II and IV, and the lowest for fruit without irradiation. The above order remained the same throughout the storage period at 20 °C (Figure 5C).

The color index (a*/b*) after 2 weeks of light exposure varied depending on the spectrum and daily treatment time. The lowest values were found for tomato irradiated with spectra I and III for 8 hpd. The average was also lowest for spectra I and III, but the highest index throughout the period at 20 °C was obtained by the control tomatoes. The time of exposure also affected the color index. After irradiation for 8 hpd, the tomatoes obtained a lower index than after irradiation for 4 hpd. As the storage period at 20 °C was prolonged, changes in the index were negligible, not exceeding 0.2, but there was a tendency for a slight decrease in subsequent weeks (

Table 1. Color index (a*/b*/) during storage at 20°C after 14 d of irradiation at 15°C.

Spectrum	Irradiation Time per d. (h)	Storage Time at 20 °C After 14 Days of Irradiation at 15 °C (d)
		0	7	14	21
Average for spectrum and irradiation time per d.
I	8	0.9 ± 0.1 a	0.9 ± 0.1 a	0.8 ± 0.1 b	0.8 ± 0.1 b
II	8	1.1 ± 0.1 b	1.1 ± 0.1 bc	1.1 ± 0.1 cde	1.0 ± 0.1 cd
III	8	0.9 ± 0.1 a	0.8 ± 0.1 a	0.7 ± 0.1 a	0.7 ± 0.1 a
IV	8	1.1 ± 0.2 bc	1.1 ± 0.1 bc	1.1 ± 0.1 def	1.1 ± 0.1 cd
I	4	1.1 ± 0.1 b	1.0 ± 0.1 bc	1.0 ± 0.1 cd	1.0 ± 0.1 c
II	4	1.2 ± 0.2 bc	1.2 ± 0.2 c	1.1 ± 0.1 ef	1.1 ± 0.1 d
III	4	1.1 ± 0.1 b	1.0 ± 0.1 b	1.0 ± 0.1 c	1.0 ± 0.1 c
IV	4	1.3 ± 0.2 c	1.2 ± 0.1 d	1.2 ± 0.1 f	1.1 ± 0.1 d
Control	0	1.4 ± 0.1 d	1.3 ± 0.1 e	1.3 ± 0.1 g	1.3 ± 0.1 e
Average for spectrum
I		1.0 ± 0.2 a	0.9 ± 0.1 a	0.9 ± 0.1 b	0.9 ± 0.1 b
II		1.2 ± 0.1 b	1.1 ± 0.1 b	1.1 ± 0.1 c	1.1 ± 0.1 c
III		1.0 ± 0.2 a	0.9 ± 0.1 a	0.9 ± 0.2 a	0.8 ± 0.1 a
IV		1.2 ± 0.2 b	1.1 ± 0.1 b	1.1 ± 0.1 c	1.1 ± 0.1 c
Control		1.4 ± 0.1 c	1.3 ± 0.1 c	1.3 ± 0.1 d	1.3 ± 0.1 d
Average for irradiation time per d
	8	1.0 ± 0.2 a	0.9 ± 0.2 a	0.9 ± 0.2 a	0.9 ± 0.2 a
	4	1.2 ± 0.2 b	1.1 ± 0.2 b	1.1 ± 0.1 b	1.0 ± 0.1 b
	0	1.4 ± 0.1 c	1.3 ± 0.1 c	1.3 ± 0.1 c	1.3 ± 0.1 c

Average from 2 experiments ± s.d. The following spectra (B:G:R) were used: I—35:11:53; II—17:12:70; III—56:23:21; and IV—7:3:88. Means followed by the different letters in the columns within the compared group are significantly different (p < 0.05, Tukey test).

).

3.4. Pro-Healthy Value of Tomato

Although both experiments were conducted on tomatoes of the same variety, harvested at the same maturity stage and grown by the same grower, the fruit in the first experiment showed higher levels of ascorbic acid and polyphenols and higher antioxidant activity than in the second experiment (Figure 6A1–D2). However, the effect of LED illumination in both experiments caused similar trends in changes in the content of the chemical components.

Illuminated tomatoes contained more ascorbic acid than non-illuminated tomatoes. The highest levels in the first and second experiments were found in objects irradiated with spectrum III, followed by spectrum I. Among the irradiated tomatoes, those treated with spectrum IV contained the least ascorbic acid.

Irradiation did not result in a higher polyphenol content in tomatoes, with the exception of irradiation with spectrum IV for 8 hpd in the first experiment. On average, the least polyphenols were found in tomatoes irradiated with spectrum I in both the first and second experiments (Figure 6B1,B2). Illumination significantly reduced lycopene synthesis in all objects in the first experiment. To the greatest extent, the reduction occurred in tomatoes irradiated with spectrum I. In the second experiment, the reduction in lycopene in the irradiated tomatoes occurred after irradiation with spectrum I (4 and 8 hpd) and in the case of longer daily irradiation (8 hpd) with spectra II and III (Figure 6C1,C2).

The antioxidant capacity measured by the ABTS test showed that in the appropriate illumination lies the potential to increase this value. In the first experiment, an increase in antioxidant activity was observed after irradiation with spectrum III for 4 and 8 hpd and spectra I and II for 8 hpd. In the second experiment, an increase in activity was found in all irradiated objects, but to the greatest extent in the objects irradiated with spectrum III. In both the first and second experiments, a higher level of antioxidant activity was found after irradiation for 8 hpd with spectra I and II than after irradiation for 4 hpd. However, in the case of spectra III and IV, higher levels were recorded after shorter (4 hpd) rather than longer (8 hpd) exposure (Figure 6D1,D2).

4. Discussion

This study was conducted on tomatoes harvested at an early stage of coloration, which is considered “vine ripeness.” With the onset of fruit coloration, nutrients stop flowing into the fruit, while internal ethylene is produced to regulate skin and flesh coloration [35,36,37]. Tomatoes are already physiologically ripe, although they are not yet fully colored. Famuyni et al. [38] and Oduntan et al. [39] recommend harvesting at an early stage of ripeness as the most suitable for storage, as tomatoes retain an optimal shelf life and high nutritional quality. In postharvest tomatoes, chlorophyll degradation and lycopene synthesis take place, i.e., the same processes occur during ripening on the plant [40,41]. In the present study, the irradiation intensity was quite high, as irradiation was carried out discontinuously for part of the day, i.e., 8 hpd or two times for 2 hpd. Appolloni et al. [42] used even more intense supplemental blue and red light (at 180 µmol m⁻² s⁻¹) during tomato development and ripening in growth chambers. Preharvest irradiation increased storability and preserved the nutritional properties in the postharvest period of tomato fruit.

In the current study, irradiation caused a slightly greater reduction in hardness, which was also reflected in a slightly higher weight loss, measured straight after exposure. As reported by Marinez-Sanchez et al. [43], Braidot et al. [18], Hasperue et al. [44], and Liu et al. [45], a general response to light in plant tissue is the opening of stomata, leading to a loss of water content. However, Lee et al. [17] and Nassarawa et al. [9] reported that higher mass losses were caused by blue light. This was not confirmed in the present study, as treatment with spectra I and III had the highest percentage of blue light and resulted in losses similar to other irradiation treatments. According to Cano-Molina [26], LED light did not influence mass losses and had no negative impact on sensory attributes. However, with prolonged storage at 20 °C, weight loss in the illuminated combinations was slower than in the non-illuminated tomatoes. In the present study, in the first 2 weeks at 20 °C, the highest weight loss occurred in tomatoes after longer daily exposure. In the following weeks, the trend changed, indicating a lower intensity of vital processes in treated than in non-treated tomatoes.

The current study provides strong evidence that irradiation with certain parameters affects fruit quality and shelf life. Even though immediately after irradiation the hardness of irradiated tomatoes was lower than that of controls, this relationship changed during further storage at 20 °C. Tomatoes treated with the highest proportion of blue light retained better firmness, as determined by finger touch, than those treated with a high predominance of red light or those in the control. In the case of eggplants, fruits treated with red + blue light tended to maintain better firmness than those treated only with red or blue light [46]. In studies with red and blue light illumination of peppers [45], the effect on hardiness varied depending on the light quality and crop genotype. According to Batu [47], firmness is strongly related to overall tomato quality, as confirmed in the present study. Firmer fruit was less susceptible to rot and showed higher commercial value when stored at 20 °C. Dhakal and Baek [7] highlighted the positive role of blue light in delaying tomato softening and extending shelf life. However, they reported that red LEDs promoted softening and accelerated ripening. In our study, no acceleration of softening and no increase in rotting were observed in objects with predominantly red light compared to control tomatoes. Ma et al. [21] and Liu et al. [45] showed that both blue and red LEDs delayed senescence in fruits and vegetables. It is conceivable that a small admixture of blue light may have helped reduce the effect of red light on the rate of tomato degradation processes.

Postharvest illumination affected fruit coloration. Although all tomatoes were red after two weeks of light exposure, there were differences in the color intensity depending on the composition of the spectrum used. Tomatoes exposed to spectra with the greatest amount of blue light had the least intense red color, followed by spectra with a high predominance of red light, and control tomatoes were the most red. Such a relationship persisted for consecutive weeks of storage at 20 °C in the dark. Additionally, according to Dhakal and Baek [7], the a* value of tomatoes treated with blue light was significantly lower, meaning that tomatoes were less red than those treated with red light and those that were not irradiated.

In the current study, similar to Messina et al. [28], the values of the C* parameter first increased and then decreased, while the L* parameter showed a decreasing trend during storage. Additionally, in Bruijn et al. [48], the L* parameter decreased during storage but, in contrast to the present study, with greater intensity in tomatoes illuminated by a visible LED. In both Messina et al. [28] and Bruijn et al. [48], the H* parameter decreased but with greater intensity in tomatoes that were illuminated, in contrast to the results of the current study.

In the presented research, the color index (a*/b*) showed the lowest percentage of red color occurred in tomatoes treated with the spectrum with the highest proportion of blue light. However, Najera et al. [8] and Baenas et al. [4] reported that treatment with red light increased the a* and b* parameters and that higher ethylene production accelerated postharvest tomato senescence. In our study, tomatoes exhibiting the lowest a*/b* ratio (treated with spectrum III) retained the highest commercial value during storage. However, in the first weeks at 20 °C, tomatoes illuminated with predominantly red light spectra, with a higher color index, were also firm and without signs of aging or rotting; thus, they also presented a high commercial value. Batu’s [49] suggestion that the color index of commercial tomatoes should be in the range of 0.60–0.95 and that a value higher than 0.95 should be considered overripe is questionable. Ripening and senescence result in complex changes in color, texture, and chemical content. In turn, the impact of light on postharvest fruit characteristics depends on the light quality, fruit genotype, cultivation method, and storage time.

In addition to color, firmness, and senescence, LEDs influence the nutritional value of horticultural commodities [4,7,8,9,10,50]. The present study showed that lycopene levels were lower in all illuminated tomatoes but the lowest in those treated with spectra with the most blue light. D’Souza et al. [6] also claimed that blue light irradiation caused a slower rate of lycopene accumulation. However, Bruijn et al. [48] demonstrated that visible LED light promoted lycopene synthesis during tomato storage. According to Xie et al. [51], blue and red light increase the lycopene content in tomato fruit by two alternative pathways. Cano Molina et al. [26] showed increased lycopene levels relative to the control in tomatoes harvested in the green stage after 7 d of LED lighting (3:1 mix of red and blue). Panjai et al. [25] showed that continuous red LED illumination effectively increased the lycopene content in tomato fruit. The similar effect of red light was confirmed by Meiramkulova et al. [52]. Arif et al. [53] emphasize the role of far-red light in decreasing lycopene content in tomatoes harvested at the green maturity stage. According to the available literature [27], phytoene synthase (PSY) is the main precursor of carotenoids, and these compounds in tomatoes are represented by lycopene at 67%. Once the photoreceptors receive the appropriate stimulus, the genetic chain for PSY synthesis is activated, resulting in an increase in carotenoids, including lycopene. Nevertheless, it is well known that red color is correlated with lycopene content, and in the current study, similar to Sipos et al. [54], less lycopene content was found in less red-colored fruit, which occurred with the highest amounts of blue light.

Only the results from the first experiment confirmed earlier findings by Panjai et al. [25], showing that red light increases the total polyphenol content of treated tomatoes. The red + blue light also increased the phenolic content in eggplant, both in peel and flesh [46]. In general, in the current study, the antioxidant activity was higher in all irradiated combinations than in the control, while it was highest in tomatoes irradiated with the highest amount of blue light. Wang et al. [55] found that LED light irradiation over 3 hpd increased the ascorbic acid in tomato fruit. In addition to polyphenols, ascorbic acid contributes to maintaining higher antioxidant activity, and most of this acid in both the first and second experiments was contained in tomatoes illuminated by a spectrum with predominantly blue light. Ntagkas et al. [56] explain the higher ascorbic acid content of irradiated tomatoes by an increase in myo-inositol (a precursor of ascorbic acid). The content of this compound can increase as much as twofold within 6 days of irradiation.

5. Conclusions

The potential of LEDs to extend the shelf life of horticultural crops is growing. Light of a certain wavelength regulates the processing of phytochemicals in vegetables and fruit and affects postharvest senescence and the rate of ripening, as well as their nutritional value.

Significantly greater inhibition of ripening and senescence occurred in tomatoes treated with LEDs containing 56 or 35% blue light and 21 and 53% red light, respectively, than in non-treated tomatoes or those treated with LEDs with lower blue:red light ratios (17:70, 7:88). The effect of illumination on the health-promoting properties of tomatoes was marked. Tomatoes illuminated with LEDs with a high proportion of blue light showed a higher ascorbic acid content and higher antioxidant activity than non-treated tomatoes. However, they showed a delay in lycopene synthesis compared to non-treated and light-treated tomatoes with a higher red band fraction.

The application of LED technology in vegetable storage requires more research, as different species and varieties have different biology and require different treatments. This method is very promising and can help reduce losses and maintain better product quality.

However, based on the results obtained in the current study, the application of LED technology seems promising, and treatment can be applied in the pre-storage period, for example, during shipment. The fruit can then be transferred to a standard storage chamber for further storage, into warehouses, or on store shelves. The technology can also be used by retailers to reduce waste and offer better products to customers. The integration of LEDs with photovoltaics is a promising solution for modern environmentally safe technology.