Vegetables Treated before Drying with Natural Antioxidants plus UV-C Improve Colour and Bioactive Compounds

Abstract

The quality of minimally processed fruits and vegetables is essential for consumers’ health and effective marketing. This study investigates the effects of UV-C irradiation, applied alone and combined with various natural antioxidants, on the preservation of bioactive compounds and the colour of dehydrated fruits and vegetables. To achieve this, red peppers, yellow peppers, tomatoes, zucchini, eggplants, and melons were subjected to pre-treatments with natural antioxidants and UV-C before processing using low-temperature airflow (50 °C). The drying kinetics showed typical curves of hygroscopic materials, while the drying time was high due to the low temperature applied. The higher drying rate was found for eggplant, due to its porosity, thus allowing a faster moisture removal. The application of antioxidants and UV-C treatments effectively preserved the colour parameters L*, a*, and b*, while in the case of untreated dried vegetables, a significant worsening of colour parameters was noted. However, most applied pre-treatments had positive effects on bioactive compound losses. The best results were obtained using a combination of UV-C with one antioxidant mix that was composed of vanillin, rosemary, and citrus extracts, and combined with a mixture of olive, onion, garlic, and citric acid extracts, which was highly effective in preserving the colour and bioactive compounds of most dried vegetables.

1. Introduction

The quality of fruits and vegetables is critical for maintaining consumer health and effective marketing. This is so because consumers expect high nutritional value in these products, an attractive appearance, and minimum processing [1,2]. Vegetables provide different phytochemicals, which have a role as powerful antioxidants and regulators of several metabolic processes [3]. Vegetables are sources of nutritionally relevant minerals, such as copper, iron, phosphorus, zinc, manganese, etc., in addition to providing a large supply of dietary fiber and vitamins [4,5]. Thus, vegetables are highly recommended to prevent important diseases such as cancer and cardiovascular disorders [6].

There is a huge amount of research indicating a correlation between the beneficial health effects (such as reduced incidence of coronary heart disease) and the colour of the edible portion of fruits and vegetables [7], attributed to the presence of pigmented phytochemicals, such as carotenoids. Moreover, consumer acceptance is closely related to colour, since natural pigments of fruits and vegetables change over time with maturation, ripening, and eventually browning [8]. Therefore, the analysis of colour parameters L*, a*, and b* is widely employed to describe the visual deterioration of the quality of fruits and vegetables [9].

Currently, the market demands high-quality standards on fruits and vegetables for longer periods, although this fact becomes a handicap for marketing, since they are highly perishable [10]. Furthermore, they are highly susceptible to mechanical damage and spoilage [11]. Among various preservation technologies, dehydration is a traditional solution for extending the shelf life of fresh fruits and vegetables worldwide, moreover, it reduces their weight and provides stability for transport, thus facilitating trading, as well as promoting food safety. Nowadays, dried fruits and vegetables are produced as healthy snacks, which increases consumers’ acceptance of these products, and thus facilitates the inclusion of vitamins in their diet [11]. The significance of this technology is clear; today, over 20% of the world’s produce is dehydrated [12], and convective air dehydration is the most used technique [13].

Although consumers accept dried vegetables easily, the process of air dehydration encompasses a loss of nutrients and flavour quality [11,14]. In this regard, vitamin C is highly labile to high temperatures and light; therefore, its concentration decreases during the dehydration process [15]. Furthermore, a relevant change in texture and flavour is noticed in air-dehydrated vegetables [16], which compromises consumers’ acceptance. Variations in colour and taste of hot air-dehydrated fruits and vegetables are partly due to chemical browning due to the Maillard reaction [17], which frequently compromises product quality.

To face the above-mentioned problems, several strategies were assayed. Low-temperature air drying seems to delay the spoilage experienced by fruits and vegetables after harvesting [18]. It was noted that the type of facility where dehydration takes place influences the vitamin C of vegetables [19,20]. Additionally, some pre-treatments (such as blanching, sulphating, and osmotic treatment) are used to improve the quality of fruits and vegetables after dehydration [11,21,22]. Nowadays, there is a trend towards natural antioxidants use, which have well-known antimicrobial effects [23] and are applied to slow down ripening [24]. Moreover, UV-C irradiation is extensively used to disinfect produce after harvesting [25,26,27,28], as well as to prevent colour fading of fresh green vegetables such as broccoli [29], and to extend shelf life in combination with modified atmosphere packaging [30] and refrigeration [31]. This treatment can improve the quality of postharvest fruits and vegetables by improving the phenolic content and defense systems in postharvest fruits and vegetables [32].

Recently, numerous studies explored different drying techniques to enhance the quality and yield of the vegetables studied in this work, such as hot air [33,34,35,36,37,38,39,40,41,42], convective hot air [40,43,44,45], low-temperature air [36,38,41,46], layer flow air [44,47], freeze-drying [34,43], greenhouse drying [45,47], infrared drying [42,43,48,49,50], microwave drying [40,43,45,51,52], microwaved-vacuum drying [34], osmotic drying [39], sun drying [34,39], solar driers [39,50,53], and drying by hybrids systems [45], as shown in Supplementary Table S1. Air drying, combined with various pre-treatments such as ultrasound [46,47,52], blanching [51,53], additives [53], freezing [33], osmosis [44], microwave [51], and pulsed electric fields, was extensively assayed due to its good effects on moisture removal and bioactive compound retention.

For eggplant, ultrasound combined with low temperature accelerated drying, and the impact on quality was negligible [46]. Other approaches, such as hot air drying with freezing pre-treatment, improved drying rates but led to colour changes and structural collapse [33]. In the case of peppers, freeze-drying was more efficient than microwave vacuum drying and sun drying in preserving colour, capsaicin, and β-carotene [34]. For melons, reports highlighted that drying temperature significantly affected bioactive compounds such as polyphenols and carotenoids, and the higher the temperature, the greater the bioactive compounds’ degradation [38]. Zucchini and tomato experiments revealed that microwave-assisted drying considerably reduced drying time and improved rehydration capacity, enhancing phenolic content retention [40,45,52]. Recent research highlights that pre-treatments and drying variables are crucial in balancing drying efficiency to maintain good nutritional and sensory qualities in dried products.

However, despite extensive detailed research on the vegetable drying process, the influence of UV-C light in conjunction with natural antioxidants to prevent browning and bioactive compound losses in dehydrated fruits and vegetables is yet to be studied. Consistently with the above, this study was designed to check the effects of UV-C irradiation as a single pre-treatment and in combination with various natural antioxidants to improve the concentration of bioactive compounds and colour quality when drying selected fruits and vegetables.

2. Materials and Methods

2.1. Samples

Unless otherwise specified, all reagents were purchased from Merck (Darmstadt, Germany). Red bell pepper fruits (Capsicum annuum L. var. Lamuyo), yellow bell pepper fruits (Capsicum annuum L. var. California), tomatoes (Solanum lycopersicum L. var. Round), zucchinis (Cucurbita pepo L. var. Consul), eggplants (Solanum melongena L. var. Long Black), and melons (Cucumis melo L. var. Canary yellow) were obtained from ZetaSeed, Almería, Spain. Upon arrival at the laboratory, the samples were washed with distilled water and stored at 4 °C until treatments started, ~3 h. Before drying, samples were sliced into 5 mm width slices, and melons were peeled before slicing.

2.2. Pre-Treatments

Before drying, vegetables underwent two different pre-treatments:

Natural Antioxidants: Samples were immersed in 2000 ppm water solutions of different plant extracts: (i) vanillin, (ii) rosemary, (iii) a mix of citrus extracts (orange, grapefruit, and mandarin), and iv) a mix of olive, onion, garlic extracts, and citric acid. Immersion was performed in 500 mL flasks for 15 min.

UV-C Light Treatment: Samples were placed in an enclosed treatment chamber (115 × 75 × 60 cm) equipped with UV lamps on the top and sides, with aluminium foil behind the lamps to reflect radiation and achieve higher intensities. Shortwave UV lamps (254 nm wavelength; ALG Germicidal model G30T8, 30 W, General Electric, Tokyo, Japan) provided a total radiation intensity of 4.4 W m⁻². UV doses of 7.9 kJ m⁻² (30 min) and 10.5 kJ m⁻² (40 min) were applied.

2.3. Drying Equipment

Drying was conducted using an adjustable hot air oven dryer (Selecta, Barcelona, Spain), which operates at 230/380 V, 50 Hz, and 59 kW with a maximum temperature of 300 °C, using 1.0 m s⁻¹ horizontal airflow over products placed in a single layer on anodized aluminium trays.

2.4. Drying Methodology

After the pre-treatment of vegetables, approximately 100 g of vegetable slices were dried in the described oven at 50 °C, with three replicates. The mass of the drying samples was monitored every 30 min during the initial stages and every hour during the later stages until a constant mass was noted. The average moisture content was used to plot drying characteristic curves, displaying the dimensionless moisture ratio against drying time.

2.5. Drying Kinetics

Drying kinetics were expressed in terms of empirical models. Experimental data at 50 °C were plotted as dimensionless moisture ratio (MR) against drying time (min):

MR = (M − M_e)/(M_o − M_e)

(1)

where M is the moisture content at any time, t (min), M_e is the equilibrium moisture content, and M₀ is the initial moisture content.

The drying rate (DR) was calculated as follows:

DR = (M_t+dt − M_t)/(dt)

(2)

where M_t+dt is the moisture content (kg water per kg dry matter) at t + dt, and t is the drying time (min).

2.6. Colour Determination

Colour measurements were performed using a CM 3500D Konica Minolta spectrophotometer (Konica Minolta, Inc., Osaka, Japan) with standard illuminant D65. Data were recorded within the CIE Lab colour space, measuring L* (lightness), a* (red-green), and b* (yellow-blue) parameters on three random points of each sample. The final data represent the mean of these measurements.

2.7. Extraction and Quantification of Vitamin C by HPLC

The extraction process for vitamin C was accomplished as described by Vinci et al. [54]. Fresh samples (15 g) were homogenized by adding distilled water, while dry samples (2 g of powder) were mixed with 10 mL of distilled water. The mixture was centrifuged at 5000 rpm for 10 min at room temperature, and the supernatant was filtered through a 0.2 µm syringe filter. A 500 µL aliquot of the filtered sample was mixed with 500 µL of 10 mM tris(2-carboxyethyl) phosphine hydrochloride to reduce dehydroascorbic acid to ascorbic acid. Vitamin C was analysed using a Finnigan Surveyor HPLC system (Thermo Finnigan, San Jose, CA, USA) with a reverse-phase column (Hypersil Gold 250 × 4.6 mm, 5 µm particle size) and UV-visible detector, with detection at 254 nm. The mobile phase consisted of tetra butyl ammonium hydroxide (2 × 10⁻³ M, pH 5.0) and acetonitrile (75:25) at a 0.8 mL min⁻¹ flow rate. Vitamin C content was expressed in mg per 100 g of fresh weight.

2.8. Carotenoids Determination

Carotenoids were extracted by adding 30 mL of diethyl ether to 1 g of freeze-dried sample, followed by shaking and centrifugation until the ethereal phase was colourless. The carotenoid solution was treated with 10 mL of methanolic sodium hydroxide (10% w/v) and stored at 4 °C for 1 h. The organic upper layer was collected, evaporated using a vacuum rotary evaporator, and dissolved in 1 mL of methyl tert-butyl ether (MTBE). HPLC-DAD analyses were performed using a binary solvent system (acetonitrile and water) in gradient mode at a 1 mL min⁻¹ flow rate and detection at 450 nm. Lycopene (75051, analytical grade, Supelco, Madrid, Spain), lutein (07168, analytical grade, Merck), and β-carotene (Y0002050, analytical grade, Merck) were quantified by using calibration curves made with pure standards.

2.9. Statistical Analysis

Results were analysed using Statgraphics Plus 5.1 for Windows (Manugistics Inc., Rockville, MA, USA). Comparisons were made using multivariate analysis of variance (ANOVA), and Duncan’s Multiple Range Test (p < 0.05) was used to test data from colour and nutritional content of each vegetable.

3. Results and Discussion

3.1. Dehydration Kinetics

Figure 1 shows the variation in moisture ratio versus drying time at 50 °C. The initial average moisture content of the vegetables was 15.75, 14.23, 10.25, 13.80, 19.77, and 6.88, g water/g dry matter for eggplant, yellow pepper, red pepper, tomato, zucchini, and melon. They were reduced, respectively, to 0.08, 0.11, 0.10, 0.11, 0.06, and 0.09 g water/g dry matter after ~420 min of drying (yellow pepper, red pepper, and zucchini) and ~480 min of drying (tomato, melon, and eggplant).

The drying rates are shown in Figure 2 (drying rate vs. time) and Figure 3 (drying rate vs. moisture content). Within the initial drying period, the rate of evaporation increases, with mostly free moisture being removed. However, because of the low temperature used for drying, the drying rate curve for all vegetables is characterised by a differential heating phase with a low increase in the drying rate (Figure 2). The higher value of drying rate was obtained for eggplant, which is a porous vegetable and thus the moisture can easily reach the surface of the product, being quickly removed and accelerating the process of drying. The first part of the drying curves can be described by applying a linear regression model, assuming that a constant drying rate occurred during the first stage of the process [55]. Then, the drying rate decreased during the second or fall drying rate period.

The second phase of drying, the constant rate period, was noted for melon, eggplant, and tomato (Figure 2), in which the average initial moisture content was relatively high. In this period, free moisture persists on the surfaces and the evaporation rate alters very little as the moisture content reduces. Thus, during this period, the rate of drying was relatively constant. The absence of a constant rate period for the remaining vegetables may be due to the thin layer of the product, which did not supply water constant for enough time, or it might be because measurements were taken at longer time intervals. Additionally, some resistance to water movement may exist due to the shrinkage of the product on the surface, which reduces the drying rate considerably [56].

The end of the first constant drying period, i.e., the point where the drying rate starts to decrease, is known as the critical moisture content. It was defined as the period for which a linear regression of the drying curve can be attained with an R² ≥ 0.99. At this point, the sample reached the critical moisture content. Such moisture had values of 4.4, 2.2, 2.4, 4.0, 2.7, and 4.5 g water g⁻¹ dry matter for eggplant, yellow pepper, red pepper, round tomato, melon, and zucchini, respectively. After that, the drying period is known as the falling rate period. This is a characteristic period because the material surface is not wetted completely, due to moisture migration. The drying rate tends to go to zero when the rate of evaporation from the surface equals the rate of moisture absorption by the material and is known as the equilibrium moisture content [57].

Since the drying rates of tomato and melon decrease to zero with a certain bound moisture, both vegetables are hygroscopic material. Furthermore, the drying rate trends showed a typical hygroscopic material curve as reported by Mujumdar and Devahastin [58].

Figure 3 illustrates the drying rate as a function of moisture content, highlighting the characteristic drying behaviour of each vegetable. Notably, zucchini and eggplant exhibit a rapid initial moisture loss, consistent with their high water content and porous structure, facilitating faster drying. As moisture decreases, the drying rate follows a declining trend, typical of the falling rate period. This behaviour agrees with observations from similar studies on porous vegetables such as carrot cubes, where a significant reduction in drying rate occurs as surface moisture is removed and bound water begins to dominate the process [56]. The sharp decline in moisture content towards the end of the drying process indicates that equilibrium moisture content is being approached, leading to a plateau in the drying curve [58].

3.2. Selection of Pre-Treatments for Drying

A crucial parameter of the quality of vegetables is the content of ascorbic acid and carotenoids, given that both affect their healthy properties. It was reported that losses of ascorbic acid occur not only during the dehydration process, but also during pre-drying treatments, e.g., irradiation affects the product structure, which facilitates water removal but decreases the ascorbic acid retention [59]. The main variables that affect this compound, temperature and time, are the most important parameters. Because of the high sensibility of this nutrient to heat, the combination of these two parameters determines its retention. Similarly to ascorbic acid, drying time and temperature are the main variables affecting β-carotene retention [60].

All vegetables were screened to discern the influence of the temperature on bioactive nutrient retention. For instance, fresh tomato contains 41.9 mg 100 g⁻¹ of ascorbic acid and 46.4 mg g⁻¹ of β-carotene. After drying, the retention of such compounds was 60.4 and 75.1% (50 °C), 50.3 and 66.9% (60 °C), and 35.3 and 59.7% (70 °C), respectively. Considering that all the remaining vegetables gave similar results, it was decided to perform drying processes at 50 °C, to maximise the concentration of bioactive nutrients.

A previous series of experiments were performed to study colour retention. Specifically, they checked brightness, L*, in eggplant, since it is the vegetable more susceptible to browning among the studied ones; a*, the green–red component, in tomato and red pepper; and b*, the blue-yellow component, in yellow pepper. Colour changes after drying were assessed for vegetables treated with several antioxidants commonly used for vegetable drying: CaCl₂ (1.5%), ascorbic acid (2000 ppm), and the antioxidant mixes described in Section 2.2, all of them diluted in distilled water. This previous processing included using UV-C and the application of antioxidants as explained in Section 2.2. The sliced vegetables were not subjected to hot water blanching to inactivate peroxidase, given that such inactivation was tried using the selected treatments. The best results, especially in eggplant, were obtained by applying the antioxidant mix described in Section 2.2 (iv), which included a mix of olive, onion, garlic extracts, and citric acid, combined with UV-C. In addition, the order of application to vegetables of both treatments, UV and antioxidants, was investigated. The colour of the freshly sliced vegetables was checked before and after drying. Neither ascorbic acid nor CaCl₂ provided comparably good results to natural antioxidants alone or combined with UV-C, so they were discarded for further experimentation. Consistent with these results, the vegetables were subjected to the following treatments: (i) drying alone, (ii) drying + antioxidant, (iii) drying + UV, (iv) drying + antioxidant + UV, and (v) drying + UV + antioxidant. The results of the analysis of the colour of such treatments are exposed in

Table 1. Colour parameters of dried vegetables affected by several treatments ^a.

		Colour Parameters
Vegetables		L*	a*	b*
	Zucchini
0	Fresh	84.6 ± 1.3 a	−2.50 ± 0.71 a	25.6 ± 3.3 a,b
1	Dehydrated	77.5 ± 1.0 c	−0.97 ± 0.54 c	29.3 ± 3.5 a
2	Dehydrated + antiox	81.0 ± 3.2 b,c	−1.97 ± 0.54 b	29.5 ± 2.6 a
3	Dehydrated + UV	81.9 ± 2.1 b	−2.17 ± 0.78 a,b	29.5 ± 1.6 a
4	Dehydrated + antiox + UV	83.4 ± 2.3 a,b	−1.77 ± 0.59 b	23.6 ± 2.2 b
5	Dehydrated + UV + antiox	82.5 ± 5.3 a,b	−2.19 ± 0.48 a,b	23.5 ± 1.3 cb
	Eggplant
0	Fresh	85.9 ± 2.9 a	−1.6 ± 0.6 d	25.2 ± 3.9 b,c
1	Dehydrated	63.4 ± 5.8 c	9.0 ± 1.8 a	28.9 ± 1.1 a,b
2	Dehydrated + antiox	73.2 ± 5.6 b	5.2 ± 2.0 b,c	23.7 ± 1.9 c
3	Dehydrated + UV	69.1 ± 3.9 b,c	6.1 ± 2.1 b,c	31.6 ± 3.8 a
4	Dehydrated + antiox + UV	73.8 ± 4.3 b	5.4 ± 1.5 b,c	26.8 ± 1.3 b,c
5	Dehydrated + UV + antiox	74.9 ± 4.3 b	4.0 ± 2.1 c	25.2 ± 2.6 b,c
	Melon
0	Fresh	74.0 ± 2.7 b,c	−0.58 ± 0.47 c	9.97 ± 1.53 c
1	Dehydrated	69.0 ± 3.0 c	5.05 ± 1.21 a	18.8 ± 4.2 a
2	Dehydrated + antiox	78.7 ± 6.6 a,b	0.24 ± 0.15 b	16.9 ± 3.9 a,b
3	Dehydrated + UV	74.9 ± 6.7 b	−1.83 ± 0.31 d	14.5 ± 3.6 b,c
4	Dehydrated + antiox + UV	82.3 ± 6.9 a	−0.56 ± 0.32 c	10.6 ± 3.3 c
5	Dehydrated + UV + antiox	81.4 ± 4.4 a	−0.59 ± 0.12 c	15.1 ± 3.0 a,b
	Tomato
0	Fresh	40.9 ± 3.0 a	18.9 ± 1.9 c	16.5 ± 2.1 c
1	Dehydrated	39.4 ± 2.6 a	29.0 ± 5.0 a	25.0 ± 2.3 a,b
2	Dehydrated + antiox	42.5 ± 3.7 a	30.4 ± 2.7 a	27.0 ± 2.9 a
3	Dehydrated + UV	37.3 ± 2.2 a,b	25.4 ± 4.2 b	20.2 ± 2.1 b,c
4	Dehydrated + antiox + UV	38.4 ± 3.1 a,b	28.1 ± 2.9 a,b	20.9 ± 2.5 b
5	Dehydrated + UV + antiox	35.2 ± 1.0 b	27.3 ± 3.0 a,b	22.9 ± 1.8 b
	Red bell pepper
0	Fresh	31.5 ± 0.9 c	25.6 ± 1.8 c	12.3 ± 1.0 b
1	Dehydrated	36.7 ± 1.8 a,b	31.4 ± 1.5 a,b	19.1 ± 0.7 a
2	Dehydrated + antiox	38.1 ± 0.7 a	31.2 ± 1.9 a,b	20.6 ± 2.0 a
3	Dehydrated + UV	36.9 ± 1.2 a,b	29.7 ± 2.2 b	22.7 ± 3.0 a
4	Dehydrated + antiox + UV	36.1 ± 1.0 a,b	34.4 ± 1.5 a	22.2 ± 1.5 a
5	Dehydrated + UV + antiox	35.8 ± 0.9 b	29.0 ± 1.6 b,c	21.2 ± 1.2 a
	Yellow bell pepper
0	Fresh	54.2 ± 2.3 a	10.3 ± 1.8 a	53.3 ± 4.4 a
1	Dehydrated	49.9 ± 1.7 b	10.1 ± 2.1 a	41.4 ± 3.0 b
2	Dehydrated + antiox	50.7 ± 2.1 b	11.4 ± 2.9 a	40.2 ± 3.7 b
3	Dehydrated+ UV	49.6 ± 1.5 b	10.5 ± 2.2 a	40.9 ± 3.2 b
4	Dehydrated+ antiox + UV	53.6 ± 1.8 a,b	10.4 ± 2.3 a	48.8 ± 3.9 a
5	Dehydrated + UV + antiox	55.0 ± 1.9 a	8.8 ± 2.6 a	49.2 ± 3.1 a

^a For each vegetable and within a column, variables sharing the same subscript letter are not statistically different according to the multivariate analysis of variance and multiple range test of Duncan (p < 0.05).

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An absence of statistical differences between the colour parameters obtained in vegetables subjected to treatments (iv) and (v) was found in most cases, which means that the order of these pre-treatments had little relevance on colour results. Since these treatments induced colour quality close to the one of fresh vegetables, treatment (iv) weas selected for further experimentation along with the remaining ones to study the content of bioactive compounds after vegetables drying.

3.3. Colour and Nutritional Quality of Oved-Dried Vegetables

Ascorbic acid and carotenoid profiles were analysed in fresh samples and after drying at 50 °C, and the results are detailed in

Table 2. Nutrient content of fresh and dried vegetables ^a,b.

	Moisture g/100 g		Vitamin C c mg·100 g−1	Main Carotenoids µg·g−1 Dry wt
	50 °C	105 °C d	Fresh wt	Lycopene	β-Carotene	Lutein	Other Carotenoids e	Total Carotenoids
Zucchini
Fresh	93.0 ± 0.1 a	94.9 ± 0.2	18.5 ± 0.5 a	-	23.3 ± 2.1 a	231 ± 14 a	-	255 ± 13 a
Dehydrated	6.0 ± 0.1 b		14.7 ± 0.2 b	-	18.9 ± 1.7 b	170 ± 12 b	-	189 ± 12 b
Dehydrated + UV	5.5 ± 0.1 b		9.8 ± 0.4 c	-	17.0 ± 1.9 b	131 ± 14 c	-	148 ± 15 c
Dehydrated + antiox + UV	5.8 ± 0.1 b		15.7 ± 0.3 b	-	21.9 ± 1.8 a,b	172 ± 14 b		194 ± 14 b
Eggplant
Fresh	92.0 ± 0.1 a	91.7 ± 0.1	4.3 ± 0.9 a	12.7 ± 1.2 a	4.3 ± 0.2 a	18.0 ± 0.9 a	-	35.0 ± 1.0 a
Dehydrated	7.3 ± 0.1 b		3.4 ± 0.5 b	9.4 ± 1.1 b	3.9 ± 0.1 b	12.7 ± 0.3 c	-	31.3 ± 0.9 b
Dehydrated + UV	7.5 ± 0.1 b		2.1 ± 0.4 c	7.5 ± 1.9 c	3.3 ± 0.3 c	12.3 ± 0.4 c	-	23.1 ± 1.8 c
Dehydrated+ antiox + UV	7.4 ± 0.1 b		3.9 ± 0.2 a,b	9.7 ± 1.3 b	3.8 ± 0.2 b	17.2 ± 0.5 a		30.7 ± 1.2 b
Melon, var. “Yellow”
Fresh	94.5 ± 0.1 a	94.9 ± 0.2	22.5 ± 2.4 a	-	40.8 ± 2.1 a	26.8 ± 1.3 a	-	67.6 ± 2.0 a
Dehydrated	10.0 ± 0.1 b		17.7 ± 1.8 b	-	30.9 ± 2.3 b	15.7 ± 3.1 b,c	-	46.6 ± 3.0 b
Dehydrated + UV	9.1 ± 0.1 c		10.2 ± 3.0 c	-	28.1 ± 5.0 b	13.2 ± 1.3 c	-	41.3 ± 4.3 b
Dehydrated+ antiox + UV	9.0 ± 0.1 c		17.2 ± 3.0 b	-	30.1 ± 3.0 b	16.6 ± 1.3 b		46.7 ± 2.8 b
Tomato
Fresh	95.0 ± 0.1 a	96.1 ± 0.2	41.9 ± 6.2 a	491 ± 12 a	46.4 ± 0.9 a	73.8 ± 11.0 a	x 245 ± 30 a	856 ± 54 a
Dehydrated	11.0 ± 0.1 b		25.4 ± 3.5 c,d	333 ± 12 c	34.8 ± 0.3 b	55.3 ± 4.8 c	x 189 ± 25 b	610 ± 67 b
Dehydrated + UV	8.5 ± 0.1 d		21.9 ± 1.8 d	202 ± 20 d	22.9 ± 0.4 c	32.1 ± 3.8 d	x 178 ± 33 b	566 ± 58 b
Dehydrated+ antiox + UV	9.6 ± 0.1 c		32.0 ± 3.8 b	445 ± 21 b	33.9 ± 0.5 b	63.5 ± 5.4 b	x 188 ± 23 b	619 ± 51 b
Red bell pepper
Fresh	93.5 ± 0.1 a	95.4 ± 0.2	60.6 ± 5.5 a	-	346 ± 7.5 a	73.9 ± 3.0 a	y 330 ± 35 a	859 ± 43 a
Dehydrated	10.0 ± 0.1 b		55.9 ± 3.1 b	-	310 ± 12 b	55.7 ± 2.1 c	y 307 ± 41 a,b	791 ± 33 a,b
Dehydrated + UV	9.5 ± 0.1 c		43.7 ± 3.3 c	-	213 ± 16 c	22.0 ± 1.4 d	y 267 ± 29 b	688 ± 46 c
Dehydrated+ antiox + UV	9.2 ± 0.1 c		58.7 ± 3.5 a,b	-	335 ± 14 a,b	62.4 ± 1.4 b	y 298 ± 23 a,b	715 ± 39 b,c
Yellow bell pepper
Fresh	94.0 ± 0.1 a	95.1 ± 0.2	86.6 ± 6.3 a	-	284 ± 16 a	18.5 ± 3.9 a	y 208 ± 19 a	510 ± 39 a
Dehydrated	11.0 ± 0.1 b		62.6 ± 7.4 b	-	229 ± 13 b	13.5 ± 2.0 b	y 178 ± 13 a,b	431 ± 28 b
Dehydrated + UV	9.6 ± 0.1 c		46.0 ± 5.2 c	-	215 ± 24 b	8.4 ± 2.5 c	y 151 ± 15 b	398 ± 42 b
Dehydrated+ antiox + UV	9.9 ± 0.1 c		62.9 ± 5.2 b	-	220 ± 15 b	13.8 ± 2.7 b	y 190 ± 15 a,b	417 ± 33 b

^a Drying pre-treatment type was selected according to colour similarity to fresh product. ^b Whitin each vegetable and treatment, means followed by the same subscript letter are not statistically different according to the ANOVA Multiple Range Test of Duncan (p < 0.05). ^c Values obtained in the dehydrated product and derived to fresh matter, considering the moisture of fresh products. ^d Verification of initial humidity by drying at 105 °C. ^e Tentative identification by retention time: ^x Phytoene; ^y Capsanthin + capsanthin ester + β-cryptoxanthin.

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The more relevant results in each vegetable are as follows:

3.3.1. Zucchini

The edible part of zucchini has a cream-white colour that becomes grey-light brown when enzymatic and non-enzymatic reactions after harvesting trigger the production of water-soluble pigments that confer a deteriorated appearance to the fruit [8]. Therefore, brightness is the more important colour parameter. L* (brightness) lacks difference among the control and samples treated with both antioxidants and UV-C (p < 0.05). However, a*, the green–red component, changed from −2.50 in fresh samples to −0.97 in dried samples, which indicates a loss of greenness. This was partially corrected by applying the remaining treatments listed in Table 1.

Concerning ascorbic acid, in agreement with previous findings, the UV-C radiation decreases ascorbic acid retention during drying [59]. In this study, 20.5 and 47% of ascorbic acid reduction was noted after drying alone and drying + UV-C. Conversely, when combining antioxidants and UV-C, this situation partially reverted to a 15.1% reduction. This reversion was also noted for lutein (without statistical significance) and especially for β-carotene (p < 0.05).

One report on SO₂-treated and 50 °C-dehydrated zucchini showed ascorbic acid and total carotenoid content at 35 and 5 µg g⁻¹ dw (Supplementary Table S2). In this work, dried zucchini using antioxidants-+ UV-C showed ascorbic acid and carotenoids at 211 mg 100 g⁻¹ fw and 194 µ g⁻¹ dw (Table 2). Ascorbic acid values in the fresh vegetable were similar in both works, thus, the procedure applied in this research induced noticeable ascorbic acid retention. However, although carotenoid content in the dried product was higher in this work, the fresh product also contained higher amounts of carotenoids, which prevented this parameter from being compared.

3.3.2. Eggplant

The edible part of eggplant is similar to the one of the zucchinis, with a creamy colour that rapidly turns into a dun colour after cutting due to browning reactions. The positive effect of the treatments on eggplant was found more than on zucchini because eggplant contains high amounts of phenolics [61]. Drying changes the brightness, L*, from 85.9 to 63.4, thus darkening the product. However, all treatments revert this change, especially treatments using antioxidants, although without statistical significance, due to that antioxidants being natural anti-browning agents against polyphenol oxidase activity [62]. The results for colour retention for eggplant agree with those of Ferrao et al. [63] and Mishra et al. [64]. Furthermore, the treatment with UV-C + antioxidants allowed the retention of ascorbic acid, β-carotene, and lycopene, being statistically significant for ascorbic acid and lutein (p < 0.05). Such results are consistent with those obtained by Siems et al. [65], who demonstrated that the breakdown of lycopene and β-carotene affected by UV-C light and various pro-oxidants was much faster than that of lutein and zeaxanthin.

Previous works on eggplant dehydration are exposed in Supplementary Table S2. Notice that drying at 50 °C without treatments induced a loss of ascorbic acid of 25.5%. In this work, the loss under the same conditions was similar at 20%. However, when using antioxidants + UV-C, the loss was only 9.3%, thus improving the nutritional value of the dehydrated vegetable.

3.3.3. Melon

There are reports that fresh cuts of fruits such as melon undergo a decrease in freshness and its indicators [27]. In this work, after drying, the brightness (L*) in melon significantly decreased (p < 0.05); however, all applied treatments improved this parameter, being higher in antioxidant, antioxidant + UV, and UV + antioxidant than that of control (p < 0.05). This disagrees with previous observations that dried vegetables are darker than fresh [66,67]. a* was also modified by drying, and the use of the last two treatments allows this parameter to return to levels similar to those of the fresh sample (p < 0.05). Artes-Hernandez et al. [68] used UV-C radiation in different doses to preserve the quality of fresh-cut melon without drying, and they found that the parameters of the CIELab colour space were slightly altered following radiation, but without a clear trend towards positive or negative changes. These results overlap with the observations made in this study since the L* parameter does improve upon receiving UV-C radiation treatment, but the parameters a* and b* in the experiment change without a clear positive effect. In line with these results, increasing brightness was also observed by Caminiti et al. [69] in apple juice and by Lante et al. [70] in different cut fruits treated with UV radiation to prevent browning.

In this fruit, ascorbic acid and total carotenoids were diminished by drying until reaching 79 and 69% of the original content, and were reduced to 45 and 41% by using UV-C treatment, which agrees with that previously noted for zucchini. β-carotene and lutein were also reduced to 76 and 59% after drying while UV-C induced a slight extra decrease in both carotenoids. However, the use of antioxidants allowed higher amounts of both lutein and β-carotene, although without significance. Artes-Hernandez et al. [68] used UV radiation to preserve the nutrients of fresh cantaloupe melon cuts and lengthen its shelf life, noting that the β-carotene content decreased by only 16% by treating the melon with radiation. Gil et al. [27] studied carotenoid loss during storage in fresh cuts of different fruits, measuring a 15% decrease in carotenoid content in cantaloupe melon cuts after 6 days at 5 °C. This indicates the sensitivity of carotenoids to the light and oxygen after cutting, even though no high temperatures are applied. As expected, in the present work, the losses of carotenoids were higher due to the drying process applied to samples.

Previous works on ascorbic acid and carotenoid content in dehydrated melon are exposed in Supplementary Table S2. Notice that both bioactive compounds had much lower concentrations in dried melon than that obtained in this work. However, data from both works cannot be compared due to the absence of data for fresh products in these works.

3.3.4. Tomato

The use of different drying methods exerts significant effects on the quality parameters of dried tomatoes. The colour and appearance changes of dried tomatoes are due to an increase in the 5-hydroxymethyl-2-furfural (HMF) content, resulting in undesirable colour [71]. In the present work, the worst results for L* values were obtained by using UV-C alone or in combination with antioxidants. Moreover, using antioxidants improves L*, reaching higher values in the dried samples than in fresh samples, although without statistical significance. Concerning a* values, all treatments improved this parameter in comparison with fresh samples, especially by using antioxidants alone (p < 0.05), which may be because the drying was accomplished at low temperatures. This finding agrees with the observations of Shi et al. [67], who stated that the retention of colour (redness) and brightness in dehydrated tomatoes is better at low temperatures.

Ascorbic acid shows relatively good preservation, i.e., ~60% is retained in dried samples, while the use of antioxidants + UV-C increases such retention until ~76% (p < 0.05). Georgé et al. [72] noted a drastic reduction (80%) in ascorbic acid in thermally processed tomatoes, and such reduction was also noticed by Kocabiyik et al. [73] when different drying techniques were applied to tomatoes. The good results obtained in this work would be due to the simultaneous use of low temperatures for drying combined with antioxidants.

It was reported that lycopene had a high stability during tomato drying [74]. In the present work, the drying without treatments induced losses of lycopene, β-carotene, lutein, and total carotenoids of 32, 25, 25, and 29%. Such losses were higher, approximately up to half of the concentration in the fresh product, when UV-C was used as a pre-treatment. Conversely, the treatment antioxidants + UV improved all bioactive compounds to values close to those found in fresh samples, although without statistical significance.

Data on ascorbic acid and lycopene for experimentation performed on drying tomatoes are exposed in Supplementary Table S2. Notice the retention in the ~20–90% range for ascorbic acid and the retention of ~60–80% for lycopene, regardless of the dehydration procedure, which agrees with the data of the present work.

3.3.5. Red Bell Pepper

The deterioration of colour in red pepper by drying was widely studied since it is one of the most relevant quality parameters that can be affected by low water activity and high temperature, mostly due to Maillard reactions [75]. In the present study, all treatments increased both L* and a* parameters (p < 0.05). This was especially relevant in the case of L*, for which an increase of 21% was obtained using antioxidants, while for a* the treatment antioxidant + UV-C induced an increase of 34%. The values for colour were found to be slightly lower than those reported by Rhim et al. [75]. Carotenoids are responsible for the colour of red pepper [76]; thus, as the drying of fruits maintains the concentration of such pigments close to values on fresh weight, this was responsible for the noted colour improvement.

Both vitamin C and total carotenoids in red bell pepper experienced a mild reduction of ~8% after drying. However, again, the use of antioxidants + UV-C allowed reducing losses of ascorbic acid, but not of lycopene.

Values of the bioactive compound most studied in dried red bell peppers, i.e., ascorbic acid, are detailed in Supplementary Table S2. Notice that all these values are much lower than those obtained in this work, regardless of the dehydration procedure.

3.3.6. Yellow Bell Pepper

The change in the colour trait undergone by yellow peppers after harvesting and processing (especially drying, which triggers enzymatic and nonenzymatic reactions) is thought to be associated with the reduction in pigments such as β-carotene and zeaxanthin [77]. In this work, the parameter L* had an 8% reduction after drying. However, the simultaneous use of antioxidants and UV-C restores L* to values without statistically significant differences from those found in fresh fruits (p < 0.05). The b* parameter indicates the yellowness–blueness. Although in dried samples b* value decreased by 22%, in line with the trend of L*, by using antioxidants and UV-C, values returned to the range of those found in fresh pepper (p < 0.05).

Ascorbic acid in yellow bell pepper was reduced by 28% after drying and the treatment with UV-C diminished such value; however, the application of antioxidants had weak effects on improving ascorbic acid concentration. Concerning carotenoids, a similar trend was found; the treatment with UV-C promotes the degradation of carotenoids, while using antioxidants had a minor impact on carotenoid preservation. Topuz and Ozdemir [78] studied the effect of γ-radiation in dehydrated peppers focusing on carotenoids. They found that the higher the radiation dose, the greater the reduction in the β-carotene content in pepper. This agrees with the results obtained in this study, in which a greater β-carotene reduction was observed when pepper samples were irradiated.

Research on yellow bell pepper dehydration is detailed in Supplementary Table S2. Notice that the amount of ascorbic acid in products dried at 50 °C was similar to that of fresh products. However, considering the moisture in fresh and dried peppers (91 and 14 g 100 g⁻¹), a reduction of about 90% of ascorbic acid is detected in the dried product. These losses were much higher than those noted in this work in any of the treatments tested. Such differences are probably due to the size of the slices subjected to drying, the airflow applied in the oven, the variety of peppers, light preservation, and other related factors.

3.4. Practical Applications of the Methodology Developed in This Work

The methodology applied in this work to produce dehydrated vegetables uses only natural antioxidants and UV-C to preserve colour and bioactive compounds, thus, it has innovative and wide-ranging practical applications, particularly in the food and health industries. Natural antioxidants such as the ones used in this work can prevent oxidative degradation, which leads to nutrient loss and colour fading in dehydrated foods. When combined with UV-C light, a non-thermal technology known for its antimicrobial properties, this method ensures that vegetables remain safe for consumption without needing chemical preservatives. This approach is ideal for producing clean-label dehydrated vegetables as ready-to-eat meals, snacks, and soups where consumers demand freshness, nutrition, and appearance. UV-C treatment further extends shelf life by reducing microbial load, making it particularly useful for producing long-lasting food supplies for emergency kits, military rations, and space missions. The developed methodology is also well-suited for organic and health-conscious markets since it uses natural ingredients and a safe, non-chemical sterilization process. Additionally, using UV-C in combination with natural antioxidants helps maintain the integrity of sensitive nutrients such as vitamins and minerals, making dehydrated products more appealing for medical nutrition and specialized diets. This technology can also reduce food waste by preserving surplus vegetables in a cost-effective and environmentally friendly way.

4. Conclusions

The findings of this study indicate that combining low-temperature drying with UV-C irradiation and natural antioxidants can improve the colour and bioactive compounds in various vegetables. Drying kinetics revealed typical hygroscopic material curves with extended drying periods due to the low temperature used, especially for eggplant, melon, and tomato. The drying rates were highest for eggplant due to its porosity, facilitating quicker moisture removal. Critical moisture content for each vegetable was identified, guiding the drying process.

Treatments that use antioxidants combined with UV-C effectively preserved colour parameters (L*, a*, and b*) for most vegetables. Natural antioxidants applied before drying, such as the mix of olive, onion, garlic extracts, and citric acid, were particularly effective on colour preservation. In most cases, UV-C treatment alone decreased ascorbic acid content, but when combined with antioxidants, such losses were reduced. Similar trends were observed for carotenoids, with significant improvements in pigment retention in tomatoes and red bell peppers.

Furthermore, the use of antioxidants alone showed promising results in maintaining ascorbic acid and carotenoid levels, indicating that this pre-treatment can be effectively used without any other synergistic treatment. The findings align with consumer demands for high-quality, minimally processed food products. Limitations of this study include the fact that only a very low temperature was explored to develop vegetable dehydration and that some bioactive compounds, e.g., tocopherols, were not analyzed in vegetables, nor were they used as natural antioxidants to improve the quality of dehydrated products. Another limitation is that the process was not scaled up to an industrial level. Future studies should explore the long-term stability of the studied dry vegetables and the potential scalability of these methods for commercial applications. In this regard, scaling up provides an opportunity to evaluate the efficiency, cost-effectiveness, and reliability of the drying process in real-world settings, ensuring it can be adapted to various vegetables while maintaining quality and safety standards.