Mild Temperature Conditions Applied to Carrot (Daucus carota L.) Waste Using Different Drying Methods: Effect on the Kinetics and Some Chemical Parameters

Abstract

The effects of different mild drying conditions using various drying methods [freeze drying (FD), vacuum drying at 15 kPa pressure (VD15), vacuum drying at 1 kPa pressure (VD1), convective drying (CD), and infrared drying (IRD)] on drying kinetics, proximate composition, yield of extracted pectin, methoxyl content, sugar content, total carotenoids content, antioxidant potential, and color parameters of carrot wastes were examined experimentally. CD was the shortest drying treatment compared to the other drying processes, at 270 min, followed by IRD, VD1, FD, and VD15. The results showed a higher retention of pectin and carotenoids in CD-dried samples. Moreover, along with VD1, CD was able to maintain sucrose and antioxidant potential to a greater extent than other methods. Based on color parameters, FD and IRD had the most significant changes in relation to CIELab values, with ∆E* values close to 33 and 34 units, whereas VD15, VD1, and CD had values (without significant differences) close to 16, 18, and 21 units, respectively. Therefore, the current findings suggest that a short period of exposure of the waste to mild drying temperature conditions is essential for obtaining high-quality waste with potential for use in the food industry.

1. Introduction

In the 21st century, the valorization of food waste is one of the most important environmental and public health issues; there is also an enormous emphasis on sustainable development in food production sectors [1]. The Food and Agriculture Organization (FAO) estimates that one-third of all edible foods produced for human consumption are discarded every year, which equates to approximately 1.3 billion tons per year, raising several serious environmental and ecological concerns and leading to global warming [2]. The food waste generated costs around USD 680 billion in developed countries and USD 310 billion in developing countries [3]. Approximately 20–25% of the fruit and vegetable mass is estimated to be lost during production. This might be caused by multiple factors such as low shelf life, product defects, diseases, low stability of their components under adverse environmental conditions, and bad agronomic practices [4].

In the last few years, the world’s production of carrots (Daucus carota L.) has increased significantly, since these root vegetables have been highlighted in the human diet due to their high nutritional profile, including carotene (particularly β-carotene) and dietary fiber contents. In addition, carrots are a good source of carbohydrates, minerals, and phenolic compounds [5,6]. Moreover, these vegetables are recognized for their pectin content, a structural hetero-polysaccharide with high functional value as a hydrocolloid due to its excellent stability and emulsifying properties, facilitating its use as a gelling agent [7]. However, a considerable quantity of carrots are discarded annually in various regions of the world due to the raw material not meeting quality standards; hence, there is considerable economic loss for producers focused on primary production. The discarded carrots also represent a relevant environmental problem [8]. Therefore, several studies have indicated the need for key technologies for the valorization of biologically active components in both edible and non-edible parts of food waste [9].

Among the key technologies, drying methods such as convective hot air, vacuum, infrared, and freeze drying have been applied to preserve carrots, with freeze-drying treatment showing low tissue shrinkage and minimal loss of color, flavor, and nutrients in the treated samples [10]. Therefore, freeze drying is typically recommended for materials that contain heat-sensitive compounds such as tocopherols, ascorbic acid, carotenoids, and phenols; however, freeze drying has high initial investment and processing costs and needs longer drying time compared to convective hot air drying. Additionally, convective hot air drying is associated with simplicity of operation [11].

The novelty of the use of different drying methods adjusted to mild operating conditions such as temperature, vacuum pressure, air velocity, etc., is based on information on the physicochemical characteristics, nutritional value, and organoleptic properties that can be recovered from food waste, in particular, carrot waste. This information, in turn, could be used for the preparation of different food products or certain recovered components could served as functional ingredients.

Therefore, the aim of the present work was to determine the effects of different drying technologies, namely (freeze drying (FD), vacuum drying at 15 kPa pressure (VD15), vacuum drying at 1 kPa pressure (VD1), convective drying (CD), and infrared drying (IRD)) adjusted to mild drying temperature conditions, on drying kinetics, color, and some chemical properties such as proximate composition, pectin content, sugar content, carotenoids content, and antioxidant capacity of carrot (D. carota) waste, and to identify the most appropriate drying method for these discards in terms of valorization for the food industry.

2. Materials and Methods

2.1. Preparation of Raw Materials and Drying Processes

Carrot (D. carota) waste, including discarded outsized, undersized, and irregularly shaped fresh carrots, as well as rejected tops and tips, was kindly donated by carrot growers in La Serena (Region de Coquimbo, Chile). Fresh waste material was selected, washed, and grated manually to construct the drying curves. A total of 30 g of grated carrots were placed in 14 × 8 cm baskets, forming a 0.4 cm thick layer of material. The sample weights were monitored at specified time intervals for each drying method using a digital balance (SP402, Ohaus, New Jersey, USA). The experiments continued until equilibrium was reached, indicated by a stable sample weight. Each drying experiment was carried out in triplicate at a temperature of 40 °C (except FD). Process conditions for each drying method are detailed in

Table 1. Drying methods, conditions and time, as well as the dryers used in the study.

Drying Method	Dryer and Drying Conditions	Drying Time
Freeze drying (FD)	The samples were pre-frozen at −80 °C prior to FD. Freeze-dryer (Friologic, Given one 5K, Santiago, Chile); condenser temperature: −55 °C; Vacuum pressure: 0.027; Secondary drying: 25 °C	840 min
Vacuum drying at 15 kPa pressure (VD15)	Vacuum dryer (Memmert VO 400, Schwabach, Germany); Vacuum pressure: 15 kPa; Drying temperature: 40 °C	870 min
Vacuum drying at 1 kPa pressure (VD1)	Vacuum dryer (Memmert VOcool 400, Schwabach, Germany); Vacuum pressure: 1 kPa; Drying temperature: 40 °C	660 min
Convective drying (CD)	Convective dryer oven (custom-made equipment); Air speed: 1.5 m/s; Drying temperature: 40 °C	270 min
Infrared drying (IRD)	Infrared dryer oven (custom-made equipment); Radiant source: 175-Watt IR lamps; Distance from radiant source: 18 cm	420 min

:

2.2. Proximate Composition Analysis and Water Activity Measurement

Proximate composition was used for chemical characterization of carrot waste based in moisture, fat, ash, crude protein and crude fiber contents, following the guidelines described by AOAC (Association of Official Analytical Chemists) [12] methodologies. Water activity was also determined for both fresh and dried carrot waste. All measurements were conducted in triplicate.

2.3. Extraction and Characterization of Pectin from Carrot Waste

Pectin extraction was carried out following an acid hydrolysis protocol as described by Jafari et al. [7], with minor modifications. Pectin was extracted from the five carrot waste treatments. Briefly, milled samples were taken and mixed with 2 N HCl at pH 2.5, in a ratio of 1:3 (carrot:solution, w/v). Subsequently, acid hydrolysis was carried out by heating, and later, the solution was stirred on a heating plate (Dragon Lab model MS-H280-Pro, Beijing, China) at 50 °C for 3 h. Then, the samples were filtered using a metal strainer, and the filtrate was centrifuged (Eppendorf 5810 R, Hamburg, Germany) at 4000 rpm for 20 min at 5 °C. The supernatant was mixed with ethanol 1:1 (supernatant:ethanol, v/v) and the precipitated pectin was re-covered and it was brought to freezing conditions at −20 °C for 24 h. After this time, it was again centrifuged at 4000 rpm for 20 min at 5 °C, where the solid was recovered. The obtained pectin was dried in a freeze dryer (Virtis, Benchtop 3L model, NY, USA), and it was stored to determine the yield and the methoxyl content.

The pectin yield was calculated from Equation (1) according to Qiu et al. [13]. Pectin was kept in a hermetically sealed tube, and kept at room temperature, protected from light, moisture and heat.

$$\mathrm{P}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=\left({\displaystyle \frac{\mathrm{d}\mathrm{r}\mathrm{y} \mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{r}\mathrm{o}\mathrm{t} \mathrm{w}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{e} \mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}}\right)\times 100$$

(1)

The determination of the methoxyl group content of commercial pectin and pectin from carrot waste was carried out using the methodology of Biltekin et al. [14], with modifications. 0.5 g of pectin (Sigma-Aldrich, St. Louis, MO, USA) or pectin from carrot waste was weighed and 100 mL of distilled water was added, and then, 25 mL of 0.1 N NaOH was added, and the solution was stirred. Later, the solution was left at room temperature for 30 min. After this time, 25 mL of 0.25 N HCl was added, and the mixture was titrated with 0.1 N NaOH until reaching a pH of 7.5. The methoxyl content was calculated by Equation (2), and expressed as % methoxyl.

$$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} (\mathrm{\%})={\displaystyle \frac{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{N}\right)\times \mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \left(\mathrm{m}\mathrm{L}\right)\times 3.1}{\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n} \mathrm{p}\mathrm{o}\mathrm{w}\mathrm{d}\mathrm{e}\mathrm{r} \left(\mathrm{g}\right)}}$$

(2)

where 3.1 represents the molecular weight of the methoxyl group (31 g/mol) normalized.

2.4. Sugar Content

Sugars (glucose, fructose, and sucrose) were extracted according to the protocol described by Djendoubi et al. [15], with some modifications. Specifically, 2 g of dried carrot waste and 8 g of fresh carrot waste were dissolved in 80% methanol (HPLC grade) and mixed for 30 min in an orbital shaker (Boeco, OS 20, Hamburg, Germany) at 200 rpm. The extracts were centrifuged for 3 min and the supernatants were filtered using a 0.45 µm membrane filter. Thus, sugar analysis was carried out by HPLC (Perkin-Elmer, FlexarLC model, Boston, MA, USA) with a refractive index detector (RID), and it includes a Flexar binary LC pump, a Flexar LC autosampler and a furnace Flexar column. Sugars were separated using a Superosil ™ LC-NH2 column, 5 µm particle sizes (25 cm × 4.6 mm) at 25 °C. The mobile phase was composed of acetonitrile:water (82.5:17.5). The data were processed using the Totalchrom®, 6.2.1 software. Quantifications were carried out using calibration curves prepared for glucose, fructose and sucrose, and their contents were expressed as mg/g dry matter (d.m.).

2.5. Total Carotenoids Contents and Antioxidant Potential of Carrot Waste

2.5.1. Total Carotenoid Content (TCC)

TCC was determined using the methodology reported by Saleh et al. [16], with modifications. Concisely, dried carrot discards (3 g) were extracted with 75 mL of a solvent mixture composed of hexane/acetone/ethanol (2:1:1, v/v) at room temperature for 1 h. The extracts were then filtered, and the filtrate volume was adjusted to 100 mL using the same solvent mixture. The diluted extracts were transferred to a separating funnel, where 25 mL of distilled water was added. The mixture was left to separate, and the absorbance (A) of the organic layer was measured at 470 nm. Hence, TCC was calculated according to Equation (3), and TCC results were expressed as mg/100 g d.m.

$$\mathrm{T}\mathrm{C}\mathrm{C}={\displaystyle \frac{\left(\mathrm{A}\times \mathrm{v}\times {10}^4\right)}{\left({\mathrm{A}}^{1\mathrm{\%}}\mathrm{w}\right)}}$$

(3)

where v is total extract volume (mL), A^1% = 2600 (molar extinction coefficient of β-carotene) and w is sample weight (g).

2.5.2. Preparation of Extracts for Antioxidant Potential Measurement

The preparation of extracts was carried out using the method of Bochnak & Świeca [17], with some modifications. 2 g of dried carrot waste and 8 g of fresh carrot waste were mixed with 10 mL of 80% methanol in an orbital shaker (BOECO, OS-20, Hamburg, Germany) for 30 min at 200 rpm (at room temperature). The samples were centrifuged (Eppendorf, 5804 R, Germany) at 5000 rpm for 15 min and the supernatants were filtered (Whatman filter paper grade No1, size 125 mm) in 50 mL glass round-bottom flask kept in the dark. After centrifugation, the pellets were extracted with 10 mL of 80% methanol, and the previously steps were repeated under the same conditions. A final extraction with 10 mL of 80% acetone was carried out and the flasks were evaporated until dryness in a rotary evaporator (Büchi, RE-210, Flawil, Switzerland). Each sample was reconstituted in methanol:formic acid (99:1, v/v), and later, transferred to a 5 mL volumetric flask. The different extract solutions were stored at −80 °C until use.

2.5.3. 2,2′-Diphenyl-2-Picrylhydrazyl (DPPH) Assay

The free radical scavenging capacity of carrot waste was assessed following the method outlined by Brand-Williams et al. [18]. Briefly, 100 μL of the sample was combined with 3.9 mL of a DPPH solution (2.0 mg of DPPH dissolved in 100 mL of acidified methanol). The mixture was then incubated in the dark at room temperature for 30 min to allow the reaction to occur and the absorbance was read at 515 nm. The DPPH results were expressed as µmol Trolox equivalent (T.E)/g d.m., with Trolox being used to establish the standard curve (y = −0.4688x + 0.4052; R² = 0.9987).

2.5.4. Oxygen Radical Absorbance Capacity (ORAC) Assay

The antioxidant activity by ORAC assay was assessed following the procedure described by Zhang et al. [19] using a Multilabel plate reader (Perkin-Elmer, Victor X3, Hamburg, Germany). In a 96-well plate, 200 μL of fluorescein solution (4 µM) in 75 mM phosphate buffer (pH 4.0) was added to each well, and immediately, 40 μL of the extract solution was added, and then, the solution was incubated at 37 °C for 20 min. After incubation, 35 μL of 0.36 M 2,2′-Azobis(2-amidinopropane)dihydrochloride (AAPH) was added and the fluorescence was measured at 37 °C with excitation at 485 nm (λex) and emission at 535 nm (λem). The ORAC results were expressed as µmol T.E/g d.m., with Trolox being used to establish the standard curve (y = 0.00003x − 25.667; R² = 0.9769).

2.6. Surface Color

The surface color of fresh and dehydrated carrot waste was determined with a colorimeter (HunterLab, MiniScan XE Plus, Reston, VA, USA). The results were expressed using the CIELab color space system (illuminant D65 and 10° observer angle). All determinations were carried out in triplicates at ambient temperature (≈22 °C). The total color difference (ΔE*) between the dried and the initial sample was calculated by Equation (4). Similarly, Chroma (C*_ab) (colorfulness relative to the brightness of its surrounding) and Hue (h*_ab) (attribute described by color name such as red, green, blue, etc.) was also estimated by Equation (5) and Equation (6), respectively.

$${∆E}^{*}=\sqrt{{\left({∆L}^{*}\right)}^2+{\left({∆a}^{*}\right)}^2+{\left({∆b}^{*}\right)}^2}$$

(4)

$$C_{ab}^{*}=\sqrt{\left[{\left(a^{*}\right)}^2+{\left(b^{*}\right)}^2\right]}$$

(5)

$$h_{ab}^{*}={tan}^{-1}\left({\displaystyle \frac{b^{*}}{a^{*}}}\right)$$

(6)

where ΔL* = (L* − L₀*), Δa* = (a* − a₀*), Δb* = (b* − b₀*), and L₀*, a₀*, and b₀* correspond to the control values for fresh carrot waste.

2.7. Statistical Analyses

Statistical analyses were conducted using Statgraphics Centurion Version 18.1.12 (Statgraphics Technologies, Inc., The Plains, VA, USA). All experimental values were measured in triplicate, and the results expressed as the average ± standard deviation. Analysis of Variance (ANOVA) was performed to examine differences among group means. In addition, Duncan’s Multiple Range Test (DMRT) was used as a post hoc test to identify specific differences between pairs of means (p < 0.05).

3. Results and Discussion

3.1. Exploring Drying Curves of Carrot Waste

Drying curves of carrot waste under FD, VD15, VD1, CD and IRD are illustrated in Figure 1.

The variations of moisture content (g water/g d.m.) as a function of time measured under different drying conditions are shown in Figure 1A. It can be seen that the moisture content decreased continuously until a mean moisture value of 0.20 g water/g d.m. was reached. At this point, the curve approaches an asymptotic value, representing the equilibrium moisture content of the carrot waste. The same outcomes have been observed across a wide range of studies on carrot drying [16,20,21,22,23,24]. Likewise, the drying time was found to vary significantly depending on the drying method and conditions used. For instance, the CD process resulted in the shortest drying time (270 min), while the VD15 method led to the longest drying time (870 min). The longest drying time observed for the carrot waste under VD15 process can be associated to the high water vapor content inside the vacuum chamber, which might cause condensation in the vacuum pump heads under prolonged operation, leading to inefficient expulsion of water vapor [25]. On the other hand, the shortest drying time in the CD process can be due to the continuous circulation of hot air inside the oven, which contributes to a faster elimination of humidity, resulting in high heat/mass transport coefficients [26].

The drying rate presented in Figure 1B shows a negligible constant rate period in all curves, indicating that drying mainly occurs during the falling rate period. This means that diffusion is the main mechanism involved in the drying of carrot waste [27]. Nonetheless, in the case of VD15 and VD1, this period it is not only controlled by moisture diffusion but it can also involve surface diffusion, capillary action, and other mechanisms driven by vacuum pressure [28]. Our results are in agreement with other authors, who observed that drying of carrot or black carrot pomace subjected to different drying methods only occurred in the falling rate period, whereas the constant rate period was not observed in any curve [16,20,21,22,23]. However, it should be noted that the slope of the curves in the falling rate period observed in our study is less prominent than those reported in the aforementioned literature. This allows us to deduce that at mild drying temperatures (40 °C), the energy available for water evaporation is reduced, which slowed down the rate of water loss during the falling rate phase. Similar findings have been reported by Kocabiyik et al. [29], who used infrared radiation at 25 °C to dry carrot slices. In the case of FD, the process transitions from sublimation (the dominant phase of primary drying) to desorption of bound water, which occurs in secondary drying phase. This change significantly reduces the rate of water removal due to the increased energy requirements and diffusion limitations [30,31].

3.2. Proximate Composition and Water Activity of Carrot Waste

The values of the proximate composition and water activity (a_w) evaluated for fresh carrot waste and dried by different drying technology are presented in

Table 2. Proximate composition analysis and a_w of carrot waste undergoing different drying methods.

Parameters (g/100 g d.m.)	Drying Methods
	Fresh	FD	VD15	VD1	CD	IRD
Moisture ‡	89.15 ± 0.28 a	4.15 ± 0.06 f	8.63 ± 0.26 b	8.02 ± 0.08 c	6.85 ± 0.09 d	6.50 ± 0.07 e
Lipid	2.14 ± 0.08 b	2.14 ± 0.10 b	1.77 ± 0.18 c	1.87 ± 0.14 c	2.75 ± 0.13 a	1.74 ± 0.10 c
Ash	7.15 ± 0.77 ab	6.84 ± 0.34 bc	6.52 ± 0.20 bc	6.13 ± 0.53 bc	6.47 ± 0.38 bc	7.80 ± 0.34 a
Protein	7.60 ± 0.30 d	9.75 ± 0.23 c	10.00 ± 0.18 bc	12.62 ± 1.23 a	11.21 ± 0.81 b	13.25 ± 0.91 a
Crude fiber	13.96 ± 0.13 a	6.24 ± 0.03 c	3.77 ± 0.25 e	3.53 ± 0.17 e	6.79 ± 0.48 b	4.71 ± 0.31 d
aw *	0.989 ± 0.001 a	0.310 ± 0.004 d	0.426 ± 0.004 b	0.420 ± 0.009 b	0.419 ± 0.006 b	0.394 ± 0.002 c

Average ± standard deviation (n = 3) of samples in the same row with different superscript letter (a–f) indicate a statistically significant difference at a 95% confidence level (p < 0.05). ^‡ values expressed in g/100 g. * dimensionless.

.

The moisture content of fresh carrot waste was 89.15%, wet basis (wb), whereas the a_w was 0.989. Lipids, ash, protein and crude fiber constitute 2.14%, 7.15%, 7.60% and 13.96% dry matter (d.m.), respectively. These results are in accordance with those values reported in literature [8,22,24,32,33,34,35].

As a result of the drying process, the dried carrot waste showed low moisture content and low a_w values (Table 2). This reduction, ranging from 4.15% to 8.63% on a wet basis for moisture and from 0.310 to 0.426 for a_w, can be attributed to water vaporization from the carrot waste. In fact, different drying mechanisms and conditions may influence on how effective water vaporizes. For example, internal transport mechanisms, such as capillary flow in the porous matrix of VD15 and VD1 samples, can add complexity to water evaporation. This may result in samples that are drier at the surface while retaining higher moisture in the central regions [28], leading to a higher overall moisture content in certain areas. FD instead was more efficient in reducing moisture content, which is difficult to achieve with other drying methods, as it allows water molecules to vaporize directly from the solid state through sublimation [31].

As a consequence, the decrease in lipid content of the dried samples could be associated with the enzymatic hydrolysis in high moisture samples (VD15 and VD1) [36], as well as oxidation reaction with unsaturated fatty acids during IR drying [37]. FD samples showed the same lipid content as in the fresh carrot waste, whereas an increase lipid content in sample under CD treatment might be due to short duration of drying process, which lead only the breakdown of cell wall, and thereby, increasing the lipid content [38]. The ash content of drying wastes was not significantly different from that of raw waste, and the content ranged from 6.13 to 7.80 g/100 g d.m., indicating that the drying treatment had little effect on the ash content of carrot waste. Regarding the protein content, fresh carrot waste contained 7.60 g/100 g d.m., significantly lower than the content in all dehydrated samples; this is probably due to destruction of protein inhibitors as a consequence of drying, whereas in the determination of crude fiber content, the samples of the CD method obtained the highest and VD1 the lowest values, which can be probably a response to heating mechanism and employed drying conditions [38].

3.3. Pectin Yield and Pectin Characterization of Carrot Waste

Pectin is a complex heteropolysaccharide found in plant cell walls. It possesses interesting functional properties, making it suitable for use as a multifunctional ingredient [7]. Figure 2 presents the pectin extraction yield and methoxyl content of the samples dehydrated by means of the five technologies.

As shown in Figure 2A, the highest pectin yield of 1.09% was achieved by CD method, followed by IRD (0.95%) and FD (0.81%), whereas significantly lower yield of pectin was observed in VD15 (0.51%) and VD1 (0.49%) samples. Gao et al. [39] explained that the drying method has a significant influence on the variation in pectin yield from carrots. For example, rapid drying processes, such as CD and IRD, may enhance pectin extraction yields by inducing structural changes that facilitate the diffusion and solubility of pectin from the food material into the extraction solvent [14]. In the case of FD, water is removed from the frozen carrot through sublimation under vacuum, causing minimal damage to the pectin content of the plant material [40]. Conversely, the combined effects of high pressure, prolonged exposure time in the VD15 and VD1 processes, and the higher moisture content of the samples likely contributed to some degradation of the pectin fraction, ultimately leading to a reduced yield.

It is noteworthy that pectin yield from dried carrot was significantly below the range reported in literature [7,14,39,41,42]. These differences are attributed to the pectin extraction methods employed in those works, which have demonstrated greater efficiency compared to the method utilized in the present study.

The degree of methylation of the pectin samples from carrot, as influenced by different drying method, is shown in Figure 2B. The methoxyl content of pectin extracted from carrot dried by FD, VD1, VD15, CD and IRD was 18.50%, 16.63%, 16.03%, 15.53% and 13.91% respectively. This indicated that all pectin obtained from dried carrot can be categorized as low-methoxyl pectin. Therefore, they can form gels with low sugar concentrations and in the presence of divalent cations such as calcium ions [14,41,43]. These findings align with the degree of methylation values reported by other researchers [7,14].

Pectin extracted from carrot dried by FD had higher degree of methylation by comparison to pectin extracted from the same vegetable, but dried by other method. This difference was attributed to the temperature of 40 °C used during drying, which could activate the natural pectin methylesterase, leading to partial demethylation of pectin [40,43,44]. In contrast, vacuum and low-temperature conditions during FD could maintain, to some extent, the pectin’s natural methoxylation level of carrot.

3.4. Soluble Sugar Content of Carrot Waste

Quantitative determination of soluble sugars in fresh and dried carrot waste by different methods was evaluated by HPLC-RID and results are shown in Figure 3.

Sucrose was the predominant sugar contributor in fresh carrot waste (110.52 mg/g d.m.), followed by glucose (69.98 mg/g d.m.) and fructose (41.36 mg/g d.m.). These results are consistent with the values of soluble sugars content (based on fresh weight [FW]) reported by other authors for fresh carrot [45,46]. After the application of drying, a significant decrease (p < 0.05) in sucrose, fructose, glucose was observed compared to fresh waste. This reduction is thought to be primarily caused by the progressive rise in the concentration of soluble sugars within cell tissues as drying progresses, which may decompose, transform, or engage in reactions with other substances, influenced by enzyme activities (specifically glycosidase), which may help reduce sugar levels during the drying process [35,47].

Despite the loss of soluble sugars in dried samples, a significant amount of fructose and glucose was maintained in FD-dried carrot. It was also observed that the CD process retained a higher amount of sucrose compared to the other methods, but there were no statistically significant differences (p > 0.05) with VD1-dried sample (Figure 3). Consequently, it is reasonable to conclude that drying at low temperatures, under vacuum conditions, or for a short duration may help minimize the Maillard browning reaction and its associated losses [37,48].

3.5. Total Carotenoid Contents and Antioxidant Potential of Carrot Waste

Data in Figure 4 depicted the total carotenoids content (TCC) and antioxidant potential (using the DPPH and ORAC assays) of both fresh and dried carrot wastes.

As shown in Figure 4A, the TCC of fresh carrot waste was 60.8 mg/100 g d.m. (or 6.60 on FW), which was within the wide range (6.0–54.8 mg/100 g FW) reported in literature [21,45,46,49]. TCC decreased after drying processes and ranged from 20.0 to 23.8 mg/100 g d.m. It is known that carotenoids are easily degraded or isomerized by various factors, including drying temperature, drying time, and moisture content [34]. Saleh et al. [16] observed that TCC started to degrade noticeably when the carrot reached a moisture content of less than 30% during drying. Moreover, the loss of turgor pressure due to the disruption of the vegetable matrix during drying, as well as exposure to light, oxygen, or metal ions, can also contribute to the degradation of these compounds [49,50]. In this study, the highest TCC retention was obtained by CD due to the short drying time, followed by FD under vacuum conditions and lower temperatures. In fact, there was no significant difference between the TCC of samples dried by CD and FD (p > 0.05). Although IRD also offer rapid drying, it significantly impacts on TCC from carrot, since exposure to radiant light can trigger the isomerization and degradation of carotenoids, resulting in the formation of reactive oxygen species (ROS). These ROS can damage carotenoid molecules, compromising their stability and reducing their concentration [2]. Likewise, VD15 samples had the lowest TCC probably due to longer effective drying time of the product, which may have intensified degradative reactions such as β-carotene isomerization in carrots [16]. While both VD15 and VD1 were set at the same drying temperature (40 °C), VD1-dried samples retained higher TCC compared to VD15, demonstrating that vacuum pressure is also a factor affecting carotenoids in carrots.

On the other hand, the antioxidant potential was assessed using 2,2′-diphenyl-2-picryl-hydrazyl (DPPH) and Oxygen Radical Absorbance Capacity (ORAC) assays, which are established assays in food research. As indicated in Figure 4B, DPPH radical scavenging capabilities decreased in general when the samples were subjected to various drying conditions and its value was found to be 174.3 μmol TE/g d.m in fresh carrots and between 128.9 and 173.9 μmol TE/g d.m in dried carrot samples. The values were consistent with a previous study conducted on the fresh carrot and dried by radio frequency-assisted hot air drying, hot air drying and freeze drying [23], with reported values of 184.45 μmol TE/g d.w in fresh carrot and between 102.32 μmol TE/g d.w and 130.24 μmol TE/g d.w in dried samples. Likewise, Keser et al. [6] reported equivalent values of 5.44 mM Trolox/g in fresh carrots and between 2.55 and 3.85 mM Trolox/g in samples dried by microwave at different power levels.

Based on the ORAC assay (Figure 4B), the fresh waste exhibited a value of 111.8 μmol TE/g d.m. This value falls within the range reported in literature. Yusuf et al. [51] determined the antioxidant activity of 12 colored carrot varieties using ORAC assay and reported a wide range of values, from 25.2 to 170.6 μmol TE/g d.m. Likewise, Singh et al. [52] reported values on fresh weight between 2.3 and 12.4 μmol TE/g for 5 carrot cultivars, which are equivalents to our results.

Regardless of the measurement assay, the potential antioxidant of fresh carrot waste was stronger than that of dried waste, but there was no significant difference (p > 0.05) with VD1-dried sample in both assays. This could be due to the fact that the vacuum pressure at 1 kPa used in VD1 restricted the respiration of plant tissues by the absence of oxygen, which contributed to the reduction of oxidative degradation of antioxidant-active compounds [53]. Instead, higher oxygen concentrations and extended drying periods in VD15 may accelerate the breakdown of such compounds. However, the lowest DPPH and ORAC values were noted in the FD- and CD-dried samples respectively, with reductions of 26.05% and 59.12% compared to the fresh samples. The difference between the results of antioxidant capacity of the two assays may be due to the complex system formed by numerous antioxidant compounds, which produces varying responses depending on the assay used. This variation arises because the radical quenching capability differs for each assay [48]. Nonetheless, this study did not conclusively determine the associated antioxidant compounds, warranting further investigation.

3.6. Surface Color of Carrot Waste

The change in vegetable color during drying results from a combination of thermal effects, isomerization, oxidation, and enzymatic browning. In the case of carrots, this change is closely linked to their carotenoid content [54]. In this study, the changes in appearance of carrot waste after drying by different methods are shown in

Table 3. Color parameters of carrot waste undergoing different drying methods.

CIELab Parameter	Drying Methods
	Fresh	FD	VD15	VD1	CD	IRD
L*	49.10 ± 0.94 f	73.57 ± 0.17 a	61.62 ± 0.16 e	62.96 ± 0.20 d	65.98 ± 0.11 c	70.40 ± 0.36 b
a*	37.68 ± 1.31 a	25.24 ± 0.28 e	33.79 ± 0.17 b	31.78 ± 0.24 c	30.82 ± 0.11 d	19.57 ± 0.35 f
b*	48.63 ± 1.45 a	30.87 ± 0.48 e	40.51 ± 0.27 b	38.36 ± 0.28 c	37.78 ± 0.25 d	28.69 ± 0.31 f
∆E*	-	32.75 ± 0.77 b	15.55 ± 0.59 e	18.36 ± 0.78 d	21.31 ± 0.66 c	34.39 ± 1.35 a
h°ab	52.23 ± 0.25 b	50.73 ± 0.14 c	50.17 ± 0.09 d	50.36 ± 0.06 d	50.80 ± 0.12 c	55.70 ± 0.21 a
C* ab	61.52 ± 1.94 a	39.88 ± 0.55 e	52.75 ± 0.30 b	49.82 ± 0.37 c	48.76 ± 0.25 d	34.73 ± 0.45 f

Average ± standard deviation (n = 3) of samples in the same row with different superscript letter (a–f) indicate a statistically significant difference at a 95% confidence level (p < 0.05).

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The CIELab values for fresh carrot waste were 49.10, 37.68, 48.63 units, for L*, a* and b* respectively, and these values specify the characteristic orange color of fresh carrots. These values are consistent with literature data [6,20,21,23].

After the drying processes, the L* parameter of the dehydrated samples increased considerably compared to the fresh sample (p ≤ 0.05), with 73.57, 61.62, 62.96, 65.98, and 70.40 units for FD, VD15, VD1, CD, and IRD treatments, respectively. These results are in accordance with Keser et al. [6], who also reported an increase in luminosity after the drying treatment of carrots. The concordance of results can be accredited to the fact that the dehydrated products have much less water and therefore the light is reflected differently [55]. All drying methods produced a decrease in a* and b* parameters in comparison to the fresh sample, indicating a decrease in redness and yellowness of the dried carrot waste. The losses in redness and yellowness of carrot can be strongly related to the decomposition of their carotenoids, flavonols, flavones, and isoflavones [22]. In particular, the IRD treatment showed the lowest a* and b* values among all samples (p < 0.05), which can be explained by the fact that exposure to radiant light exposure promotes the isomerization of carotenoids and their degradation [2]. The difference in ΔE* values of dried carrot waste was significant (p < 0.05), ranging from 15.55 to 34.39. This indicates that the drying process caused a visible color difference compared to fresh waste, as a ΔE* value greater than 3.5, according to the International Commission on Illumination (CIE), signifies a noticeable color change [56]. Among the dried samples, both IRD and FD presented the highest ΔE*. This is explained by the increase in L* and the reduction of b* and C*_ab, meaning a lighter overall color and less vibrant orange. In fact, the higher h°_ab and lower C*_ab values in the IRD- and FD-dried samples confirmed that they turned more yellowish, while the VD15-dried carrot exhibited a more vivid orange character compared to the other dried samples. Although the VD15 sample has the lowest amount of carotenoids (Figure 4A), other pigments in carrots may have been enhanced by this drying method.

4. Conclusions

The results obtained in the present study indicate that FD-dried samples had the least impact on carrot waste, preserving a brighter color and retaining higher levels of fructose and glucose. At the same time, their degree of methylation was higher than that of the other analyzed methods, but required longer drying time. The drying time of CD was the shortest, resulting in higher retention of sucrose and carotenoids, as well as an improved yield in pectin extraction. Thus, CD stands out as the most viable drying process for carrot waste. Nonetheless, it is important to note that the VD1 treatment demonstrated better performance in terms of retaining antioxidant activity of the carrot waste. Future studies should aim to identify and quantify these antioxidant compounds to enhance the valorization of waste in food and pharmaceutical industry.