Comparative Analysis of the Physicochemical Properties of Fresh, Solar-Dried, Oven-Dried and Commercial Royal Gala Apple Snacks

Abstract

This study compares the physicochemical properties of Royal Gala apple snacks fresh and processed using different methods: solar-dried (SD), oven-dried at 65 °C (OD65°) and 85 °C (OD85°) and two commercial brands (CC—commercial apple C and CF—commercial apple F). Evaluated parameters included color, microstructure, acidity, sugar content, phenolic compounds, antioxidant activity, and the presence of heat-induced compounds such as 5-hydroxymethylfurfural (5-HMF) and acrylamide. Commercial samples showed more pronounced color changes and a denser microstructure, with higher browning index (BI) values. The ratio of soluble solids to titratable acidity, an indicator of sensory acceptance, was more influenced by drying methods than temperature. Total phenolic content was highest in fresh apples (123.68 mg GAE/100 g d.m.) and decreased across all drying methods, particularly in solar-dried (SD) samples (78.57 mg GAE/100 g d.m.). Antioxidant activity followed a similar trend, although SD samples performed better than expected, likely due to the retention of certain bioactive compounds. Fresh apples had the highest sugar content (43.25 mg/100 g d.m.), followed by CC (33.81 mg/100 g d.m.), OD65° (33.37 mg/100 g d.m.), CF (31.56 mg/100 g d.m.), OD85° (25.92 mg/100 g d.m.) and SD (25.01 mg/100 g d.m.). Commercial samples were sweeter and darker, with detectable levels of 5-HMF and acrylamide. The findings highlight that drying method significantly affects snack quality. While solar- and oven-dried samples better preserve bioactive compounds, industrial processes enhance sweetness and visual appeal but increase the formation of potentially undesirable compounds. These results are valuable for both consumers and producers aiming to balance sensory quality with nutritional and food safety considerations in dried apple products.

1. Introduction

The apple (Malus domestica x Borkh) is an important fruit crop, widely produced in both fresh and processed forms, holding significant economic importance [1]. Among the numerous apple cultivars identified, Royal Gala, Golden Delicious and Fuji red apples are the most commonly consumed worldwide [2], although over 7000 varieties are known [3]. While the harvest season typically occurs towards the end of summer and during the fall, apples remain available year-round in various processed forms, including frozen products [4].

Using dried apples as a snack is an interesting way to improve fruit consumption and is becoming more popular due to their phytochemical components that contribute significantly to their nutritional value [2], as they are very rich in water (>80%), sugars (fructose > glucose > sucrose), vitamins (mainly vit. C, 2.3–31.1 mg/100 g of dry matter), minerals and dietary fibers [3]. Additionally, they have attractive organoleptic qualities, are beneficial to human health and used therapeutically, because they promote gastric secretion absorption, lung function, increase weight loss, eliminate toxins and have diuretic effects [4,5].

In recent years, global market tendencies have indicated a surge in the consumption of refined foods. However, in the past few years, consumers have become increasingly informed and have consequently shifted their purchasing and consumption patterns towards high-quality food products [6]. Snack foods made from fruits and vegetables are witnessing a rise in popularity due to their lower oil and fat contents, as well as reduced sugar levels compared to other common choices such as chips and cookies [7]. There is a noticeable increase in demand for dried products that preserve most of their original characteristics [8].

The food industry strives to enhance its products by incorporating ingredients that closely resemble “natural” foods while meeting consumer demands for quality at minimal costs [9]. Conventional hot air drying, accounting for over 85% of industrial fruit dehydration methods [10], is cost-effective but carries drawbacks, such as nutrient and bioactive compound loss, changes in physicochemical characteristics and prolonged exposure to high temperatures [11]. Consequently, many companies are now offering home dehydrators and some individuals use their own ovens on a small scale for food preservation to minimize waste [12]. The resurgence of food dehydration has become a trend again, although it is essential to prepare food properly to mitigate changes in its chemical and physical composition, as quality parameters undergo various alterations during drying and storage [10].

Color analyses, microstructure evaluation, assessment of acidity, including pH and titratable acidity, the determination of soluble solids and sugars, antioxidant activity and phenolic compounds are all parameters that contribute to the market value of apples as well as the consumer’s choices [13].

Despite the abundance of information and publications on the subject, there is a lack of comparative studies examining apple snacks obtained through different methods, whether homemade or commercial. Therefore, the objective of this study is to compare fresh and dried Royal Gala apple snacks produced using processing techniques, including solar-dried (SD), oven-dried (OD) at 65 °C and 85 °C, as well as two commercial brands (CC and CF), in terms of their physical and chemical properties.

This analysis aims to evaluate how different drying methods affect the physicochemical properties of Royal Gala apple snacks and the formation of potentially harmful compounds, ultimately supporting more informed choices by consumers and producers regarding snack quality and safety.

2. Materials and Methods

2.1. Apple Samples

The Royal Gala apples were acquired from a local market, in Vila Real, Portugal, for the study. Solar-dried samples were obtained using an indirect cabinet solar drier previously described [14], for a 6 h drying period. Oven-dried samples (Captain Jerky 110, Klarstein, Berlin, Germany) were processed for 6 h and 4 h at the temperatures of 65 °C and 85 °C, respectively. For both solar- and oven-dried procedures, empty trays were weighed before the start of each drying operation. The standard sample preparation procedure involved selecting the product, washing it, removing the core and cutting it into rings using an electric slicer to ensure a uniform thickness of approximately 3.0 ± 0.1 mm. The prepared samples were evenly distributed in a single layer on the trays. Relative to commercial snacks, we used a white-label product of one of the most important national supermarkets (CC) and a registered trade specialist in dried food (CF). Both were chosen because they specifically describe the utilization of Royal Gala in their ingredients list. Parts of the samples were lyophilized and ground for further utilization in chemical analyses.

2.2. Color Analysis (L*a*b*) and Browning Index

The values of L*, a* and b* were extracted from the surface of the apple snacks. Total color differences, ΔE, between fresh and dried apple were calculated according to the following formula [6]:

$$\mathsf{\Delta }{E}^{*}=\sqrt{{(a_0^{*}-a_1^{*})}^2+{(b_0^{*}-b_1^{*})}^2+{(L_0^{*}-L_1^{*})}^2}$$

(1)

where a₀* is the degree of redness/greenness of fresh apple snacks, b₀* is the degree of yellowness/blueness of fresh apple snacks, L₀* is the degree of lightness/darkness of fresh apple snacks, and a₁*, b₁*, and L₁* are the color values of dried apple snacks.

Another parameter used to determine color changes on the surface of the apple was the browning index (BI) [7,8]:

$$\mathrm{B}\mathrm{I}={\displaystyle \frac{100(x-0.31)}{0.17}}, \mathrm{where} x={\displaystyle \frac{a^{*}+1.75L^{*}}{5.645L^{*}+a^{*}-3.012b^{*}}}$$

(2)

The BI is derived from the L*, a* and b* values obtained from the color analysis. It quantifies the degree of browning or discoloration of the apple surface.

2.3. Microstructure-Scanning Electron Microscopy

Food microstructure study has gained significance owing to its impact on the texture attributes of food products. Scanning electron microscopy studies (FEI Quanta 400 SEM/ESEM, Phillips at CIDE-UME, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal) were carried out using fresh and dried apples (2 cm × 2 cm size), which were placed on aluminum stubs and glued with carbon double-sided adhesive tape. For the images acquisition, we used a low vacuum mode with a pressure inside the chamber of 6 mbar and an acceleration voltage of 20 kV, with different resolutions.

2.4. pH, Titratable Acidity (TA) and Soluble Solids Content (SSC)

The apple powder (0.5 ± 0.1 g) was mixed with 20.0 ± 0.5 mL of distilled water. Soluble solids (Brix°) were determined using a digital handheld refractometer (Vee Gee PDX-1, VeeGee Scientific, Inc.; PDX series; Model PDX-1; Vernon Hills, IL, USA). The pH of samples was measured using a Jenway 3310 pH meter (Bibby Scientific Ltd., Stone, UK). For titratable acidity determination, the prepared mixtures were titrated with 0.1 M NaOH solution to pH 8.2, using phenolphthalein as an indicator [10,11,15]. The experiments were performed in triplicate.

The results were expressed in g of malic acid per 100 g of dry sample according to the following equation:

$$TA={\displaystyle \frac{N\times V\times f}{m}}\times 100$$

(3)

where N is the normality of the NaOH solution, V is the volume of the NaOH solution, f is the correction factor of malic acid and m is the mass of the sample.

2.5. Sample Extract Preparation

To prepare the extracts, 1.0 ± 0.1 g of lyophilized grounded fresh and dried snacks was weighed, placed in a Falcon tube and mixed with a total of 20.0 ± 0.5 mL of extraction solution (80% aqueous methanol, acidified with 0.1% HCl) in a vortex, for 2 min. Afterwards, the extracts were shaken at 150 rpm for 2 h at room temperature and centrifuged at 3000× g for 15 min. The supernatants were used for determination of total phenolic content and antioxidant capacity. For phenolic profile analysis the same method was used but with 50% methanolic extraction solution.

2.6. Phenolic Compounds

2.6.1. Total Phenolic Content

Total phenolic content was determined according to the Folin–Ciocalteu colorimetric method with gallic acid used for the generation of the standard curve. The extract (0.5 mL) was mixed with 1.0 mL Folin–Ciocalteu reagent, 7.5 mL distilled water and vortexed. After 3 min incubation, at room temperature, 1.0 mL of aqueous sodium carbonate solution (7.5%) was added, tubes were vortexed and the total phenolic content was determined after 90 min of incubation at room temperature. The absorbance of the mixtures was measured at 765 nm using a spectrophotometer (Genesys^TM50, Thermo Fisher Scientific, Waltham, MA, USA). The absorbance of the samples was quantified with respect to the standard curve of gallic acid and the results were expressed as gallic acid equivalents (GAE), mg/100 g of dry mass [12,16]. All experiments were performed in triplicate.

2.6.2. Phenolic Profile

For the determination of the phenolic profile and 5-HMF, the method of Golding et al. (2001) [17] was used with some modifications. The apple extract was analyzed by high-performance liquid chromatography (DIONEX ICS 3000 liquid chromatography system (Dionex, Sunnyvale, CA, USA)). For the analysis, a C-18 column (ACE, length 250 mm, diameter 45 mm, particle size 5 μm) was used. The column was kept at 35 °C during separation. The optimal analysis conditions were achieved using a 5% aqueous formic acid solution (A): methanol (B) (95:5, v/v) as the mobile phase for 5 min. The mobile phase was changed to 35% (A) at 65 min and maintained until 67 min, and from 67 min to 75 min, the mobile phase was 95% (A). The flow rate was 1.0 mL/min, and the absorbance was monitored between 200 and 600 nm. The absorbances at 280 nm, 325 nm and 525 nm were used for analysis. For quantification purposes standard solutions were prepared for each compound or a related one. 5-HMF was identified based on its characteristic retention time and UV spectrum, distinct from those of phenolic compounds, ensuring accurate quantification without interference. All experiments were performed in triplicate.

2.7. Assessment of Acrylamide Content

Ground samples were defatted by washing with 10 mL of n-hexane. A total of 10 mL of water and 75 µL of 50 mg/L ¹³C-acrylamide solution (Internal Standard, I.S.) were added to the defatted samples, which was then swelled in a water bath at 70 °C, for 30 min. After cooling, 10 mL of 1-propanol was mixed and then centrifuged at 10,000 rpm for 10 min. A total of 7 mL of the supernatant was transferred to a glass centrifuge tube and it was evaporated until dryness, using a centrifugal evaporator at 30 °C. A total of 2 mL of water was added to the tube and the sample resuspended. A clean-up was made, adding to the tube 50 mg of PVPP and 50 mg of PSA. The tube was centrifuged, and the supernatant was taken.

Bromination and extraction of acrylamide were carried out [18]. After the clean-up, 50 µL of H₂SO₄ was added to the tube to decrease the pH of the solution and then 1.5 g of KBr and 16.7 mg of KBrO₃ were added. The orange solution obtained was placed on an ice bath for 1 h. The reaction was then stopped through the addition of 0.1 M NaS₂O₄ until the orange color totally disappeared. For the extraction of 2,3-DBP, 1 g of NaCl was added, followed by the addition of 2.5 mL of an ethyl acetate/n-hexane mixture (4:1). The solution was mixed, and the aqueous layer was transferred. This process was repeated three times. The resulting ethyl acetate phase was evaporated using a centrifugal evaporator and, when dry, 1 mL of ethyl acetate was added to the tube and a final clean-up was made, using 50 mg of PSA and 150 mg of anhydrous NaSO₄. The sample was centrifuged and the supernatant was transferred to a 2 mL screw cap glass vial. 2,3-DBP was converted to 2-dibromopropionamide (2-BP) by adding 100 µL of triethylamine to the vial, immediately before the injection. The sample was then ready for the GC-MS analysis (Trace GC Ultra, Thermo Fisher Scientific, Waltham, MA, USA). All experiments were performed in triplicate.

2.8. Antioxidant Activity

2.8.1. DPPH (2,20-Diphenyl-1-picrylhydrazyl) Assay

The 1,1-diphenyl-2-picrylhydrazyl radical (DPPH^•) scavenging was determined to measure the antioxidant capacity. Dried and fresh sample extract solution (0.1 mL) and freshly prepared 3.9 mL of DPPH radical solution (0.06 mM) in methanol were mixed and vortexed for 15 s. After incubation in the dark for 30 min, the absorbance was measured at 517 nm using a spectrophotometer. The control was considered the DPPH radical without sample. All experiments were performed in triplicate [15,19]. The DPPH free radical scavenging rate of apple samples was calculated by the following equation:

$$DPPH free radical scavenging rate \left(\%\right)={\displaystyle \frac{A_c-A_t}{A_c}} \times 100$$

(4)

where A_c is the absorbance of the control and A_t is the absorbance of the sample.

2.8.2. ABTS (2,20-Azino-bis-3-ethylbenzothiazoline-6-sulfonic Acid) Assay

The free-radical scavenging activity was determined by ABTS radical cation decolorization assay. ABTS (7 mM) was dissolved in water. The radical was produced by reacting ABTS stock solution with potassium persulfate (2.45 mmol/L), and the mixture was left in the dark, at room temperature, for 14–16 h. The obtained solution was diluted with water to an absorbance of 0.70 ± 0.02, at 734 nm. After the addition of 30 µL of methanol extracts to 3.0 mL of diluted ABTS^•+ solution, the absorbance was read exactly 6 min after initial mixing. All experiments were performed in triplicate. The results were expressed in μmol Trolox equivalents per Kg (μmol TE/Kg) of the dry sample [20,21].

2.9. Determination of Sugars

A high-performance anion-exchange liquid chromatography system with pulse amperometric detection (HPAEC-PAD) was used to analyze glucose, fructose, sucrose and sorbitol. Chromatographic measurement was performed using DIONEX ICS 3000 liquid chromatography system (Dionex, Sunnyvale, CA, USA) equipped with a quaternary gradient pump and electrochemical detector, which consisted of Au as the working electrode and Ag/AgCl as the reference electrode. All separations were performed on Carbo Pac PA-20 (30 × 3 mm; Dionex, Sunnyvale, CA, USA) thermostated to 30 °C. The flow was constant (0.7 mL/min). A new extraction was made with apples and extraction solution with ethanol and water (50:50) [22]. All experiments were performed in triplicate.

2.10. Vitamin C Content

The vitamin C content of apples was determined using a validated hydrophilic interaction chromatography (HILIC) method [23]. For the analysis of samples, 1.0 g was weighed and 10.0 mL of the extraction-reduction solution (5% MPA, 2 mmol/L tris(2-carboxyethyl)-phosphine (TCEP) and 2 mmol/L EDTA) was added. After standing for 90 min at 40.0 ± 2.0 °C, the suspension was centrifuged for 5 min at 4427× g (Hettich EBA 8 S, Tuttlingen, Germany). A Dionex liquid chromatographic system (Sunnyvale, CA, USA) was used, which included an Ultimate 3000 pump, PDA-100 photodiode array detector, a WPS-3000TSL Analyt auto sampler and column oven (Model TCC-3200, Thermo Fisher Scientific, Waltham, MA, USA). The chromatographic analyses were conducted using a TSK-gel Amide-80 (4.6 i. d. ×100 mm, 5 μm, Tosoh Bioscience, Tokyo, Japan) maintained at 20 °C. The optimal analysis conditions involved the use of a mobile phase consisting of acetonitrile (B) and aqueous 0.1% trifluoroacetic acid (A) in a ratio of 90:10, v/v for 7 min. The mobile phase was then changed to 80% (A) at 8 min and maintained until 13 min, followed by 10% (A) at 14 min, maintained until 20 min. The flow rate was set at 0.7 mL/min and the absorbance was monitored between 210 and 350 nm, with the analysis performed at 244 nm. Quantification was performed through external calibration and a calibration curve of ascorbic acid was constructed in the range of 7.5 mg/L to 100 mg/L. All experiments were performed in triplicate.

2.11. Statistical Analysis

The data were expressed as mean plus/minus standard deviation. Significant differences (p < 0.05) and their interaction were identified using one-way analysis of variance and the unequal N Tukey post hoc test. All statistical treatments were performed using Statistica 12 software (StatSoft, Inc., Tulsa, OK, USA).

3. Results and Discussion

3.1. Color Analysis (L*a*b*) and Browning Index

Color is an important quality parameter of apples and influences consumers [13], stimulating appetite and affecting the desirability of the product [10]. The apple pulp color varies among different varieties and undergoes immediate changes when cut and exposed to air, primarily due to enzymatic browning [24]. During the drying process, enzymatic browning, Maillard reactions, ascorbic acid oxidation and pigment degradation can cause color alteration in the apple [25], which can be temperature-dependent. Surface browning is one of the most visible alterations in color and is linked to a decrease in lightness L* and an increase in the values of a* (greenness) and b* (yellowness) [26]. These color changes can be quantitatively measured using indicators such as ΔE and BI (

Table 1. L*a*b*, ΔE, BI, BI/BI₀ and color simulation of fresh apple, samples dried with different methods (SD—solar; OD65°—oven at 65°; OD85°—oven at 85°) and commercial samples (CC—commercial white label; CF—commercial label).

Sample	L*	a*	b*	ΔE	BI	BI/BI0	Color Simulation
fresh	92.8 ± 1.5 a	−7.6 ± 0.5 c	32.5 ± 1.6 d	-	31.3 ± 1.1 e	-
SD	82.3 ± 2.4 b	−4.8 ± 1.0 c	43.2 ± 1.0 c	15.3 ± 1.8 b	62.5 ± 1.4 d	2.0 ± 0.1 b
OD65°	81.7 ± 3.3 b	−4.7 ± 1.9 c	42.5 ± 1.2 c	15.5 ± 2.3 b	63.6 ± 1.8 d	2.0 ± 0.1 b
OD85°	77.7 ± 1.8 b	1.4 ± 1.0 b	50.8 ± 2.8 b	25.4 ± 1.1 a	100.5 ± 1.3 c	3.2 ± 0.3 a
CC	66.6 ± 3.6 c	16.0 ± 3.7 a	60.5 ± 2.8 a	-	219.9 ± 2.5 b	-
CF	58.9 ± 2.0 d	17.3 ± 2.0 a	58.2 ± 1.9 a	-	268.1 ± 2.2 a	-

Data are expressed as mean ± standard deviation (n = 6). Values with different lowercase letters in same parameter column are significantly different (p < 0.05).

).

The drying process reduces lightness (L*), with the highest loss observed in OD85°, in the samples dried by us. This is expected due to browning reactions (e.g., Maillard reactions) and pigment degradation at higher temperatures. In relation to the a* parameter, fresh samples are particularly greenish, but this decreases with the drying process. OD85° shifts to a slightly red tone, likely due to chemical changes caused by heat. The yellow tone (b*) increases in all dried samples, with OD65° and OD85° showing similar intense yellow. This might be related to caramelization or other heat-induced reactions (Tables S1–S6).

The absolute color difference ΔE is quite similar in SD and OD65° samples, which is probably related to the utilization of the same drying temperature. The greatest deviation is observed in OD85°, showing that higher drying temperatures lead to more pronounced visual changes (Tables S7 and S8).

The browning index is used to characterize the overall changes in browning color and is one of the most common indicators in food products containing sugar [27]. Browning of apples results from both enzymatic and non-enzymatic reactions and is related with cultivar [28]. In our work BI increases the same way as ΔE, fresh < SD < OD65° < OD85°. A big difference can be observed between the commercial and the other dried samples in the BI values. Again, SD and OD65° have similar results and all the other samples are significantly different, which is also represented in BI/BI₀, compared with fresh apples (Tables S9–S12).

3.2. Scanning Electron Microscopy

Changes in the physical properties of apples can result from microstructural alterations caused by exposure to varying temperatures, leading to either beneficial or adverse effects based on the intended final use [29]. These alterations may involve modifications in cell structure, cell integrity, moisture distribution and other structural characteristics. To assess the differences in internal structure among the various samples of apple snacks, scanning electron microscopy was employed (Figure 1).

The microstructural analysis of the apple snacks reveals a honeycomb-like formation in all considered samples. It was noted that the cell structure contracted after dehydration, with visible changes in turgor and a slight reduction in cell wall stretching. These modifications could lead to tissue collapse and surface cracking, particularly at higher temperatures, where the microstructure becomes denser and more compact. Although some deformation is inherent to the analysis method, the most significant structural alterations were observed in the commercial samples, particularly in the CC samples [30,31,32,33].

3.3. pH, Titratable Acidity (TA) and Soluble Solids Content (SSC)

The pH readings of fresh, SD, OD65°, OD85°, CC and CF samples are listed in

Table 2. pH, titratable acidity (TA) and soluble solids content (SSC) of fresh apple, samples dried with different methods (SD—solar; OD65°—oven at 65°; OD85°—oven at 85°) and commercial samples (CC—commercial white label; CF—commercial label).

Sample	pH	TA (% Malic Acid)	SSC (° Brix)	SSC/TA Ratio
fresh	3.8 ± 0.1 b,c	0.8 ± 0.1 b	14.3 ± 0.6 f	18.9 ± 1.2 c
SD	3.8 ± 0.1 c	0.8 ± 0.1 b	22.1 ± 0.1 e	29.3 ± 2.9 b
OD65°	3.9 ± 0.1 a,b	1.3 ± 0.1 a	28.0 ± 0.2 c	21.6 ± 1.2 b,c
OD85°	3.9 ± 0.1 a,b	1.3 ± 0.2 a	26.7 ± 0.3 d	20.9 ± 3.4 c
CC	4.0 ± 0.2 a	0.4 ± 0.1 c	33.8 ± 0.1 a	24.5 ± 1.5 b,c
CF	4.0 ± 0.1 a	0.7 ± 0.1 b	31.5 ± 0.5 b	44.3 ± 5.4 a

Data are expressed as mean ± standard deviation (n = 3). Values with different lowercase letters in same parameter column are significantly different (p < 0.05). The samples were used dehydrated before liophilization.

. All the samples, except SD, had a pH value higher than fresh ones (3.82) and there are significant differences between them (p < 0.000078) (Tables S13 and S14).

Titratable acidity is considered amongst the best predictors of sensory attributes, acid taste, overall flavor and apple flavor [34] and it is a quality feature of the fruit, distinguishing sweet from sour apples [35]. For the “normal acidity” apple category, TA can range between 0.4 and 1.0% [36], but the sweet taste of an apple perceived during the eating experience depends also on the amount of acids present. Sadler and Murphy [37] considered that the typical malic acid content for apple fruit is between 0.27 and 1.02%. The TA of fresh and SD samples was 0.8% and the highest percentages were 1.3% in both OD65° and OD85°. In commercial samples we have registered acidity of 0.4% and 0.7% for CC and CF samples, respectively. There are significant differences between them (p < 0.000001) (Tables S15 and S16).

The increase in TA values is probably due to the organic acids in apples that become more concentrated with the high temperature [38]. The importance of SSC and TA in the prediction of consumers’ acceptance of products leads to the development of some indices that show their correlation [39]. The increase in SSC and the reduction in TA result in an increase in the SSC/TA ratio, which is associated with consumer acceptance of the fruits. This index is used to determine the taste [40].

The soluble solids content (SSC) is the best objective predictor of sweetness [41] and in our analysis varies from 14.3 to 33.8 °Brix (all the values are significantly different), which for fresh samples is in agreement with the literature on the Royal Gala cultivar [42,43]. As expected, the levels of soluble solids of dried apple are higher than fresh because of sugar concentration during the process. The differences of SSC between our dried samples and the commercial ones could also be related to the growing season and storage conditions [44] (Tables S17 and S18).

There are also significant differences between SSC/AT ratio (p < 0.000005) and the obtained values, ranging from 18.9 in fresh to 44.3 in CF (Tables S19 and S20).

3.4. Phenolic Compounds

3.4.1. Total Phenolic Content (TPC)

Phenolic compounds, naturally present in foods, are good antioxidants and have beneficial effects on health [29,45]. The impact of different drying processes can be seen in Figure 2.

In our study, the TPC of fresh apple has the highest value of gallic acid equivalents (GAE)/100 g of dry matter (d.m.), due to the absence of processing that could degrade phenolic compounds. The solar drying process results in the lowest TPC among all treatments. Oven drying at 65 °C preserves phenolic content significantly better than solar drying. Oven drying at 85 °C shows even higher retention of phenolic compounds compared to SD and OD65 °C, potentially due to shorter drying times reducing the degradation. CC samples have a moderate TPC value, which is lower than oven drying samples but higher than solar drying. CF samples retain phenolic content almost as high as the fresh samples. This suggests that commercial chips undergo processing that moderately affects phenolic content, possibly involving heat exposure or additives; however, the characterization of the apple from which these snacks originate is unknown.

In summary, fresh and OD85° samples are statistically similar; OD65° and CF have similar values but differ from fresh and OD85°. SD is significantly lower than all other tested samples, which could be related to the type of dryer, which provokes a higher exposure to oxygen due to the circulation of the air, resulting in greater degradation [46]. The same results were otained by Baeghbali et al. (2019) [25] in cabin-dried samples. In the oven-dried samples, TPC values do not seem to be clearly affected by the temperature diference.

According to the bibliography the TPC values can present a high variation [19,20,25,47], which could be related to which part of the apple is utilized, whether the whole apple, with or without peel; the apple peel is more rich in phenolic compounds [19,47,48,49,50]. It is common for TPC to become concentrated during dehydration, due to the loss of water, although partial degradation of sensitive compounds or conversion into oxidized or modified forms can also occur. Exposure to certain temperatures can promote the loss of phenolic compounds in dried food [45], because they are sensitive to high temperatures [19,30]. Vega-Gálvez et al. [51] also reported that an increase in drying temperature produced higher degradation of total phenolics with respect to content in fresh apples, but Ertekin and Seydim [52] and Al Juhaimi et al. [4] showed an increase in TPC in dried apples of the varieties they studied. Thermal treatment can provoke the disruption of cell compartments and enzymatic degradation, denaturation and inactivation [53], which decreases the content and bioactivity of phenolics of a big variety of fruits and vegetables [54] (Tables S21 and S22).

3.4.2. Phenolic Profile

The separation of the phenolic compounds and 5-HMF of the samples under study, at 280 nm, is shown in Figure 3.

According to our results, the same compounds were obtained in all considered samples. We were able to identify six different classes of phenolics, in a total of 15 compounds: hydroxybenzoic acids (gallic acid and protocatechuic acid), hydroxycinnamic acids (chlorogenic acid and coumaric acid), flavanols (procyanidins and catechins), anthocyanidins (cyanidin-3-glucoside), flavonols (rutin and quercetins) and dihydrochalcones (phloridizin and phloretin) (

Table 3. Content of phenolics and 5-HMF (g/kg d.m.) in the different apple samples under study.

	Fresh	SD	Oven 65°	Oven 85°	CC	CF
Gallic acid	0.21 ± 0.03 b,c	0.07 ± 0.03 c	0.40 ± 0.16 a,b,c	0.64 ± 0.06 a,b	0.59 ± 0.15 a,b	0.66 ± 0.02 a
Protocatechuic acid	0 d	0 d	0.12 ± 0.01 c	0.33 ± 0.03 b	0.47 ± 0.07 a	0.36 ± 0.02 b
Total Hydroxybenzoic Acids	0.21	0.07	0.52	0.97	1.06	1.02
Chlorogenic acid	2.96 ± 0.12 a	1.78 ± 0.11 b	2.60 ± 0.17 a	2.98 ± 0.09 a	1.85 ± 0.21 b	1.08 ± 0.14 c
Coumaric acid	0.62 ± 0.01 a	0.29 ± 0.01 c,d	0.54 ± 0.01 b	0.61 ± 0.03 a	0.27 ± 0.02 d	0.33 ± 0.02 c
Total Hydroxycinnamic Acids	3.58	2.07	3.14	3.59	2.12	1.41
Procyanidin B1	0.36 ± 0.01 a	0.20 ± 0.03 c	0.29 ± 0.02 b	0.17 ± 0.01 c	0.03 ± 0.01 d	0.06 ± 0.01 d
Procyanidin B2	0.77 ± 0.01 a	0.15 ± 0.03 c	0.60 ± 0.06 b	0.06 ± 0.02 c,d	0 d	0.07 ± 0.05 c,d
Procyanidin trimer	0.16 ± 0.03 a	0.15 ± 0.01 a,b	0.10 ± 0.03 b,c	0.09 ± 0.03 b,c	0.08 ± 0.01 c	0.05 ± 0.01 c
Total Procyanidins	1.29	0.50	0.99	0.32	0.11	0.18
(+)-catechin	0.36 ± 0.07 a	0.17 ± 0.03 b	0.36 ± 0.05 a	0.10 ± 0.01 b,c	0 d	0.02 ± 0.01 c,d
(−)-epicatechin	0.06 ± 0.01 a	0.03 ± 0.01 c	0.06 ± 0.01 a	0.06 ± 0.01 a	0.05 ± 0.01 a,b	0.04 ± 0.01 b,c
Other Flavanols	0.42	0.20	0.42	0.16	0.06	0.06
Total Flavanols	1.71	0.70	1.41	0.48	0.16	0.24
Cyanidin-3-glucoside	0.24 ± 0.01 a,b	0.17 ± 0.05 b,c	0.27 ± 0.06 a	0.15 ± 0.01 c	0.03 ± 0.01 d	0.22 ± 0.02 a,b,c
Total Anthocyanidins	0.24	0.17	0.27	0.15	0.03	0.22
Rutin	0.45 ± 0.06 a	0.10 ± 0.03 c	0.17 ± 0.03 b,c	0.25 ± 0.02 b	0.22 ± 0.04 b	0.17 ± 0.02 b,c
Quercetin I	0.13 ± 0.02 b	0.05 ± 0.01 c	0.07 ± 0.01 c	0.09 ± 0.01 b,c	0.18 ± 0.02 a	0.21 ± 0.03 a
Quercetin II	0.07 ± 0.01 a	0.06 ± 0.02 a	0.07 ± 0.02 a	0.07 ± 0.01 a	0.07 ± 0.02 a	0.07 ± 0.01 a
Total Flavonols	0.65	0.21	0.31	0.41	0.47	0.45
Phloridizin	0.51 ± 0.02 b	0.69 ± 0.06 a	0.43 ± 0.03 b	0.45 ± 0.03 b	0.23 ± 0.03 c	0.36 ± 0.03 b,c
Phloretin	0.34 ± 0.02 a,b	0.42 ± 0.06 a	0.22 ± 0.01 b	0.27 ± 0.03 a,b	0.31 ± 0.05 a,b	0.23 ± 0.02 b
Total Dihydrochalcones	0.85	1.11	0.65	0.72	0.54	0.59
Total Polyphenols	7.24	4.33	6.30	6.32	4.38	3.93
OTHER IDENTIFIED COMPOUND
5-HMF	0 a	0 a	0 a	0.18 ± 0.03 a	0.56 ± 0.07 a	4.30 ± 0.58 b

Data are expressed as mean ± standard deviation (n = 3). Values with different lowercase letters in same parameter line are significantly different (p < 0.05).

).

The predominant compound in all samples is chlorogenic acid (OD85° > fresh > OD65° > CC > Solar > CF), constituting, with coumaric acid, the most abundant group, the hydroxycinnamic acids. Similar results were obtained by Juhart et al. [55] and Wojdylo et al. [21].

In fresh apple, the total phenolic content was 7.24 g/kg d.m., consisting of 49.45% hydroxycinnamic acids, 23.62% flavanols, 11.74% dihydrochalcones, 8.98% flavonols, 3.31% anthocyanidins and 2.90% hydroxybenzoic acids. Then oven-dried samples, at 65 °C and 85 °C, had higher phenolic content, with 6.30 and 6.32 g/kg d.m., respectively. For OD65 °C samples, hydroxycinnamic acids (49.84%) were followed by flavanols (22.38%), dihydrochalcones (10.32%), hydroxybenzoic acids (8.25%), flavonols (4.92%) and anthocyanidins (4.29%). In OD85 °C samples, there was a slight variation compared to OD65 °C samples, with hydroxybenzoic acids (15.35%), dihydrochalcones (11.39%) and flavanols (6.49%).

For solar-dried samples, the total phenolic content was 4.33 g/kg d.m., consisting of 47.81% hydroxycinnamic acids, 25.64% dihydrochalcones, 16.17% flavanols, 4.85% flavonols, 3.93% anthocyanidins and 1.62% hydroxybenzoic acids. In the case of commercial samples, CC registered 4.38 g/kg d.m. and CF registered 3.93 g/kg d.m. The order of almost all compounds was the same: hydroxycinnamic acids (48.40% for CC and 35.88% for CF), hydroxybenzoic acids (24.20% for CC and 25.95% for CF), dihydrochalcones (12.33% for CC and 15.01% for CF) and flavonols (10.73% for CC and 11.45% for CF). The difference was in flavanols (3.65% for CC and 6.11% for CF) and anthocyanidins (0.68% for CC and 5.60% for CF). Our results are in accordance with other authors [16,56,57,58].

It is also consistent across all samples that the groups with the lowest concentration of phenolics are anthocyanidins and flavonols. Regarding anthocyanidins, this may be due to the fact that we were only able to identify one compound, cyanidin-3-glucoside. In the case of flavonols, although we identified three different compounds—rutin, quercetin I, and quercetin II—the low concentration found can be attributed to the fact that quercetins and their derivatives are primarily located in the peel, as noted by Wojdylo et al. [21] and Joshi et al. [58]. Thus, while these compounds are present because we used the entire apple, they have low representation due to the higher proportion of pulp [20].

Comparing the phenolic profile with the total phenolic content results, a correlation between the concentrations can be observed. Fresh apples have the highest amount of phenolics in both analyses. They are followed by the oven-dried samples at 85 °C, 65 °C and solar-dried samples, suggesting that the variation in concentration is not only related to temperature but also with the drying method. For the commercial samples, the total phenolic content in CF is higher than in CC. However, in the phenolic profile analysis, CC shows a higher phenolic concentration than CF.

For the confirmation of the presence of 5-HMF, HPLC was conducted again, with a standard compound. The samples from a new extraction were evaluated.

Considering the obtained results, the identification of the 5-HMF compound (0.56 ± 0.07 g/kg d.m. for CC and 4.30 ± 0.58 g/kg d.m., for CF) explains the difference in the phenolic compound quantification found in the commercial samples. This compound is formed either by acid-catalyzed degradation of reducing sugars or via the Maillard reactions and is often used as an indicator of heat processing and storage conditions [59,60]. Several studies say that the presence of 5-HMF in samples is normally related to the heat or prolonged storage, impacting the overall phenolic content [61,62,63,64,65]. The increase in compounds like 5-HMF in higher-temperature treatments is an indicator of degradation and the formation of undesirable by-products [66]. Their actual impact on the human diet has not yet been fully elucidated, but it has been shown that HMF at high concentrations is cytotoxic, irritating to the eyes, the upper respiratory tract, skin and mucous membranes.

3.5. Acrylamide Content

The principal factors influencing acrylamide formation are temperature and time, sugar and asparagine content and industrial processing [67]. It has been added to the list of food-borne toxicants since 2002 by the Swedish National Food Administration. Acrylamide typically forms at temperatures above 120 °C during the Maillard reactions [60,61] and longer times or higher temperatures in commercial processing can significantly increase its levels [62,63]. Regarding sugar and asparagine content, apples with a high quantity of reducing sugars, such as glucose and fructose and asparagine, which are precursors of acrylamide formation, are particularly susceptible. Differences in sugar content and processing conditions may explain variations between samples. Additionally, industrial processing often involves more severe conditions, such as higher temperatures and longer drying times, compared to laboratory or homemade drying methods, which contributes to elevated acrylamide levels in commercial products.

Table 4. Acrylamide content of fresh apple, samples dried with different methods (SD—solar; OD65°—oven at 65°; OD85°—oven at 85°) and commercial samples (CC—commercial white label; CF—commercial label).

Sample	Acrylamide Content (µg/kg d.m.)
fresh	n.d.
SD	n.d.
OD65°	25.4 ± 12.7 b
OD85°	n.d.
CC	147.2 ± 57.3 a
CF	98.7 ± 39.4 a,b

Data are expressed as mean ± standard deviation (n = 3). Values with different lowercase letters are significantly different (p < 0.05) (Tables S23 and S24).

presents data on the presence of acrylamide in Royal Gala apples processed under different methods. The values are expressed in micrograms per kilogram of dried matter (µg/kg d.m.) and “n.d.” indicates “not detected”.

Acrylamide was not detected (n.d.) in fresh or solar-dried (SD) apple samples, which is consistent with expectations, as acrylamide formation typically requires exposure to high temperatures, conditions not reached during solar drying or in raw, unprocessed apples. Although oven-drying at 65 °C occurs well below the typical threshold temperature for acrylamide formation (120 °C), the presence of trace amounts may be attributed to the prolonged drying time, which could promote limited formation [68]. The relatively low value observed likely reflects restricted Maillard reaction activity, due to the mild thermal conditions [69].

In samples dried at 85 °C (OD85°), acrylamide was also not detected. This may be due to insufficient overall thermal input or other limiting factors such as shorter drying duration or higher moisture content, which can inhibit acrylamide development despite the elevated temperature compared to OD65° [70].

Notably, commercial sample CC exhibited the highest acrylamide content (147.2 ± 57.3 µg/kg d.m.), accompanied by considerable variability. This discrepancy suggests inconsistencies in industrial processing conditions [71]. In contrast, commercial sample CF showed a lower but still appreciable level of acrylamide. These findings indicate that the acrylamide levels in commercial snacks are likely influenced by high-temperature industrial drying processes, which favor its formation.

Overall, this analysis underscores the need to balance sensory attributes and food safety when developing dried apple snacks, particularly in relation to processing conditions that affect acrylamide content [72].

3.6. Antioxidant Activity (AA)

Two tests were conducted to determine the antioxidant properties of the apple snacks, DPPH and ABTS radical scavenging assays. The antioxidant activity values and their bioaccessibility rates in the fresh and dried samples are shown in Figure 4.

According to the DPPH radical scavenging inhibition assay, fresh apple exhibited a higher value of antioxidant ability with 83.31%. In the dried apples it was demonstrated in the order OD65° > SD > OD85° > CC > CF (p < 0.000001) (Tables S25 and S26). Ergun [19] obtained similar results in solar-dried apples of the Amasya and Starking varieties, where high AA values were registered, comparing five cultivars grown in the same location and revealing the differences between peel and pulp in the fresh, oven-dried and solar-dried samples.

Based on this evaluation it was found that the drying process resulted in the degradation of antioxidants in the apple, which is in agreement with the reports of Tomaino et al. [73] and Ma et al. [20]. The reduction in AA registered in OD85°, compared with OD65° and SD, is not significant but can be associated with termal degradation related to temperature. The results may suggest that CC and CF were dried at high temperature, as the AA value is the lowest compared with the other samples. However, it could also be related to the presence or absence of the peel [19,47,74], and in the case of commercial snacks, we do not have such clear information.

For the ABTS assay, the samples had similar behavior to the DPPH analysis. The highest scavenging ability shows the same trend of fresh apple (40.33 µmol Trolox/g d.m.), followed by, in decreasing order, SD > OD65° > OD85° > CC > CF (p < 0.000020) (Tables S27 and S28).

Wojdyło et al. [75] and Uğurlu and Bakkalbaşı [56] found different results from ours in their apple drying studies, where antioxidant activity increased with temperature. On the contrary, Ma et al. [20] in apple peel dehydration, Kidoń and Grabowska [16] in red-fleshed apple and Sonawane and Arya [76] in Jambhul and wood apple had results similar to ours, where antioxidant activity decreased with temperature. Once again, the utilization of the whole apple could interfere with the obtained results. Many authors compare the analyses of TPC and AA, and their increasing correlation has been reported during food dehydration [20,49,51]. However, in our study this relatioship is not obvious. It was found that the antioxidant ability of samples treated at different temperatures had no positive correlation with phenolic content, especially in SD samples, which might be because the free radical scavenging ability of the apple after drying was not only related to the total content of phenolic compounds, but also might be related to the non-heat-sensitive antioxidant components and the concentration of sugars. Other reasons could include the varieties studied and the means of preparation of the extracts, which are different among studies [77,78].

3.7. Sugar Content

The predominant sugars found in apples are glucose, fructose, sucrose and sorbitol, which contribute to the sweetness and flavor of the fruit [79]. Figure 5 shows the composition of soluble sugars (mg/100 g d.m.) of the samples studied.

The most abundant sugar in all tested samples was fructose, followed by sucrose or glucose and sorbitol. In apples, only a small fraction of fructose is incorporated into starch. Instead it accumulates in the vacuoles of apple cells, and as a result, fructose is always higher than glucose in fruit tissue [80].

In terms of glucose, fresh apple has the biggest amount followed by CF, OD65°, SD, OD85° and CC (p < 0.001175) (Tables S29 and S30). For fructose, the highest value was also registered in fresh apple, then CF, OD65°, OD85°, CC and SD, with no significant differences (Tables S31 and S32). This tendency is switched for sucrose content, where the highest value was registered in CC, followed by fresh apple, OD65°, OD85°, SD and CF (p < 0.000010) (Tables S33 and S34).

In relation to sorbitol, the results are 1.18 mg/100 g d.m. for CF and 0.95 mg/100 g d.m. for CC, showing that commercial samples have the highest amount. For fresh samples the sorbitol content was 0.78 mg/100 g d.m., followed by OD65°, SD and OD85° (p < 0.000519) (Tables S35 and S36).

Comparing the sum of the sugars in the different samples, fresh Royal Gala has a total of 43.25 mg/100 g d.m., followed by CC with 33.81 mg/100 g d.m., OD65° with 33.37 mg/100 g d.m., CF with 31.56 mg/100 g d.m., OD85° with 25.92 mg/100 g d.m. and SD with 25.01 mg/100 g d.m. Despite the variation of sugar content related to the drying process, there is no clear relationship with browning index. We expected to obtain a concentration effect because the drying process typically concentrates sugars by removing water, leading to an increase in sugar content [81]. However, it does not occur in our study, which can be explained by several factors, such as the exposure of sugars to heat and air during drying, which can cause its degradation. The degradation processes, like caramelization or Maillard reactions, can cause a reduction in the measurable sugar content and contribute to the color and flavor changes in dried apple [82].

3.8. Vitamin C Content

Apples are generally poor in vitamin C, and according to several studies, the Royal Gala is one of the cultivars with the lowest amount [83,84,85,86]. Some authors also show that the content of vitamin C decreases with the storage duration [87,88], and Arora et al. [89] found that Royal Gala was the least stable, with almost 83% degradation.

Associating these factors with the exposure to different drying temperatures and the use of apples with a high ripeness index, since conservation samples purchased from a local supermarket were used, it is justified that we only find vitamin C in fresh apples, but with very reduced concentration (1.7 ± 0.2 mg/100 g d.m.), and we were unable to identify it in any of the dehydrated or commercial apples.

4. Conclusions

In this study, the color, microstructure, acidity, sugars, phenolic compounds, antioxidant activity, 5-HMF, acrylamide and vitamin C content of fresh, solar-dried, oven-dried and two commercial Royal Gala apple snacks were evaluated. The ΔE and BI analyses proved the observable differences in the color of the studied samples, especially in commercial ones, which are significantly browner. Similar variations were observed in microstructure, with denser and more compact surfaces in CC and CF samples. The results indicated that apple snacks maintained some acidity, despite a slight increase in malic acid percentage in OD65°, OD85° and CC samples. The drying process led to a concentration of sugars in the samples compared to fresh apples, observable in soluble solids content but not in individual sugar quantification. Antioxidant activity, in both DPPH and ABTS tests, shows the same tendency, with low values in dried snacks compared with fresh apple. Also, phenolic compound content decreased across all drying methods due to the drying process, temperature increases and processing time, particularly in SD samples, but the same compounds were obtained in all considered samples. 5-HMF, which is not present in fresh, SD and OD85°, was found in OD65° and commercial samples. Due to its toxicity and potential carcinogenic activity, its content in food should be monitored. Acrylamide is a potential health concern due to its classification as a probable human carcinogen by the International Agency for Research on Cancer (IARC). The high variability of quantification values in commercial samples, especially CC, emphasizes the need for better control over processing conditions. The dried samples with undetected or lower acrylamide levels, such as SD, OD65° and OD85°, are preferable for the consumer. Research in these areas is improving and continues to be assessed to minimize acrylamide formation, which should be mitigated.