From Sun to Snack: Different Drying Methods and Their Impact on Crispiness and Consumer Acceptance of Royal Gala Apple Snacks

Abstract

This study explores the acoustic, mechanical and sensory characteristics of Royal Gala dried apples, with a special focus on the potential of solar drying as a sustainable processing method. Apple samples were subjected to different drying techniques, being solar dried (SDA) or oven dried (ODA), with two industrially processed commercial products (CCA—commercial apples C and CFA—commercial apples F) included. The samples were analyzed using acoustic measurements, X-ray diffraction (XRD) and sensory evaluation to assess textural properties and consumer perception. Acoustic analysis revealed that crispier samples produced louder and higher-frequency sounds upon fracture, showing strong alignment with sensory assessments. X-ray diffraction indicated an increase in crystallinity during dehydration, with a shift in the amorphous peak toward lower angles, and reduced intensity, reflecting progressive water removal. Sensory evaluation showed varying degrees of crispiness among the samples, in the following order: CFA > SDA > CCA > ODA. Consumer testing highlighted greater acceptance and consensus for SDA and ODA samples in terms of texture and overall appeal, whereas CCA and CFA received more polarized opinions. These findings demonstrate how different drying methods influence the structural and textural properties of dried apples. Solar drying was shown to be a promising sustainable alternative; as it uses a renewable energy source, it has a low operating cost and simple maintenance. It allows farmers and small producers to process their own food, adding value and reducing post-harvest losses, preserving desirable textural attributes and achieving high consumer acceptance.

1. Introduction

Drying is a complex process that involves simultaneous heat and mass transfer, supplemented by physical, chemical and phase change transformations [1].

The temperature and duration of the drying process play a crucial role in determining the quality of the final product. Typically, fruits are dried at temperatures between 50 °C and 70 °C [2], while vegetables may require slightly lower temperatures to preserve nutrients [3]. The drying time can vary from several hours to more than a day, depending on the type and size of the food and the drying method used (e.g., air drying, oven drying, or using a dehydrator) [4].

Despite its effectiveness, drying can cause some damage to food. High temperatures or prolonged drying may lead to the degradation of heat-sensitive vitamins such as vitamin C and some B vitamins. Additionally, drying can cause changes in texture, color, and flavor, which may affect consumer acceptance [5]. However, when performed carefully, drying can retain much of the food’s nutritional value while extending its shelf life [6].

In the food industry, drying is a widely used method for producing apple snacks, as it reduces logistics costs and lowers water activity [7].

Dried apple snacks have gained popularity as a convenient and nutritious alternative to traditional processed snacks. They retain the natural sweetness and beneficial nutrients of fresh apples while offering portability [8]. However, the drying process significantly alters the structural properties of apples, affecting their texture, crispness and associated sound profile. Different drying methods can lead to variations in moisture content, porosity and mechanical strength, ultimately influencing the way these snacks break and sound when bitten [9,10].

Solar dehydration stands out as a sustainable and energy-efficient technique for food preservation, such as for dehydrated apples. This method operates with renewable solar energy, reducing the consumption of non-renewable resources and minimizing the carbon footprint associated with conventional industrial processes [4]. Additionally, solar dehydration has been shown to better preserve the structural integrity and moisture balance of foods, resulting in products with a crispy texture and higher sensory acceptance by consumers [11]. In solar dehydration, temperatures typically range from 40 °C to 70 °C, depending on the design of the solar dryer, the intensity of sunlight and ambient conditions [12]. These temperatures are ideal for preserving fruits and vegetables, allowing for moisture removal without excessive nutrient loss. To achieve a crispy texture—a key factor for higher sensory acceptance—drying times must be carefully controlled. In sunny conditions, it usually takes between 6 and 10 h to dehydrate thin apple slices to a crispy state using an indirect solar dryer. Thicker slices or foods with higher moisture content may require longer. Proper air circulation and consistent temperature are essential to ensure uniform drying and to develop the desired texture that appeals to consumers [13]. Thus, plus being an eco-friendly solution, solar drying contributes to the production of high-quality fruit snacks, aligning with current demands for more sustainable and environmentally respectful agricultural practices [14].

Texture properties of food are fundamental sensory attributes that considerably influence consumer perception and acceptance [15]. Consumers often trust texture as a key quality indicator, associating specific textural characteristics with freshness, processing methods and overall product quality [16]. For instance, crispness and crunchiness in snacks are linked to freshness and desirability, whereas softness or excessive hardness may lead to negative evaluations [17].

In the case of dried apple snacks, texture is particularly important, as the dehydration process alters the cellular structure, influencing crispness, porosity and mechanical resistance [18]. The way these snacks fracture, break or produce sound upon biting directly affects consumer expectations and satisfaction [19]. Studies have shown that acoustic feedback from food can reinforce positive sensory experiences and increase perceived quality [20]. This knowledge can guide food manufacturers in optimizing drying processes to enhance the sensory appeal of their products, ensuring a desirable balance between texture, taste and overall eating experience [21].

The sensory experience of food extends beyond taste and aroma, incorporating visual, textural and auditory indications that influence consumer perception and acceptance [22]. Among these, sound plays a crucial role in shaping the perceived crispness and freshness of snacks, particularly those that rely on a crispy texture for consumer appeal. The auditory properties of food have been extensively studied in different products such has cheese [23], biscuits [24], French fries [25], bakery products [26] and apple [27,28,29], yet their impact on dried fruit snacks remains underexplored [30,31,32]. By examining the frequency and intensity of the sound produced upon biting, a correlation between acoustic properties, textural attributes and consumer acceptance is sought [33].

This study aims to compare the mechanical, acoustic and sensory characteristics of Royal Gala apples dried with solar, oven and industrially processed methods. By analyzing the acoustic properties of different drying methods, it is aimed to determine how sound contributes to the sensory experience and overall product desirability. Understanding these auditory cues could provide valuable insights and practical implications for food manufacturers in optimizing the textural and sensory qualities of dried fruit snacks, enhancing consumer satisfaction and market competitiveness.

2. Materials and Methods

2.1. Apples and Drying Process

Royal Gala apples for this study were sourced from a local market. Solar-dried samples (SDA) were prepared using a custom-built indirect cabinet solar dryer [14], with the temperature of 65 °C for 6 h, while oven-dried samples (ODA) were processed with a Captain Jerky 110 food dehydrator (Klarstein, Berlin, Germany), with the temperature of 85 °C for 4 h. Prior to each drying session, empty trays were weighed to ensure precise measurements. The preparation involved selecting, washing, coring and slicing the apples into rings using an electric slicer, maintaining a consistent thickness of approximately 3.0 ± 0.1 mm. These rings were then arranged in a single layer on the trays. During the drying process, the trays were periodically unloaded and weighed until the weight remained consistent in two consecutive measurements, indicating the end of the drying process. Weight loss values were normalized relative to the initial sample weight [34].

For comparison, two commercial products were included: CCA—commercial apples C, a store-brand snack from a major national supermarket, and CFA—commercial apples F, a specialized dried food brand, both of which explicitly list Royal Gala apples as principal ingredient. The thickness of the CCA dried snacks is similar to the SDA and ODA (1.0 ± 0.1 mm); however, the CFA samples are slightly thicker (1.5 ± 0.1 mm).

Despite the absence of information on the drying method, hot air drying continues to be the most widely adopted on an industrial scale due to its balance between cost, energy efficiency and mass production capacity [35,36,37].

2.2. Crispiness Evaluation

The crispiness parameter was evaluated using puncture tests that is a standard method to assess the textural properties of food [38]. For that purpose, the dried apple snacks were measured using a texture analyzer (TA. XTplus; Stable Micro Systems, Godalming, UK). The equipment was set with an 8 mm spherical ball probe. The heavy-duty platform was positioned in such a way that the ball probe passed centrally through the crisp support rig, which had a diameter of 1.5 cm. During the puncture test, the probe ran at a speed of 1 mm s⁻¹ for 5 mm into the sample. The test was triggered by a force of 0.049 N. Ten measurements were performed for each drying treatment, ensuring a sufficient sample size for statistical analysis. This test measures the force required to puncture the sample, providing insights into its mechanical resistance and crispiness [10].

2.3. Sound Measurements Apparatus

The sound was acquired during the breaking of each dried sample during puncture tests, simulating one bite (Figure 1).

The technique consisted of using a microphone to capture the sound produced by the breaking of each dried snack and transferring it to a computer. The microphone was directed at a right angle to the test piece, considering the angle at which the microphone is sensitive, while the distance was standardized. The data were analyzed with the free software Audacity (Version 3.7.3) with no manipulation or filtering. Sound assessment was carried out in terms of amplitude/time plots and sound intensity and frequency analysis graphs of each sample [17].

2.4. X-Ray Diffraction

X-ray diffraction (XRD) patterns were acquired, at room temperature, by a PANalytical X’Pert Pro diffractometer, equipped with X’Celerator detector and secondary monochromator in Ꝋ/2Ꝋ Bragg–Brentano geometry. The measurements were carried out using 40 kV and 30 mA, a CuKα radiation (λꝊ₁ = 1.54060 Å and λꝊ₂ = 1.54443 Å), 0.017°/step, 100 s/step, in a 10–60° 2Ꝋ angular range. The resulting diffractograms were analyzed using HighScore 4.8 software through baseline and peak deconvolution.

2.5. Sensorial Analyses

The sensory evaluation of the dried apple samples was conducted following a protocol approved by the Ethics Committee of the Universidade de Trás-os-Montes e Alto Douro (Doc100-CE-UTAD-2024). A consumer study was carried out with participants recruited from the university student population, who were selected based on their interest in participating in the project. The study included 100 participants (94 Portuguese, 3 Brazilians, 2 Chinese and 1 Spanish) young adults aged 18–25 (88%) and 66% were female. All participants provided informed consent before taking part. The session began with a questionnaire collecting demographic information, such as gender and age, along with details about intolerances and allergies. The evaluations were performed in a standardized testing room (ISO 8589:2007), with each session lasting approximately 15 min.

All dried apple samples were presented at the same time on white plates divided into four sections, with each having a three-digit code (SDA, ODA, CCA and CFA). Data were collected via mobile phones using an online descriptive questionnaire (Google Forms^®), which included a Check-All-That-Apply (CATA) section featuring 5 descriptors. The CATA test is a sensory evaluation method where participants are asked to check the texture attributes that apply to a given sample.

Consumers were additionally asked to rate the products based on overall liking and intention to consume using a 9-point hedonic scale (1 = “dislike extremely,” 9 = “like extremely”). To ensure accurate evaluations, participants were provided with water to refresh their palate between samples [39,40].

2.6. Statistical Analysis

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

3. Results and Discussion

3.1. Drying Kinetics

The drying kinetics curves for apple slices (SDA and ODA) are shown in Figure 2.

The graphs show the weight loss trends for solar-dried (SDA) and oven-dried (ODA) apple samples over the drying period until constant weight. Both drying methods exhibit a steep decline in weight within the first hour, indicating high moisture evaporation at the beginning of the process [41]. The trend is similar for both SDA and ODA, suggesting that the early drying phase is ruled by surface moisture removal [42,43]. Similar results were obtain in other studies of dried apple [44,45,46].

After the initial drop, the weight loss rate decreases gradually, reflecting the transition from surface moisture evaporation to water removal. By the third hour, weight loss stabilizes, indicating that most free water has been removed [47,48].

Both drying methods reach a similar final weight loss. ODA shows slightly higher moisture loss at the first hour, possibly due to temperature (approximately 85 °C for ODA samples and 65 °C for SDA samples) or the efficiency of the air circulation within the oven. Also, the total weight loss is higher in ODA samples and the drying process is quicker (6 h for SDA samples and 4 h for ODA samples before a constant weight is reached).

The drying dynamics of SDA and ODA are comparable, with both showing rapid early moisture loss followed by a slower stabilization phase. The choice between methods may depend on energy efficiency, product texture and consumer preference rather than significant differences in dehydration performance [49,50].

3.2. Crispiness

For definition, crispiness is a property that describes how a food breaks when the force that is applied exceeds the sample’s resistance. Figure 3 shows the differences in crispiness in the dried samples under study.

The average crispiness of the snacks was approximately 4, 5, 6 and 9 newtons for ODA, CCA, SDA and CFA samples, respectively.

The results indicate that SDA, ODA and CCA do not differ significantly from each other (p > 0.05), while CFA presents a significantly higher crispiness value (p < 0.05). The absence of statistical differences between SDA, ODA and CCA suggests that, despite the different drying conditions, such as temperature, the impact on final crispiness was similar. This may indicate that other factors, such as the original composition of the raw material or the storage history (in the case of CCA), had no influence on the final product. These results reinforce the idea that less controlled drying methods, such as SDA, can still produce products with acceptable textural characteristics, comparable to those obtained by more technological methods, such as ODA, and even like the commercial samples analyzed (CCA).

CFA samples were significantly (p < 0.05) crispier than those from any other drying method, suggesting that this specialized brand employs advanced drying techniques that enhance crispiness.

With the highest crispiness values, CFA snacks are the best option for consumers who prioritize crispy texture. In contrast, SDA, CCA, and ODA samples, which exhibited lower crispiness, retained higher moisture content with less structural damage, making them more suitable for those who prefer a chewier or not-so-hard texture [16].

3.3. Sound Emission Analyses

Textural properties are perceived through a combination of visual, tactile, kinesthetic and auditory sensations [51,52]. The sounds produced during food consumption are crucial sensory indicators that influence consumer perception and acceptance. Acoustic properties, such as crunchiness and crispiness, contribute to the overall eating experience and play a vital role in determining the quality of dried apple snacks [53].

3.3.1. Sound Amplitude

During the application of a force to a noisy product, a sound wave is produced [54]. The amplitude of displacement of the molecules is proportional to the amount of force that is applied to the sample [55]. This can be graphically illustrated by plotting the amplitude of the sound wave vs. time (Figure 4).

The amplitude represents the loudness or intensity of the sound, while the time axis shows the duration over which the sound is produced. The vertical scale (amplitude) ranges from approximately −1.0 to +1.0, likely normalized for comparison. The horizontal scale (time) is in milliseconds, showing the temporal evolution of the sound.

SDA samples have low amplitude peaks, indicating a softer, less intense sound, with short-lived sound events with minimal sustained vibrations. The low amplitude and short duration suggest that SDA samples have moderate crispiness but do not produce a very loud or prolonged sound.

ODA samples registered moderate amplitude peaks, higher than SDA but lower than CCA and CFA, with shorter bursts of sound, like SDA but slightly more pronounced. Oven-dried apples produce a louder sound compared to solar-dried apples, indicating slightly higher crispiness, but still relatively soft.

CCA samples showed higher amplitude peaks compared to SDA and ODA, suggesting a louder sound, with more sustained sound events, indicating longer-lasting vibrations. CCA dried apples exhibit greater crispiness, as evidenced by the louder and more prolonged sound.

CFA samples indicated high amplitude peaks, substantially louder than the other drying methods, with long-lasting sound events with sustained vibrations. The loudest and most prolonged sound indicates extremely high crispiness.

The results show some differences compared to the crispiness chart. The fracture sound depends on the physical characteristics of the material, like rigidity, density and cellular structure, while the crispiness is influenced by mechanical properties [17]. It involves not only the sound, but also the mechanical resistance of the food to compression and fracture, as well as the residual texture after breaking [56].

The compression and puncture testing with acoustic envelope detector (AED) [26,57,58,59] and impact testing [24,29,60] are ideal for assessing the acoustic response of food. However, it requires specialized equipment (compression/puncture tests require a texturometer and an AED) or may not provide detailed information about progressive deformation (impact testing). In line with this, some authors developed ultrasonic testing [61] and resonance frequency analysis [62] methods that are suitable for non-destructive evaluation of internal structure and material properties. These techniques are adequate for samples with relatively uniform shapes and which require precise calibration and coupling of the transducers. The most simple and practical methods for rapid quality control and product development are the drop test with sound recording [63], microphone acoustic assessment [23,25,32,64,65], used in the present study, and the use of a sound-attenuated booth [66,67]. Their limitations can be related to their lower precision and the need for data processing software.

Although the sound of breaking is often used as an indirect indicator of crispiness, they are not synonymous. A food may make a loud, crisp sound when breaking but not be perceived as “crispy” by consumers if, for example, it is too hard or produces a sticky residue in the mouth [68]. On the other hand, foods with low sound emission can be considered crispy if they have a crumbly texture that is pleasant to the palate [67].

3.3.2. Sound Frequency

Sound frequency is a fundamental characteristic in acoustics, essential for understanding the nature and impact of sound. It provides insights into the origin of sound sources, how sound propagates through different media and its effects on human perception [69]. Based on the sound frequency analysis graphs, distinct patterns can be observed for each Royal Gala apple dehydration method (Figure 5).

The x-axis represents frequency in Hertz (Hz), ranging from approximately 100 Hz to 10,000 Hz. The y-axis represents the sound level in decibels (dB), with lower values indicating lower sounds. Each spectrum shows peaks and troughs that reflect the distribution of sound energy at different frequencies.

The overall spectrum of SDA samples is relatively smooth, with moderate energy levels across mid-to-high frequencies. The presence of peaks suggests some crispiness, but the overall energy distribution is not very pronounced. There is a significant frequency peak above 10,000 Hz, indicating considerable crunchiness. The smooth transition in the curve suggests a more uniform crispiness structure.

Energy is concentrated at lower frequencies, with a gradual decline toward higher frequencies in ODA samples, which show similar behavior to that of SDA, with strong peaks at high frequencies, which also suggests, although less pronounced, a crispy texture. Small variations may indicate differences in cellular structure due to the controlled oven temperature.

More prominent peaks are seen in CCA samples with higher-frequency components (>5000 Hz), showing increased energy compared to SDA and ODA. They present a less uniform pattern, with oscillations before the main peak. This may indicate a less homogeneous texture, with variations in crispiness throughout the snack.

High-frequency components (>5000 Hz) show significantly more energy in CFA samples compared to the others, with sharp, pronounced peaks observed around 1000 Hz and 3000 Hz, that indicate very high crispiness. It has a similar behavior to CCA, but with a more defined peak pattern, which may suggest a denser microstructure.

For a more detailed analysis of the crispiness of the apple snacks, once is generally associated with high-frequency sounds, typically above 5000 Hz, a closer look of the spectra at higher frequencies was made (Figure 6).

SDA and ODA samples exhibit well-defined peaks in the range of 10,000 Hz to 12,000 Hz, which suggests a crispy texture and uniform breakdown of the snack structure. They have smoother transitions at the beginning and a swift increase at the maximum peak, suggesting a sharper crisp snap. Our results are in concordance with the bibliography, once the measured frequencies range of crispy products should extend at least 12,000 Hz, according to Duizer (2001) [54]. Similar results were obtain in apple by Piazza and Giovenzana (2015) [57], Banaszak and Pawlowski (2018) [59], Marzec et al. (2009; 2010) [61,64] and Demattè et al. (2014) [70].

On the other hand, CCA and CFA samples also present peaks in the same frequency range, but with more variations before the maximum peak, indicating possible differences in texture and breakdown pattern. They show oscillations before the maximum peak, which may indicate irregular fragmentation of the snack structure, resulting in less consistent crispiness.

In terms of the impact of the final product, SDA preserves some structural integrity, resulting in moderate, high-frequency sound. The smoother drying process can better preserve the apple’s cellular structure, leading to a well-defined and crispy characteristic sound. ODA degrades the structure more, leading to lower-frequency dominance sound, similar to solar dehydration, but which may have a slightly different crispiness due to more uniform temperature and humidity control. CCA achieves moderate crispiness through controlled drying processes, and CFA uses techniques to maximize high-frequency sound, producing the crispiest texture. Industrial processing can affect the final texture, making the crispiness less uniform and variable between brands [71].

The analysis of the graphs confirms that dehydrated apple snacks present distinct crispiness patterns depending on the drying method used. SDA and ODA appear to maintain a more uniform and intense crispness, while CCA and CFA may present variations in texture.

3.4. X-Ray Analyses

The X-ray diffraction (XRD) pattern provides insights into the structural differences among the different dried Royal ala apple samples [72]. In Figure 7, the diffraction patterns of fresh and dried apples are shown.

All diffraction patterns show broad peaks, indicating the absence of long-range order characteristic of an amorphous material. However, peak positions change substantially from fresh to dehydrated apples and smaller differences can be observed between samples with different dehydration techniques. Usually, in amorphous materials such as silica glass, two peaks can be observed [73]. The first one, around 2Ꝋ ~ 21–24° (d ~ 3.7–4.2 Å) corresponds to the scattering from the nearest neighbor lattice points, i.e., the average distance between silicon atoms. The diffractograms were analyzed using HighScore 4.8 software to deconvolute the peaks, calculating peak positions and calculate integral breadth (

Table 1. XRD peak positions and crystallite sizes of fresh and dried samples.

	Peak 1		Peak 2		Peak 3		Peak 4		Crystallite Size
Sample	(2Ꝋ)	d (Å)	(2Ꝋ)	d (Å)	(2Ꝋ)	d (Å)	(2Ꝋ)	d (Å)	(Å)
Fresh	27.74°	3.214	39.22°	2.295	72.51°	1.303	-	-	7 ± 2
SDA	16.43°	5.389	20.60°	4.308	37.83°	2.376	43.90°	2.061	18 ± 2
ODA	17.58°	5.040	20.36	4.359	25.08	3.548	34.75°	2.579	16 ± 4
CCA	16.17°	5.476	19.55°	4.536	-	-	-	-	18 ± 2
CFA	17.46°	5.075	21.66°	4.097	38.16°	2.357	44.86°	2.019	24 ± 4

SDA—solar-dried apples; ODA—oven-dried apples; CCA—national supermarket apples; CFA—specialized dried food brand apples.

).

Crystallite size was calculated using the Scherrer model (Equation (1)).

$$t_c={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(1)

where t_c is the crystallite size, K = 0.9, λ = 1.5418 Å, and β is the integral breadth corrected with instrumental broadening (Equation (2)) and Ꝋ is the peak position (°).

$${\beta }^2={\beta }_{\exp }^2-{\beta }_{inst}^2$$

(2)

The XRD patterns of fresh fruit show broad amorphous peaks due to water and non-crystalline components. The long range order, in general represented by crystallite size, is very short (7 Å), typically related to highly amorphous material. The first peak is located at 27.7° (d = 3.2 Å), which could represent average distances between organic molecules (e.g., starch or sugars) intermediated by water molecules through hydrogen bonds. In dehydrated fruit the amorphous peak position shifts toward lower angles (higher d values) due to changes in intermolecular distances and diminishing intensity (the removal of water molecules replaces the stronger hydrogen bonds by weaker Van der Waals forces). Consequently, the first peaks shift towards 2θ ~ 16.1–17.5° (d ~ 5.06–5.50 Å). At the same time, sugars became more concentrated, leading to increases in the crystallinity denoted by the increase in the crystallite size (18 Å). Clearly, in CFA samples, the drying process was taken to a different level, leading to a further increase in the crystallite size (24 Å).

3.5. Relation Between Crispiness, Sound and X-Ray Diffraction

The relation between crispiness, sound and X-ray diffraction in dried apples provides valuable insights into their texture and structural properties (

Table 2. Relation between crispiness, sound and x-ray diffraction of the samples under study.

	Acoustic Signal	Acoustic Analyses	X-Ray
SDA	low amplitude and short duration signal; weak and brief sound	sound energy concentrated at lower frequencies, with little energy at higher frequencies	increase in crystallinity, comparing to fresh
ODA	moderate amplitude and short duration signal; louder than SDA, but still relatively weak sound	sound energy like SDA, but slightly more concentrated at low frequencies	increase in crystallinity, comparing to fresh
CCA	moderately high amplitude and prolonged duration signal; more intense and long-lasting sound	more distributed sound energy, with significant peaks at mid-high frequencies	increase in crystallinity, comparing to fresh
CFA	high amplitude and long duration signal; very intense and prolonged sound	greater energy at high frequencies, with well-defined peaks	highest increase in crystallinity, comparing to fresh and other drying processes
Principal Conclusions	crispness is directly related to the intensity and duration of the acoustic signal	crispness is related to the presence of high frequency components in the sound spectrum	crispiness is associated with crystallinity, but it is largely influenced by drying process

SDA—solar-dried apples; ODA—oven-dried apples; CCA—national supermarket apples; CFA—specialized dried food brand apples.

).

Underlying structural properties are central to understanding how crispiness, sound, and X-ray diffraction are interconnected. [74]. Crispier foods are associated with higher crystallinity, reflected in XRD peaks and produce louder, higher-pitched sounds upon fracture [75]. Generally, as drying processes reduce crystallinity, the food can become less crispy and the associated sounds become lower. However, the X-ray diffraction pattern may indicate changes in crystallinity in dehydrated apples, as the formation of more crystalline or amorphous structures is related to the degree of hydration and the transformations of cellular components during the drying process [76]. The dehydrated apple is crispy because its mechanical structure becomes rigid and brittle after the water is removed, allowing it to break abruptly and produce characteristic sounds [64]. This difference reflects the distinction between mechanical properties (perceived sensorially) and structural properties (observed by techniques such as XRD). Similar results were obtained by Bhat et al. [77] and Qadri et al. [78] in dried apple powder. There is also a strong correlation (Pearson’s r = 0.969) between crispiness (Figure 3) and crystallite size (Table 1) that is presented in Figure 8.

3.6. Sensory Analyses: CATA Tests, Quality and Acceptance

3.6.1. CATA Test

The results provided in

Table 3. Frequencies for each texture descriptor of the check-all-that-apply questionnaire.

		SDA	ODA	CCA	CFA
Texture	Dry	30	35	41	51
	Crispy	73	84	63	80
	Soft/elastic	14	9	24	10
	Hard	20	29	22	31
	Firm	34	36	26	27

SDA—solar-dried apples; ODA—oven-dried apples; CCA—national supermarket apples; CFA—specialized dried food brand apples.

summarize the percentage of respondents who selected each texture attribute for the four types of dried apples.

The selected texture attributes for the CATA test were as follows: dry—the absence of moisture or juiciness; crispy—indicates a brittle, easily breakable texture with a sharp sound upon biting or crushing; soft/elastic—describes a flexible or springy texture; hard: refers to a firm, resistant texture that requires significant force to bite or crush and firm—indicates a solid, compact texture but not as extreme as “hard”.

In relation to dryness, CFA has the highest percentage (51%) for the “dry” attribute, indicating that it is perceived as the driest among the samples. ODA and CCA follow closely, suggesting they are also relatively dry. SDA has the lowest percentage (30%), implying it retains more moisture compared to the others. The crispy parameter reveals that ODA has the highest percentage (84%) for the “crispy” attribute, indicating it is perceived as the crunchiest or most brittle. CFA has 80%, suggesting it also has a very crispy texture. SDA and CCA have lower percentages, at 73% and 63%, respectively, indicating they are less crispy compared to ODA and CFA. Considering the soft/elastic factor, CCA has the highest percentage (24%), suggesting it retains some flexibility or springiness. SDA, ODA and CFA have much lower percentages (14%, 9% and 10%, respectively), indicating they are predominantly hard or brittle rather than soft or elastic. In terms of hardness, CFA has the highest percentage (31%), indicating that it is perceived as the firmest or most resistant to biting. ODA follows closely with 29%, suggesting it is also quite hard. SDA and CCA have lower percentages (20% and 22%, respectively), indicating they are less hard compared to ODA and CFA. In firmness, ODA has the highest percentage (36%), indicating it is perceived as the most solid or compact. SDA registered 34%, suggesting it is also relatively firm. CCA and CFA have lower percentages (26% and 27%, respectively), indicating they are slightly less firm compared to SDA and ODA.

Summary, the CATA test results provide insights into how consumers perceive the texture of the dried apples, based on different drying methods. SDA balances crispiness and firmness, providing a moderate texture. ODA is characterized by high crispiness and hardness, making it ideal for those who prefer a very crunchy texture. CCA is the most flexible and least dry, appealing to those who prefer a softer texture. CFA is extremely dry and hard, offering a distinct texture profile.

3.6.2. Texture Evaluation

The provided funnel graphs represent the results of hedonic tests regarding the texture of dried apples using a 9-point scale (1 = extreme dislike, 9 = like extremely much) (Figure 9).

The analysis of the funnel plots reveals that the SDA and ODA samples present predominantly high evaluations, concentrated between 7 and 9, with greater consistency in SDA, while ODA shows slight variability in opinions. CCA samples exhibit a more uniform distribution, with medium scores (4 to 6) being more frequent, indicating less emphasis on texture in comparison to the other samples. CFA, on the other hand, resembles ODA, with scores also concentrated in the upper range, but less frequently in the maximum values (9). In terms of overall average, SDA and ODA lead as the preferred ones, followed by CFA, while CCA demonstrates less positive consensus among participants.

The statistical analyses showed that SDA samples had the highest mean ± SD (7.20 ± 1.71), followed by ODA (7.06 ± 1.46) and CCA (7.13 ± 2.14). The CFA samples had the lowest mean ± SD (6.54 ± 1.89). The CFA samples are significantly different (p < 0.05) from the others.

This analysis highlights the importance of drying methods in consumers’ perception of texture in dried apples. Solar-dried apples were the most favored by participants, with the strongest consensus and highest scores. The oven-dried apples (ODA) were also highly appreciated, closely following SDA in terms of appeal. CCA samples showed the most variability in participant opinions, suggesting they may not meet the same texture standards as the other options and CFA offered good texture but did not stand out as distinctly as SDA or ODA. The texture appears to be well-liked, but it does not match the strong consensus seen by SDA.

3.6.3. Overall Quality

The provided funnel graphs represent the results of hedonic tests evaluating the overall quality of dried apples using a 9-point scale (1 = extreme dislike, 9 = like extremely much) (Figure 10).

The analysis reveals that SDA and ODA have scores concentrated in the high range (7–9), with SDA showing the greatest consensus (8 and 9) and ODA following behind but with slight dispersion. CCA is more evenly distributed, with more average ratings (4–6) and less overall prominence. CFA is concentrated in the high scores (6–9) but not as much as SDA or ODA. On average, SDA leads as the highest rated, followed by ODA, while CFA is consistent but garners less enthusiasm and CCA shows greater variability and lower consensus.

The statistical analyses showed that SDA samples had the highest mean ± SD (7.07 ± 1.24), followed by ODA (6.58 ± 1.43) and CCA (6.27 ± 1.55). The CFA samples had the lowest mean ± SD (5.04 ± 1.84). There are no significant differences between the ODA and CCA samples. However, the SDA samples show significant differences (p < 0.05) compared to all other samples, being associated with the highest acceptance, while the CFA samples show significant differences linked to the lowest acceptance.

The best overall quality was registered in SDA samples, which were the most favored by participants, with the strongest consensus and highest scores. ODA samples were also highly appreciated, closely following SDA in terms of appeal. In turn, CCA samples showed the most variability in participant opinions, suggesting they may not meet the same quality standards as the other options. The CFA samples offered good overall quality, but not as distinctly as SDA or ODA.

3.6.4. Acceptance of the Snacks Under Study

The acceptability of the different types of dried apples based on participants’ responses to their overall quality were assessed. The categories of acceptance were as follows: certainly consuming, possibly consuming, perhaps consuming, possibly not consuming and certainly not consuming. Each category represents how likely participants are to consume each type of dried apple, with higher values indicating stronger acceptance (Figure 11).

SDA samples had the highest acceptance across all categories; 40% of consumers considers that would “certainly consume” this product. This indicates strong and consistent acceptance of solar dried apples. On the other hand, the commercial samples, CCA and CFA, showed the lowest acceptance compared to the other options.

4. Conclusions

The research conducted demonstrated that the drying method influences mechanical, acoustic and sensory properties of dried apples. Acoustic measurements showed that not only does the crispiness parameter cause louder, higher-pitched sounds during fracture, but it is highly related with physical properties.

X-ray diffraction (XRD) analysis highlighted the transition from a short-range molecular order, water mediated, controlled by hydrogen bonds, to a longer molecular order in dehydrated samples. The amorphous peak position shift toward lower angles and diminish in intensity due to water removal and increased crystallinity during dehydration.

Sensory evaluations showed that dried apples retained varying degrees of crispiness, with solar-dried (SDA) and oven-dried (ODA) samples exhibiting superior textural qualities compared to industrially processed alternatives (CCA and CFA). In both overall quality and consumers’ acceptance, the most preferable samples were SDA, followed by ODA.

Specifically, SDA relies on renewable solar energy, which significantly reduces carbon emissions compared to industrial drying methods such as CCA and CFA that depend on fossil fuels or high-consumption electric systems. According to the bibliography, industrial drying can emit approximately 1.5–3.0 kg CO₂ per kg of dried product, whereas SDA has near-zero operational emissions. Despite its lower energy input, SDA was shown to preserve superior textural properties—including crispness and fracture acoustics—when compared to industrial alternatives, as evidenced by sensory, mechanical, and XRD analyses. This dual benefit positions SDA not only as a more environmentally friendly option but also as a method capable of maintaining key sensory attributes that influence consumer acceptance.

These results underscore the importance of drying techniques in preserving desirable textural attributes, offering advantages in maintaining consumer appeal. Sensory tests confirmed a strong correlation between perceived crispiness, acoustic properties and drying methods, highlighting the role of both structural integrity and auditory feedback in consumer preferences.

This study provides a comprehensive analysis of the structural, mechanical and sensory properties of dried apples, emphasizing the impact of drying methods on their quality, ultimately improving their marketability and consumer satisfaction.