From Apple By-Product to Shortbread Cookies: Drying Conditions and Their Impact on Product Quality

Abstract

Apple pomace, a by-product of juice production, is a rich source of dietary fiber and bioactive compounds, making it a promising functional ingredient for bakery applications. This study evaluated the physicochemical and sensory properties of shortbread cookies enriched with apple pomace dried under different conditions, while also analyzing the drying process, focusing on drying kinetics and powder characteristics. Pomace dried by either contact drying or freeze-drying was ground and used to replace 20% of wheat flour in the cookie formulation. Drying kinetics were best described by the modified Page model, and freeze-dried pomace showed higher grindability than contact-dried samples. Cookies enriched with pomace exhibited similar overall composition, with differences mainly observed in fiber content (9.82–11.75%). Those containing freeze-dried pomace were lighter, with reduced red and increased yellow tones, and were firmer, requiring approximately 30% higher cutting force. Despite differences in physical properties, enriched cookies were consistently rated higher in overall acceptability than the controls. The results indicate that the drying method and temperature influence the physicochemical properties of apple by-product and the resulting cookies, while having mainly minor effects on sensory acceptance, confirming the potential of apple pomace as a functional ingredient in bakery products.

1. Introduction

Pomace, a by-product of fruit and vegetable processing, represents a valuable source of nutritionally and functionally important compounds. It is rich in dietary fiber and bioactive compounds, including polyphenols, flavonoids, and carotenoids, which exhibit antioxidant, anti-inflammatory, and a range of potentially health-promoting properties [1,2]. Growing consumer interest in functional foods and efforts to valorize food industry by-products within circular economy strategies have stimulated research into the use of pomace as a natural ingredient for food enrichment. The addition of pomace can enhance nutritional value by increasing fiber and antioxidant content, while also affecting the technological and sensory properties of the final product [3,4,5]. Despite their potential as functional ingredients, fruit and vegetable pomaces contain high levels of moisture, which makes them susceptible to microbial growth and unstable for direct application in the food industry. Fresh pomace often contains more than 60% moisture [5]. Drying is commonly employed to extend pomace shelf life, enable long-term storage, and allow practical use in food processing. Water reduction prevents microbial proliferation, concentrates valuable components such as dietary fiber and bioactive compounds [6], and influences subsequent processing methods. Lower moisture facilitates grinding processes, directly influencing particle size distribution and thereby the physical properties of the resulting powder [7,8,9]. Therefore, selecting appropriate drying conditions is a critical step in processing pomace for food enrichment. Convective drying is one of the most established and widely applied methods for preserving fruits and their by-products. The principle of this technique involves exposing the material to a stream of heated air, which promotes moisture evaporation and consequently reduces water activity. This reduction effectively inhibits microbial growth and slows enzymatic reactions that contribute to spoilage. Key advantages of convective drying include its technological simplicity, relatively low capital requirements, and broad industrial applicability. In addition, it ensures efficient and relatively fast dehydration of a variety of raw materials, ranging from fresh fruits to seeds and by-products such as peels, which may subsequently serve as valuable sources of dietary fiber and bioactive compounds. Despite these benefits, the method has certain limitations. The most significant disadvantages relate to the deterioration of product quality [10,11,12]. Prolonged exposure to elevated temperatures can cause undesirable changes in sensory attributes, including fading of natural color, loss of aroma, and alterations in texture [13]. Moreover, sensitive nutritional and bioactive components are prone to thermal degradation [14]. It is also important to note that convective drying is generally unsuitable for liquid or semi-liquid materials, such as fruit pulps, because the method requires a stable solid structure to ensure effective heat and mass transfer. For these types of substrates, contact drying is more advantageous, as it relies on direct heat conduction from a heated surface to the product, allowing more efficient moisture removal from semi-liquid or paste-like matrices, including fruit pulps and moist pomace [15]. Among available drying methods, freeze-drying (FD) stands out due to its ability to minimize the degradation of thermolabile compounds and preserve the functional quality of the final product [16]. Freeze-dried products are characterized by high porosity, low bulk density, and excellent solubility, making them attractive additives for functional foods [17]. However, its long processing time, high energy demand, and need for specialized equipment make freeze-drying one of the most expensive drying methods [18]. As a result, its industrial use is mainly limited to high-value products or cases where preserving the full profile of health-promoting compounds is essential. Therefore, convective and contact drying (CD) are more commonly applied. Although previous studies have investigated the effects of individual drying methods on fruit pomace properties, most have focused on a single technique or temperature in isolation, without systematically comparing multiple methods or examining their combined effects on final product quality [19,20,21]. As a result, there is a lack of comprehensive understanding of how the choice of drying method and temperature interacts to influence powder characteristics and the technological and sensory properties of enriched foods. Addressing this gap is essential to guide the effective use of pomace in food formulations, particularly when considering the trade-offs between processing cost and quality preservation.

Understanding the drying kinetics of a product is essential for the design and simulation of drying equipment. Simulating the drying process of food is crucial for gaining insight into heat and mass transfer mechanisms and provides a foundation for developing a comprehensive mathematical model of the process [22,23]. Determining these kinetics provides valuable information on the rate and extent of moisture removal under various operating conditions. This knowledge facilitates the optimization of the drying process, reducing both drying time and energy consumption while maintaining product quality. Furthermore, it enables accurate process modeling and the scale-up of laboratory results to industrial applications, which is crucial for designing efficient dryers and predicting the properties of the final product. In addition, the analysis of drying kinetics allows for the comparison of different drying methods and the identification of optimal process parameters for a wide range of raw materials, including fruits [22], and pomace [15,24].This study aimed to compare the drying kinetics, physicochemical properties, and grinding behavior of apple by-product (ABP) subjected to freeze-drying and contact drying at different temperatures. It also investigated how powders obtained under different drying conditions affect the physicochemical and sensory properties of shortcrust cookies. Apple pomace was selected because of its wide availability from apple juice and concentrate production, and its high content of bioactive compounds such as polyphenols, dietary fiber, vitamins, and minerals [25]. This raw material offers strong potential for sustainable valorization within closed-loop food production systems.

2. Materials and Methods

2.1. Raw Materials

Champion apples (purchased from a local market in Lublin, Poland) were carefully selected, discarding any bruised or blemished fruits. After thorough washing, the stems, calyx, seeds, and cores were removed using a clean, sharp knife. After this preparation, the cleaned apples were pressed using a twin-screw juicer (Angel Juicer, Angel Co., Ltd., Busan, Republic of Korea), which separated the juice from the solid residues. The remaining fraction, consisting mainly of peel, pulp, and minor core remnants, is commonly referred to in the literature as apple pomace or ABP, and this terminology is used throughout the manuscript. Apples from commercial orchards in the Lubelskie region were harvested in 2024 and stored under standard refrigeration until analysis. The obtained ABP was collected and kept for further processing and analysis. A portion of the ABP intended for FD was placed in a laboratory freezer (GTL-4905, Liebherr, Gothenburg, Sweden) and frozen at −30 °C.

2.2. Drying Processes

Samples of ABP, each weighing approximately 200 g, were subjected to FD using an Alpha 1–4 freeze-dryer (Martin Christ, Osterode am Harz, Germany). The FD process was conducted at three plate temperatures—20 °C, 40 °C, and 60 °C—which cover the operational range of the equipment, with 60 °C being the maximum temperature, allowing assessment of low, medium, and high temperature effects [15]. The drying was performed under a controlled vacuum pressure of 100 Pa.

In parallel, another set of samples was dried by means of CD using a Promis-Tech dryer (Promis-Tech Sp. z o.o., Wrocław, Poland) at temperatures of 40 °C, 60 °C, and 80 °C. For CD, the temperatures 40 °C, 60 °C, and 80 °C represent typical and technologically relevant ranges for fruit drying, with 80 °C being the upper limit to avoid undesirable changes in the product [20,23]. Both drying devices were equipped with integrated mass measurement systems that allowed continuous monitoring of the sample weight throughout the drying process. Mass measurements were recorded at five-minute intervals, enabling detailed tracking of moisture loss over time. The drying process for each sample was concluded once the moisture content reached an average value of 9% ± 0.25%, confirming the target dryness level. For comparison purposes, a control sample consisting of undried ABP was also included in the study.

2.3. Drying Kinetics and Mathematical Modeling

Drying kinetics of ABP were analyzed through the moisture ratio (MR) over time, a dimensionless indicator of drying progress. The MR was calculated using the following equation:

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{{\mathrm{u}}_{\mathrm{t}}}{{\mathrm{u}}_0}}$$

(1)

where

u_t is the water content at a given time during the drying process (expressed in H₂O·kg⁻¹ dry weight),

u₀ is the initial water content of the sample (kg H₂O·kg⁻¹ dry weight).

The equilibrium moisture content was neglected, as it is negligible compared to u₀ and u_t, an assumption commonly adopted in drying studies with minimal impact on kinetic modeling accuracy [26].

Seven thin-layer drying models frequently cited in the literature were selected to determine the best fit for ABP drying behavior under different conditions. The mathematical forms and parameters of these models are listed in

Table 1. Equations (drying models) used for the freeze-drying and contact-drying of ABP.

Number	Model	Equation
1	Newton [27]	$\mathrm{M}\mathrm{R}=\exp (-\mathrm{k}·\mathsf{\tau })$	Equation (2)
2	Page [28]	$\mathrm{M}\mathrm{R}=\mathrm{e}\mathrm{x}\mathrm{p}\left(-\mathrm{k}·{\mathsf{\tau }}^{\mathrm{n}}\right)$	Equation (3)
3	Modified Page [29]	$\mathrm{M}\mathrm{R}=\mathrm{a}·\mathrm{e}\mathrm{x}\mathrm{p}\left(-\mathrm{k}·{\mathsf{\tau }}^{\mathrm{n}}\right)$	Equation (4)
4	Handerson and Pabis [30]	$\mathrm{M}\mathrm{R}=\mathrm{a}·\exp \left(-\mathrm{k}·\mathsf{\tau }\right)$	Equation (5)
5	Logarythmic [31]	$\mathrm{M}\mathrm{R}=\mathrm{a}·\exp \left(-\mathrm{k}·\mathsf{\tau }\right)+\mathrm{b}$	Equation (6)
6	Wang and Singh [32]	$\mathrm{M}\mathrm{R}=1+\mathrm{a}·\mathsf{\tau }+\mathrm{b}·{\mathsf{\tau }}^2$	Equation (7)
7	Logistic [33]	$\mathrm{M}\mathrm{R}=\mathrm{b}/(1+\mathrm{a}·\mathrm{e}\mathrm{x}\mathrm{p}(\mathrm{k}·\mathsf{\tau }\left)\right)$	Equation (8)

k—drying coefficient (min⁻¹); τ—time (min); n—exponent; a, b—equation coefficients.

. Their performance in fitting the experimental data was compared using statistical criteria such as the coefficient of determination (R²), root mean square error (RMSE), and reduced chi-square (χ²).

2.4. Particle Size Characterization Following the Grinding Process

Dried ABP samples were milled using a Grindomix GM-200 mill (Retsch GmbH, Haan, Germany). Milled samples were analyzed for particle size using laser diffraction (Mastersizer 3000, Malvern Instruments Ltd., Malvern, UK). A 5 g sample was automatically examined, and the dispersion index (Span) was calculated as:

$$\mathrm{S}\mathrm{p}\mathrm{a}\mathrm{n}={\displaystyle \frac{{\mathrm{d}}_{90}-{\mathrm{d}}_{10}}{{\mathrm{d}}_{50}}}$$

(9)

where d₁₀, d₅₀, d₉₀ correspond to the particle diameters at 10%, 50%, and 90% of the cumulative volume distribution, respectively [34]. For further analyses, the samples were sieved, and any particles larger than 0.3 mm were re-ground using the same mill to ensure a uniform particle size below 0.3 mm.

2.5. Cookies Preparations

Seven cookie variants were prepared in this study. The control sample was made without any addition of ABP. In addition, six experimental samples were produced by substituting 20% of the wheat flour with ABP powder. The pomace powders used differed based on the drying method (FD or CD) and drying temperature (20 °C, 40 °C, or 60 °C for FD; 40 °C, 60 °C, or 80 °C for CD), resulting in distinct variants of enriched cookies.

All cookie formulations contained the same basic ingredients: wheat flour (300 g), sugar (50 g), butter (160 g), and egg (70 g). The dough was prepared using a shortbread-type method [35] to ensure consistency across samples. The cookie dough was prepared without the addition of water.

The dough was rolled to ~5 mm thickness, and 40 mm diameter circles were cut using a cutter. Cookies were baked in a preheated oven at 200 °C for 15 min and cooled to 20–21 °C before analysis. Once baked, the cookies were cooled at room temperature until reaching 20–21 °C before any further analysis.

2.6. Water Content and Activity

Moisture content of apple by-product was determined gravimetrically: 5 g samples were dried at 105 °C until constant weight. Water activity was assessed with a LabMaster instrument (Novasina, Lachen, Switzerland).

2.7. Basic Composition

The chemical composition of apple by-product powders and cookies was determined according to AACC (2021) and AOAC (2021) official methods [36,37]. Ash content was measured by AACC Method 08-01. Protein content was determined using the Kjeldahl method (AACC Method 46-08), with a Kjeltec 2300 system (FOSS, Höganäs, Sweden), and the protein content was calculated from total nitrogen using a conversion factor of 5.7. Free fat was quantified by continuous hexane extraction with a Soxtec™ 8000 system (FOSS, Sweden). Total dietary fiber, including insoluble and soluble fractions, was determined enzymatically according to AACC Methods 32-05 and 32-21 and AOAC Methods 991.43 and 985.29. A 1 g portion of dried samples was enzymatically treated in sequence with heat-stable α-amylase, protease, and amyloglucosidase (Megazyme International Ireland Ltd., Wicklow, Ireland), following the procedures outlined in the official methods. Available carbohydrates were calculated by difference from 100 minus the sum of the measured components.

2.8. Color Evaluation

Color parameters of both fresh and dried ABP, as well as of the final cookie samples, were determined using the reflection method within the CIELab color space system. In this system, the L* value corresponds to lightness (ranging from black to white), the a* coordinate represents the red-green axis (with positive values indicating redness and negative values indicating greenness), and the b* coordinate reflects the yellow-blue axis (with positive values indicating yellowness and negative values indicating blueness). Measurements were performed using a NR20XE colorimeter (Shenzhen Threenh Technology Co., Shenzhen, China).

2.9. Cutting Test

Cookie texture was analyzed using a Zwick universal testing machine (model Z020/TN2S, Ulm, Germany), with maximum cutting force as an indicator of hardness. For each measurement, a single cookie was placed on the testing platform and subjected to a cutting test using a 1 mm thick blade, moving at a constant speed of 20 mm·min⁻¹. The test was continued until the blade reached a distance of 0.1 mm from the base plate, ensuring full penetration of the sample.

2.10. Sensory Assessment

Consumer acceptance of the shortbread cookies was evaluated by 45 untrained participants recruited from regular consumers. The evaluation was performed using a 9-point hedonic scale, where 1 meant “dislike extremely,” 2 “dislike very much,” 3 “dislike moderately,” 4 “dislike slightly,” 5 “neither like nor dislike,” 6 “like slightly,” 7 “like moderately,” 8 “like very much,” and 9 “like extremely” [38]. Participants rated several attributes including appearance, color, aroma, taste, texture, and overall acceptability. Before the test, participants were informed about the study’s purpose and gave their consent to participate voluntarily. Participants were informed about the study and gave voluntary consent. Evaluation was conducted under natural daylight at 20 °C, with approval from the institutional ethics committee.

2.11. Statistical Evaluation

Statistical analyses were performed on data from three independent replicates for water content and activity, basic composition, and five replicates for color and hardness. Differences between means were assessed by one- or two-way ANOVA with Tukey’s post hoc test (α = 0.05). Sensory data were analyzed using the Kruskal–Wallis test (α = 0.05). Analyses were conducted with Statistica 14.0 (StatSoft, Tulsa, OK, USA).

3. Results

3.1. Modeling Drying Kinetics of Apple By-Product

Table 2. Coefficients of determination, root mean square errors, and chi-square test values for the models describing the freeze-drying process of ABP.

Model	Sample
	20 °C			40 °C			60 °C
	R2	RMSE	χ2	R2	RMSE	χ2	R2	RMSE	χ2
1	0.971	0.015	1.488 × 10−4	0.928	0.040	1.687 × 10−3	0.939	0.025	6.665 × 10−4
2	0.999	0.002	3.673 × 10−8	0.999	0.001	5.214 × 10−7	0.994	0.003	7.490 × 10−6
3	0.999	0.001	3.253 × 10−8	0.999	0.001	3.596 × 10−7	0.996	0.002	3.407 × 10−6
4	0.986	0.007	6.208 × 10−5	0.959	0.023	5.706 × 10−4	0.954	0.019	3.973 × 10−4
5	0.995	0.002	6.803 × 10−6	0.982	0.010	1.146 × 10−4	0.996	0.002	2.880 × 10−6
6	0.995	0.003	2.823 × 10−6	0.975	0.014	2.164 × 10−4	0.996	0.002	3.032 × 10−6
7	0.889	0.059	3.867 × 10−3	0.836	0.092	9.629 × 10−3	0.846	0.033	1.177 × 10−3

Models: 1—Newton, 2—Page, 3—Modified Page, 4—Handerson and Pabis, 5—Logarythmic, 6—Wang and Singh, 7—Logistic; R²—the determination coefficient; RMSE—mean-square error; χ²—chi-square test.

and

Table 3. Coefficients of determination, root mean square errors and chi-square test values for the models describing the contact-drying process of ABP.

Model	Sample
	20 °C			40 °C			60 °C
	R2	RMSE	χ2	R2	RMSE	χ2	R2	RMSE	χ2
1	0.977	0.017	2.865 × 10−3	0.989	0.007	4.582 × 10−5	0.983	0.009	8.646 × 10−5
2	0.999	0.001	1.262 × 10−7	0.998	0.001	1.137 × 10−7	0.999	0.001	8.163 × 10−7
3	0.999	0.001	4.923 × 10−8	0.999	0.001	7.248 × 10−7	0.998	0.001	7.819 × 10−7
4	0.987	0.009	8.920 × 10−5	0.996	0.003	7.497 × 10−6	0.992	0.005	2.184 × 10−5
5	0.996	0.003	8.279 × 10−6	0.996	0.002	5.094 × 10−6	0.995	0.003	9.923 × 10−6
6	0.998	0.001	1.164 × 10−6	0.995	0.003	1.031 × 10−5	0.984	0.002	5.983 × 10−6
7	0.987	0.009	9.543 × 10−5	0.996	0.003	7.663 × 10−6	0.902	0.055	3.237 × 10−3

Models: 1—Newton, 2—Page, 3—Modified Page, 4—Handerson and Pabis, 5—Logarythmic, 6—Wang and Singh, 7—Logistic; R²—the determination coefficient; RMSE—mean-square error; χ²—chi-square test.

present coefficients characterizing the goodness of fit of the analyzed models describing ABP drying kinetics, while

Table 4. Values of parameters in the models describing freeze-drying process of ABP.

Temperature of Heating Plates	Model	Coefficient
		a	k (min−1)	n	b
20 °C	1		0.006850
	2		0.001149	1.353876
	3	1.005718	0.001231	1.341492
	4	1.106479	0.007606
	5	1.246105	0.005374		−0.183377
	6	−0.005222			0.000007
	7	0.000024	0.000001		−0.999980
40 °C	1		0.009664
	2		0.000338	1.716085
	3	1.015958	0.000425	1.670972
	4	1.169251	0.011266
	5	1.440053	0.006628		−0.330720
	6	−0.006998			0.000011
	7	0.000020	0.000001		−0.999984
60 °C	1		0.010988
	2		0.000827	1.572410
	3	0.955788	0.000400	1.719416
	4	1.100717	0.012184
	5	2.26321	0.00353		−1.24166
	6	−0.007381			0.000009
	7	0.000015	0.000001		−0.999987

Models: 1—Newton, 2—Page, 3—Modified Page, 4—Handerson and Pabis, 5—Logarythmic, 6—Wang and Singh, 7—Logistic; R²—the determination coefficient; RMSE—mean-square error; χ²—chi-quadrate test; k—drying coefficient (min⁻¹); a, b—equation coefficients; n—exponent.

and

Table 5. Values of parameters in the models describing contact-drying process of ABP.

Drying Conditions	Model	Coefficient
		a	k (min−1)	n	b
40 °C	1		0.003927
	2		0.000669	1.313040
	3	0.983331	0.000535	1.349242
	4	1.096148	0.004305
	5	1.193991	0.003190		−0.142006
	6	−0.002976			0.000002
	7	−210.9870	0.004000		−193.4260
60 °C	1		0.005529
	2		0.002061	1.185464
	3	1.024774	0.002629	1.144678
	4	1.077641	0.005963
	5	1.094867	0.005518		−0.030441
	6	−0.004255			0.000005
	7	−3102.390	0.010000		−2879.800
80 °C	1		0.006409
	2		0.001833	1.243122
	3	1.010475	0.002049	1.223719
	4	1.083984	0.006959
	5	1.129610	0.005937		−0.069461
	6	−0.004989			0.000007
	7	0.000027	0.000001		−0.999977

Models: 1—Newton, 2—Page, 3—Modified Page, 4—Handerson and Pabis, 5—Logarythmic, 6—Wang and Singh, 7—Logistic; R²—the determination coefficient; RMSE—mean-square error; χ²—chi-quadrate test; k—drying coefficient (min⁻¹); a, b—equation coefficients; n—exponent.

provide the corresponding regression coefficients. In both freeze-drying and contact drying, the best results were obtained using the Page model and the modified Page model to describe changes in water content in ABP samples. These models exhibited the highest coefficients of determination along with the lowest root mean square error (RMSE) and chi-square values. The very high coefficients of determination (R² > 0.99) indicate that both models accurately capture the variation in water content in the samples under both drying methods and across different process temperatures. Conversely, the low values of the root mean square error and chi-square test confirm the minimal deviations of the models from the experimental data. The Page model and its modification are frequently cited as versatile empirical equations for describing drying kinetics of biological materials [29,39,40,41]. The present study demonstrates that these models can also be successfully applied to describe the kinetics of freeze-drying and contact drying of apple pomace. These findings further suggest their potential for predicting drying time, improving final product quality, and enhancing process energy efficiency. By contrast, the Logistic model exhibited the poorest fit to the experimental data, particularly in the case of freeze-drying. ABP drying kinetics are influenced by several factors, primarily temperature [8], and in freeze-drying also chamber pressure, which govern heat and mass transfer intensity [42]. Physicochemical properties such as porosity, particle size, and initial water content also play a crucial role [43]. These processes are highly nonlinear and often difficult to reproduce accurately with simple theoretical models.

Analysis of ABP drying curves (Figure 1 and Figure 2) showed that both the drying method and temperature significantly affected the process duration. The shortest drying time was observed for FD ABP at 60 °C (~160 min), whereas the longest occurred for CD samples at 40 °C (~650 min). Higher process temperatures enhanced heat and mass transfer, substantially reducing drying time. Freeze-drying of apple pomace at 60 °C was approximately twice as fast as at 20 °C. For CD, increasing the temperature from 40 to 80 °C reduced drying time by approximately 1.7-fold.

FD proceeded considerably faster than CD in this study. Similar results were obtained during drying of pear pomace [44]. This phenomenon can be explained by the different mechanisms of water removal. In CD, moisture transport is limited by heat conduction and by the diffusion of water in the liquid phase, which slows down the overall process. In contrast, during lyophilization, water is removed directly through ice sublimation under reduced pressure. The application of a vacuum, together with the porous structure of the material formed during freezing, facilitates more efficient release of water vapor. Process efficiency may also be strongly influenced by the condenser temperature in the lyophilizer, maintained at approximately –50 °C. This low temperature ensures effective trapping of water vapor and maintains a sufficient pressure gradient between the product and the condenser. As a result, sublimation occurs continuously and at a high rate, which may further contribute to the reduction in total drying time compared with contact drying.

3.2. Particle Size of Ground ABP

The parameters characterizing the particle size of ground ABP are presented in

Table 6. Particle size distribution parameters of apple by-product obtained under various drying conditions.

Drying Method	Drying Temperature (°C)	d10 (µm)	d50 (µm)	d90 (µm)	Span
FD	20	45.59 ± 0.46 a	182.12 ± 1.34 b	315.84 ± 2.37 a	1.48 ± 0.00 ab
	40	43.47 ± 1.33 a	176.37 ± 1.14 a	314.43 ± 1.92 a	1.54 ± 0.02 bc
	60	43.57 ± 1.03 a	173.63 ± 1.97 a	315.38 ± 7.64 a	1.57 ± 0.04 c
CD	40	54.13 ± 1.22 b	188.75 ± 1.72 c	333.73 ± 1.30 c	1.48 ± 0.01 a
	60	55.01 ± 0.24 b	189.30 ± 0.91 c	328.93 ± 1.27 bc	1.45 ± 0.01 a
	80	53.01 ± 0.86 b	188.81 ± 2.58 c	323.79 ± 1.69 ab	1.43 ± 0.01 a

d₁₀, d₅₀, d₉₀—diameters corresponding to the 10th, 50th, and 90th percentiles of the total particle volume, assuming spherical particle shape; Span—a particle size dispersion index; CD—contact-drying, FD—freeze-drying. The values are presented as mean ± SD. Means labeled with different superscript letters indicate significant differences at α = 0.05. Lowercase letters indicate differences among apple by-product powders.

. Slightly lower d₁₀, d₅₀, and d₉₀ values were observed for apple pomace dried by FD. In contrast, ABP samples obtained after CD at 40 °C and 60 °C exhibited significantly lower Span values compared with lyophilized samples at the same plate temperatures, indicating a somewhat higher uniformity of particle size distribution in contact-dried ABP. Overall, the analysis suggests that particle size of ground dried ABP samples produced under different drying conditions was generally comparable.

Differences in particle size distribution can be attributed to structural changes occurring during drying. Freeze-drying produces a highly porous structure due to ice crystal sublimation, which upon grinding can result in a broader particle size distribution with a lower average particle size. In contrast, conventional drying involves gradual water removal and collapse of cellular structures, producing denser, more compact material with larger particle sizes [45].

Particle size of ground dried material influences several physical properties, including bulk density, flowability, and water absorption capacity. When used as a food ingredient, it also affects the final product’s textural and sensory characteristics, as demonstrated in baked goods such as shortbread cookies [46]. Therefore, standardizing particle size across samples helps eliminate its influence on product properties and allows a clearer assessment of the effects of drying method and conditions on the functional quality of apple pomace powders.

3.3. Basic Composition of Pomace and Cookies

All dried ABP samples showed similar final moisture content (8.70–9.42%) and water activity (0.516–0.549), as targeted during processing (Table S1). Moisture content and water activity of the cookies were within typical ranges for low-moisture baked goods (3.51–4.24% and 0.233–0.294, respectively), with no substantial variation between control and ABP-enriched samples (Table S1).

The drying method and conditions had only a minor impact on the basic chemical composition of the ABP (

Table 7. Basic chemical composition of apple by-product powders (g·100 g⁻¹ d.m.).

DM	DT	Fat	Ash	Protein	IDF	SDF	TDF	Carbohydrates
CABP	-	2.72 ± 0.06 a	1.45 ± 0.03 a	4.20 ± 0.03 a	27.22 ± 0.12 b	10.92 ± 0.26 a	37.14 ± 0.22 b	54.50
CD	40	2.67 ± 0.09 a	1.42 ± 0.03 a	4.21 ± 0.12 a	29.47 ± 0.36 c	11.79 ± 0.11 b	39.44 ± 0.46 c	52.26
	60	2.75 ± 0.11 a	1.48 ± 0.04 a	4.19 ± 0.14 a	26.79 ± 0.16 b	10.83 ± 0.08 ab	37.63 ± 0.23 b	53.96
	80	2.70 ± 0.07 a	1.46 ± 0.02 a	4.14 ± 0.05 a	25.39 ± 0.12 a	10.20 ± 0.50 a	35.59 ± 0.62 a	56.10
FD	20	2.68 ± 0.08 a	1.44 ± 0.02 a	4.10 ± 0.06 a	26.30 ± 0.18 ab	11.46 ± 0.25 b	37.76 ± 0.11 b	54.03
	40	2.69 ± 0.05 a	1.45 ± 0.03 a	4.20 ± 0.13 a	25.25 ± 0.49 a	11.55 ± 0.11 b	36.80 ± 0.40 ab	54.85
	60	2.75 ± 0.04 a	1.46 ± 0.02 a	4.18 ± 0.06 a	25.87 ± 0.44 a	11.94 ± 0.26 b	37.81 ± 0.18 b	53.81

DM—drying method, DT—drying time, IDF—insoluble dietary fiber, SDF—soluble dietary fiber, TDF—total dietary fiber, CABP—control apple by-product, CD—contact-drying, FD—freeze-drying. The values are presented as mean ± SD. Means labeled with different superscript letters indicate significant differences at α = 0.05. Lowercase letters indicate differences among apple by-product powders.

). However, significant differences were observed in total dietary fiber (TDF), soluble dietary fiber (SDF), and insoluble dietary fiber (IDF) contents. Among all samples, the highest TDF content was recorded in the CD sample at 40 °C, while the lowest was observed in the CD sample dried at 80 °C. In CD samples, both IDF and SDF contents decreased with increasing temperature, although the reduction in SDF was minor and not statistically significant. Elevated drying temperatures for lignocellulosic materials reduce average pore size, decrease mesopore volume, and diminish the specific surface area of fibers [47]. These structural modifications are irreversible and may result in reduced extractability and diminished functional properties of the fiber. The limited changes in SDF content suggest that this fraction in ABP powder is more thermally stable than IDF. The minimal effect of the drying method on the overall chemical composition indicates that both techniques effectively preserve the key nutrients in dried pomace, which is beneficial for their potential application. Drying temperatures in the range of 20–80 °C appear to be safe for maintaining the nutritional quality of ABP.

The fiber fraction represented the predominant component of the ABP powder, averaging 37.5 g of total dietary fiber per 100 g of dry matter across the CD and FD samples, which confirms its potential as a functional and prebiotic ingredient (Table 6). Soluble and insoluble fiber contents averaged 11.10 and 26.61 g/100 g d.m., respectively, indicating that the insoluble fiber fraction of the ABP powder was approximately 2.4 times higher than the soluble fraction. This is consistent with previous findings reporting IDF levels to be 1.85 to 2.71 times higher than SDF [48]. Such variation can be attributed to factors such as apple variety and harvest time [49]. The fat content of the ABP powder was 2.71 g/100 g d.m., which falls within the range reported in the literature (0.6–4.2 g/100 g d.m.) [50]. This relatively low fat content supports its suitability for use in low-fat food products and enhances product stability by minimizing the risk of lipid oxidation during storage. The ash content was 1.45 g/100 g d.m., closely aligning with previously reported values such as 1.5 g/100 g d.m. [51], indicating that the mineral content of the powder was preserved effectively during processing. On average, the ABP powder contained 4.17 g of protein per 100 g d.m. While apple pomace is not typically considered a significant protein source, the residual protein may still contribute to its functional performance in food systems—particularly by enhancing water-binding capacity and promoting Maillard reactions during thermal processing.

Cookies enriched with 20% ABP powder showed an average increase of 58.91% in total dietary fiber content, including a 93.61% increase in IDF and a 29.98% increase in SDF compared to cookies without the addition (

Table 8. Proximate composition of cookies enriched with apple by-product dried under various conditions (g·100 g⁻¹ d.m.).

DM	DT	Fat	Ash	Protein	IDF	SDF	TDF	Carbohydrates
CC	-	24.82 ± 0.04 A	0.36 ± 0.02 A	8.69 ± 0.73 B	2.99 ± 0.02 A	3.58 ± 0.07 A	6.57 ± 0.09 A	59.56
CD	40	24.44 ± 0.28 A	0.50 ± 0.01 AB	7.67 ± 0.04 A	6.72 ± 0.31 D	5.03 ± 0.19 C	11.75 ± 0.14 E	55.64
	60	24.66 ± 0.03 A	0.44 ± 0.02 B	7.60 ± 0.11 A	5.20 ± 0.12 B	4.79 ± 0.40 BC	9.99 ± 0.28 BC	57.31
	80	24.29 ± 0.35 A	0.52 ± 0.02 AB	7.67 ± 0.26 A	6.02 ± 0.05 C	4.54 ± 0.13 BC	10.56 ± 0.08 D	56.96
FD	20	24.44 ± 0.28 A	0.51 ± 0.03 AB	7.61 ± 0.08 A	5.61 ± 0.31 BC	4.72 ± 0.17 BC	10.33 ± 0.15 CD	57.11
	40	24.72 ± 0.12 A	0.51 ± 0.03 AB	7.62 ± 0.23 A	5.46 ± 0.09 B	4.35 ± 0.04 B	9.82 ± 0.10 B	57.34
	60	24.60 ± 0.43 A	0.59 ± 0.18 B	7.48 ± 0.06 A	5.69 ± 0.07 BC	4.51 ± 0.15 BC	10.20 ± 0.08 BCD	57.13

DM—drying method, DT—drying temperature, IDF—insoluble dietary fiber, SDF—soluble dietary fiber, TDF—total dietary fiber, CC—control cookies, CD—contact-drying, FD—freeze-drying. The values are presented as mean ± SD. Means labeled with different superscript letters indicate significant differences at α = 0.05. Uppercase letters indicate differences among samples.

). The increased IDF in the final product, may have positive health effects for consumers, such as improving intestinal peristalsis, preventing constipation, and maintaining regular bowel movements [52]. Studies have consistently identified hemicellulose, lignin, and cellulose as the primary constituents of IDF in apple pomace. Quantitative analyses indicate that apple pomace contains approximately 11.6% cellulose and 10.0% hemicellulose, with lignin content at 18.9%, based on unwashed samples [53]. With respect to SDF, by forming a gel in the digestive tract, it contributes to lowering cholesterol levels, stabilizing blood glucose, and better appetite control. Additionally, the enriched cookies exhibited slightly elevated ash content relative to the control samples, reflecting a higher mineral content in the ABP powder compared to wheat flour; however, statistically significant differences in ash levels were only observed in cookies fortified with by-product dried by CD at 60 °C and FD at 60 °C. Conversely, cookies containing ABP powder showed a reduction in protein content as well as in available carbohydrates compared to the control cookies. Despite these variations, the fat content remained consistent across all cookie variants.

3.4. Color of Apple By-Products and Enriched Cookies

The brightness (L* value) of ABP powders was higher in samples subjected to FD compared to those dried by CD, as presented in

Table 9. Color coordinates of apple by-product dried under different conditions.

DM	DT	L*	a*	b*
CABP	-	79.99 ± 0.13 d	4.21 ± 0.19 b	22.48 ± 0.31 b
CD	40	74.18 ± 0.53 c	9.78 ± 0.08 d	27.57 ± 0.35 e
	60	72.63 ± 0.41 b	9.62 ± 0.08 d	23.60 ± 0.34 c
	80	67.40 ± 0.58 a	8.90 ± 0.12 c	24.54 ± 0.41 d
FD	20	80.04 ± 0.33 d	4.26 ± 0.15 b	22.54 ± 0.26 b
	40	80.19 ± 0.57 d	3.61 ± 0.15 a	21.14 ± 0.07 a
	60	80.41 ± 0.31 d	3.48 ± 0.28 a	20.72 ± 0.06 a

DM—drying method, DT—drying temperature, L*—lightness, a*—redness, b*—yellowness, CABP—control apple by-product, CD—contact-drying, FD—freeze-drying. Means labeled with different superscript letters indicate significant differences at α = 0.05. Lowercase letters indicate differences among apple by-product powders.

. This difference can be largely attributed to the gentle nature of FD, which effectively prevents oxidation and the degradation of natural pigments responsible for the product’s color. By sublimating ice directly from the frozen state under low temperature and pressure conditions, FD preserves the integrity of color compounds, resulting in a lighter and more visually appealing powder.

Furthermore, freeze-drying helps maintain a more porous and intact cellular structure within the ABP. This porous microstructure enhances light scattering and reflection on the powder surface, contributing to the increased brightness observed in FD samples [17]. In contrast, contact drying exposes the material to higher temperatures for longer periods, promoting structural collapse and pigment alteration, which together diminish brightness. Interestingly, variations in the drying temperature during freeze-drying—from 20 °C up to 60 °C plate temperatures—did not significantly influence the brightness of the ABP powders. This suggests that within the operational range of FD, temperature changes are not critical enough to cause notable pigment degradation or structural changes affecting color.

On the other hand, contact drying at elevated temperatures led to a clear decrease in brightness. The reduction in lightness observed in ABP powders dried by CD is likely a result of non-enzymatic browning reactions, primarily the Maillard reaction, which can begin at relatively low temperatures, around 50–60 °C [54]. These reactions cause the formation of brown pigments and darkening of the material, negatively impacting brightness. Such findings are consistent with previous research on pear pomace drying, where similar temperature-dependent decreases in brightness were reported [8].

Moreover, ABP powders obtained via contact drying exhibited more intense red (a*) and yellow (b*) hues compared to FD powders. This shift in chromaticity reflects the typical color evolution observed during non-enzymatic browning, which progresses from yellow to red and eventually to brown under heat treatment [55]. However, as the drying temperature increased in both drying methods, a gradual decrease in redness and yellowness was observed. This decline is likely due to the thermal degradation of natural pigments such as carotenoids and flavonoids, which are sensitive to heat and degrade when exposed to higher temperatures for extended periods [56].

The drying method of the ABP had a clear impact on the color of the cookies enriched with ABP. Cookies enriched with FD powders were noticeably lighter, less red, and more yellow compared to those containing ABP powders dried by contact drying (

Table 10. Color coordinates of cookies enriched with apple by-product dried under different conditions.

DM	DT	L*	a*	b*	Cutting Force [N]
CC	-	75.15 ± 0.06 D	5.08 ± 0.01 A	26.59 ± 0.02 A	39.14 ± 2.32 BC
CD	40	60.74 ± 0.16 A	13.65 ± 0.32 D	28.76 ± 0.60 BC	31.62 ± 2.78 A
	60	60.70 ± 0.72 A	13.30 ± 0.40 D	28.45 ± 0.30 B	32.54 ± 3.16 A
	80	60.02 ± 0.35 A	12.41 ± 0.27 C	28.12 ± 0.63 B	34.62 ± 2.16 AB
FD	20	65.05 ± 0.34 C	11.54 ± 0.06 B	30.21 ± 0.47 E	42.60 ± 1.29 CD
	40	64.36 ± 0.09 BC	11.29 ± 0.44 B	29.79 ± 0.07 DE	41.08 ± 1.95 CD
	60	64.25 ± 0.39 B	11.35 ± 0.56 B	29.39 ± 0.21 CD	45.38 ± 2.36 D

DM—drying method, DT—drying temperature, L*—lightness, a*—redness, b*—yellowness, CC—control cookies, CD—contact-drying, FD—freeze-drying. Means labeled with different superscript letters indicate significant differences at α = 0.05. Uppercase letters indicate differences among apple by-product powders.

). In the case of contact drying, higher thermal loads promoted the formation of darker pigments, resulting in a more intense coloration of the cookies. On the other hand, the influence of temperature within each drying method (both FD and CD) was less pronounced, suggesting that the primary mechanism driving color differences is related mainly to the mode of heat transfer and the associated thermal effects, rather than temperature value alone.

Compared to controls, enriched cookies were less bright but richer in red and yellow hues, reflecting the chromatic properties of the incorporated ABP powders. Such color changes are consistent with the color coordinates of the added powders and typically intensify as the contrast between the base flour and the enrichment ingredient increases. The greater the difference in L*, a*, and b* values between the flour and the ABP powder, the more pronounced the shift in the color of the dough and baked cookies, due to the direct transfer of pigment-rich components into the matrix of the final product. Similar trends in color modification, including decreased lightness and enhanced redness and yellowness, have been observed in cookies enriched with other fruit pomaces such as grapes, rosehip, rowanberry, blackcurrant, and elderberry, supporting the generality of these effects [4,57].

3.5. Hardness of Cookies

The drying method of ABP was the primary factor influencing the hardness of enriched cookies, whereas variations in drying temperature within each method had only a minor effect (Table 10). Cookies enriched with FD pomace were significantly harder than those containing CD pomace powders, as assessed by cutting force measurements, regardless of the applied temperature. Within the FD group, cookie hardness did not differ significantly between temperatures, and most FD samples were harder than the control, except for the sample freeze-dried at 60 °C, which was comparable to the control. In contrast, cookies containing CD by-product powder at 40 °C and 60 °C exhibited a markedly softer texture than the control, whereas the sample dried at 80 °C did not differ significantly from the control. Overall, increasing the drying temperature—from 40 °C to 80 °C for CD and from 20 °C to 60 °C for FD—did not significantly affect final hardness, confirming that drying method was the dominant factor. The observed variations in cutting force, which serve as an indicator of cookie hardness, can be attributed to several interrelated factors. Chief among them are the physical characteristics of the added pomace powders, including particle size distribution and fragmentation degree [58]. The FD sample produced a fine and highly uniform powder, which can be attributed to the porous dried structure with fine internal voids—an effect resulting from the sublimation process in freeze-drying that preserves the microstructure, reduces brittleness, and facilitates easier milling [59,60].

The presence of smaller particle sizes in freeze-dried powders likely promotes enhanced interaction and binding within the dough matrix, improving the cohesion between flour, fat, and other ingredients. This improved integration can lead to an increase in the overall dough density and rigidity, which is then reflected in the greater cutting force required to break the baked cookies. Such phenomena have been documented in studies on shortbread-type products, where finer particles from freeze-dried ingredients contributed to increased hardness and firmness of the final product [61]. Conversely, coarser particles from contact-dried powders may interfere with the dough network differently, yielding softer textures in the baked cookies. Overall, these findings demonstrate that the observed differences in cookie hardness are consistent with the physical properties of the pomace powders and their interactions within the dough matrix, highlighting the dominant effect of drying method while acknowledging the marginal role of temperature.

3.6. Sensory Results

The sensory analysis of the cookies revealed that replacing wheat flour with ABP improved their overall acceptability compared to the control (Figure 3). Mean overall acceptability scores increased from 5.77 in the control cookies to between 6.86 and 7.70 in ABP-enriched samples (Table S2, Supplementary Material), with median scores reflecting a similar trend. Literature indicates that a higher proportion of very fine particles may reduce sensory acceptability, whereas larger particles often enhance product perception. Zlatanović et al. [62] demonstrated that adding particles approximately 0.16 mm in diameter (“fine”) above 25% decreased acceptability, while a 25% addition of particles around 0.5 mm in size (“coarse”) improved it. In our study, where particle size was below 0.3 mm, a 20% addition increased acceptability, suggesting that the effect of particle size on sensory properties may depend on both the precise particle dimensions and the substitution level.

Although instrumental measurements revealed differences in cutting force among the biscuit variants (Table 10, Section 3.4), these differences did not significantly affect texture acceptance by the sensory panel (Figure 3), indicating limitations of instrumental parameters as predictors of consumer perception. The texture perceived during consumption is modified by the action of saliva and enzymes, such as α-amylase, which break down food components and influence sensory sensations, highlighting the complex relationship between physical properties and texture perception [63]. Cookies enriched with ABP were generally rated higher in terms of taste and appearance compared to the control. Significant correlations were observed between some sensory attributes and instrumental or compositional parameters: appearance scores were strongly correlated with color scores (r = 0.9833, p < 0.001) and inversely correlated with the a* color parameter (r = −0.9720, p = 0.001), while sensory color ratings were also inversely correlated with a* (r = −0.9451, p = 0.004). Additionally, perceived texture was negatively correlated with total dietary fiber content (r = −0.8566, p = 0.029), suggesting that higher fiber levels slightly reduced the perceived softness of the cookies.

The appearance scores for the control cookies ranged from 2 (“dislike very much”) to 9 (“like extremely”), whereas the scores for the enriched cookies ranged from 3 (“dislike moderately”) to 9. In terms of color, cookies enriched with FD by-product received significantly higher scores than the control, regardless of the drying temperature, while color ratings for cookies containing CD ABP did not differ significantly from the control, also irrespective of drying temperature. These trends were further reflected in the median scores, which reached 8 for cookies with FD ABP, 6–7 for CD ABP, and 6 for the control. The improvement in color scores for FD ABP-enriched cookies is likely associated with the milder processing conditions that better preserve the natural pigments and reduce thermal browning. The absence of significant differences in aroma ratings between enriched and control cookies suggests that the addition of ABP did not introduce undesirable odor notes that could negatively affect product acceptability. At a constant substitution level of 20%, neither the drying method nor processing temperature substantially affected overall sensory acceptability, despite measurable differences in instrumental color and texture (Table 10, Section 3.4). In contrast to these findings, Usman et al. [20] reported a decline in sensory quality when conventionally dried apple pomace exceeded 15% in the formulation. These discrepancies may be attributed to differences in formulations, including types and amounts of additives (e.g., shortening, sodium bicarbonate, sodium chloride, milk, and water), as well as the apple varieties used for pomace production. It should be emphasized that neither the drying method nor the drying temperature of ABP had a significant effect on the overall acceptability of the enriched products, indicating a high level of consumer tolerance for technological differences in the processing of this ingredient.

4. Conclusions

The study demonstrated that the drying method of apple pomace significantly influences both its drying kinetics and the characteristics of the resulting powder, with freeze-dried pomace exhibiting superior grindability compared to contact-dried material. Incorporation of 20% pomace into shortbread cookies exerted minimal effects on their overall composition, aside from a notable increase in dietary fiber content. Cookies enriched with freeze-dried pomace were characterized by higher lightness, reduced red tones, enhanced yellow hues, and increased firmness, requiring approximately 30% greater cutting force than cookies containing contact-dried pomace. Despite these physical differences, consumer acceptance remained only slightly affected and consistently higher for enriched cookies compared to non-enriched controls. Significant correlations indicated that compositional and colorimetric properties influenced some sensory parameters of cookies, with appearance linked to color scores and a* coordinate values, and texture inversely related to total dietary fiber. Furthermore, variations in the drying temperature of the pomace had only a minor effect on cookies enriched with the pomace, whereas the drying method remained the primary factor determining their properties, including color, hardness, fiber content, and particle size. These results indicate that apple pomace is an effective ingredient for enhancing the nutritional quality of cookies, while the choice of drying technique allows targeted modulation of specific textural and color attributes without compromising overall sensory appeal. Overall, this study provides a systematic comparison of freeze-drying and contact drying at multiple temperatures and their combined effects on enriched cookies, addressing a gap in previous research and highlighting its innovative aspect. Future studies will focus on the biochemical properties of dried apple pomace and enriched products, particularly antioxidant activity and phenolic content, to further clarify their functional potential.