The Impact of Drying Method on the Physicochemical, Bioactive Compounds and Antioxidant Properties of Common Quince Fruit (Cydonia oblonga Mill.)

Abstract

The fruits of the common quince (Cydonia oblonga) have wide-ranging health benefits due to their valuable composition. However, consumers usually do not welcome the hard flesh and astringency, so the fruit is not eaten raw. Therefore, it is important to choose the proper method for processing, including heat treatment, to preserve the high quality of the common quince fruit. The study examined the effects of freeze drying and convection drying at two temperatures (40 °C and 60 °C) on selected physicochemical, bioactive, and antioxidant properties of Cydonia oblonga fruits. It was found that freeze drying allowed the processed fruit to retain properties most similar to fresh fruit. This variant had the highest rehydration rate (3.53 ± 0.04), the lowest shrinkage rate (9.87 ± 0.29%) and the lowest bulk density (0.41 ± 0.01 g/cm³). Freeze drying preserved the brightest fruit colour (L* = 75.70 ± 1.71). These samples also had the highest total acidity (1.34 ± 0.01 g/100 g DM). Drying reduced the fruit’s tannin content, but no statistically significant differences were detected between freeze-dried and convection-dried samples at 40 °C and 60 °C. Freeze-dried quince fruits retained a high content of polyphenols (233.56 ± 5.96 mg GEA/100 g DM), flavonoids (36.79 ± 0.74 mg EPI/100 g DM), and antioxidant activity against ABTS (364.51 ± 9.12 µM Trolox/100 g DM) and DPPH (258.78 ± 5.16 µM Trolox/100 g DM). The highest losses of the mentioned bioactive compounds, and thus of antioxidant activity, were recorded in samples dried by convection at 60 °C.

1. Introduction

Cydonia oblonga Mill. is a small-sized shrub in the Rosaceae family, with yellow pome fruit known as quince. Due to their hard flesh and sour taste, these fruits are not popularly eaten raw, but they are processed in the food industry into jellies, marmalades, jams, compotes, wines, and liqueurs, and dried into slices. The unusual aroma and valuable composition of quince make it suitable for various products, including yoghurt or beer [1,2]. Unfortunately, fresh quince fruits are predisposed to browning and rotting, limiting their long-term storage [3]. One of the most common processes used to improve food stability is dehydration, which reduces water activity in the material, improves microbiological stability, and minimises physicochemical changes during storage. Drying is a complex process that can lead to significant changes in the structure (e.g., pore formation and shrinkage), physical properties (e.g., surface hardening and colour change), and chemical composition of the preserved material. Therefore, choosing the proper drying method and conditions minimises potential product losses. The most common food dehydration process is convection drying, which results in products with significant colour change, low porosity, and poor rehydration capacity. Significantly improved properties of the dehydrated material are obtained by sublimation drying. After this process, the product retains its colour and aroma, has a porous structure, exhibits little to no shrinkage, and possesses good rehydration properties [4,5,6]. Due to their rich phytonutrient content, wide availability, and low cost, quince fruits are a valuable ingredient for preparing health-promoting products [2]. Quince fruits have been shown to exhibit antioxidant, anti-inflammatory, immunomodulatory, antibacterial, antiviral, anti-hemolytic, anti-tumoral, and anti-ulcer effects [2,7]. They also exhibit protective effects against lipid and carbohydrate metabolism disorders and, thus, the cardiovascular system [7]. Quince fruits are rich in fibre (1.10–2.6%), vitamins (mainly vitamin C, 15.32 g/100 g), and minerals [8]. Quince is a better source of mineral elements than the popularly consumed apples. The fruit is particularly rich in potassium (107.97–251.99 mg/100 g FW), phosphorus (12.85–23.11 mg/100 g FW), and calcium (7.27–19.07 mg/100 g FW) [3]. Notably, the fruit also contains a high amount of pectin. These compounds are used in the food industry as gelling agents. Once consumed, pectin exhibits various physiological benefits, acting as a prebiotic and lowering glucose and cholesterol levels [9]. Rop et al. [3] tested 22 quince varieties with pectin content ranging from 1.75 to 3.51 g/100 g FW. Thus, quince fruits are used to make fruit spreads [1]. Quince fruits also demonstrate high polyphenol content, contributing to their antioxidant activity [2]. There are about twice as many compounds in C. oblonga fruit as in apple fruit [7]. Phenolic compounds in quince fruit, such as quercetin-3-O-rutinoside and quercetin-3-O-glycosides, are considered potent antioxidants responsible for the fruit’s quality characteristics, including its colour, tart taste, and texture [1]. Another phenolic compound is 5-O-cavoylquinic acid, the primary substrate of the enzyme polyphenoloxidase, which contributes to the enzymatic browning of quince fruit [9]. It should be noted that fruit’s phytochemical composition depends on several factors, including the variety, growing conditions (soil and weather), the length of the growing season, and the plants’ physiological state [7]. The maturity of the fruit and the time it was stored are also important. With increasing ripeness, the content of phenolic compounds decreases, as does the total content of soluble solids. The decrease in the astringency and bitterness of fruits at maturity is precisely due to the degradation and use of phenolic compounds in the biosynthesis of other compounds [1,7]. The analytical procedure used, as well as the extractant, have a significant effect on the number and proportions of compounds extracted. The storage process of the fruit also affects the content of individual bioactive compounds. The individually selected extractant significantly impacts the number and proportions of compounds extracted. In addition, regarding dried fruits, choosing the proper drying method, including time and temperature, also plays a significant role [7].

Quince fruits are characterised by their hard flesh and a tart, sour taste, so they are not commonly consumed unprocessed. On the other hand, they are rich in the previously mentioned bioactive ingredients that positively affect health. This leads the search for methods of processing these fruits to minimise the impact on the content of valuable substances found in quince. Therefore, this study aimed to evaluate the effects of convection drying at 40 °C and 60 °C and freeze drying on C. oblonga fruits’ physicochemical and health-promoting properties. Convection drying is the most common method on an industrial scale and in domestic conditions, so choosing the appropriate process temperature is important. The results presented in the paper can provide valuable information, especially for fruit processors, who can obtain high-quality, health-promoting dried fruit by selecting the proper drying method and temperature.

2. Materials and Methods

2.1. Raw Material

The quince (Cydonia oblonga) fruits used in this study were bought from a supermarket in Lublin, Poland. The fruit came from the same place and variety (cv. Leskovacka). All quinces were selected by subjective visual inspection based on colour, size (average weight 221.7 ± 18.32 g), and lack of external damage. The fruits were refrigerated at 4–6 °C for 12 h until analysis.

2.2. Drying Methods

The fruits were washed with distilled water, dried on a towel on the counter, then cut into thin slices about 3–4 mm thick, and the seeds were removed. Three methods were used to carry out the drying process: (1) convection at 40 °C in a laboratory convection oven with forced air circulation (at an air speed of 1.5 m/s) (SUP-65W, Wamed, Warsaw, Poland), drying time 16 h; (2) convection at 60 °C in a laboratory convection oven with forced air circulation (at an air speed of 1.5 m/s), drying time 7 h; (3) freeze drying (Delta 2–24 LSCplus, Christ, Osterode am Harz, Germany); process parameters: pressure 0.521 mbar, shelf temperature 20 °C, freeze-drying chamber temperature −65 °C, drying time 48 h.

In the case of convection drying, an effort was made to select the drying time so that the raw materials retained a similar water content (about 10–12%). The freeze-dried and convection-dried test material was sealed in plastic containers and stored until analysis.

2.3. Measurement of Quantitative and Qualitative Characteristics

2.3.1. Shrinkage

The sample’s shrinkage was determined according to the methodology described by Yang et al. [10]. The millet was divided into two parts: one was poured into a 100 mL beaker, and the other was left in a 50 mL measuring cylinder. Tested quince fruit slices (fresh and dried by different methods) were placed in separate beakers, to which millet from the cylinder was added. The volume of millet remaining in the cylinder was obtained as the volume of dried and fresh quince slices. The shrinkage rate (VR) was calculated according to the formula:

VR = (1 − V_d/V) × 100%

where V is the volume of fresh quince (cm³), and V_d is the volume of dried quince (cm³).

2.3.2. Rehydration Ratio

The dried quince samples were weighed, poured into 100 mL of distilled water at 25 °C, and soaked for 30 min. After this time, the samples were removed, and the excess water was gently dried with blotting paper. Then, the samples were weighed, and the rehydration rate (RR) was calculated using the following formula [11]:

RR = M_r/M_b

where M_r is the sample weight after rehydration (g), and W_b is the weight before rehydration (g).

2.3.3. Bulk Density

The bulk density of dried fruit was calculated as the ratio between the weight of the sample and its total volume using the following formula [11]:

ρ_b = M_t/V_t

where M_t is the sample weight (g), and V_t is the sample volume (cm³).

2.4. Colour Measurements

The flesh colour of fresh and dried quince samples was measured with an EnviSense NH310 colourimeter (EnviSense, Lublin, Poland). The CIE L*a*b* scale was used to record the colour differences between the samples, in terms of brightness (L*) and colour (a*—red; b*—yellow). Then, the total colour difference (ΔE) was calculated using the formula:

$$∆\mathrm{E}=\sqrt{{∆\mathrm{L}*}^2+{∆\mathrm{a}*}^2+{∆\mathrm{b}*}^2}$$

Additionally, the browning index (BI) was calculated using the measured L*, a*, and b* values using the formula [12]:

BI = [100 × (X − 0.31)]/0.17

where X:

X = (a* + 1.75 L*)/(5.645 L* + a* − 0.012 b*)

2.5. Determination of Total Acidity

The total acidity of quince samples was determined following the Polish Standard (PN-EN 12147:2000) [13]. To 25 g of each quince sample, 100 mL of distilled water was added. The mixture was then boiled, cooled, and quantitatively transferred to a 250 mL volumetric flask, which was filled to the mark with distilled water. The volumetric flasks were set aside for 15 min. The contents were then filtered, 10 mL was taken from the filtrate, and a few drops of phenolphthalein were added and titrated with a 0.1 M NaOH solution until the colour changed. Total acidity was converted to malic acid as required by the standard and expressed in g/100 g of product.

2.6. Bioactive Properties

2.6.1. Determination of Tannin Content

The tannin content of the quince samples was determined using the titration-weighing method [14]. Five grams of each sample were placed in beakers, 250 mL of boiling distilled water was added, the beakers were covered, and the mixture was left for 10 min. After this time, the contents were filtered through a tissue paper filter. From the filtrate, 175 mL was taken and heated to a boil. Then, 20 mL of 4% copper acetate solution was added, cooled, made up to 200 mL with distilled water, and filtered. To 100 mL of the resulting filtrate, the following was then added: 2% aqueous starch solution (1.5 mL), 10% potassium iodide solution (20 mL), and 50% acetic acid (25 mL). The resulting filtrate was titrated with 0.1 M sodium thiosulfate solution until the colour change of the solution persisted for a minimum of 30 s. Considering the dilutions and sample weights, the tannin content was calculated based on the following formula and expressed in g/100 g DM of quince:

X = (V1 − V2) × 0.01039

where

• V1—volume of 0.1 M thiosulfate consumed to titrate the control sample;
• V2—volume of 0.1 M thiosulfate consumed to titrate the control sample;
• 0.01039—the number of grams of tannins corresponding to 1 mL of 0.1 M thiosulfate.

2.6.2. Bioactive Compounds Extraction

The quince samples (fresh and dried by different methods) were taken at 1 g each, then crushed and placed in Falcon tubes. Next, 10 mL of ethanol (99.8%) and distilled water were added to the samples in a ratio of 4:1 (v/v). The mixture was shaken for 120 min in a laboratory shaker. After that, the samples were filtered on filter paper, and the filtrates were centrifuged at 4500 rpm for 8 min and stored at −20°C for further analysis. The prepared extract determined total polyphenols, flavonoids, and antioxidant activity against ABTS and DPPH.

2.6.3. Total Polyphenol Content (TPC)

TPC was determined according to the Singleton and Rossi method [15]. The prepared quince extract (0.04 mL), distilled water (3.16 mL), and Folin–Ciocalteu reagent (0.2 mL) were mixed in glass tubes. After 3 min, a 10% sodium carbonate solution (0.6 mL) was added, mixed, and heated at 40 °C for 30 min. After this time, the absorbance of the samples relative to the blank was measured at 765 nm (Cary 50 Scan spectrophotometer, Varian, Palo Alto, CA, USA). TPC was expressed as gallic acid equivalent per 100 g dry weight (mg GEA/100 g DM).

2.6.4. Total Flavonoid Content (TFC)

TFC was determined according to the procedure of Karadeniz et al. [16]. The prepared extract (0.5 mL), 5% water solution of sodium nitrate (III) (0.15 mL), and distilled water (2.5 mL) were mixed in glass tubes. After 5 min, a 10% water solution of aluminium chloride hexahydrate (0.3 mL) was added and remixed. After another 5 min, 1 M sodium hydroxide solution (1 mL) and distilled water (0.55 mL) were added to the tubes. The samples’ absorbance at 510 nm relative to the blank sample was measured. TFC was expressed as the amount of epicatechin per 100 g dry weight (mg EPI/100 g DM).

2.6.5. ABTS Radical Scavenging Activity

ABTS scavenging activity was defined using the methods of Re et al. [17]. The radical solution was prepared from ABTS•+ and potassium persulfate, dissolved in distilled water to a concentration of 2.45 mM. To develop the radical, the solution was allowed to stand in the dark (20 h) and then diluted to give an absorbance of about 0.7–0.8 at 734 nm. In glass tubes, 0.1 mL of quince extract and 2.9 mL of ABTS+ solution were mixed. The tubes with the mixture were left for 30 min in the dark. The absorbance of samples was measured against the blank at 734 nm. The results were expressed as Trolox equivalent antioxidant activity (TEAC) values (µM Trolox/100 g DM).

2.6.6. DPPH Radical Scavenging Activity

DPPH scavenging activity was defined using the methods of Brand-Williams et al. [18]. In glass tubes, 0.1 mL of quince extract and 2.9 mL of a DPPH solution in 75% methanol (6 µM) were mixed. The tubes were left for 30 min in the dark. The absorbance of the samples was measured at 515 nm against the blank sample. The results were expressed as Trolox equivalent antioxidant activity (TEAC) values (µM Trolox/100 g DM).

2.7. Statistical Analysis

All trials were conducted in at least three replicates, and results are presented as mean ± SD (standard deviation). A statistical analysis of the results was performed with Statistica software (version 13.3, StatSoft, Krakow, Poland) using one-way analysis of variance (ANOVA) with Tukey HSD (honestly significant difference) post hoc test, at the significance level of p < 0.05.

3. Results and Discussion

3.1. Quantitative and Qualitative Characteristics

Table 1. Rehydration ratio, shrinkage rate, and bulk density of dried quince fruits.

Characteristics	Freeze-Dried	Conv-40	Conv-60
rehydration ratio	3.53 ± 0.04 a	2.58 ± 0.06 b	1.84 ± 0.04 c
shrinkage rate [%]	9.87 ± 0.29 c	57.33 ± 0.55 b	66.03 ± 0.8 a
bulk density [g/cm3]	0.41 ± 0.01 c	0.63 ± 0.02 b	0.71 ± 0.02 a

Freeze-Dried—freeze-dried fruit; Conv-40—convection-dried fruit at 40 °C; Conv-60—convection-dried fruit at 60 °C; within each line, values with the same letter are not significantly different at p < 0.05 (Tukey’s post hoc test).

shows the rehydration, shrinkage, and bulk density values obtained for freeze-dried and convection-dried quince fruits at 40 °C and 60 °C. Rehydration is a complex process of reabsorption of water by a material after it has already been removed. However, lost water cannot be restored to its original state. Many factors, including processing conditions and product properties, among them the characteristics of the pore network, affect the rehydration process [19]. This study found that the rehydration coefficient of convection-dried quince was significantly lower than the freeze-dried sample. Quince fruits dried at 60 °C had a lower rehydration coefficient (1.84 ± 0.04) compared to samples dried at 40 °C (2.58 ± 0.06). Izli and Polat [20] noted a similar correlation, reporting a significantly lower rehydration rate for convection-dried quince cubes compared to freeze-dried, as well as Caliskan and Dirim [5] for drying pumpkin slices. Yang et al. [10] also noted a significantly lower rehydration rate of convection-dried apricot slices at 60 °C compared to sublimation-dried samples. This may be due to the formation of a “shell” on the surface of the dried fruit and the collapse of the inner cell matrix due to high temperatures, which in turn affects the penetration of external water into the slices [10]. Convection drying disrupts the structure of food cells due to high shrinkage levels. This shrinkage reduces the hydrophobicity of the sample [20]. Product shrinkage is a common physical occurrence during crop drying. The action of heat causes stresses in the cellular structure of the product, leading to shrinkage and deformation. Food shrinkage during drying reduces rehydration capacity and bulk density [11]. In this study, convection-dried fruit had the highest shrinkage percentage, at 66.03 ± 0.8 for quince dried at 60 °C and 57.33 ± 0.55 for fruit dried at 40 °C, respectively. The freeze-dried sample had the lowest percentage of shrinkage (9.87 ± 0.29%), indicating that its volume remained nearly unchanged during the water removal process. According to the literature, ice sublimation leaves an extensive pore network, so the material has slight shrinkage. Yang et al. [10] obtained similar results by comparing different methods of drying apricot slices, finding the lowest percentage of shrinkage for freeze-dried samples and the highest for hot air-dried samples. In the study presented here, freeze-dried samples had the lowest values of bulk density (0.41 ± 0.01 g/cm³), since freeze drying allows for ice sublimation, leaving voids in the structure without significant shrinkage. Samples dried at 40 °C (0.63 ± 0.02) and 60 °C (0.71 ± 0.02) had significantly higher bulk density values. Bulk density depends on the shrinkage rate, which strongly depends on the chosen drying method [4]. Bulk density testing is significant in food product packaging, storage and transportation [11].

3.2. Colour Measurements

The appearance of dried quince samples is shown in Figure 1. The obtained results measured in the L*a*b* system, the total colour difference (ΔE) and browning index (BI) of C. oblonga flesh, are shown in

Table 2. Colour parameters in L*a*b* space of fresh and dried quince.

Parameter	Fresh	Freeze-Dried	Conv-40	Conv-60
L*	75.70 ± 1.71 b	79.66 ± 1.85 a	57.56 ± 1.82 c	58.45 ± 3.78 c
a*	5.21 ± 0.95 d	10.48 ± 1.80 c	17.42 ± 0.86 b	18.92 ± 1.14 a
b*	27.26 ± 1.77 d	30.28 ± 2.22 c	33.91 ± 2.22 b	37.85 ± 1.84 a
ΔE	-	7.25	22.85	24.25
BI	5.03	9.40	22.28	20.90

Fresh—fresh fruit; Freeze-Dried—freeze-dried fruit; Conv-40—convection-dried fruit at 40 °C; Conv-60—convection-dried fruit at 60 °C; within each line, values with the same letter are not significantly different at p < 0.05 (Tukey’s post hoc test).

. In evaluating the flesh of the quince fruits studied, the value of the L* parameter was statistically significant, the highest in freeze-dried quince fruit (79.66 ± 1.85), which means that these samples were thebrightest most promising among all those analysed. The lowest L* parameter was observed in convection-dried quince, with no statistically significant differences between drying temperatures of 40 °C and 60 °C. This indicates a darker colour compared to fresh and freeze-dried fruit. The values of the a* and b* parameters were highest in quince dried at 60 °C, followed by 40 °C. This is related to the approximation of the colour of the fruit flesh to red and yellow. At the same time, these samples also had the highest browning index and total colour difference. Larger values of ∆E show more significant changes in colour between fresh and dried fruit. A colour difference of ∆E > 5 is interpreted as very pronounced [21].

The literature data report that the colour of vegetables and fruits is influenced by several factors, including storage and transport conditions, during which the degradation of colouring substances (particularly carotenoids and chlorophylls) can occur. In addition, enzymatic and non-enzymatic browning reactions and raw material processing methods (especially heat treatment) significantly affect the colour [22]. The physicochemical properties of the product are also important, such as total acidity, pH, sugar content (especially reducing sugars, which are substrates for the Maillard reaction), polyphenolic compound content, and time of exposure to light and atmospheric oxygen [23].

Colour is one of the most essential quality characteristics influencing consumer purchase. Incorrect colour is associated with food deterioration and causes consumers to reject the product [22]. Therefore, this characteristic must be as attractive and similar to the fresh product [21]. Quince is a fruit that turns brown rather quickly after cutting, related to the oxidation of polyphenolic compounds and the high activity of the enzyme polyphenol oxidase in the flesh [24]. Therefore, fruit treated with low-pressure sublimation drying is lighter in colour compared to raw and convective-dried fruit. The oxidation of dyes can explain significant colour changes during convection drying due to extended exposure to oxygen and higher drying temperatures. Maillard reactions occur more rapidly at higher temperatures, which can be explained by the dark colour of convection-dried fruits. In addition, as the heat treatment time increases, the intensity of browning of the dried fruit also increases. Therefore, for the extended drying time at 40 °C (16 h), there were no statistically significant differences in the colour of quince flesh dried at this temperature compared to drying at 60 °C.

3.3. Total Acidity

The results of the total acidity of quince fruits are shown in Figure 2. Fresh quince fruits had the lowest total acidity per malic acid (0.84 ± 0.03 g/100 g), while freeze-dried fruits had the highest values of this parameter (1.34 ± 0.01 g/100 g). The total acidity of convection-dried quince at 40 °C (1.01 ± 0.02 g/100 g) and 60 °C (1.09 ± 0.06 g/100 g) were similar and not statistically significantly different.

Fruit acidity is a critical quality parameter and a key determinant of taste [25]. When the acidity is between 0.08 and 1.95%, the fruit can be classified as mild in flavour and generally more appreciated by consumers when fresh [26]. The literature data report that this parameter depends on several factors, including cultivar and genotype, growing conditions, agricultural practices, harvesting date (related to fruit maturity), and even extraction and analytical procedures [27,28,29]. Therefore, the data available in the literature on the acidity of fresh quince fruit are quite disparate. For example, in a study by Rasheed et al. [27], the fruit’s acidity ranged from 1.11% to 1.25%, depending on the crop’s location. Sharma et al. [30] and Ali et al. [25] report similar results, stating that quince fruit acidity is 1.20% and 1.33–1.35%, respectively. Curi et al. [26] obtained analogous results for the acidity of fresh fruit compared to those obtained in the present study. They found acidity in 10 varieties of common quince, ranging from 0.81 to 1.00 g malic acid/100 g. Different results were obtained by Najman et al. [31], stating that the total acidity of fresh quince fruit was 0.26 g/100 g. On the other hand, the researchers’ conclusions agree with the results obtained in the present study and confirm that drying quince fruit increases its acidity, especially in the case of freeze drying. Mir et al. [32] obtained similar results, finding an increase in the acidity of tunnel-dried quince (1.39%) compared to fresh fruit (1.18%).

3.4. Bioactive Properties

3.4.1. Tannin Content

The results of the tannin content determination are shown in Figure 3. The study shows fresh quince had the highest tannin content (3.31 ± 0.83 g/100 g DM). In the case of convectively dried and freeze-dried fruit, the content of tannins was significantly lower. No statistically significant differences were detected between freeze-dried (1.17 ± 0.28 g/100 g DM) and convection-dried samples at 40 °C (0.84 ± 0.21 g/100 g DM) and 60 °C (1.1 ± 0.47 g/100 g DM).

Tannins are bitter-tasting compounds that impart a brown colour to products, making them undesirable organoleptic properties. Nevertheless, they are valuable compounds that exhibit antioxidant, anti-inflammatory, antimicrobial, and antiviral activity. Tannins protect against various biotic and abiotic stressors [33,34]. Due to the bitterness and astringent taste imparted by, among other things, tannins, quince fruits are rarely eaten fresh [35,36]. In the literature, various data on the tannin content of fresh quince fruit can be found. For example, in a study by Sharma et al. [30], tannin was only 0.8%, while in the work of Djilali et al. [36], the content of these compounds was high, amounting to 9.66% and 7.33%, depending on the extraction method. These differences may be due to different extraction methods and the solvents used [36], but may also depend on the variety, the part of the fruit (skin, pulp), or the degree of ripeness of the fruit [1].

3.4.2. TPC and TFC

The phenolic and flavonoid content of fresh and dried quince fruits shown in

Table 3. Bioactive properties of fresh and dried quince.

	Fresh	Freeze-Dried	Conv-40	Conv-60
Total polyphenol content (mg GEA/100 g DM)	259.68 ± 23.26 a	233.56 ± 5.96 ab	205.87 ± 12.90 b	141.79 ± 9.83 c
Total flavonoid content (mg EPI/100 g DM)	40.17 ± 0.48 a	36.79 ± 0.74 b	25.99 ± 0.21 c	22.41 ± 0.70 d
ABTS scavenging activity (µM Trolox/100 g DM)	448.62 ± 4.23 a	364.51 ± 9.12 b	283.56 ± 8.26 c	232.12 ± 11.47 d
DPPH scavenging activity (µM Trolox/100 g DM)	328.85 ± 16.98 a	258.78 ± 5.16 b	172.85 ± 15.51 c	144.24 ± 5.83 d

Fresh—fresh fruit; Freeze-Dried—freeze-dried fruit; Conv-40—convection-dried fruit at 40 °C; Conv-60—convection-dried fruit at 60 °C; within each line, values with the same letter are not significantly different at p < 0.05 (Tukey’s post hoc test).

. Fresh fruits had the highest TPC and TFC of 259.68 ± 23.26 mg GEA/100 g DM and 40.17 ± 0.48 mg EPI/100 g DM, respectively. Freeze drying preserved a high content of phenolic compounds (233.56 ± 5.96 mg GEA/100 g DM), which were only about 10% less in freeze-dried samples than in fresh fruit. Similarly, convection drying at 40 °C preserved a relatively high content of polyphenols (205.87 ± 12.90 mg GEA/100 g DM). Significantly, the lowest TPC was found for quince dried at 60 °C (141.79 ± 9.83 mg GEA/100 g DM). Sublimation drying also preserved a high content of flavonoids (36.79 ± 0.74 mg EPI/100 g DM), the loss of which, compared to fresh fruit, was about 11%. The amount of these compounds was significantly lower in convection-dried samples at 40 °C (25.99 ± 0.21 mg EPI/100 g DM) and 60 °C (22.41 ± 0.70 mg EPI/100 g DM).

The present study did not investigate the qualitative composition of bioactive compounds, focusing on their quantity and change during the selected method or drying parameters. However, the available literature shows quince fruits’ qualitative composition of polyphenols or flavonoids. They contain important phenolic compounds, including 3-O—claviloylquinic acid (chlorogenic acid), 4-O—clavoylquinic acid (crypto chlorogenic acid), 5-O—clavoylquinic acid (neochlorogenic acid), and 3,5—diclavoylquinic acid. The dominant flavonoids in the fruit of C. oblonga are epigallocatechin and rutin, with catechins and quercetin present in lower concentrations. In addition, lucenin, vicenin, stelarin, and apigenin were also found [37]. The importance of flavonoids in quince was proven in a study by Umar et al. [38]. Using hyperlipidemic rat models, researchers studied the effects of total flavonoids from C. oblonga (from the leaves and fruit) on blood lipid levels and antioxidant potential. They demonstrated that total quince flavonoids can effectively regulate lipid metabolism and scavenge oxygen free radicals, indicating their potential value in preventing and treating hyperlipidemia.

In general, freeze drying helps protect the product’s quality characteristics, since a low temperature reduces the likelihood of degradation of many sensitive compounds, such as polyphenols and flavonoids [5]. For example, in a study by Turkiewicz et al. [39] on Japanese quince, they also found that freeze drying allows for high content of bioactive compounds, including polyphenols, whose loss was only slightly more than 4% compared to the polyphenolic extract before drying. A study by Shofian et al. [40], evaluating the effect of freeze drying on the phenolic compound content of selected tropical fruits, showed that freeze-dried melon had only about 11% fewer polyphenols than fresh fruit. The same study showed just over 20% loss of phenolic compounds in freeze-dried mango and carambola compared to fresh counterparts and as much as a 40–50% reduction in TPC in melon and watermelon after freeze drying. On the other hand, in the Bayram et al. [41] study, the TPC of freeze-dried groundnut fruit was significantly higher (more than 40%) than that of fresh fruit. The authors explain this increase by the higher extractability of polyphenols into the extraction solvent due to the mild microstructural changes induced by the formation of ice crystals during freeze drying and higher rehydration with solvent due to the porous structure obtained. In addition, drying can change the fractional contribution of free and bound phenolic compounds [41]. A study by Stamenkovic et al. [42] found losses of bioactive compounds in raspberries convection-dried at 60 °C, 70 °C, and 80 °C compared to fresh fruit: total phenols at 32–40%, while flavonoids were in the range of 3–25%. Concerning polyphenols, these are similar losses to those obtained in this study.

3.4.3. Antioxidant Activity Against ABTS and DPPH

The highest antioxidant activity against both ABTS and DPPH was found for fresh fruit and was 448.62 ± 4.23 µM Trolox/100 g DM and 328.85 ± 16.98 µM Trolox/100 g DM, respectively (Table 3). The freeze-dried samples had a high antioxidant activity of 364.51 ± 9.12 µM Trolox/100 g DM against ABTS and 258.78 ± 5.16 µM Trolox/100 g DM against DPPH. Indeed, convection-dried fruit had the lowest free radical scavenging capacity. Drying temperature also played a significant role, as quince dried at 60 °C had the lowest activity against ABTS (232.12 ± 11.47 µM Trolox/100 g DM) and DPPH (144.24 ± 5.83 µM Trolox/100 g DM).

Quince fruits are a good source of natural phenolic antioxidants. These antioxidants play an essential protective role in the plant against ultraviolet radiation, act as a defence against pathogens and predators, act as an attractant in fruit spreading. Quince is a natural and inexpensive source of potent antioxidants, such as phenolic acids and flavonoids. However, the antioxidant capacity is attributed not only to the content of phenolic and organic acids but also to the action of numerous other compounds present in quince and possible synergistic and antagonistic effects [1]. However, when exposed to elevated temperatures and oxygen, many bioactive compounds responsible for antioxidant activity are degraded [42,43]. Therefore, the present study showed a loss of free radical scavenging capacity due to convection drying. Similar correlations, but for dried raspberry fruit, were reported in a study by Stamenković et al. [42], which found lower radical scavenging capacity in convection-dried samples compared to fresh or freeze-dried. To preserve the high antioxidant activity, freeze drying quince fruit appears to be a good processing method. Another study by Antoniewskaya et al. [44] proved that such fruits can then be used to produce functional foods. The researchers developed a cookie recipe with freeze-dried Japanese quince fruit. The cookies had 2–3.5 times higher antioxidant activity and fewer secondary lipid oxidation products than the control cookies. In addition, the product enriched with natural antioxidants showed microbiological stability for as long as 16 weeks of storage.

4. Conclusions

Methods such as freeze drying and convection drying help extend the shelf life of fruits. Choosing a method for drying a food product is a crucial step, as the drying procedure and its conditions significantly impact the quality and cost of the dried product. To meet the expectations of modern consumers, the processing method must be selected so that the final product resembles its fresh counterpart in appearance and composition. The drying method also significantly affects the colour of quince fruit, which may also be important to consumers. Choosing the proper method of drying quince fruit significantly impacts the content of its bioactive compounds. In the present study, the most significant quality changes occurred, especially during convection drying at 60 °C, while freeze drying allowed the processed fruit to retain properties most similar to fresh fruit. However, it is a rather expensive method of preserving food products, so it is in the interest of fruit processors and consumers to choose a more appropriate convection method. In the case of the presented study, lower losses were found when 40 °C was used. Compared to fresh samples, 55–79% of total phenols and 56–65% of flavonoids were retained in convection-dried quince. Accordingly, lower free radical scavenging capacity was also found in convection-dried samples compared to fresh or freeze-dried samples. Dried quince slices, especially freeze-dried, can become an interesting alternative for snacks with health-promoting properties. They fit perfectly into the functional food segment.