Convective Drying of Fresh and Frozen Raspberries and Change of Their Physical and Nutritive Properties

Raspberries are one of Serbia’s best-known and most widely exported fruits. Due to market fluctuation, producers are looking for ways to preserve this fresh product. Drying is a widely accepted method for preserving berries, as is the case with freeze-drying. Hence, the aim was to evaluate convective drying as an alternative to freeze-drying due to better accessibility, simplicity, and cost-effectiveness of Polana raspberries and compare it to a freeze-drying. Three factors were in experimental design: air temperature (60, 70, and 80 °C), air velocity (0,5 and 1,5 m·s−1), and state of a product (fresh and frozen). Success of drying was evaluated with several quality criteria: shrinkage (change of volume), color change, shape, content of L-ascorbic acid, total phenolic content, flavonoid content, anthocyanin content, and antioxidant activity. A considerable influence of convective drying on color changes was not observed, as ΔE was low for all samples. It was obvious that fresh raspberries had less physical changes than frozen ones. On average, convective drying reduced L–ascorbic acid content by 80.00–99.99%, but less than 60% for other biologically active compounds as compared to fresh raspberries. Convective dried Polana raspberry may be considered as a viable replacement for freeze-dried raspberries.

In Serbia, raspberries are often processed to jelly, diary, or confectionery products able to be stored for a longer time [6,[14][15][16][17], however it is difficult to preserve this berries with high level of natural properties. In industry, this problem is commonly tackled with some form of drying, where one of

Experimental Design and Statistical Analysis
Convective raspberry drying was three-factor experiment with one qualitative and two quantitative factors. The qualitative factor represented the initial state of the raspberry fruit before drying with two levels (fresh and frozen raspberries). Quantitative factors of the experiment were drying temperatures (60, 70, to 80 • C) and air velocity (0.5 to 1.5 m·s −1 ). The absolute air humidity was approximately constant in the experiments with average value 0.011 ± 0.002 kg w /kg d.air . For each experimental run, the initial mass of the raspberries was approximately 500 g. Raspberry samples were set on perforated sieve in a thin stagnant layer and placed in a drying chamber. Air flew along or across the surface of the material in the dryer, and the samples were dried until the same value of moisture content of approximately X d.b = 0.152 kg w /kg d.b. The experiment was conducted with three repetitions for each experimental run.

Measuring of Volume and Shrinkage Determination
When drying some biomaterial, volume shrinkage (V sh ) is one of the most common physical and quality indicators of the final product. Shrinkage is expressed by the ratio between the change of volume after drying and volume of sample before drying. The samples volume (n = 45) were measured by immersing the raspberries into 96% concentration of ethanol [27] according to V 0 = (m 0 − m l )/ρ t (1) where m 0 is the mass of liquid and the immersed sample (kg), m l is the mass of liquid, and ρ t is the liquid density (kg/m 3 ). The volumetric shrinkage of raspberries (V sh ) was based on the following equation [28].
where V 0 is initial average volume and V i is volume of each raspberry after drying.

Determination of Heywood Shape Factor
If observed independently, the volumetric shrinkage is not a sufficient indicator of the changes in dried material. For this reason, an additional indicator was used to monitor changes of shape, i.e., Heywood shape factor (k), able to detect the changes after drying [29][30][31]. This factor k = 0.523 and was calculated from the relation where V p is the particle volume with equivalent diameter of the projected area of the particles. This was obtained by assigning the area of an equivalent circle with the same greater diameter as that of the fruit [32].

Color Measurement and Total Color Difference
Before and after each drying regime, CIELab color parameters were assessed for raspberry samples (n = 45) by colorimeter Konica Minolta CR400(C-light source and the observer angle of 2 • ). Where L* was whiteness/brightness, a* was redness/greenness, and b* represented yellowness/blueness. The total color difference (∆E), hue angle (h o ) and chromaticity (C*) were calculated by Equations (4)-(6) [33,34]: where, L 0 , a 0 , and b 0 are the color values before drying, while L*, a*, and b* are the color values after drying.

Extraction Procedure
Methanol extract was prepared from the fresh/dried raspberry samples to determine the contents of total phenols, flavonoids, and radical scavenging capacity. Briefly, 50 mL of an extraction solvent (methanol, 99.8%) (Fisher Scientific, UK) was poured over the raspberry sample into an Erlenmeyer flask. The flasks were covered and placed on a laboratory stirrer for 24 h (in a dark place). After the extraction, the samples were transferred to volumetric flasks of determined volume, filtered, and stored in a dark and cool place until the analysis was carried out.

Determination of Total Phenolic Content (TPC)
The TPC in methanol extracts of fresh and dried raspberries was determined by Folin-Ciocalteu spectrophotometric method [35]. In a 50 mL volumetric flask with V = 0.5 mL of extract, 0.25 mL of Folin-Ciocalteu was mixed and 0.75 mL of 20% Na 2 CO 3 (m/v). After 3 min of stirring, distilled water was added and made up to volume of 50 mL. Reaction mixture was left to stand at room temperature for 2h and absorbencies were measured at 765 nm by UV-Vis spectrophotometer. Based on the measured absorbance, the concentration (mg/mL) of TPC was calculated from the calibration curve of the standard solution of gallic acid. The results are expressed in g of gallic acid equivalents (GAE) per 100 g of fruit dried basis (gGAE/100g d.b ).

Determination of Total Flavonoids Content (TFL)
The TFL was determined by previously described colorimetric method [36]. In short, the reaction mixture was prepared by mixing 1 mL of an extract with 4 mL of distilled water and 0.3 mL of a 5% NaNO 2 solution (m/v). Then mixture was incubated at room temperature for five minutes, and then 0.3 mL of 10% AlCl 3 (m/v) was added. After six minutes, when the solution became very yellow, 2 mL of NaOH was added. Distilled water was added to the reaction mixture and made up the volume to 10 mL in a volumetric flask. The absorbance was measured at 510 nm. The TF were calculated according to the catechin standard calibration curve and expressed in mg of catechin equivalents (CAE) per 100 g of fruit dried basis (mgCAE/100g d.b ).

Determination of Radical Scavenging Capacity
The free radical scavenging capacity (RSC) of raspberry extracts was determined using a simple and fast spectrophotometric method described by Espin et al. [37]. Briefly, the prepared extracts were mixed with methanol (95%) and 90 µM 2,2-diphenyl-1-picryl-hydrazyl (DPPH) to give different final concentrations of extract. After 60 min at room temperature, the absorbance was measured at 517 nm. RSC was calculated according to Equation (7) and expressed as IC50 value, which represents the concentration of extract solution required for obtaining 50% of RSC.
where A blank is the absorbance of the blank and A sample is the absorbance of the sample. The obtained results were presented as a mass of dry sample material that is necessary for inhibition of 50% of DPPH (IC50 (mg d.b /mL)).

Determination of Monomeric Anthocyanin Content (AC)
The sample preparation for the content of total AC was conducted by previously described method [38]. Here an extraction solvent (ethanolic acid solution) [39] was poured over the samples (fresh or dried raspberries) and the mixture was thoroughly homogenized in a glass beaker. Afterwards, the beaker was covered with paraffin film and left to sit at 4 • C. After 24 h, the extraction mixture was kept at room temperature, filtered, and transferred to volumetric flask and made up to the volume of 100 mL with an extraction solvent. An aliquot of an extract was transferred into two volumetric flasks with added buffers at pH = 1.0 and pH = 4.5. After 15 min, the absorbencies were measured at 510 and 700 nm against distilled water as a blank. The content of AC was recalculated to cyanidin-3-glucoside by where AC = anthocyanin content (mg/100g); A 510 = sample absorbance at λ = 510nm; A 700 = sample absorbance at λ = 700nm; M w = molecular weight of cyanidin-3-glucoside (449.2), D f = dilution factor = original solution volume; = molar extraction coefficient of cyanidin-3-glucoside (26900); and m = sample weight (g). The content of AC was expressed in mg per 100 g of fruit dried basis (mg/100g d.b ).

Determination of Vitamin C Content
Separations and quantifications of vitamin C were performed by HPLC equipment (Thermo Scientific™ UltiMate 3000) on Nucleosil 100-5C 18 , 5 µm (250 × 4.6 mm I.D.) column (Phenomenex, Los Angeles, CA). Separation was performed with standard method BS EN 14130:2003 (Foodstuffs. Determination of vitamin C by HPLC). The content of AC was expressed in mg ascorbic acid (the sum of ascorbic acid and its oxidative form of dehydroascorbic acid) per 100 g of fruit dried basis (mg/100g d.b ).

Statistical Analysis
For the purposes of statistical tests, analysis of variance was performed (ANOVA) with Statistica13 (Stat Soft, Inc., Oklahoma, United States). In order to define homogenous groups of samples an additional Duncan test was performed with statistical significance at p < 0.05.

Volume Shrinkage
Comparison of convective and freeze-drying technique for fresh vs. frozen samples revealed that the least changes in volume had freeze-drying (V sh = 16.49 ± 2.75%). For convective drying, the least changes in volume had fresh raspberry samples dried at T = 60 • C and air velocity of 1.5 m·s −1 (V sh = 35.74 ± 6.78%). Pavkov et al. 2017 [17] reported results of air drying red Polana raspberry, dried at air temperature of 50, 60, 70, and 80 • C and constant air velocity of 1 m·s −1 . Judging by the volume shrinkage during convective drying, air temperature of 50 • C will lead to a total collapse and loss of the product shape. Interestingly, the least volume shrinkage (23.17%) was achieved with air temperature at 70 • C. Drying with air temperature at 60 • C provoked shrinkage of 28.74%, what is still lower than it was obtained in the current study. Samples dried with air temperature of 80 • C reached volume shrinkage of 43.13%. Results obtained from these two experiments revealed that higher air temperatures do not necessarily lead to higher changes in volume shrinkage. This may be explained by the fact that higher air temperature have tendency to lean towards mechanical stabilization of the raspberry surface, thus limiting the degree of shrinkage.
Air temperature and initial state of the raspberry, prior to convective drying significantly changed the volume of dried raspberry. On the other hand, the air velocity did not have any impact on the change in raspberry volume ( Table 1). The most considerable changes in volume occurred when drying frozen raspberries at T = 80 • C and air velocity of 0.5 m·s −1 (V sh = 79.07 ± 4.07%). As previously reported, drying temperature of 80 • C had similar trend on volume shrinkage [17]. Additional for convective drying, some reports indicated variations in volume shrinkages with regards to raspberry varieties. Sette et al. [29] dehydrated with convective drying previously frozen Autumn Bliss raspberry, at T = 60 • C and air velocity of 1-1.5 m·s −1 . Here they found higher volume shrinkage (V sh = 81 ± 3%) than what was reported in the current study.
Initial raspberry state effected the shrinkage, which was not surprising as creation of the ice crystals tends to destabilize cellular structures and this is particularly emphasized with drying air velocity of 0.5 m·s −1 . On the contrary, Duncan's test revealed that there are no significant differences in the volume shrinkage of the samples which were dried at the same temperature and with velocity of1.5 m·s −1 . The reason for this may be the faster drying rate in the first drying period, which can lead to a faster mechanical stabilization of the surface, hence the preservation of the volume. As expected, initial raspberry state had an effect on volume shrinkage, and the results after drying are presented in Figure 1. Figure 2 shows the shrinkage and the changes in fruit size after convective and freeze-drying of fresh raspberry at T = 60 • C and air velocity of 1.5 m·s −1 . Additional for convective drying, some reports indicated variations in volume shrinkages with regards to raspberry varieties. Sette et al. [29] dehydrated with convective drying previously frozen Autumn Bliss raspberry, at T = 60 °C and air velocity of 1-1.5 m·s −1 . Here they found higher volume shrinkage (Vsh = 81 ± 3%) than what was reported in the current study.
Initial raspberry state effected the shrinkage, which was not surprising as creation of the ice crystals tends to destabilize cellular structures and this is particularly emphasized with drying air velocity of 0.5 m·s −1 . On the contrary, Duncan's test revealed that there are no significant differences in the volume shrinkage of the samples which were dried at the same temperature and with velocity of1.5 m·s -1 . The reason for this may be the faster drying rate in the first drying period, which can lead to a faster mechanical stabilization of the surface, hence the preservation of the volume. As expected, initial raspberry state had an effect on volume shrinkage, and the results after drying are presented in Figure 1. Figure 2 shows the shrinkage and the changes in fruit size after convective and freeze-drying of fresh raspberry at T = 60 °C and air velocity of 1.5 m·s -1 .

Heywood Shape Factor Results
The referent Heywood shape factor before drying of fresh raspberry was k = 0.3323 (Figure 3), and all three experimental factors were significant for the changes in Heywood shape factor. As compared to k of a fresh raspberry, the factor after freeze-drying equaled to k = 0.27. In case of

Heywood Shape Factor Results
The referent Heywood shape factor before drying of fresh raspberry was k = 0.3323 (Figure 3), and all three experimental factors were significant for the changes in Heywood shape factor. As compared to k of a fresh raspberry, the factor after freeze-drying equaled to k = 0.27. In case of convective drying, the lowest deviation from k occurred for fresh raspberries at T = 60 • C and air velocity of 1.5 m·s −1 (k = 0.2694 ± 0.003). Results showed that under the same experimental conditions, the convective dried frozen raspberries had greater deviation of size as compared to the fresh raspberries. Hence, Heywood shape factor corresponded with the results for volume shrinkage. Figure 2. Red raspberry, variety Polana: (a) fresh (af = 20.74±2.45mm; bf = 20.04 ± 1.96 mm; cf = 18.88 ± 1.72 mm), (b) after convective drying of fresh raspberry at T = 60 °C and air velocity of 1.5 m·s −1 (acd = 16.97 ± 1.86 mm; bcd = 15.37 ± 1.65 mm; ccd = 15.34 ± 1.43 mm), and (c) after freeze-drying (afd = 20.62 ± 1.98 mm; bfd = 20.62 ± 2.66 mm; cfd = 18.58 ± 1.78 mm).

Heywood Shape Factor Results
The referent Heywood shape factor before drying of fresh raspberry was k = 0.3323 (Figure 3), and all three experimental factors were significant for the changes in Heywood shape factor. As compared to k of a fresh raspberry, the factor after freeze-drying equaled to k = 0.27. In case of convective drying, the lowest deviation from k occurred for fresh raspberries at T = 60 °C and air velocity of 1.5 m·s -1 (k = 0.2694 ± 0.003). Results showed that under the same experimental conditions, the convective dried frozen raspberries had greater deviation of size as compared to the fresh raspberries. Hence, Heywood shape factor corresponded with the results for volume shrinkage.

Color Change
CIE Lab color parameters L*, a*, b*, C*, h*, and ΔE* measured on fresh and dried raspberries at different drying conditions are shown in Table 2. Any considerable influences on color caused by the drying of raspberry was not detected, as total color change was roughly 10, except for freeze dried

Color Change
CIE Lab color parameters L*, a*, b*, C*, h*, and ∆E* measured on fresh and dried raspberries at different drying conditions are shown in Table 2. Any considerable influences on color caused by the drying of raspberry was not detected, as total color change was roughly 10, except for freeze dried samples, and essentially lightness (L) remained similar to those of a fresh samples. Hence, changes in color were driven by the parameters a* and b*. Generally, convective dried raspberry samples slightly shifted towards maroon color, which can originate from decomposition of carotenoid pigments. Moreover, high temperature induces nonenzymatic Maillard browning with formation of brownish pigmentations [40]. Alternatively, this may be the consequences of high concentrations of preserved anthocyanins in dried samples [3]. A slight increase of a* and b* will have positive repercussions, as it will lean towards more saturated color of products, which corresponds well with increased chroma values.
Air temperature, air velocity, and initial state of raspberry before drying had statistically significant effect on color change (p < 0.05) ( Table 1). The least color change (∆E = 5.18) was observed with convective drying at T = 60 • C and air velocity of 1.5 m·s −1 . This temperature remained optimal choice regarding ∆E, as it was not modified by different air velocities and initial state of material (fresh and frozen). The largest color change for this drying type was at T = 80 • C and air velocity of 0.5 m·s −1 for both frozen and fresh raspberry when ∆E was 11.17 and 10.12, respectively. Unexpectedly, freeze-dried raspberries had the largest color changes (∆E = 19.60) that were caused by increase in all of the three-color parameters, and especially for a* (∆a = 18.04). Bustos et al. [25] reported similar findings for freeze-dried berries with higher values for redness (∆a = 25.62) as compared to convective samples. Also, study by Sette et al. [3] reported an increase of a* and emphasized that besides pigmentation, differences of internal structures should be considered among convective and freeze dried raspberries. For instance, after freeze-drying, free water from raspberry is replaced by air, so shifts in red color and lightness can be a consequence of different diffusion of light that passes throughout a material. This effect is likely more pronounced for fruits with defined and vibrant hues, as for the raspberries [3,41]. During the conventional air-drying, increasing drying temperatures reduce the drying time, whereas shorter drying times may result in reduced risks of food quality deterioration [23]. Increasing hot air temperature for convective drying of Cassia alata from 40 • C to 60 • C reduced drying time from 180 min to 120 min [42]. Consequently, from data obtained, it can be assumed that as the temperature and the drying time increase, the color change of dried raspberries will increased too.

Ascorbic Acid Reduction
The average amount of L-ascorbic acid in fresh samples before drying was 118.27 mg/100g d.b. (18.92 mg/100g w.b. ) (Table 4), which was similar to quantities reported by Bobinaite et al. [12]. This content of L-ascorbic acid was significantly reduced during convective drying under all experimental conditions. This is expected as prolonged exposure to heightened temperatures and oxygen has tendency to reduce the content of this acidin fruits [21,[43][44][45][46][47]. Figure 4 shows temperature kinetics of raspberry samples during convective drying from a fresh state. Type K thermocouple probes were used to monitor and control product temperature during the process, by placing probes inside the drupelet. For all experiments, the temperature at the beginning of the process is approximately 35 • C, but after 10 minutes of the drying, the raspberry temperature can reach 50 • C. Due to the reduced moisture content, during the last quarter of drying all samples have the same temperature as drying air. This means that drying time at T = 80 • C is 6-8 h, and depending of the air velocity can last almost three times longer at T = 60 • C. However, the highest content of L-ascorbic acid was after the shortest convective drying with air velocity of 1.5 m·s −1 . This was regardless of the fact that the raspberry temperature reached T = 80 • C, and equaled to 27.46 ± 1.12 mg/100g d.b. and 22.54 ± 1.28 mg/100 d.b. , after drying of frozen and fresh raspberry, respectively (Table 4). Conversely, the degradation of 99% L-ascorbic acid was detected for longest drying with T = 60 • C and air velocity of 0.5 m·s −1 . Accordingly, this might mean that L-ascorbic acid degradation is more induced by longer exposure to higher oxygen levels during the convective drying than to the drying temperature itself.
This reasoning is in accordance with Verbeyst et al. [43] research with thermal and high-pressure effects on vitamin C degradation in strawberries and raspberries. Here it was shown that ascorbic acid degradation from strawberry and raspberry is slightly temperature dependent for temperature range of 80 to 90 • C, and that oxygen presence plays the key role. As expected, the highest levels of L-ascorbic acid preservation was achieved by freeze-drying (115.48 ± 2.29 mg/100g d.b .), since there was neither thermal nor oxygen degradation involved. 4). Conversely, the degradation of 99% L-ascorbic acid was detected for longest drying with T = 60 °C and air velocity of 0.5 m·s -1 . Accordingly, this might mean that L-ascorbic acid degradation is more induced by longer exposure to higher oxygen levels during the convective drying than to the drying temperature itself.
This reasoning is in accordance with Verbeyst et al. [43] research with thermal and high-pressure effects on vitamin C degradation in strawberries and raspberries. Here it was shown that ascorbic acid degradation from strawberry and raspberry is slightly temperature dependent for temperature range of 80 to 90 °C, and that oxygen presence plays the key role. As expected, the highest levels of L-ascorbic acid preservation was achieved by freeze-drying (115.48 ± 2.29 mg/100gd.b.), since there was neither thermal nor oxygen degradation involved. Figure 4. Raspberry temperature kinetics during convective drying from a previously fresh state.

Total Phenols Reduction
The average values for relevant nutritive profile of fresh raspberries are presented in Table 3. Average total amount of polyphenols in fresh raspberry was 1.63 g GAE/100gd.b. All three individual experimental factors had influence on the content of total phenols (Table 1).
When these samples were dried convectively the best preserved polyphenolic content was at T = 70 °C and air velocity of 1.5 m·s −1 (1.28 gGAE/100gd.b.). On the contrary, they were least preserved at 60 °C and air velocity of 0.5 m·s −1 (0.92 gGAE/100gd.b.). Freeze-drying preserved 1.10 g GAE/100gd.b. of total phenols, and, as expected, convective drying reduced polyphenolic content in the samples. The exceptions were the samples freshly dried at air velocity of 1.5 m·s −1 and drying temperatures of 70 ℃ and 80 ℃ in which higher total phenolic content was observed in comparison to freeze-dried samples. Similar results were recently reported where higher phenolic content was found in

Total Phenols Reduction
The average values for relevant nutritive profile of fresh raspberries are presented in Table 3. Average total amount of polyphenols in fresh raspberry was 1.63 g GAE/100g d.b . All three individual experimental factors had influence on the content of total phenols (Table 1).
When these samples were dried convectively the best preserved polyphenolic content was at T = 70 • C and air velocity of 1.5 m·s −1 (1.28 gGAE/100g d.b .). On the contrary, they were least preserved at 60 • C and air velocity of 0.5 m·s −1 (0.92 gGAE/100g d.b .). Freeze-drying preserved 1.10 g GAE/100g d.b . of total phenols, and, as expected, convective drying reduced polyphenolic content in the samples. The exceptions were the samples freshly dried at air velocity of 1.5 m·s −1 and drying temperatures of 70 • C; and 80 • C in which higher total phenolic content was observed in comparison to freeze-dried samples. Similar results were recently reported where higher phenolic content was found in convectively hot air-dried Cassia alata in comparison to freeze-dried samples [42]. Hossain et al. has suggested that freeze-drying may not have completely deactivated degradative enzymes due to the low-temperature process. Therefore, reactivation of this degradative enzymes could be further occurred in freeze-dried samples thus result in lower phenolic content [48].
Vasco et al. [49] made classification of 17 fruit types from Ecuador based on their content of total phenols and according to this classification there are three main groups: one with low levels of total phenols (<0.1 gGAE/100g w.b. ), one with medium level (0.2-0.5 gGAE/100g w.b. ), and the third with high levels (>1.0 gGAE/g100g w.b. ). This classification was accepted by others [50,51], and states that fresh Polana raspberry belongs to a high content group, as do freeze-dried and convectively dried samples from this study (under all experimental conditions).

Total Anthocyanin Reduction
Temperature had significant influence on the content of anthocyanin (Table 4), however air velocity had no effect on this group of compounds. Amount of anthocyanin in fresh raspberry was 511.7 mg/100g d.b. After convective drying, anthocyanin content was preserved from 40-56%. Anthocyanin content (287.0 mg/100 d.b. ) was best preserved with drying of fresh raspberries at T = 70 • C and air velocity of 0.5 m·s −1 . Their least retention occurred after convective drying of frozen samples at T = 70 • C and air velocity of 0.5 m·s −1 (205.3 mg/100 d.b. ). Thermal degradation of anthocyanins and complementary oxidization is the origin of the maroon color that was detected with the CIELab analysis. After freeze-drying, the content of anthocyanin in raspberry was 410.4 mg/100g d.b. which is expected due to minimized considerable influence of temperature and oxygen.

Radical Scavenging Capacity
As expected, all experimental factors influenced the radical scavenging capacity ( Table 4). As a smaller IC50 means higher radical scavenging capacity, the majority of convectively dried raspberries exhibited lower radical scavenging capacity in comparison to fresh or freeze-dried samples. The IC50 value of fresh raspberry was IC50 = 0.0534 mg d.b. /mL. Freeze-dried samples had highly preserved radical scavenging capacity that was equal to 0.0641 mg d.b. /mL, likely due to high preservation of all bioactive compounds. The lowest IC50 value (e.g. the highest radical scavenging capacity) had convective drying for frozen samples, of IC50 = 0.0845 mg d.b. /mL which was obtained at T = 80 • C and air velocity of 0.5 m·s −1 . The main reason for this may be the high preservation of total flavonoid content (0.97%) in dried samples with same convective drying regime. Raspberry belongs to a group of biomaterial with high radical scavenging capacity [12,50]. It is also believed that almost 20% of its total radical scavenging capacity is secured by the content of L-ascorbic acid [52]. As previously reported, heat and oxygen have influence on almost all bioactive compounds with some form of degradation, so it is not surprising that to find the loss of radical scavenging capacity due to convective drying.

Conclusions
Using physical properties, contents of various biologically active compounds and radical scavenging capacity proved to be useful in selecting alternatives for preservation of raspberries as in the case of convective and freeze-drying. For Polana variety, the most desirable results against freeze-drying as standard in terms of color, volume shrinkage, and Heywood shape factor change was achieved with convective drying of fresh raspberry at T = 60 • C with air velocity of 1.5 m·s −1 . Convective drying of raspberry had influenced all measured biologically active compounds. In comparison to fresh samples, in convectively dried raspberries 60-78% of total phenols was preserved as well as 75-97% of flavonoids and 40-56% of anthocyanins. Consequently, lower radical scavenging capacity was found in convectively dries samples as compared to fresh or freeze-dried. The largest shortcoming for convective drying was observed in difference between freeze-dried for preservation levels of L-ascorbic acid. Freeze-drying preserved more than 97% of L-ascorbic acid, while convective drying samples had degradation of over 80% of this compound. This might not be as relevant where L-ascorbic acid is added in processing of raspberries (e.g., confectionery products, biscuits, cookies, dairy product etc.). In conclusion, Polana raspberry dried convectively with air temperature of 60 • C and air velocity of 1.5 m·s −1 , may be considered as sufficient alternative to freeze-drying.