The effects of different drying methods on some physical and chemical properties of ‘Eksikara’ grape cultivar grown in Karaman region

Abstract

In this work, four different drying methods, namely natural drying in sun, natural drying in shade, natural drying in sun using a dipping solution and natural drying in shade using dipping solution were used to drying Ekşikara grapes growing in Karaman, Turkey. While a_w values of all grapes were decreased during drying, total soluble solids content increased. The values of pH and total acidity in grape samples significantly increased during the drying period due to the increase in dry matter content. The results showed that all the drying treatments significantly increased the total phenolic content, total antioxidant activity, trans-resveratrol, organic acid and sugar compounds. Trans-resveratrol amounts were slightly higher in the samples dried in the shade and in the dipping solution treatment and decreased as the drying time increases. An increase in organic acid and sugar contents was observed depending on the drying time. The change of colour was comparatively faster in dried samples with dipping solution. It was found that dipping solution treatment (7 days) reduced the drying time by half when compared to the samples without dipping solution treatment (15 days). This result is important for the grape producers of Karaman/Turkey, where the drying period under sun generally take 15 days.

Keywords

Ekşikara grape solar drying sun drying physical properties chemical properties

1 Introduction

Grape is a non-climacteric fruit that grows on perennial and deciduous woody vines of the genus Vitis [1, 2]. Grape is composed of water, proteins, lipids, carbohydrates, vitamins, minerals, and important biological compounds such as fiber, vitamin C [3]. Grape contains large amounts of phytochemicals including phenolics, flavonoids, anthocyanins and resveratrol, which offer health benefits [1]. Grape skins possess a compound named resveratrol, which is not present in other parts of the grape. Resveratrol is a phytoestrogen that takes preventive action against cardiovascular diseases [3]. After drying process, the physical properties as well as some chemical components such as antioxidant activity, phenol content, sugars and organic acids of dried products exhibit a significant importance for the final products of food and beverages, mainly for raisins [4].

However, grapes must be consumed within a few days after harvesting, due to the high water content and highly perishable tissue. Thus, drying can form an effective solution for the preservation [5, 6]. The preservation of grapes by drying is a major industry in many parts of the world where grapes are grown. Drying the grape, either by open sun drying, shade drying or mechanical drying, produces raisins [7, 8]. Sun drying has been used around the world for centuries. The use of solar dryers in developing countries may minimize losses of crops and enhance the quality of the dried product over classical drying process. However, sun drying is not possible in all countries since it requires adequate climatic conditions [6 , 10]. The sun drying process causes grape skin to gradually shrink, darken and become less elastic and more fragile; accordingly, berry pulp is also shrunken and darkened [11]. This method is used successfully in grape-producing countries since the most important cost in the drying process is energy [8].

Traditional drying methods are successfully employed in almost all grape producing countries, due to lower costs [8]. Grapes need to be treated with certain chemical solutions to enhance the drying rate before they are dried. The drying time required for natural and pretreated grapes are 20 and 8–10 days, respectively. Solar energy is used as either the sole source of the required heat or as a most important resource in solar drying [12]. There are two methods of using solar energy for crop drying: sun and solar drying [13]. Around the world, most raisins are dried, directly (sun) or indirectly (solar) in the sun. Sun drying comprehends the direct exposure of products to the sun. In solar drying, the solar energy is captured by some process to rise the temperature of the drying air and air flows through the product by natural or forced convection [14]. Natural drying (drying in the shade) is still most widely used due to its low cost [15 –17]. In this drying method, during the middle of the day, the products to be dried are shaded by the roof or by the racks above them [18]. Some times, side curtains are provided to protect from rain and dust [17]. The solar energy absorbed by the fruit in the morning and evening is subsequently utilised for water vaporisation during the shaded mid-day period, as well as after sunset [18].

Grape berry surface is covered by a thin-layer of wax, which is the main obstacle for moisture diffusion during drying. Generally, chemical pre-treatment is frequently used before drying to dissolve the cuticular wax layer and improve the permeability of the peel [14 , 20]. There are chemical pre-treatments (potassium carbonate+olive oil (potasa), potassium carbonate+ethyl oleate, odium hydroxide, sodium carbonate, soy lecithin, carbonic maceration) and physical pre-treatments (abrasion, steam, microwave, hover electric current, freezing, ohmic heating) that facilitate drying of grapes [21, 22]. Among those surface active agents, methyl oleate-K₂CO₃, ethyl oleate-K₂CO₃, NaOH-ethyl oleate and K₂CO₃-olive oil solution have been found to increase drying rates of grapes [22].

In the chemical pretreatments, often, an alkaline solution such as potassium carbonate is combined with other components such as olive oil [20, 23]. The main disadvantage of solar dryer is the availability of solar radiation for a limited period of time. The chemical additive residue in the raisins may cause food safety problems and how to deal with larger quantities of corrosive chemicals is a serious problem [1].

Ekşikara grape cultivar, a classic grape, is available in the Konya and Karaman regions of Turkey. This cultivar is considered one of the most important Vitis vinifera grape which are used to produce raisins with high quality and specific features. There have been many studies concerning the different kinds of grapes; however, there is a lack in data regarding fresh and dried Ekşikara grapes. In this context, studies on the chemical properties of fresh and dried Ekşikara grapes are relevant for the dried food producers, since this cultivar presents a great drying potential.

The aim of this study was to determine the effects of sun and shade drying (solar drier; especially procuded for grape drying) on the pretreated Ekşikara grapes with and without K₂CO₃-olive oil solution.

2 Material and methods

2.1 Material

Ekşikara grape samples (Vitis vinifera), approximately reached to 20 brix value (total soluble solids), were collected from a single vineyard located in the village of Damlapınar (37° 5” 6.26” N, 32° 50” 48.21” E, height 1272 m, Karaman/Turkey) on August 20st 2018. The grapes were harvested from different parts of the vineyard and vinestock. Approximately, 150 kg of grapes were randomly collected from different parts of vinestocks and transported to the drying area in insulated containers in polyethylene bags within two hours.

All reagents used were analytical or HPLC grade. Standard trans-resveratrol (Catalog No: R5010), sugars (glucose, fructose, sucrose) and organic acid standarts (L-tartaric, L-malic, citricacid) were provided by Sigma Aldrich Co. (Sigma-Aldrich Chemie, Steinheim), methanol, acetonitrile and glacial acetic acid were from Merck Milipore (Darsmtadt, Germany).

2.2 Drying equipment

Harvested grapes were dried up to 16% –18% moisture content in a custom made solar dryer. In the study, a custom-made drying system (100×100×28.5 cm) was used. In order to obtain raisins, two different materials were used. In order to obtain raisins, the fresh grapes with or with out dipped into the solution (5% K₂CO₃ and 0.5% high acid natural olive oil) were dried in four different ways. Following the preparation, samples were dried by using four different methods: natural drying in the sun (ND1), drying with dipping solution in the sun (DS1), natural drying in the shade (ND2), drying with dipping solution in the shade (DS2). The custom made solar dryer was covered with thickcardboard for shade drying. Dipping time with the solutions (Dipping Solution: DS) was about 20–25 sec and approximately 30–35 kg of grape samples were used for each drying method. Depending on the drying methods, the average temperature was 24°C and the relative humidity was 35% (26% to 43%), during the drying period of 7–21 days. These data were obtained from the Meteorology Unit which is established in Karaman. The samples were exposed to sunlight for about 14 hours during a day. Moisture content, total soluble solids content, pH, total acidity (as tartaric acid), water activity, hunter color values, total phenolic content, total antioxidant activity, trans-resveratrol value, sugars (glucose, fructose, sucrose) and organic acids (tartaric acid, malic acid, citric acid) contents were analyzed daily during drying. Drying treatments and analyses were carried out in duplicate and triplicate, respectively.

2.3 Determination of moisture content and water activity

The moisture content of grape samples was identified with an OHAUS MB45 (Ohaus Corparation, Nänikon, Switzerland) electronic moisture analyzer [24]. At the same time, oven (Nuve Corparation, Ankara, Turkey) drying method was also used for the confirmation of the results [25]. The moisture content was calculated by using the Eq: 1. $% Moisture content (g) = [(M_{i} - M_{s}) / M_{i}] \times 100$ (1)

where M_i and M_s are the masses of initial and dried samples, respectively. Water activity values were measured at room temperature using a water activity (labmaster a_w novasina, Lachen, Switzerland) device [26, 27]. Calibration was made with Salt-T relative humidity standards [27]. The a_w value is calculated by using the Eq: 2. $a_{w} = ERH / 100$ (2)

2.4 Colour measurements

Color measurement was conducted by using a Color Flex s/n CX2733 Hunter Lab date (Hunter Associates Laboratory, Reston, Virginia, USA) [28]. The instrument was standardized prior to each measurement with white and black ceramic plates. The lightness value (L^*) indicates the darkness/lightness of the sample, a^* indicates the greenness/redness and b^* indicates the extent of blueness/yellowness of the samples. Colour measurements were realized according to the CIELAB (Commission Internationale de l'Eclairage) colour space system [1].

2.5. Determination of total soluble solids content value

The amount of total soluble solids content in fresh and raisin grape samples was determined by using a table type refractometer (Kyoto Electronics MFG. CO. LTD. RA-600 Kyoto, Japan) [29]. The refractometer was calibrated with distilled water prior to measurement. °Brix value was measured during the drying time of the grape samples. Total water-soluble dry matter (TWSDM) content of raisins and fresh grape samples was calculated using the Eq: 3 [30, 31]: $% TWSDM = (B \times V) / M$ (3)

where B: the degree of °Brix determined in diluted sample,

V: volume to which the sample was diluted (mL),

M: are sample weight (g).

2.6 Determination of total acidity

Following the washing of fresh grape samples and after the raisin samples rehydrated with distilled water, and homogenized in a high-speed blender (LB20EG, Waring Commercial, Connecticut, USA), then by passing through filter paper and performed for the analyses of pH and total acidity. The potentiometric determination of pH value [32] was performed by using a digital pH-meter with glass electrode (PL-700PV, Taipei, Taiwan). The pH meter was calibrated with buffer solutions (pH 4, pH 7) before measurement. Total acidity was determined after the addition of 20 mL of distilled water on 10 mL of grape juice and titrating with 0.1 N NaOH to pH 8.1. The results were given in grams per liter of tartaric acid denominated according to the Eq. 4 [11, 33]. $% Total acidity (tartaric acid, g) = [V \times 0, 007505 / M] \times 100$ (4)

where V: the amount of NaOH consumed,

M: sample weight (g),

0,007505: are milliequivalent weight of tartaric acid.

2.7. Extraction of total phenolic content, total antioxidant activity and trans-resveratrol analyses

The extraction method proposed by Jeandet [34] was used with some modifications. A 6 g of fresh grape and raisin sample was homogenized in a blender by adding 94 mL of 90% methanol solution. The resulting mixture was transferred to 300 mL flask and allowed to stand in the ultrasonic bath for 15 min after they were sealed. The resulting mixture was shaken with a rotary shaker (Rotator, Dragon Laboratory Instruments, Hangzhou Shi, Zhejiang Sheng, China) at 250 rpm for 2 h and filtered through filter paper. A 50 mL of methanol was added to the remaining pulp and left to the ultrasonic bath during 5 minutes. The solution was shaked at 250 rpm in a circular shaker for 1 hour and filtered with filter paper. All the filtrates were combined and completed to a volume of 150 mL with methanol solvent. After the extraction process, the extracts were centrifuged (Eppendorf, Centrifuge 5804 R, Hamburg, Germany) at 10.000 rpm at 10°C for 10 min. Phase separation was done in the centrifuge tubes and 5 mL of the supernatant sample was taken with a pasteur pipette. Then, it was filtered through 0.45μm PTFE syringe-type membrane and filled into 5 mL vials covered with aluminum foil. The samples were stored at –20°C until analysis. The extracts obtained after centrifugation were also used in total phenolic content and total antioxidant activity analyses. All extractions were conducted in triplicate.

2.8 Determination of total phenolic content

Total phenolic content of grape extracts was determined by employing the Folin-Ciocalteu assay described by Singleton and Rossi [35]. A 100μl of extract was transferred into a test tube. Later, 500μL of Folin-Ciocalteu reagent and 1.5 mL of saturated sodium carbonate were added and then completed to volume of 10 mL with distilled deionized water. After standing the mixture in a dark place at room temperature for 2 h, absorbance was measured at 760 nm against solvent using a UV-Vis spectrophotometer (Shımadzu, UV-1800, Duisburg, Germany) [36]. The standard calibration (5 mg/L–2000 mg/L) curve was plotted using gallic acid. The content of grape phenolics in extracts was expressed in terms of gallic acid equivalent (mg of GAE/kg extract of dry matter).

2.9 Determination of total antioxidant activity

The ability of the plant extract to scavenge DPPH (2,2-Diphenyl-1-picrylhydrazyl) free radicals was assessed by the standard method, adopted with some modifications [37]. A 3.9 mL DPPH solution (0.1 mM) prepared in methanol was added to the 100μL grape extract. The tubes containing reaction mixture were incubated at ambient temperature for 30 min in a dark place. The DPPH absorption values at 515 nm were measured relative to methanol. Total Antioxidant Activity (TAC) was calculated as percent inhibition using equation [36, 38]; $% DPPH = [(Methanol Absorbance - Sample absorbance) / Methanol Absorbance$ × 100

2.10 Quantification of trans-resveratrol

The retention time of trans-resveratrol was determined by experiments with a series of standard solutions under the following chromatographic conditions. The trans-resveratrol concentration corresponding to each grape extract was determined in milligrams per liter (ppm) using the calibration graph. The trans-resveratrol was weighed 0.0025 g and dissolved in 100 mL of methanol. The calibration solutions were diluted from this stock solution and prepared at seven points in the range of 0.2 mg/L to 5 mg/L. The prepared calibration solutions were used after waiting for 15 min in an ultrasonic water bath [39]. The reliability of the method was confirmed by recovery experiments. Recoveries for trans-resveratrol varied between 95% –96%. The detection limit for each trans-resveratrol, based on a signal-to-noise ratio (S/N) of 3, were 0.032 mg/kg. Results are given on dry basis.

HPLC conditions of trans-resveratrol analaysis were as follows: Liquid chromatograpy pump (Shimadzu, Model LC-20 AT-VP, Kyoto, Japan) with photodiode array detector (Shimadzu, Model SPD-M20A-UVVIS, Kyoto, Japan), a degasser (Shimadzu, Model DGU 14A, Kyoto, Japan) and a Bio Rad Aminex HPX-87 ion exclusion column (300 mm×7.8 mm) were used. The mobile-phase flow was 0.6 mL/min. The eluents were (A) acetonitrile (65%) and (B) ultra pure water (35%). The column oven temperature was 30°C and thermostatically controlled. Injection volume was 20μL. The gradient flow was conducted by using elution with solvent A from 0 to 18 min then from B 100% to a 100% in 1 min and from A % 100 to B 100% in 6 min. The eluent was monitored at 310 nm.

2.11 Extraction for sugars and organic acids

Organic acid and sugar extraction of fresh grapes and raisins were conducted by the methods of Soyer et al. [40] and Sturm et al. [41]. After addition of 1/1 (w/w) ultrapure water to 20 g fresh grape and raisin samples, they were homogenized by shredding with a laboratory blender (Waring-USA) for 3 min. A 10 g of homogenized mixture was added with 50 mL of ultrapure water and then centrifuged (Eppendorf, Centrifuge 5804 R, Hamburg, Germany) for 15 min at 10.000 rpm. A 5 mL of supernatant was passed through a 0.45μm PTFE (Sartorius, SM16555Q, Germany) syringe-type filter and transferred to 5 mL vials. The extracts were stored at –20°C until the analysis of organic acids (malic acid, tartaric acid, citric acid) and sugars (glucose, fructose, saccharose). The extractions were carried out in triplicate.

2.12 Chromatographic determination of sugars

Sugar contents of the samples were determined by using the glucose, fructose and sucrose calibration curves. For this purpose, calibration solutions were prepared for glucose, fructose and sucrose standards at 5 different concentrations. The reliability of the method was confirmed by recovery experiments. Recoveries for glucose, fructose and sucrose varied between 96.3% –97.1%, 97.2% –98.5% and 95.8% –97.3%, respectively. The detection limits for each sugar based on a signal - to-noise ratio (S/N) of 3, were 0.2 g/L for glucose, 0.15 g/L for sucrose and 0.3 g/L for fructose [42]. Results are presented on dry basis.

HPLC conditions of sugars were as follows: a liquid chromatograpy pump (Shimadzu, Model LC-20 AT-VP, Kyoto, Japan) with refractive index detector (Shimadzu, Model RID-10A, Kyoto, Japan), a degasser (Shimadzu, Model DGU 14A, Kyoto, Japan) and a Bio Rad Aminex HPX-87 ion exclusion column (300 mm×7.8 mm) were used fot the sugar analysis. The mobile phases (isocratic) were (A) acetonitrile (80%) and (B) ultra pure water (20%). The column oven temperature was 80°C and thermostatically controlled. Injection volume was 20μL.

2.13 Chromatographic determination of organic acids

Five different amounts of calibration solutions of the standards of malic, tartaric and citric acid were prepared for the determination of the organic acid amounts in the samples. The reliability of the method was confirmed by recovery experiments. Recoveries for malic acid, tartaric acid and citric acid varied between 94.8% –96.9%, 97.6% –98.5% and 97.4% –98.6%, respectively. The detection limits for each organic acid based on a signal - to-noise ratio (S/N) of 3, were 0.005 g/L for tartaric acid, 0.035 g/L for malic acid and 0.040 g/L for citric acid [42]. Results are presented on dry basis.

A liquid chromatograpy pump (Shimadzu, Model LC-20 AT-VP, Kyoto, Japan) with photodiode array detector (Shimadzu, Model 20AD PDA, Kyoto, Japan), a degasser (Shimadzu, Model DGU 14A, Kyoto, Japan) and a Bio Rad Aminex HPX-87 ion exclusion column (300 mm×7.8 mm) were used for the organic acid analysis. The mobile phase (isocratic) was 0.01 N H₂SO₄. The column oven temperature was 25°C and thermostatically controlled. Injection volume was 20μL and the eluent was monitored at 214 nm.

2.14 Drying conditions

Drying was terminated when moisture content reached approximately 18% in all samples [43, 44]. The desired moisture levels were obtained on the 7th day, 11th day, 15th day and the 21st day in DS1, DS2, ND1 and ND2 samples respectively. Drying conditions were carried out as shown in Fig. 1 Statistical evaluations were performed considering the results of the 7th day of all samples.

Fig. 1

Drying conditions of Ekşikara grapes.

Ekşikara grape samples were harvested at 22-23 °Brix and the maturity index has been identified as 39%. Maturity index is one of the most important parameters in determining the optimum harvest time [45]. Maturity index in grape samples was calculated using the equation 5 [46]. Cangi et al. [47] determined the maturity index value in black grape varieties (Pinot Noir and Syrah) as 31.90 and 31.63, respectively. Cooke and Berg [48] reported that the optimum maturity index for 20–24°Brix value in black grape varieties to be in the range of 23.5–34.3. ${Maturity Index =}^{\circ} Bx / TA$ (5)

where °Bx and TA are the masses of water soluble dry matter and titratable amount of acid, respectively.

2.15 Statistical analysis

Among the drying methods, the earliest drying took place on the seventh day and in the DS1 samples. In order to make comparisons between drying methods, statistical analyses were performed on the results up to the seventh day for all samples.

Data were analyzed by the analysis of variance (ANOVA) and factorial experimental design was used. Tukey’s multiple comparison test was used as a post ANOVA technique to determine significant differences among the means. Minitab (Minitab, LLC, State College, Minitab 16.1.1, Coventry, United Kingdom) software (ver. 18.0) was used for statistical analyses [49].

3 Results and discussion

3.1 Changes in the moisture content, water activity and total soluble solids content during drying

Statistical analysis showed that changes in the moisture, water activity and total soluble solids content values were found to be significant (p < 0.01) depending on the drying period and drying method. As shown in Table 1, the moisture content of fresh grape samples was approximately 69 g/100 g. As expected, the moisture content decreased during drying. It is determined that the application of dipping solution was effective on moisture content and the most effective method in drying was the application of DS1.

Table 1
Moisture, water activity, °Brix, pH and total acidity values of the Ekşikara grape samples during drying with different drying methods

Drying methods Drying time (Day) Moisture (g/100 g)^^y Water Activity^^y °Brix (g/100 g)^^y pH^^y Total acidity (%)^*^y

Natural drying 0 69.32±0.04^a 0.983±0.004^ab 22.53±0.25^p 3.29±0.04^l 0.59±0.01^s

inthe sun (ND1) 1 68.72±0.09^ab 0.977±0.002^abc 23.84±0.25^op 3.32±0.08^l 0.72±0.00^qr

2 66.60±0.66^bc 0.974±0.003^a - d 24.17±0.38^nop 3.59±0.03^ij 0.77±0.01^l - q

3 62.89±0.61^de 0.967±0.006^a - g 25.75±1.15^mno 3.71±0.02^hij 0.81±0.02^j - o

4 58.96±0.28^g 0.965±0.001^a - h 28.10±0.01^klm 3.74±0.09^hi 0.84±0.02^h - l

5 56.35±0.94^hi 0.947±0.002^d - j 28.70±0.46^kl 3.94±0.05^fg 0.88±0.01^f - j

6 55.77±1.04^hij 0.943±0.005^f - j 31.81±0.95^g - j 4.05±0.08^ef 0.92±0.02^efg

7 53.99±0.64^ijk 0.933±0.003^ijk 35.90±0.50^f 4.59±0.07^c 0.95±0.03^ef

Drying with dipping 0 69.35±0.06^a 0.987±0.004^a 22.70±0.20^p 3.25±0.06^l 0.58±0.01^s

solution in the sun (DS1) 1 63.69±0.57^de 0.983±0.002^ab 26.07±0.06^mno 3.34±0.02^l 0.76±0.01^n - r

2 60.08±1.87^fg 0.973±0.006^a - e 27.86±0.06^lm 3.73±0.03^hij 0.83±0.02^i - n

3 55.72±1.26^hij 0.967±0.014^a - g 31.51±0.80^hij 3.93±0.01^fg 0.87±0.01^g - k

4 45.07±1.78^mn 0.933±0.007^ijk 34.76±1.25^f 4.01±0.02^fg 0.91±0.02^e - h

5 37.66±1.06^o 0.904±0.027^klm 49.93±0.29^c 4.49±0.02^c 1.11±0.05^d

6 24.16±1.09 0.876±0.029^mn 62.55±2.43^b 4.97±0.12^a 1.52±0.07^b

7 16.33±0.51^q 0.695±0.004^o 73.57±1.40^a 5.12±0.04^a 1.79±0.03^a

Natural drying 0 69.27±0.07^a 0.982±0.004^ab 22.68±0.08^p 3.31±0.03^l 0.59±0.01^s

in the shade (ND2) 1 64.31±0.27^cde 0.987±0.003^a 24.93±0.12^nop 3.41±0.02^kl 0.69±0.01^r

2 61.95±0.67^ef 0.983±0.006^ab 26.51±0.10^lmn 3.57±0.03^jk 0.73±0.00^pqr

3 59.81±0.68^fg 0.971±0.002^a - f 28.77±0.32^kl 3.85±0.05^gh 0.76±0.00^m - q

4 57.84±0.19^gh 0.960±0.003^a - i 29.75±1.14^ijk 3.91±0.04^fg 0.80±0.01^k - p

5 55.95±0.46^hij 0.952±0.005^c - i 32.02±1.16^ghi 3.96±0.01^fg 0.83±0.02^i - n

6 53.51±0.45^jk 0.944±0.006^e - j 33.88±0.11^fgh 4.03±0.02^ef 0.85±0.00^g - k

7 51.87±0.95^kl 0.938±0.009^g - j 35.97±0.61^f 4.23±0.06^d 0.86±0.03^g - k

Drying with dipping 0 69.37±0.03^a 0.986±0.004^a 23.02±0.08^p 3.27±0.06^l 0.58±0.01^s

solution in the shade (DS2) 1 66.31±0.12^bc 0.987±0.004^abc 23.91±0.02^op 3.31±0.02^l 0.75±0.01^o - r

2 65.35±0.12^cd 0.977±0.005^abc 26.41±0.20^lmn 3.63±0.01^ij 0.81±0.02^j - o

3 60.02±0.21^fg 0.956±0.005^b - i 29.36±1.25^jk 3.95±0.02^fg 0.84±0.01^i - m

4 50.45±0.13^l 0.937±0.003^hij 30.10±0.44^ijk 4.17±0.05^de 0.90±0.01^e - i

5 47.08±0.69^m 0.920±0.002^jkl 34.16±0.27^fg 4.48±0.09^c 0.97±0.01^e

6 43.69±0.69ⁿ 0.891±0.021^lmn 39.74±0.15^e 4.79±0.04^b 1.07±0.02^d

7 39.53±1.33^o 0.864±0.005ⁿ 44.82±0.78^d 4.98±0.02^a 1.28±0.02^c

Drying methods	Drying time (Day)	Moisture (g/100 g)^*^y	Water Activity^*^y	°Brix (g/100 g)^*^y	pH^*^y	Total acidity (%)^*^y
Natural drying	0	69.32±0.04^a	0.983±0.004^ab	22.53±0.25^p	3.29±0.04^l	0.59±0.01^s
inthe sun (ND1)	1	68.72±0.09^ab	0.977±0.002^abc	23.84±0.25^op	3.32±0.08^l	0.72±0.00^qr
	2	66.60±0.66^bc	0.974±0.003^a - d	24.17±0.38^nop	3.59±0.03^ij	0.77±0.01^l - q
	3	62.89±0.61^de	0.967±0.006^a - g	25.75±1.15^mno	3.71±0.02^hij	0.81±0.02^j - o
	4	58.96±0.28^g	0.965±0.001^a - h	28.10±0.01^klm	3.74±0.09^hi	0.84±0.02^h - l
	5	56.35±0.94^hi	0.947±0.002^d - j	28.70±0.46^kl	3.94±0.05^fg	0.88±0.01^f - j
	6	55.77±1.04^hij	0.943±0.005^f - j	31.81±0.95^g - j	4.05±0.08^ef	0.92±0.02^efg
	7	53.99±0.64^ijk	0.933±0.003^ijk	35.90±0.50^f	4.59±0.07^c	0.95±0.03^ef
Drying with dipping	0	69.35±0.06^a	0.987±0.004^a	22.70±0.20^p	3.25±0.06^l	0.58±0.01^s
solution in the sun (DS1)	1	63.69±0.57^de	0.983±0.002^ab	26.07±0.06^mno	3.34±0.02^l	0.76±0.01^n - r
	2	60.08±1.87^fg	0.973±0.006^a - e	27.86±0.06^lm	3.73±0.03^hij	0.83±0.02^i - n
	3	55.72±1.26^hij	0.967±0.014^a - g	31.51±0.80^hij	3.93±0.01^fg	0.87±0.01^g - k
	4	45.07±1.78^mn	0.933±0.007^ijk	34.76±1.25^f	4.01±0.02^fg	0.91±0.02^e - h
	5	37.66±1.06^o	0.904±0.027^klm	49.93±0.29^c	4.49±0.02^c	1.11±0.05^d
	6	24.16±1.09	0.876±0.029^mn	62.55±2.43^b	4.97±0.12^a	1.52±0.07^b
	7	16.33±0.51^q	0.695±0.004^o	73.57±1.40^a	5.12±0.04^a	1.79±0.03^a
Natural drying	0	69.27±0.07^a	0.982±0.004^ab	22.68±0.08^p	3.31±0.03^l	0.59±0.01^s
in the shade (ND2)	1	64.31±0.27^cde	0.987±0.003^a	24.93±0.12^nop	3.41±0.02^kl	0.69±0.01^r
	2	61.95±0.67^ef	0.983±0.006^ab	26.51±0.10^lmn	3.57±0.03^jk	0.73±0.00^pqr
	3	59.81±0.68^fg	0.971±0.002^a - f	28.77±0.32^kl	3.85±0.05^gh	0.76±0.00^m - q
	4	57.84±0.19^gh	0.960±0.003^a - i	29.75±1.14^ijk	3.91±0.04^fg	0.80±0.01^k - p
	5	55.95±0.46^hij	0.952±0.005^c - i	32.02±1.16^ghi	3.96±0.01^fg	0.83±0.02^i - n
	6	53.51±0.45^jk	0.944±0.006^e - j	33.88±0.11^fgh	4.03±0.02^ef	0.85±0.00^g - k
	7	51.87±0.95^kl	0.938±0.009^g - j	35.97±0.61^f	4.23±0.06^d	0.86±0.03^g - k
Drying with dipping	0	69.37±0.03^a	0.986±0.004^a	23.02±0.08^p	3.27±0.06^l	0.58±0.01^s
solution in the shade (DS2)	1	66.31±0.12^bc	0.987±0.004^abc	23.91±0.02^op	3.31±0.02^l	0.75±0.01^o - r
	2	65.35±0.12^cd	0.977±0.005^abc	26.41±0.20^lmn	3.63±0.01^ij	0.81±0.02^j - o
	3	60.02±0.21^fg	0.956±0.005^b - i	29.36±1.25^jk	3.95±0.02^fg	0.84±0.01^i - m
	4	50.45±0.13^l	0.937±0.003^hij	30.10±0.44^ijk	4.17±0.05^de	0.90±0.01^e - i
	5	47.08±0.69^m	0.920±0.002^jkl	34.16±0.27^fg	4.48±0.09^c	0.97±0.01^e
	6	43.69±0.69ⁿ	0.891±0.021^lmn	39.74±0.15^e	4.79±0.04^b	1.07±0.02^d
	7	39.53±1.33^o	0.864±0.005ⁿ	44.82±0.78^d	4.98±0.02^a	1.28±0.02^c

^y: Statistical analysis was performed on the results up to the seventh day for all samples ^*: The means marked with different letters in the same column are statistically (p < 0.01) different from each other.

It was determined that the moisture content dropped below 20 g/100 g (16.33±0.51 g/100 g) on the 7th day of of drying with DS1. As shown in Fig. 2, 18 g/100 g and lower humidity values were determined on the 11th day with DS2 drying (16.81±0.56 g/100 g), on the 15th day with ND1 drying (17.13±0.72 g/100 g) and on the 21st day with ND2 drying (17.16±0.98 g/100 g). The samples dried with DS treatment decreased the drying time by half-compared to the samples dried without DS treatment. Drying time was found to be 4–6 days longer than ND1 dried samples in ND2 dried samples. It was determined that drying type has a significant effect on the decrease in moisture content. The drying process was completed faster due to the breaking of the waxy layer in the DS dried samples. It has been determined that the most effective and short-term drying method was the DS1 drying method. Karakuş dried Öküzgözü, Boğazkere, Mevji and Abderi grape varieties using different drying methods and reported that the ratio of water removed during drying was 75%, and the drying time in the shade was two times higher than in the sun. Seçkin [51] conducted an experiment, where 10 different grape varieties with two different drying systems (solar collector drying and tray drying) and applications (with and without dipping) and determined that the dipping solution of raisins had significant effects on moisture content. The authors dried Dimrit grapes in a laboratory type drying oven with the dipping solution for 13.5 h, and Tekirdağ seedless grapes in the solar collector without the dipping solution in 336 h.

Fig. 2

Interaction graphics of moisture, water activity, °Brix and total acidity values of Ekşikara grapes during drying with different drying methods.

Water activity values of fresh Ekşikara grape samples were determined to be between 0.982±0.004 and 0.987±0,004, and °Brix values between 22.53±0.25 g/100 g and 23.02±0.08 g/100 g As expected, water activity values decreased during drying, while °Brix values increased. The a_w and °Brix values of grape samples, which were dried with different drying methods, were determined to be between 0.657±0.005 (ND1) to 0.695±0.004 (DS1) and 71.76±0.35 g/100 g (DS2) to 73.57±1.40 g/100 g (DS1), as minimum and maximum values. It was determined that the DS was effective in the change of a_w and °Brix as in moisture content, and drying with DS application was found to be the most effective drying method. Sen [52] reported that the a_w values of raisins produced in Turkey to be in the range of 0.505–0.694. Yalçınkaya [53] stated that Besni grapes, which were dipping in the dipping solution and dried in the sun, increased from 20.16±0.76 to 55.00±2.75 and 73.70±2.91 of °Brix values due to the increase in solution concentration and drying time.

Akdeniz [54] stated that the sugar content of the grapes at the beginning of drying of seedless Sultani grapes, which were at the harvest maturity level (22-23°Brix), was around 20 g/100 g and increased to 85 g/100 g at the end of drying.

The majority of the Ekşikara grapes grown in the Karaman region are consumed as dried [44]. Drying operations in the region are carried out in the ground- exhibition and in the ND1 natural conditions. Drying processes are generally not applied under DS and ND2 conditions. In our study, Ekşikara grape variety was dried in with the appliation of DS2 in a short time. Grapes were dried undamaged adverse environmental conditions such as dust, soil, insects etc. It was determined that Ekşikara grapes can be dried in a significantly shorter time and in a safely drying under shade conditions with the application of DS.

3.2 Changes in the acidity (pH and total acidity) during drying

As shown in Table 1, the change in pH and total acidity during drying was found to be statistically significant (p < 0.01) depending on the drying method. The pH value of fresh grape samples was determined to be between 3.25±0.06 and 3.31±0.03 before drying. The pH values of the grape samples dried with different methods were determined to be between 5.11±0.03 (ND1) and 5.32±0.07 (ND2) as minimum and maximum values, respectively. De Lerma et al. [55] found that the pH values of Pedro Ximénez grapes increased after drying in the sun for 9 days. Constantinou et al. [11] stated that sun drying had no effect on pH values and an increase in the percentage of the acid salification may be considered responsible for non-concomitant change of pH. These researchers reported that the pH values of grapes before and after drying were 3.4±0.1 and 3.4±0.1, 3.6±0.2 and 3.6±0.1 in ‘Mavro’ and ‘Xynisteri’ grapes, respectively. Seçkin [51] stated that grape dipping solution was a basic solution and increased the pH value before drying.

While total acidity values of fresh grape samples were determined to be in the range of 0.58±0.01 to 0.59±0.01% before drying, an increase in total acidity values in the samples dried with different drying methods was observed. As seen in Fig. 2, after drying, total acidity values were determined to be in the range of 1.74±0.05% (ND1) and 1.79±0.04% (ND2) as minimum and maximum values, respectively. As seen in Fig. 2, the total acidity values of grape samples were found close to each other during the four-day drying period. After the fourth day of drying, a marked increase in total acidity values of the samples dried with the application of DS was determined. It is most likely that the main reason for the increase in total acidity (TA) was due to the loss of water and the increase in dry matter. Peinado et al. [56] explained that total acidity in grape samples increased during the drying process and this increase was caused by drying. De Lerma et al. [55] and Constantinou et al. [11] are also stated that drying in the sun caused an increase in the amount of titratable acid due to dehydration.

3.3 Changes in the color values during drying

As shown in Table 2, the change in L^*, a^* and b^* color values of grapes during drying was found to be statistically significant depending on the drying method (p < 0.01). It was determined that the matte black color of the grapes turned into a darker and lower L^* values after the application of DS, which is likely caused a darker colour and lowered lightness in the samples. This color change is due to the fact that the dark black color becomes more apparent due to the removal of the waxy layer in the skin of the grapes with the application of DS. As shown in Fig. 3, a decrease in L ^* occurred in the first four days until the waxy layer completely dissolved. L ^* value of the samples remained constant until the end of drying with the application of DS after the fourth day. In samples that were dried without dipping to the DS, no significant change was observed in the L ^* values from beginning to the end of drying. It is obvious from the Fig. 3 that this color change in samples dried with the application of DS is faster and the decrease in L ^* value is more evident.

Table 2
L^, a^ and b^* color values of the Ekşikara grapes during drying with different drying methods

Drying methods Drying time (Day) L^y a^y b^*y

Natural drying 0 18.69±0.76^f –22.00±1.00^fgh –2.77±0.42^ab

in the sun (ND1) 1 19.48±0.32^c - f –43.33±1.53 –2.44±0.31^ab

2 20.66±0.76^bcd –32.00±1.00^ghi –3.13±0.01^ab

3 19.34±0.47^c - f –25.67±1.53f^gh –3.59±0.03^ab

4 21.99±0.12^ab –18.67±2.52^fg –3.47±0.11^ab

5 20.46±0.74^b - e –19.33±1.53^fg –3.46±0.04^ab

6 23.55±0.75^a –16.00±1.00^f –3.85±0.13^ab

7 20.99±0.41^bc –16.00±2.00^f –3.15±0.35^ab

Drying with dipping 0 18.87±0.67^ef –24.67±1.53f^gh –2.77±0.25^ab

solution in the sun (DS1) 1 13.80±0.19^ijk –33.67±3.06^hi –81.67±5.03^g

2 13.60±0.81^jk 86.33±1.53^a –78.67±2.08^g

3 12.66±0.03^k 47.00±3.00^bc –62.33±2.52^ef

4 14.63±0.25^g - j 44.00±3.61^bc –57.67±1.53^ef

5 14.05±0.71^h - k 41.67±5.13^c –42.67±4.16^cd

6 15.18±0.59^g - j 27.00±7.21^de –37.00±4.58^c

7 15.69±0.13^g 22.00±2.65^e –17.33±7.77^b

Natural drying 0 18.66±0.75^f –22.67±2.31f^gh –2.60±0.53^ab

in the shade (ND2) 1 19.11±0.12^def –35.33±3.22^hi –3.07±0.44^ab

2 20.97±0.91^bc –22.33±3.06^fgh –2.89±0.05^ab

3 18.18±0.37^f –27.00±3.61^fgh –2.50±0.43^ab

4 20.87±0.05^bc –61.67±3.06^j –2.27±0.04^a

5 21.53±1.08^b –32.00±3.61^ghi –2.94±0.26^ab

6 21.73±0.15^b –24.67±1.16f^gh –2.51±0.31^ab

7 20.42±0.15^b - e –33.00±1.00^hi –2.34±0.25^a

Drying with dipping 0 18.89±0.54^ef –23.67±2.52f^gh –2.83±0.32^ab

solution in the shade (DS2) 1 15.51±0.43^gh –74.00±10.82^j –.67±3.22^fg

2 14.02±0.69^h - k 82.67±8.62^a –50.67±18.58^cde

3 13.95±0.34^h - k 55.33±10.02^b –57.33±5.69^def

4 15.53±0.33^gh 44.67±3.51^bc –50.67±7.37^cde

5 15.13±0.28^g - j 39.00±3.00^cd –48.00±3.61^cde

6 15.33±0.09^ghi 35.67±2.08^cd –58.00±3.60^ef

7 15.52±0.28^gh 22.00±4.34^e –39.00±8.89^c

Drying methods	Drying time (Day)	L^*y	a^*y	b^*y
Natural drying	0	18.69±0.76^f	–22.00±1.00^fgh	–2.77±0.42^ab
in the sun (ND1)	1	19.48±0.32^c - f	–43.33±1.53	–2.44±0.31^ab
	2	20.66±0.76^bcd	–32.00±1.00^ghi	–3.13±0.01^ab
	3	19.34±0.47^c - f	–25.67±1.53f^gh	–3.59±0.03^ab
	4	21.99±0.12^ab	–18.67±2.52^fg	–3.47±0.11^ab
	5	20.46±0.74^b - e	–19.33±1.53^fg	–3.46±0.04^ab
	6	23.55±0.75^a	–16.00±1.00^f	–3.85±0.13^ab
	7	20.99±0.41^bc	–16.00±2.00^f	–3.15±0.35^ab
Drying with dipping	0	18.87±0.67^ef	–24.67±1.53f^gh	–2.77±0.25^ab
solution in the sun (DS1)	1	13.80±0.19^ijk	–33.67±3.06^hi	–81.67±5.03^g
	2	13.60±0.81^jk	86.33±1.53^a	–78.67±2.08^g
	3	12.66±0.03^k	47.00±3.00^bc	–62.33±2.52^ef
	4	14.63±0.25^g - j	44.00±3.61^bc	–57.67±1.53^ef
	5	14.05±0.71^h - k	41.67±5.13^c	–42.67±4.16^cd
	6	15.18±0.59^g - j	27.00±7.21^de	–37.00±4.58^c
	7	15.69±0.13^g	22.00±2.65^e	–17.33±7.77^b
Natural drying	0	18.66±0.75^f	–22.67±2.31f^gh	–2.60±0.53^ab
in the shade (ND2)	1	19.11±0.12^def	–35.33±3.22^hi	–3.07±0.44^ab
	2	20.97±0.91^bc	–22.33±3.06^fgh	–2.89±0.05^ab
	3	18.18±0.37^f	–27.00±3.61^fgh	–2.50±0.43^ab
	4	20.87±0.05^bc	–61.67±3.06^j	–2.27±0.04^a
	5	21.53±1.08^b	–32.00±3.61^ghi	–2.94±0.26^ab
	6	21.73±0.15^b	–24.67±1.16f^gh	–2.51±0.31^ab
	7	20.42±0.15^b - e	–33.00±1.00^hi	–2.34±0.25^a
Drying with dipping	0	18.89±0.54^ef	–23.67±2.52f^gh	–2.83±0.32^ab
solution in the shade (DS2)	1	15.51±0.43^gh	–74.00±10.82^j	–.67±3.22^fg
	2	14.02±0.69^h - k	82.67±8.62^a	–50.67±18.58^cde
	3	13.95±0.34^h - k	55.33±10.02^b	–57.33±5.69^def
	4	15.53±0.33^gh	44.67±3.51^bc	–50.67±7.37^cde
	5	15.13±0.28^g - j	39.00±3.00^cd	–48.00±3.61^cde
	6	15.33±0.09^ghi	35.67±2.08^cd	–58.00±3.60^ef
	7	15.52±0.28^gh	22.00±4.34^e	–39.00±8.89^c

Fig. 3

Interaction graphics of L^*, a^* and b^* color values during the drying of the Ekşikara grapes samples dried with different drying methods.

L^* value was between 18.66±0.75 and 18.89±0.54 in fresh grape samples. On the 7th, 11th, 15th and 21st days, the samples dried with DS1, DS2, ND1 and ND2 treatments had the L^* values of 15.96±0.13, 14.95±0.05, 20.18±0.06 and 18.65±0.12, respectively. The color was brighter and the time for darkening of the color was slower in samples that were dried without dipping to DS since the beginning of the drying process. In the shade dried samples, the decrease in L^* values depending on the drying conditions occurred in a longer time and a brighter color was observed. A brighter appearance with the dipping process and more matte appearance without the dipping process in the color of the grapes was reported by Seçkin [51]. şen [52] stated that there was a significant decrease in L^* values during the drying of grape samples and the darkening of the grapes was due to drying. In addition, the same researcher also pointed out that the L^* values of the dipping samples were significantly lower than those of the naturally dried samples.

The a^* values of fresh grape samples were determined between –22±1.00 and –24.67±1.53 and identified that the green color was dominant. As shown in Fig. 3, the green color was dominant in the samples dried with DS1 on the first day of drying and the maximum redness value was reached on the second day. DS is thought to be effective in this color change that occurs from green to red. Maximum green color value was obtained in samples dried with DS2 (–74.00±10.82). In both samples dried with DS, a decrease in red color value was observed depending on the drying time. In the samples dried with DS1 and DS2, fixed values were obtained towards the end of the drying period. When drying was completed, the values of 22.00±2.65 and 17.33±8.74 a^* were determined and at the end of drying, the a^* values were determined in red. In samples that were dried without DS dipping the green color was dominant from the beginning to the end of drying. The a^* values of the samples dried by ND1 and ND2 methods were determined as –13.67±3.69 and –15.33±1.53, respectively. There was no significant fluctuation in the samples dried with ND1 and ND2. It was determined that red color was dominant in grape samples dried with DS, and green color was dominant in grape samples dried without DS. şen [52] reported an increase in a^* values during the drying of grape samples with potasa.

The b^* values were determined between –2.60±0.53 and –2.83±0.32 in fresh grape samples and it was identified that the blue color was dominant. As shown in Fig. 3, an intense blue color was dominant in the samples dried with DS, and the blue color was decreases during the drying process. DS had an effect on the blueness of b^*. The b^* values of the samples dried with DS1 and DS2 from the beginnig to the end of drying were determined as –81.67±0.33, –71.67±3.22 and –17.33±7.77 (7th day), –17.33±8.74 (11th day), respectively.

3.4 Changes in the total phenolic content, total antioxidant activity and trans-resveratrol during drying

As shown in Table 1, the change of total phenolic content (TP), total antioxidant (TA) activity and trans-resveratrol (TR) content in the grape samples during drying were found to be statistically significant (p < 0.01) depending on drying time, drying method and their interactions.

As seen in Fig. 2, the TP content was determined to be similar in all drying methods during the first three-day drying period. Drying process was completed earlier in the samples dried with DS. TP contents of the samples dried by DS1 (7th day), DS2 (11th day), ND1 (15th day) and ND2 (21st day) methods were detected as 11486 mg GAE/kg dry matter, 10913 mg GAE/kg dry matter, 10724 mg, GAE/kg dry matter and 11338 mg GAE/kg dry matter, respectively, at the end of drying methods. An increase in TP content of the samples was obtained with all the drying methods.

Seçkin [51] determined that the total phenolic content was the highest (15546 mg GAE/kg dry matter) in Besni grape, a local cultivar in Turkey. He stated that although the color of Besni grape is white, the high amount of TP was caused by the seed of grape. Constantinou et al., [11] (‘Mavro’ and ‘Xynisteri’ grapes) and Fabani et al. [5] reported that the amount of phenolic compounds increased due to the sun drying of fresh grapes and there was a high correlation between the phenolic content and antioxidant activity of raisins. They also stated higher total phenolic content in raisins than fresh grapes.

TP content of Ekşikara grapes increased in four different drying methods depending on drying time. DS application accelerated the increase in total phenolic content of the samples. The effects of drying processes on phenolic material can vary depending on factors such as grape variety, maturity, drying medium, drying type and drying time.

The rapid change in total antioxidant activity (TA) can be clearly seen in the samples dried with DS (Fig. 2). Total antioxidant activity of the samples dried with the methods of DS1 (at the end of the 7th day), DS2 (at the end of the 11th day), ND1 (at the end of the 15th day) and ND2 (at the end of the 21st day) were determined as 85.38% ±0.49, 85.23% ±0.25, 84.04% ±0.25 and 86.17% ±0.20, respectively. The total antioxidant activity obtained with DS1 on the seventh day was achieved with ND2 after approximately three weeks of drying. It is obvious that drying conditions of DS and shade significantly affect the drying time on the samples.

Antioxidant activity of Shiraz grape variety were reported to be in the range of 0.94 mg/ml–1.14 mg/ml [57]. Konuk & Korel [58] reported high antioxidant activities in fresh and stable after drying in Alfons genus red grape seeds. The free radical removal effect of grape pulp powders dried with different drying methods were stated to be between 12.586 mg/L to 16.781 mg/L. [59]. Gülcü et al., [60] were stated that antioxidant activity in black grapes varie in proportion with total phenolic concentration. Similarly, in our study, it was observed that antioxidant activity values increased depending the phenolic content. This situation is likely to be caused by the phenolic compounds and color substances found in the bark and kernels of Ekşikara grapes.

As seen in Fig. 2, the values of trans-resveratrol in the samples dried with and without DS were determined to be similar during the first seven-day drying period. Samples dried with DS1, where the drying process was completed sooner, the amount of trans-resveratrol increased significantly at the end of fifth day of drying, and was found to be 0.694±0.006 mg/100 g dry matter at the end of the seventh day of drying process. As seen in samples dried with DS2, similar fluctuations in samples dried with DS1 were also observed, and 0.836±0.010 mg/100 g dry matter trans-resveratrol value was determined in the DS2 dried samples on the eleventh day of drying. Trans-resveratrol values were observed to be close to each other in DS dried samples. In samples that were dried with DS, DS caused a significant increase in the amount of restveratrol after the first four or five days of dissolution. When the drying process of the samples dried with ND1 and ND2 ended on the 15th and 21st days, the trans-resveratrol values were determined as 0.776±0.004 mg/100 g dry matter and 0.814±0.011 mg/100 g dry matter, respectively. Trans-resveratrol amounts were determined to be slightly higher in the shade and DS dried samples than in the sun and without DS dried samples. It was observed that trans-resveratrol values decreased as the drying time increased and DS application accelerated the drying process and had a significant effect (P < 0.01) on trans-resveratrol values (Table 3).

Table 3
Total phenolic content, total antioxidant activity and trans-resveratrol contents with different drying methods

Dryingmethods Drying time (Day) Total Phenolic Content (mg of GAE/Kg)^y Total Antioxidant Activity (%)^y Trans-Resveratrol (mg/100 g)^*y

Natural drying 0 2212±137.50^mn 34.58±1.09^p 0.443±0.005^kl

in the sun (ND1) 1 2452±275.02^lmn 41.39±0.33^o 0.448±0.005^kl

2 2564±187.52^k - n 47.87±0.25^lm 0.501±0.004ⁱ

3 2788±87.50^i - n 50.86±0.81^jk 0.508±0.004ⁱ

4 2886±112.53^h - n 52.97±0.89^ij 0.511±0.004ⁱ

5 3464±162.52^f - l 56.57±0.15^gh 0.523±0.007^ghi

6 3813±62.51^e - j 58.89±1.58^fg 0.575±0.007^de

7 4076±50.03^e - h 61.91±0.94^de 0.571±0.004^de

Drying with dipping 0 2194±116.05ⁿ 34.58±1.09^p 0.309±0.008^l

solution in the sun (DS1) 1 2602±275.02^j - n 40.56±0.15^o 0.341±0.004^j

2 2703±100.14^i - n 50.24±0.69^jkl 0.342±0.006^j

3 3688±337.50^e - k 56.77±0.15^gh 0.371±0.003ⁱ

4 4826±50.03^de 64.43±2.97^d 0.394±0.005^f

5 7350±975.00^c 78.41±0.45^b 0.389±0.014^fg

6 10052±975.01^b 80.29±0.84^b 0.567±0.005^b

7 11486±715.505^a 85.38±0.49^a 0.694±0.006^a

Natural drying 0 2214±133.01^mn 34.58±1.09^p 0.422±0.005^kl

in theshade (ND2) 1 2651±125.01^j - n 41.26±0.64^o 0.478±0.005ⁱ

2 2674±250.01^j - n 45.21±1.03^mn 0.510±0.005^fgh

3 3424±200.01^f - m 48.96±0.74^kl 0.530±0.006^ef

4 3703±250.05^e - k 52.42±0.74^ij 0.550±0.008^cde

5 3925±1507^e - i 55.24±0.40^hi 0.548±0.006^de

6 4213±262.50^efg 56.47±0.84^gh 0.558±0.006^cd

7 4314±162.52^def 58.40±0.20^fg 0.554±0.005^cd

Drying with dipping 0 2191±119.50ⁿ 34.58±1.09^p 0.367±0.011^kl

solution in theshade (DS2) 1 2213±112.50^mn 42.69±0.19^no 0.393±0.005^jk

2 3076±275.01^g - n 48.82±0.39^kl 0.443±0.005^hi

3 3388±62.51^f - n 52.47±0.20^ij 0.445±0.006^ghi

4 3802±75.08^e - j 56.42±0.70^gh 0.495±0.008^de

5 4203±150.09^efg 60.52±0.45^ef 0.500±0.003^cd

6 4404±1125.02^def 64.77±0.44^d 0.502±0.002^cd

7 5524±225.01^d 72.19±0.74^c 0.518±0.009^c

Dryingmethods	Drying time (Day)	Total Phenolic Content (mg of GAE/Kg)^*y	Total Antioxidant Activity (%)^*y	Trans-Resveratrol (mg/100 g)^*y
Natural drying	0	2212±137.50^mn	34.58±1.09^p	0.443±0.005^kl
in the sun (ND1)	1	2452±275.02^lmn	41.39±0.33^o	0.448±0.005^kl
	2	2564±187.52^k - n	47.87±0.25^lm	0.501±0.004ⁱ
	3	2788±87.50^i - n	50.86±0.81^jk	0.508±0.004ⁱ
	4	2886±112.53^h - n	52.97±0.89^ij	0.511±0.004ⁱ
	5	3464±162.52^f - l	56.57±0.15^gh	0.523±0.007^ghi
	6	3813±62.51^e - j	58.89±1.58^fg	0.575±0.007^de
	7	4076±50.03^e - h	61.91±0.94^de	0.571±0.004^de
Drying with dipping	0	2194±116.05ⁿ	34.58±1.09^p	0.309±0.008^l
solution in the sun (DS1)	1	2602±275.02^j - n	40.56±0.15^o	0.341±0.004^j
	2	2703±100.14^i - n	50.24±0.69^jkl	0.342±0.006^j
	3	3688±337.50^e - k	56.77±0.15^gh	0.371±0.003ⁱ
	4	4826±50.03^de	64.43±2.97^d	0.394±0.005^f
	5	7350±975.00^c	78.41±0.45^b	0.389±0.014^fg
	6	10052±975.01^b	80.29±0.84^b	0.567±0.005^b
	7	11486±715.505^a	85.38±0.49^a	0.694±0.006^a
Natural drying	0	2214±133.01^mn	34.58±1.09^p	0.422±0.005^kl
in theshade (ND2)	1	2651±125.01^j - n	41.26±0.64^o	0.478±0.005ⁱ
	2	2674±250.01^j - n	45.21±1.03^mn	0.510±0.005^fgh
	3	3424±200.01^f - m	48.96±0.74^kl	0.530±0.006^ef
	4	3703±250.05^e - k	52.42±0.74^ij	0.550±0.008^cde
	5	3925±1507^e - i	55.24±0.40^hi	0.548±0.006^de
	6	4213±262.50^efg	56.47±0.84^gh	0.558±0.006^cd
	7	4314±162.52^def	58.40±0.20^fg	0.554±0.005^cd
Drying with dipping	0	2191±119.50ⁿ	34.58±1.09^p	0.367±0.011^kl
solution in theshade (DS2)	1	2213±112.50^mn	42.69±0.19^no	0.393±0.005^jk
	2	3076±275.01^g - n	48.82±0.39^kl	0.443±0.005^hi
	3	3388±62.51^f - n	52.47±0.20^ij	0.445±0.006^ghi
	4	3802±75.08^e - j	56.42±0.70^gh	0.495±0.008^de
	5	4203±150.09^efg	60.52±0.45^ef	0.500±0.003^cd
	6	4404±1125.02^def	64.77±0.44^d	0.502±0.002^cd
	7	5524±225.01^d	72.19±0.74^c	0.518±0.009^c

Table 4

Organic acids and sugar values of the Ekşikara grapes dried with different drying methods

Dryingmethods	Drying time (Day)	Citric acid (mg/100 g)^*y	Malic acid (g/100 g)^*y	Tartaric acid (g/100 g)^*y	Glucose (g/100 g)^*y	Fructose (g/100 g)^*y	Sucrose (mg/100 g)^*y
Natural drying	0	61.79±0.822^qr	0.56±0.009^o	1.37±0.011ⁿ	25.05±0.014^t	28.62±0.108^q	3.17±0.025^pq
in the sun (ND1)	1	61.16±0.959^r	0.59±0.011^o	1.40±0.015^lmn	25.89±0.015^s	29.51±0.162^p	3.28±0.055^nop
	2	61.44±0.119^r	0.65±0.024ⁿ	1.44±0.019^klm	26.18±0.069^s	30.23±0.143^o	3.42±0.054^klm
	3	62.31±0.327^pqr	0.69±0.005^j - m	1.46±0.010^ijk	26.77±0.040^r	30.94±0.264ⁿ	3.43±0.044^i - l
	4	63.05±0.188^o - r	0.70±0.009^i - l	1.47±0.011^ijk	27.43±0.026^pq	31.46±0.188^mn	3.48±0.098^h - k
	5	63.85±0.246^n - q	0.70±0.005^h - l	1.50±0.012^ghi	27.84±0.054^o	31.90±0.162^lm	3.49±0.052^g - k
	6	65.69±0.505^mn	0.74±0.016^d - g	1.53±0.016^e - h	31.15±0.078ⁱ	34.29±0.281^k	3.48±0.045^g - k
	7	64.67±0.469^no	0.78±0.008^cde	1.58±0.015^cde	32.81±0.094^g	36.91±0.202^hi	3.47±0.029^h - k
Drying with	0	44.78±0.397^pqr	0.40±0.0115^o	0.98±0.006ⁿ	17.97±0.032^t	20.51±0.081^q	2.26±0.040^q
dipping solution	1	47.23±0.170^mn	0.46±0.008ⁿ	1.01±0.003^lmn	19.87±0.043^op	22.88±0.078^lm	2.38±0.014^lmn
in the sun (DS1)	2	48.18±0.337^lm	0.48±0.010^lmn	1.03±0.009^klm	20.26±0.110ⁿ	22.72±0.081^m	2.47±0.019^jkl
	3	51.50±0.674^ij	0.50±0.007^i - l	1.05±0.004^ijk	23.49±0.050^g	26.72±0.135^h	2.60±0.026^def
	4	51.86±0.496^fg	0.53±0.005^f - i	1.06±0.016^ijk	25.03±0.047^e	28.97±0.117^g	2.66±0.027^cde
	5	72.10±0.917^c	0.71±0.012^b	1.15±0.011^c	28.16±0.049^c	33.62±0.114^c	2.72±0.039^c
	6	76.05±0.71^b	0.73±0.003^b	1.27±0.013^b	31.53±0.046^b	37.62±0.148^b	2.84±0.052^b
	7	80.83±0.665^a	0.78±0.006^a	1.38±0.009^a	35.41±0.117^a	43.57±0.043^a	2.96±0.027^a
Natural drying	0	58.93±0.383^pqr	0.53±0.011^o	1.30±0.009ⁿ	23.76±0.156^t	27.21±0.300^q	3.01±0.037^opq
in the shade (ND2)	1	59.56±0.113^o - r	0.63±0.020^lmn	1.37±0.009^jkl	25.43±0.079^r	29.25±0.195ⁿ	3.13±0.014^lmn
	2	60.91±0.701^nop	0.64±0.012^k - m	1.41±0.007^hij	25.87±0.038^pq	29.81±0.037^mn	3.22±0.050^klm
	3	63.84±0.235^lm	0.68±0.004^g - j	1.39±0.022^ijk	27.90±0.049^l	33.16±0.085^j	3.30±0.025^g - k
	4	64.85±0.296^kl	0.73±0.011^c - f	1.40±0.008^ijk	28.71±0.048^k	34.45±0.071ⁱ	3.35±0.025^f - j
	5	66.55±0.295^jk	0.72±0.008^c - f	1.42±0.004^ghi	29.10±0.107^j	38.99±0.338^f	3.41±0.012^efg
	6	66.84±0.529^jk	0.70±0.008^e - h	1.46±0.012^d - g	30.48±0.025^h	40.57±0.186^e	3.38±0.036^e - h
	7	66.64±1.341^jk	0.73±0.010^cde	1.44±0.016^fgh	31.39±0.107^f	41.75±0.154^d	3.53±0.037^cd
Drying with	0	53.36±0.183^pqr	0.48±0.008^o	1.17±0.008ⁿ	21.39±0.116^t	24.51±0.270^q	2.70±0.022^q
dipping solution	1	63.28±0.480^hi	0.56±0.017^mn	1.20±0.012^mn	22.88±0.039^r	26.49±0.168ⁿ	2.81±0.023^nop
in the shade (DS2)	2	63.96±0.625^gh	0.57±0.012^lmn	1.27±0.011^ijk	23.42±0.002^q	27.79±0.105^l	2.79±0.023^n - q
	3	65.96±1.059^f	0.60±0.006^i - l	1.26±0.018^ijk	24.37±0.077ⁿ	29.03±0.190^k	2.84±0.022^mno
	4	68.42±0.291^e	0.61±0.013^g - k	1.30±0.019^ghi	24.95±0.045^m	30.34±0.192^j	2.94±0.35^jkl
	5	71.52±0.446^d	0.66±0.012^c - f	1.34±0.02^c - f	27.38±0.080^h	31.20±0.146ⁱ	2.96±0.015^h - k
	6	72.45±0.127^d	0.68±0.013^c	1.36±0.01^cd	28.20±0.105^g	34.88±0.168^fg	3.05±0.011^f - i
	7	72.85±0.859^d	0.67±0.012^cd	1.35±0.013^cd	31.04±0.204^d	37.29±0.080^e	3.20±0.012^cd

Fabani et al. [5] determined the amount of trans-resveratrol as 2.6±0.5μg/100 g dry matter in fresh grapes of ‘Sultanina’ and 27±8μg/100 g dry matter in raisins. Definition of Trans-resveratrol glycosides of Mavro’ (colored black) and ‘Xynisteri’ (colored white) grapes and slightly increased amount of this compound after drying is stated by Constantinou et al. [11]. In our study, the amount of resveratrol was determined to be higher in the samples dried with DS. Resveratrol level was found to be higher in the shade-dried samples than in the sun-dried samples. It was determined that drying time and DS application had significant effects on sample contents (phenolic, antioxidant activity, reveratrol) during the drying process. Increase in trans-resveratrol concentration due to drying is likely to be the result of synthesis reactions caused by stress conditions during drying in the sun and in the shade.

3.5 Changes in the organic acid contents during drying

The main organic acids found in grapes are tartaric acid and malic acid, and small amount of citric and succinic acid [61]. The distribution and quantities of organic acids may vary depending on factors such as variety, degree of maturity, growing conditions, ecology, geographical location grown, climate and cultural practices [52]. Citric acid is the third most common organic acid in grapes, constituting 5% –10% of the total acidity [62]. Malic acid, a weak organic acid, is one of the grape acids with an alpha-hydroxy acid structure. Malic acid is usually synthesized from pyruvates and phosphoenolpyruvates in grapes [61]. The amount of malic acid in grapes can vary between 1 g/L -10 g/L depending on the climatic conditions. The malic acid concentration of grapes grown in cool climates is higher [63]. Tartaric acid is the most important organic acid in grape juice or raisins [61]. The fruits and leaves of grapes are responsible for the production of tartaric acid and malic acid [63]. Ribereau et al. [64] observed that tartaric acid was initially synthesized in the first period of grape during the development phase and reached to a maximum concentration level before grape was greened. While determining the acid amount of ripe grapes, the amounts of potassium salts of tartaric, malic and citric acid are determined [45].

As shown in Table 2, the changes in organic acids (tartaric acid, malic acid, citric acid) of grapes during drying was found statistically significant (p < 0.01) depending on drying time, drying method and their interaction.

As seen in Fig. 3, similar values for organic acid contents were obtained at the beginning of drying and a rapid change after the fourth day, especially in grape samples dried with DS, were observed. The complete dissolution of dippeing solution is considered to be effective during this four-day period. It were determined that grape samples showed an increase in their organic acid values depending on the drying time. The difference in the change of citric acid during drying was identified to be more pronounced in samples dried with DS. Organic acid amounts were determined to be higher in the samples dried with DS than that of the samples without DS. The difference in citric acid change during drying is more pronounced in samples dried with the DS. In the samples dried by DS1 (7th day), DS2 (11th day), ND1 (15th day) and ND2 (21st day) methods, citric acid amounts were detected as 80.83±0.665 mg/100 g, 103.61±0.995 mg/100 g, 71.44±0.547 mg/100 g and 96.34±0.899 mg/100 g, respectively, at the end of drying. The amounts of malic and tartaric acid were found higher in grape samples dried with DS than in samples dried without DS. Malic and tartaric acid amounts in grape samples dried by DS1 (day 7), DS2 (day 11), ND1 (day 15) and ND2 (day 21) methots were determined as 0.78±0.006 g/100 g –1.38±0.006 g/100 g, 1.00±0.008 g/100 g –1.00±0.008 g/100 g, 1.00±0.009 g/100 g –1.67±0.003 g/100 g and 0.98±0.009 g/100 g –1.72±0.017 g/100 g, respectively. It was determined that DS application after drying caused an increase in organic acid amounts. Moreover, the grape samples dried under the shade conditions had slightly higher organic acid contents than the sun-dried samples. It is likely that the increase in organic acid amounts of all samples during drying is caused by the increase in dry matter concentration.

Organic acid concentration of naturally dried grapes was reported lower than that of dipping solution applied ones [45]. The dominant acid in grapes is tartaric acid, followed by malic acid and the amount of organic acid in black grape varieties is higher than that of white grape varieties [65]. Soyer et al., [40] determined the citric acid, tartaric acid and malic acid amounts of 11 different white grape varieties between 30 mg/L –164 mg/L, 4.98 g/L –7.48 g/L and 1.43 g/L –3.40 g/L, respectively.

3.6 Changes in the sugar contents during drying

The sugar content of grapes is significant in terms of determining the quality of the grapes as well as their organoleptic properties. Carbohydrate is required for growing and developing of grape fruit. Sugar metabolism plays an important role in the development of grapes [66]. Sugar composition of grapes can vary depending on grape type and degree of maturity [61]. As shown in Table 2, the changes in sugars (glucose, fructose, sucrose) during drying was found statistically significant (p < 0.01) depending on drying time, drying method and their interaction.

As shown in Fig. 4, similar results were obtained in the sugar content of Ekşikara grape samples for four different drying methods at the end of drying. Glucose, fructose and sucrose amounts of grape samples dried by DS1 (day 7), DS2 (day 11), ND1 (day 15) and ND2 (day 21) methods were determined as 35.41±0.117 g/100 g–42.41±0.195 g/100 g–48.13±0.104 g/100 g–45.81±0.085 g/100 g, 43.57±0.043 g/100 g–52.13±0.527 g/100 g–58.58±0.113 g/100 g–55.87±0.107 g/100 g and 2.96±0.019 mg/100 g –3.73±0.023 mg/100 g –4.37±0.023 mg/100 g –4.17±0.014 mg/100 g, respectively. A significant increase in the sugar amounts of the grape samples dried with different drying methods were determined due to drying. It was determined that the sugar amount of the DS dried samples was slightly higher than the samples dried without DS. As a result of drying processes, it was found that the dominant sugar in grape samples was fructose similar to the fresh grape samples. In addition, a rapid increase in fructose amounts in all drying methods after 3-4 days of drying time was also determined. During 3-4 day drying period, the complete dissolution of dipping solution was considered effective. In our study, it is most likely that the drying method and DS have important effects on the rapid change in fructose amount of the samples dried by DS1 method.

Fig. 4

Interaction graphics of total phenolic content, total antıoxidant activity and trans-resveratrol values during drying with different drying methods.

Fig. 5

Interaction graphics of organıc acıds of the Ekşikara grapes dried with different drying methods drying methods.

Fig. 6

Interaction graphics of sugars of the Ekşikara grapes dried with different drying methods.

Aslan et al. [61] determined the amount of glucose, fructose and sucrose in the fruit juice of black raisin as 53.52 g/L –99.127 g/L, 45.04 g/L –104.80 g/L and 0 g/L –16.67 g/L, respectively. Constantinou et al. [11] stated that there is an increase in the amount of glucose and fructose in the ‘Mavro’ (black colored) and ‘Xynisteri’ (white colored) grapes dried in the sun. Duran [65] determined the glucose and fructose amounts of 8 different grape varieties in the range of 89.4±0.51 g/L to 164.88±2.21 g/L and 92.1±0.59 g/L to 151.65±4.27 g/L, respectively. Peinado et al., [56] reported that the amount of sugar increased from 211 g/L to 478 g/L during drying of grapes. Otağ stated that glucose, fructose and sucrose amounts increased in naturally dried grape samples. He also reported that glucose amount was higher in the samples that were dried naturally in the sun than the samples dried in dipping solution and the main reason of such increase was found to be dipping solution application.

4 Conclusion

A decrease in water activity value and an increase in °Brix content was determined in grape samples dried with different methods, due to the decrease in water content. Samples of DS1 were dried in a very short time such as 7 days. The drying time of grapes, which cannot be performed in the sun in less than 15 days under natural conditions in Grape drying in Karaman region, was significantly shortened with dipping solution. It was determined that DS application significantly was affects the drying of the grapes and was allows the grapes to dry in a shorter time. It was determined that samples dried with DS were darker and better preserved its original color compared to samples without DS due to the waxy layer in the shell. In addition, grapes retained their red color better in the samples dried with DS and blue color was dominant in all dried samples. Application of dipping solution provided shorter drying time and obtaining darkness color grapes. A significant increase in the pH and total acidity values in grape samples during the drying period was found. It is most likely that the main reason for the increase in total acidity value is due to the increase in dry matter concentration. An increase in total phenolic content and antioxidant activity concentration occurred during drying in the grape samples. The amount of trans-resveratrol, an important component of grapes, was increased significantly at the end of drying. An increase in organic acid and sugar contents were observed in all samples dried by different drying methods, due to the increase in the amount of dry matter. Organic acid losses were lower in the samples dried with DS than that of without DS. This change was found to be more pronounced in the samples dried with DS. The most ideal drying method in terms of both drying time and grape components was DS1.

Grapes soaked in the Potasa solution and dried in the shade provide also an advantage in terms of contamination and growth of microorganisms due to shortening the drying time compared to sun dried grapes. In addition, drying with the potasa solution positively affected the color and other properties (drying time, trans-resveratrol, organic acids) of the grape. It is recommended to develop solar assisted drying systems to use renewable energy sources more effectively and efficiently in fruit and vegetable drying.

Footnotes

Acknowledgments

This study was supported by the Scientific Research Projects (BAP) of Karamanoğlu Mehmetbey University, Karaman-Turkey (07 - YL-19). This manuscript was prepared from the master’s thesis. The authors thank to village people of Damlapınar for their helps during providing the grapes.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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