PAPER

Drying characteristics and some quality parameters of whole jujube (Zizyphus jujuba Mill.) during hot air drying

Begüm Tepe*, Raci Ekinci

Department of Food Engineering, Faculty of Engineering, Pamukkale University, Denizli, Turkey

Abstract

Drying kinetics, water-soluble vitamins, total phenolic content (TPC), antioxidant capacity (AC) of the jujube fruits dried at 50, 60, and 70°C, and degradation kinetics of the quality parameters were investigated. The models fitted to drying were determined as Page at 50 and 70°C, Parabolic at 60°C. Increment in the drying temperature increased the drying rate and decreased the drying time. Water-soluble vitamins, TPC, and AC were significantly reduced by the drying process. Degradation of water-soluble vitamins increased with the drying temperature, although TPC and AC were not significantly affected by temperature. Thermal degradations of quality parameters were fitted to first-order kinetic.

Key words: antioxidant capacity, degradation kinetics, drying kinetics, total phenol content, water-soluble vitamins

*Corresponding Author: Begüm Tepe, Department of Food Engineering, Faculty of Engineering, Pamukkale University, Denizli, Turkey. Email: begumotag@gmail.com

Received: 20 August 2020; Accepted: 05 October 2020; Published: 01 February 2021

DOI: 10.15586/ijfs.v33i1.1947

© 2021 Codon Publications

This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0). License (http://creativecommons.org/licenses/by-nc-sa/4.0/)

Introduction

Jujube fruit (Zizyphus jujuba Mill.), belonging to the Rhamnaceae family, is a drupe, which has a round-elliptic shape, apple-like taste, and is rich in various bioactive compounds and nutrients such as vitamin C, polysaccharides, minerals (especially potassium), and phenolic compounds (Chen et al., 2015; Gao et al., 2011; Wojdylo et al., 2016). The jujube fruit has higher vitamin C content than the fruits that are known as sources of vitamin C such as kiwi, strawberry, and lemon (Frenich et al., 2005; Wu et al., 2012). In addition, the jujube fruit is a good source for thiamine, riboflavin, niacin, and pyridoxine as B complex vitamins (Gao et al., 2013). B complex vitamins have an important role as coenzymes for enzymatic reactions in different biological systems (Calderón-Ospina and Nava-Mesa, 2020). Moreover, the jujube fruit has been considered as a good source of phenolic compounds compared to common fruits, which are widely known for being a source of phenolic compound such as berries (Gao et al., 2011). In traditional Chinese medicine, the jujube fruits have been used as a crude drug for analeptic, palliative, and antibechic purposes for thousands of years (Li et al., 2007; Rostami and Gharibzahedi, 2017). The jujube fruits are also used for pharmaceutical benefits such as antioxidant, anticancer, anti-inflammatory, antiepileptic, hepatoprotective, and neuroprotective effects (Choi et al., 2012; Ji et al., 2017). The pharmaceutical benefits of the jujube fruit have been associated with chemical ingredients, which mainly consist of vitamin C, phenolics, polysaccharides, triterpenic acids, and nucleosides (Ji et al., 2017).

The jujube fruits have been consumed as fresh, dried, tea, alcoholic beverages, pickle, jam, compote, or candy (Elmas et al., 2019; Wang et al., 2016; Wojdylo et al., 2016). Although the jujube fruits are generally consumed as fresh, the postharvest shelf-life of the jujube fruit is very short. Thus, the commercial value of the jujube fruits is less (Wang et al., 2016; Zozio et al., 2014). Chemical reactions, microbiological activity, and physical alterations in the pre- or post-harvest period of many plant-based foods mostly require high water content (Tepe and Tepe, 2020). Therefore, some preservation methods such as drying can be suggested to extend their shelf-life.

Drying, which is also called dehydration, is one of the oldest methods of food preservation, used since ancient times. The beneficial properties of drying can be ordered as reducing the water activity to microbiological safety zone and transport costs, providing easier process and extending shelf life (Elmas et al., 2019). Selection of drying methods has great importance for the preservation of quality parameters such as the content of phenolic compounds, vitamins, and antioxidant capacity (AC). Sun drying and hot air drying are traditional methods for jujube drying (Wang et al., 2016). The main mechanism of hot air drying is mass and heat transfer and phase transition (Tepe and Tepe, 2020). Hot air drying provides some advantages such as being free from the climate effects, reducing the drying cycle, and hygienic conditions in comparison to sun drying (Elmas et al., 2019). However, long drying time, loss of nutritional and bioactive value, and changes in sensory properties are disadvantages of hot air drying (Elmas et al., 2019; Onwude et al., 2017; Wang et al., 2019). In addition to the hot air drying method, microwave, vacuum, and freeze drying methods have been regarded as alternative drying methods. Condurso et al. (2019) notified that decrement in drying time and better product quality could be provided by microwave drying. In addition, vacuum drying is useful for drying of easily oxidizing foods. Besides, freeze drying has also many advantages such as preserving food quality, especially in heat-sensitive foods. These drying methods can be used alone or combined with each other such as microwave–hot air or vacuum–microwave drying. The combined drying methods may be useful for increasing the efficiency (energy efficiency, environment friendly, product quality) of these drying methods (Sun et al., 2019). To select the most appropriate drying method, mathematical modeling is needed along with the determination of product quality, energy efficiency, etc. In this context, thin-layer drying has been used for the determination of the drying kinetics of fruits and vegetables. The most appropriate drying conditions can be selected by using thin-layer drying technology, a kind of mathematical modeling. Thus, a drying process can be designed and optimized (Onwude et al., 2016).

There are some recent researches on the drying of jujube fruits in the literature. Elmas et al. (2019) have reported that moisture content, water activity, and the total phenolic content (TPC) decreased as the drying temperature increased in hot air drying. In addition, Anjum et al. (2020) have investigated the effects of drying methods (sun and hot air drying) on several physical properties, AC, content of vitamin C, and TPC. In that study, it was notified that the highest AC and TPC were observed in jujube fruits dried at 70°C, while the highest vitamin C content was obtained at 50°C (Anjum et al., 2020). The effects of different drying methods (convective, vacuum-microwave, convective pre-drying, and vacuum–microwave combination) on phenolic compounds, AC, and the color of jujube fruits were investigated by Wojdylo et al. (2019). Convective drying carried out at 50°C has been reported to be the best method in terms of polyphenol content, AC, and color parameters.

In this study, it was aimed to: (i) determine the drying characteristics of whole jujube fruits at different air temperatures (50, 60, 70°C); (ii) investigate the effect of drying on vitamin C and B complex content, TPC, and AC of fresh and dried whole jujube fruits; and (iii) determine thermal degradation kinetics of these bioactive compounds during the drying process.

Materials and Methods

Sample preparation

Fresh jujube fruits (Zizyphus jujuba Mill.) were provided from a local producer in Denizli, a province in Turkey. The fresh jujube fruits were carefully selected in terms of the same ripening stage (fully mature) and the same size. Before the analysis, fresh jujube fruits were washed to remove foreign materials. The fresh jujube fruits were stored at 4°C in a refrigerator. Determination of the initial moisture content of the samples was carried out in a drying oven at 105°C till any changes in the sample weight. The initial moisture content of whole jujube fruits was 65.26% ± 0.6.

Drying experiment

A cabinet dryer (Yücebas¸ Makine Ltd. Inc., Izmir, Turkey) was used for the drying experiments. For the condition stabilization, the cabinet dryer was turned on approximately 30 min before drying. Samples (500 g) were weighted on a drying tray and placed in a cabinet dryer for each drying experiment. Drying temperatures were selected as 50, 60, and 70°C, similar to other researches on the drying of jujube fruits (Fang et al., 2009a; Motevali et al., 2012; Wojdylo et al., 2016, 2019). Besides, air velocity and relative humidity were 2 m s−1 and 20%, respectively. In practical application, the moisture content of the dried jujube fruits needs to be below 25% on wet basis (WB) (Fang et al., 2009a). Therefore, the drying experiments were continued until the moisture content of the samples achieved to 21% on WB, similar to the results of the studies by Fang et al. (2009a) and Yi et al. (2012). All of drying experiments were performed in triplicate.

Drying characteristics of whole jujube fruits

To design the best drying conditions, thin-layer drying models are very important. The thin-layer mathematical models selected in the current study are listed in Table 1. Significant information about drying temperature and time can be provided with these models (Demiray et al., 2017).

Table 1. Thin-layer mathematical models.

Model name Model References
Logaritmic aexp(-kt) + c Demiray et al. (2017)
Lewis exp(-kt) Demiray et al. (2017)
Henderson and Pabis aexp(-kt) Tepe and Tepe (2020)
Page exp(-ktn) Demiray et al. (2017)
Parabolic a + bt + ct2 Bi et al. (2015)
Wang and Sing 1 + at + bt2 Tepe and Tepe (2020)

Moisture ratio (MR) of whole jujube fruits was calculated using Eq. (1):

MR=MtMeMiMe      (1)

Mi: initial moisture content of the samples (g water g−1 dry matter);

Mt: moisture content at any point of time (g water g−1 dry matter);

Me: equilibrium moisture content (g water g−1 dry matter).

Me can be ignored because of its insignificant value in comparison to Mi and Mt (Fang et al., 2009a).

Drying rate (DR) was determined using Eq. (2):

DR=Mt+ΔtMtΔt     (2)

Mt+Δt: moisture content at time difference;

Δt: difference of time between two measuring points.

The relation between the predicted and the experimental data of whole jujube fruits dried at different drying temperatures is explained with determination coefficient (R2), root-mean square error (RMSE), and reduced chi-square (χ2). RMSE is a statistical parameter, which expresses the deviation between the predicted and the experimental values. The best equation predicting experimental data is determined according to the lower values of χ2 and RMSE, and the higher value of R2. The RMSE (Eq. 3) and chi-square (χ2) (Eq. 4) values were calculated as follows:

RMSE=1Ni=0N(MRpre,iMRexp,i)212      (3)
x2=i=0N(MRpre,iMRexp,i)2Nn     (4)

MRpre,i: predicted MR;

MRexp,i: experimental MR;

N: number of observation data;

n: constants of thin layer drying models.

Thin-layer modeling and statistical parameters were calculated using the MATLAB software (R2015a, version 8.5) non-linear curve fitting toolbox with the trust-region algorithm.

Determination of effective moisture diffusivity and activation energy in hot-air drying

Fick’s diffusion equation has been accepted to describe the drying characteristics of biomaterials. Crank (1975) has suggested a solution to this equation, which can be used for spherical products. Eq. (5) has been recommended for spherical products by assuming constant effective diffusivity and no shrinkage (Doymaz, 2006).

MR=6π2n=11n2expn2π2Defftr2     (5)

Deff: effective moisture diffusivity (m2 s−1);

r: arithmetical average of radius of samples at measured intervals (m).

Eq. (5) can be simplified for the first term of the series (Saravacos and Raouzeos, 1986). The new equation is written as given below, Eq (6):

In(MR)=In6π2π2Deffr2t      (6)

The plot gives a straight line with a slope as follows, Eq. (7):

Slope=π2r2Deff    (7)

Arrhenius equation in hot air drying process was used for the calculation of activation energy (Fang et al., 2009a):

Deff=D0expEaRT     (8)

R: universal gas constant (8.314 J mol−1 K−1 or 1.987 cal mol−1 K−1);

T: absolute temperature (K);

Ea: activation energy (kJ mol−1 or kcal mol−1);

D0: pre-exponential constant (m2 s−1).

After regulation of the natural logarithm of Eq. (8), Eq. (9) can be written as given below:

InDeff=InD0EaRT     (9)

Natural logarithm of effective moisture diffusivity versus T−1 gives a straight line with a slope, which represents activation energy.

Analysis of water-soluble vitamins

An extraction method, recommended by Dönmez (2015), was used for water-soluble vitamins. In order to determine the water-soluble vitamins, 5 g of each sample was weighted. After homogenization with distilled water (1:9, w:v), the homogenate was centrifuged at 2355 × g for 10 min (Nüve NF 800R). The supernatant obtained from centrifugation was filtrated using a 0.45 µm filter to be injected into the high performance liquid chromatography (HPLC).

A micro syringe was used for injecting 20 µL of the last filtrate into the HPLC column. Mobile phase consisted of 0.1 M HPLC grade KH2PO4 at pH 7. An HPLC device (SHIMADZU), column oven at 25°C (SHIMADZU CTO-20A), Column ACE C18 (7.8 × 300 mm), pump (SHIMADZU LC-20AD), degasser (SHIMADZU DGU-20A3), photo-diode array (PDA) detector (SPD-M20A) at 254, 261, 324, 234 nm for ascorbic acid, niacin, pyridoxine, and thiamin, respectively were used for analysis. The mobile phase was isocratic with 0.7 mL min−1 flow rate. For riboflavin analysis, the column is Macherey-Nagel NH2 (4.6 × 250 mm), column oven temperature is 40°C, and the wave length is 266 nm. The same mobile phase was isocratic with 1 mL min−1 flow rate.

The content of water-soluble vitamins was calculated using an equation obtained from a calibration curve consisting of different concentrations of stock solutions (5, 10, 25, 50, 75, and 100 ppm) with a high R2 (0.9999). Results were given as mg 100 g−1 in dry weight (DW) for vitamin C and µg 100 g−1 for niacin, pyridoxine, thiamine, and riboflavin. Each analysis was performed in triplicate.

Analyzes of TPC and AC

Analyses of TPC and AC were performed with methanolic extraction with a slight modification, as suggested by Choi et al. (2012). Jujube fruit samples (5 g) and 45 mL of 90% methanol were homogenized using a laboratory-type blender. The homogenate was centrifuged at 2355 × g for 10 min. After centrifugation, the supernatants were collected and filtrated using a filter paper.

TPC analysis was performed according to Singleton and Rossi (1965) with a slight modification. Folin-ciocalteu solution (1500 µL) (10% v/v) was added into 300 µL of the extract, and the mixture was kept in a dark place for 5 min. After adding 1200 µL of aqueous 7.5% Na2CO3 into the mixture, the mixture was incubated at room temperature in a dark place for 2 h. At the end of the incubation, the absorbance of samples was measured at 760 nm using a spectrophotometer (T80, PG Ins. UK.). Each analysis was carried out in triplicate, and TPC was expressed as mg gallic acid equivalent (GAE) 100 g−1 in DW.

The AC analysis was carried out using a method suggested by Thaipong et al. (2006) with slight modification. Extracts (150 µL) and DPPH methanolic solution (2850 µL), whose absorbance is 1.1 at 515 nm, were mixed. After incubation for 60 min at room temperature in a dark place, the absorbance of samples was measured at 515 nm. Each sample was analyzed in triplicate, and AC was expressed as mmol trolox equivalent (mmol TE) g−1 in DW.

Color measurement

Reflectance color value of the whole jujube fruit skin was measured using Hunter Lab Color Miniscan XE (45/0-L, USA). The samples were placed on a white background, and the measurement was done by covering a transparent glass. ΔE was calculated with the equations below (Horuz et al., 2017):

ΔE=(L0L)2+(a0a)2+b0b2     (10)

ΔE: Total color differences;

L: Lightness (0 = black, 100 = white) value at the end of the drying process;

L0: Initial lightness value of the fresh jujube fruits;

a: Redness (a+ = red, a- = green) value at the end of the drying process;

a0: Initial redness value of the fresh jujube fruits;

b: Yellowness (b+ = yellow, b- = blue) value at the end of the drying process;

b0: Initial yellowness value of the fresh jujube fruits.

Calculation of kinetic parameters

Labuza and Riboh (1982) and Kadakal et al. (2017) have suggested Eq. (11) as the general equation to describe the reaction rate of compounds degrading or forming:

dCdt=kCm    (11)

For the zero-order kinetic model, Eq. (12) can be written as below:

C=Ckt     (12)

When Eq. (1) is integrated, if m equals one, Eq. (3) is written as follows:

InC=InC0kt      (13)

ln C: natural logarithm of the residual vitamin C, B complex vitamins, TPC, and AC;

ln C0: initial content of vitamin C, B complex vitamins, TPC, and AC;

k: rate constant (h−1);

t: time.

Temperature dependence of vitamin C, B complex vitamins, TPC, and AC can be calculated with Eq. (14) (Kadakal et al., 2017; Labuza and Riboh, 1982):

k=k0xeEaRT     (14)

When the Eq. (14) is regulated, Eq. (15) is written as follows:

lnk=EaRx1T+lnk0     (15)

k0: frequency factor (h−1);

R: universal gas constant (8.314 × 10−3 kJ mol−1 K−1 and 1.987 × 10−3 kcal mol−1 K−1);

T: absolute temperature (K);

Ea: activation energy (kcal mol−1 or kJ mol−1).

Quotient indicator (Q10) expresses the temperature-dependence of reaction rate and is calculated with Eq. (16) (Kadakal et al., 2017):

Q10=(k2k1)10T2T1      (16)

Half-life time, a time required for half of the concentration, for each temperature is calculated with Eq. (17) for the first-order kinetic (Kadakal et al., 2017);

t1/2=ln(0.5)xk1=0.693xk1     (17)

D represents the time that it takes for the compound or quality criterion to lose 90% of its quality and is calculated for first-order kinetics as written below (Eq. 18):

D=2.303xk1    (18)

Statistical analysis

All of the data were statistically analyzed using the SPSS software (ver. 22 SPSS Inc., Chicago, IL, USA) and expressed as mean ± standard deviation (SD). Analysis of variance (ANOVA) was used to evaluate differences between treatments, with a significance level of p = 0.05. The differences between groups were determined using the Duncan test.

Results and Discussion

Drying characteristics of whole jujube fruits during hot air drying

MR and DR of whole jujube fruits during hot air drying are shown in Figure 1. As understood from Figure 1, drying temperature has statistically affected DR and the drying time of whole jujube fruits. It was clearly observed that DR increased with the increment in drying temperature. Accordingly, the drying time was reduced and found to be 48, 30, and 18 h for 50, 60, and 70°C, respectively. Likewise, Yi et al. (2012) reported that drying times of whole jujube fruits at 45, 55, and 65°C for constant air velocity (2 m s−1) were about 45, 25, and 20 h, respectively. It could be explained with increasing of the heat transfer coefficient by increment in drying temperature. Generally, two periods as constant rate and falling rate are the main constituents of the drying process of agricultural products such as fruits and vegetables. In the current study, the falling rate period was observed. This statement was found to in sync with other studies, in which the jujube fruits were dried with hot air, by Fang et al. (2009a), Yi et al. (2012), Baomeng et al. (2014), and Chen et al. (2015).

Figure 1. Moisture ratio and drying rate of whole jujube fruits during hot air drying.

MR of whole jujube fruits during hot air drying was used to be fitted mathematical models that are listed in Table 1. Statistical parameters to describe the most suitable model are presented in Table 2. Demiray et al. (2017) reported that the lower RMSE and χ2 and the higher R2 are required for goodness of the fit. As seen from Table 2, the parabolic model was the best model for predicting the experimental MR of whole jujube fruits for 60°C, while experimental MRs of jujube dried at 50 and 70°C were described with Page model with the lowest RMSE and χ2 and the highest R2 values.

Table 2. Models, constants, and statistical parameters of thin-layer drying curves.

Models Temperature Model constants χ2 RMSE R2
Lewis 50°C k = 0.03638 0.00043784 0.020710 0.9936
60°C k = 0.06996 0.000355573 0.018550 0.9953
70°C k = 0.10140 0.000841207 0.028230 0.9899
Page 50°C k = 0.02349 n = 1.135 2.00099E–05 0.004381 0.9997
60°C k = 0.05734 n = 1.074 0.00022413 0.014480 0.9972
70°C k = 0.06731 n = 1.182 5.10587E–05 0.006759 0.9995
Henderson and Pabis 50°C k = 0.03820 a = 1.041 0.000225899 0.014720 0.9968
60°C k = 0.07249 a = 1.032 0.00024309 0.015080 0.9970
70°C k = 0.10720 a = 1.049 0.000524835 0.021670 0.9944
Logaritmic 50°C k = 0.05256 a = 0.9309 c = 0.1427 0.001495566 0.037470 0.9794
60°C k = 0.10390 a = 0.9326 c = 0.1428 0.001457264 0.036280 0.9825
70°C k = 0.14750 a = 0.9333 c = 0.1439 0.002635481 0.047110 0.9733
Wang and Singh 50°C a = −0.03016 b = 0.0002595 2.90427E–05 0.005278 0.9995
60°C a = −0.05971 b = 0.0010510 4.66771E–05 0.006608 0.9994
70°C a = −0.08348 b = 0.0019720 0.000105507 0.009716 0.9988
Parabolic 50°C a = 1.003 b = −0.03039 c = −0.0002634 2.92706E–05 0.005242 0.9996
60°C a = 1.008 b = −0.06080 c = 0.0010800 3.94992E–05 0.005973 0.9995
70°C a = 1.017 b = −0.08723 c = 0.0021410 6.1355E–05 0.007188 0.9994

RMSE, root-mean square error.

Effective moisture diffusivity and activation energy of whole jujube fruits during hot air drying

Deff and Ea values of whole jujube fruits are presented in Table 3. Deff values of whole jujube fruits were calculated in the range of 6.43 × 10−11 and 1.80 × 10−10 m2 s−1. In comparison to the other drying temperatures, the highest value of Deff was obtained from the drying process performed at 70°C. It is a fact that drying temperature is one of the most important factors affecting the Deff value. Increment in Deff value means more easy evaporation of moisture content of the sample and consequently an increment in DR. In addition, a proportional relationship between Deff and DR was reported by Demiray et al. (2017). Elmas et al. (2019) and Fang et al. (2009a) notified Deff values of jujube fruits ranging from 1.27 × 10−9 to 3.55 × 10−9 and 5 × 10−11 to 2 × 10−10 m2 s−1, respectively. When compared to these studies, the result of the current study was less than that reported by Elmas et al. (2019) and very similar to the findings notified by Fang et al. (2009a). This difference might be because of drying conditions, equipment, and the shape of dried fruits (sliced or whole). Arrhenius relation between Deff and T−1 is presented in Figure 2. Ea value of whole jujube fruits was found to be 47.11 kJ mol−1 and 11.33 kcal mol−1. Various Ea values were reported for drying of the jujube fruit. Fang et al. (2009a) have reported that the Ea value of the jujube fruit was 54.51 kJ mol−1. On the contrary, Elmas et al. (2019) have found the Ea value of sliced jujube fruit to be 28.183 kJ mol−1. Besides, Motevali et al. (2012) have notified that the Ea value of the jujube fruit ranged from 34.97 to 74.20 kJ mol−1.

Table 3. Deff and Ea values of whole jujube fruits.

Temperature Deff (m2 s–1) Ea (kJ mol−1) Ea (kcal mol−1)
50°C 6.43 × 10−11
60°C 1.11 × 10−10 47.41 11.33
70°C 1.80 × 10−10

Figure 2. Arrhenius type relation between Deff and T−1.

Color properties of whole jujube fruits during hot air drying

Color properties of fresh and dried whole jujube fruits are shown in Table 4. When compared to the initial L, a, and b values of whole jujube fruits, these parameters were significantly decreased depending on the drying process (p < 0.05). The lowest L, a, and b values were obtained at 50°C. It could be because of the longer time of the drying process at 50°C. ΔE represents differences between the colors of the samples (Horuz et al., 2017). ΔE values of dried whole jujube fruits depending on drying temperature and time ranged from 16.84 to 20.17.

Table 4. The color properties of whole jujube fruits.

L a b ΔE
Fresh 21.92 ± 0.01a 15.95 ± 0.12a 9.91 ± 0.12a 0.00
50°C 10.31 ± 0.05b 2.04 ± 0.04b 1.04 ± 0.02b 20.17
60°C 11.09 ± 0.03c 2.23 ± 0.02c 1.34 ± 0.07c 19.47
70°C 12.25 ± 0.04d 4.57 ± 0.02d 2.12 ± 0.08d 16.84

*The different letters in the same column are significantly different (p < 0.05).

The effect of the drying process on the water-soluble vitamins, TPC, and AC

The effect of the drying process on the water-soluble vitamins of whole jujube fruits is given in Table 5. Water-soluble vitamins were significantly affected by the drying process. Vitamin C content of the jujube fruit differs depending on some factors such as geographical conditions and cultivars. In the current study, vitamin C content of fresh whole jujube fruits was determined as 78.90 ± 0.96 mg 100 g−1 DW. Vitamin C content of the whole jujube fruit was considerably reduced by the drying process, mainly the drying temperature (p < 0.05). Fang et al. (2009b) have similarly reported decrement in vitamin C content of the whole jujube fruit during hot air drying. Reduction in vitamin C content of sliced jujube fruit during hot air drying was also notified by Chen et al. (2015). Vitamin C is a heat-sensitive compound and, thus, might be degraded by a heating process such as drying (Chin et al., 2015). In addition, vitamin C oxidation can occur more rapidly at higher temperatures (Orikasa et al., 2014; Santos and Silva, 2008). The highest loss of vitamin C content of whole jujube fruits was 81.91% at 70°C, while the lowest loss was 55.51% at 50°C. Loss of vitamin C content of whole jujube fruits increased with the increment in drying temperature. This result was similar to Chen et al. (2015) but contrary to Fang et al. (2009b). In other fruits, vitamin C has been reported to be more degraded with the increasing air temperature (Chin et al., 2015; Kaya et al., 2010; Orikasa et al., 2014; Vega-Galvez et al., 2009).

Table 5. The effect of the drying process on water-soluble vitamins of whole jujube fruits.

Vitamin C % Loss Thiamine (B1) %Loss Riboflavin (B2) %Loss Niacin (B3) % Loss Pyridoxine (B6) %Loss
Fresh 78.90 ± 0.96a 0 27.33 ± 1.52a 0 41.00 ± 1.00a 0 883.33 ± 15.27a 0 80.33 ± 2.08b 0
50°C 35.10 ± 0.36b 55.51 19.93 ± 0.21b 27.07 19.73 ± 0.15b 51.88 761.67 ± 4.04b 13.77 ND* 100
60°C 20.93 ± 0.64c 73.47 18.63 ± 0.15b 31.83 18.13 ± 0.15c 55.78 689.33 ± 4.16c 21.96 ND 100
70°C 14.27 ± 0.55d 81.91 16.47 ± 0.21c 39.74 15.43 ± 0.15d 62.37 631.00 ± 2.00d 28.57 ND 100

*ND, not detected; Vitamin C was expressed as mg 100 g−1; DW, B complex vitamins were expressed as µg 100 g−1 DW. The different letters in the same column are significantly different (p < 0.05).

Initial content of thiamine (B1), riboflavin (B2), niacin (B3), and pyridoxine (B6) in fresh whole jujube fruits were determined as 27.33 ± 1.52, 41.00 ± 1.00, 883.33 ± 15.27, and 80.33 ± 2.08 µg 100 g−1 DW, respectively. Yaşa (2016) had reported thiamine (B1), riboflavin (B2), niacin (B3), and pyridoxine (B6) content of jujube cultivated in Denizli, a province of Turkey, to be 0.018, 0.036, 0.82, and 0.076 mg 100 g−1, respectively. Results of the current study were in good agreement with those reported by Yaşa (2016). In addition, Li et al. (2007) have reported thiamine and riboflavin content of five Chinese jujube cultivars in the range of 0.04–0.09 mg 100 g−1 and 0.05–0.09 mg 100 g−1, respectively. These values were also similar to those reported by Gao et al. (2013), Pareek (2013), Yaşa (2016), and Li et al. (2007). Drying temperature has a great impact on B complex vitamins. As seen from Table 5, B complex vitamins of whole jujube fruits remarkably decreased at the end of the drying process (p < 0.05). The highest losses in thiamin (B1), riboflavin (B2), and niacin (B3) content occurred at 70°C as 39.74, 62.37, and 28.57% (p < 0.05), respectively, whereas the drying process at 50°C resulted in the lowest losses with the percentages of 27.07, 51.88, and 13.77 (p < 0.05), respectively. On the other hand, pyridoxine was the highest affected compound among B complex vitamins. No pyridoxine content of whole jujube fruits was determined at the end of the drying process (p < 0.05). It could be under the limit of detection. It was similarly notified by Yaşa (2016) that thiamin (B1), riboflavin (B2), and niacin (B3) content of whole jujube fruits decreased during drying. In addition, no pyridoxine content was reported at the end of the drying process by Yaşa (2016).

The effect of the drying process on TPC and AC of whole jujube fruits are presented in Table 6. As seen in Table 6, TPC and AC of fresh whole jujube fruits were found to be 1911.4 ± 47.32 mg GAE 100 g−1 DW and 0.214 ± 0.001 mmol TE g−1 DW. TPC and AC of whole jujube fruits significantly decreased with hot air drying (p < 0.05). The loss percentages of TPC in whole jujube fruits dried at 50, 60, and 70°C were calculated as 78.10, 76.26, and 74.68, respectively. In the current study, the increment in the drying temperature has no significant effect on the reduction in TPC (p > 0.05). Likewise, Vega-Galvez et al. (2009) noted no significant change in the TPC of red pepper during hot air drying. Similarly, TPC of sour cherries was also notified to have decreased during drying; however, no there were no significant differences between the drying temperatures (Horuz et al., 2017). On the contrary, Yaşa (2016) has reported that the TPC of whole jujube fruits was reduced by hot air drying and the loss of TPC increased with an increment in the drying temperature. Likewise, Elmas et al. (2019) notified more decrement in the TPC of sliced jujube fruits based on an increment in the drying temperature. Furthermore, AC of whole jujube fruits dried at 50, 60, and 70°C decreased with the percentage of 61.22, 60.75, and 59.35, respectively. No significant difference was found between the drying temperatures (p > 0.05). Long drying times might decrease AC of foods (Garau et al., 2007). On the contrary, Wojdylo et al. (2016) have notified that AC of three different jujube cultivars decreased with hot air drying, and an increment in the drying temperature increased the reduction of AC in these cultivars. A decrement in AC of hot-air-dried red pepper was reported by Vega-Galvez et al. (2009). However, no significant differences between drying temperatures were notified in the same study.

Table 6. The effect of the drying process on TPC and AC of whole jujube fruits.

TPC %Loss AC %Loss
Fresh 1911.40 ± 47.32a 0 0.214 ± 0.001a 0
50°C 418.59 ± 12.18b 78.10 0.083 ± 0.001b 61.22
60°C 453.71 ± 4.61b 76.26 0.084 ± 0.001b 60.75
70°C 484.03 ± 6.21b 74.68 0.087 ± 0.001b 59.35

*TPC was expressed as mg GAE 100 g−1 DW; AC was expressed as mmol TE g−1 DW. The different letters in the same column are significantly different (p < 0.05).

TPC, total phenolic content; AC, antioxidant capacity.

Kinetic parameters of vitamin C

To the best of our knowledge, degradation of vitamin C in whole dried jujube fruits was investigated for the first time. Thermal degradation of vitamin C in whole dried jujube fruits is shown in Figure 3. As seen in Figure 3, thermal degradation of vitamin C in whole dried jujube fruits followed the first-order kinetic model. Thermal degradation of vitamin C is reported to frequently fit to the first-order reaction model in different dried foods by Demiray et al. (2013), Kadakal et al. (2017), Orikasa et al. (2014), and Kurozawa et al. (2014). Kinetic parameters of vitamin C are listed in Table 7. The rate constant of vitamin C thermal degradation increased depending on an increment in the drying temperature. Accordingly, the values of t1/2 and D decreased. Likewise, Demiray et al. (2013) and Akdaş and Başlar (2015) have reported an increment in the degradation rate constant of vitamin C in tomato and mandarin, respectively, as drying temperatures were raised. In addition, the values of t1/2 decreased with an increase in the degradation rate constant. Kadakal et al. (2017) have notified an increment in degradation rate constant and decrement in the values of t1/2 and D of vitamin C thermal degradation in rosehip nectar during thermal treatment. Also, in another study, vitamin C degradation in hot-air-dried kiwi fruits showed an increment with an increase in the drying temperature (Orikasa et al., 2014). Kurozawa et al. (2014) have indicated a rate constant of vitamin C thermal degradation in papaya with an increment in temperature during the drying process. The result of the current study is in good agreement with other reports.

Figure 3. First-order plots of (A) vitamin C, (B) thiamine, (C) riboflavin, (D) niacin and (E) pyridoxine during drying of the whole jujube fruits

Table 7. First-order kinetic parameters of water-soluble vitamins, TPC, and AC of whole dried jujube fruits.

Compound Temperature k (h−1) t1/2 (h) D (h) R2 Ea (kcal mol−1) Ea (kJ mol−1) Q10 (50–60°C) Q10 (60–70°C)
Vitamin C 50°C 0.0170 40.76 135.47 0.9865
60°C 0.0445 15.57 51.75 0.9894 19.33 80.87 2.62 2.21
70°C 0.0983 7.05 23.43 0.9890
Thiamine 50°C 0.0066 105.00 348.94 0.9798
60°C 0.0125 55.44 184.24 0.9776 16.23 67.90 1.89 2.31
70°C 0.0289 23.98 79.69 0.9869
Riboflavin 50°C 0.0162 42.78 142.16 0.9810
60°C 0.0275 25.20 83.75 0.9817 13.23 55.37 1.70 1.96
70°C 0.054 12.83 42.65 0.9918
Niacin 50°C 0.0034 203.82 677.35 0.9733
60°C 0.0084 82.50 274.17 0.9856 18.83 78.78 2.47 2.24
70°C 0.0188 36.86 122.50 0.9922
Pyridoxine 50°C 0.0505 13.72 45.60 0.9898
60°C 0.0886 7.82 25.99 0.9886 15.63 65.41 1.75 1.72
70°C 0.1524 4.55 15.11 0.9999
TPC 50°C 0.0332 20.87 69.37 0.9815
60°C 0.0503 13.77 45.78 0.9648 9.33 39.02 1.51 1.54
70°C 0.0775 8.94 29.72 0.9796
AC 50°C 0.0194 35.72 118.71 0.9619
60°C 0.0314 22.07 73.34 0.9683 10.68 44.68 1.62 1.63
70°C 0.0512 13.54 44.98 0.9684

TPC, total phenolic content; AC, antioxidant capacity.

Activation energy reflects the reaction’s temperature sensitivity. Higher Ea indicates higher sensitivity to temperature changes. Besides, higher Ea means higher stability to thermal degradation (Bell, 2020; Kadakal et al., 2017). Arrhenius equation, which was used for the calculation of the Ea of vitamin C thermal degradation, is given in Figure 4. The Ea of vitamin C thermal degradation in whole dried jujube fruits was calculated as 80.87 kJ mol−1. This value was higher than 46.248 and 46.99 kJ mol−1 by Akdaş and Başlar (2015) and Demiray et al. (2013), respectively, meaning that vitamin C was more stable to thermal degradation, and its thermal degradation reaction was more sensitive to temperature changes in whole dried jujube fruits. Q10 value, a criterion reflecting the effect of raising temperature by 10°C on the rate of reaction, is also used as an indicator of the reaction’s temperature sensitivity. Higher Q10 values denote greater temperature sensitivity (Bell, 2020; Kadakal et al., 2017). In the current study, the value of Q10 from 50 to 60°C was found to be slightly higher than from 60 to 70°C. This means that the thermal degradation of vitamin C was more affected by an increment of temperature from 50 to 60°C than from 60°C to 70°C. Kadakal et al. (2017) and Demiray et al. (2013) have similarly reported that the Q10 value of vitamin C thermal degradation decreased with an increment in the process temperature.

Figure 4. Arrhenius plots of water-soluble vitamins of whole dried jujube fruits.

Kinetic parameters of B complex vitamins

To the best of our knowledge, no data on B complex vitamin degradations in jujube fruits during hot air-drying process have been published as yet. Thermal degradation kinetics of B complex vitamins in jujube fruits during hot air-drying process was investigated for the first time in this study. Thermal degradation of B complex vitamins is given in Figure 3. However, no pyridoxine content was determined at the end of the process. Therefore, kinetic modeling of pyridoxine thermal degradation was conducted until the last point where pyridoxine was detected. Thermal degradation of B complex vitamins followed the first-order kinetic model. Likewise, Kadakal et al. (2017) have reported that thermal degradation of thiamine and riboflavin in rosehip nectar fitted to the first-order kinetic model during thermal treatment for 30 min. Thermal degradation of niacin in cooked potato for 60 min in the temperature range of 50–120°C was notified to follow the first-order kinetic model by Nisha et al. (2009). Nisha et al. (2005) and Rekha et al. (2004) have reported a first-order kinetic of thermal degradation of riboflavin and thiamin in cooked spinach and red gram splits at a temperature range of 50–120°C for 60 min, respectively. Kinetic parameters of B complex vitamins’ thermal degradation are presented in Table 7. In the current study, degradation rate constants of B complex vitamins increased with the increment in drying temperature. Accordingly, degradation rapidly occurred at higher temperatures. In consequence, the values of t1/2 and D of B complex vitamins decreased. In the current study, degradation rate constants of thiamine, riboflavin, niacin, and pyridoxine ranged from 0.0066 to 0.0289 h−1, 0.0162 to 0.054 h−1, 0.0034 to 0.0188 h−1, and 0.0505 to 0.1524 h−1, respectively. Kadakal et al. (2017), Nisha et al. (2009), Nisha et al. (2005), and Rekha et al. (2004) have also reported an increment in degradation rate constant of B complex vitamins in different foods with an increase in process temperatures.

Figure 4 shows Arrhenius equation of B complex vitamins’ thermal degradation. The Eas of thiamine, riboflavin, niacin, and pyridoxine were found to be 67.90, 55.37, 78.78, and 65.41 kJ mol−1, respectively. Sensitivity of a reaction to temperature change can be explained with the Ea as stated before. Accordingly, niacin was more sensitive to temperature changes but shows the highest stability due to the highest Ea value in comparison to other B complex vitamins in jujube fruits. Nisha et al. (2005) have found the Ea of riboflavin thermal degradation in cooked spinach to be 21.72 kJ mol−1. Kadakal et al. (2017) have also reported that the Eas of thiamine and riboflavin thermal degradation in rosehip were 36.38 and 37.15 kJ mol−1, respectively. Nisha et al. (2009) have notified the Ea of niacin in cooked potato cubes to be 16.70 kJ mol−1. The Eas of thiamine, riboflavin, niacin, and pyridoxine of jujube fruits were found to be higher than those reports. This means that thiamine, riboflavin, niacin, and pyridoxine in jujube fruits were more sensitive to temperature changes; however, they were more stable to thermal degradation when compared to those reports. On the other hand, niacin was the highest affected compound by 10°C temperature increment from 50 to 60°C due to the highest Q10 value when compared to Q10 values.

Kinetic parameters of TPC and AC

The current study presented the first data on TPC thermal degradation in whole dried jujube fruits. TPC thermal degradation is shown in Figure 5. It was found that TPC thermal degradation followed the first-order kinetic model. Akdaş and Başlar (2015) have similarly reported a first-order reaction of TPC of mandarin during the oven drying process. Sarpong et al. (2018) have notified that TPC thermal degradation in banana samples during convective drying was described using the first-order kinetic model. The result of the current study was in good agreement with reports by Akdaş and Başlar (2015) and Sarpong et al. (2018). Kinetic parameters of TPC thermal degradation are listed in Table 7. Rate constant of TPC thermal degradation showed an increment as drying temperatures increased. It was obvious that this increment caused a decrement in the values of t1/2 and D. Rate constant of TPC thermal degradation ranged from 0.0332 to 0.0775 h−1. Akdaş and Başlar (2015), Sarpong et al. (2018), and Kadakal and Duman (2018) have reported an increment in TPC thermal degradation in different foods with an increase in temperature. TPC thermal degradation was slightly affected by 10°C temperature increment. Ea was calculated using the Arrhenius equation as given in Figure 6. The Ea of TPC thermal degradation was calculated as 39.02 kJ mol−1. Sarpong et al. (2018) have reported the Ea of convective dried banana at 60, 70, and 80°C as 14.29 kJ mol−1. Likewise, The Eas of TPC thermal degradation in Starking Delicious, Golden Delicious, and Granny Smith apple cultivars dried at 65, 70, and 75°C were notified as 27.52, 29.84, and 32.48 kJ mol−1, respectively (Ertekin Filiz and Seydim, 2018). Akdaş and Başlar (2015) have reported the Ea as 55.037 kJ mol−1 for oven-dried mandarin at 55, 65, and 75°C. The result of the current study was higher than those reported by Sarpong et al. (2018) and Ertekin Filiz and Seydim (2018); however, it was lower than those reported by Akdaş and Başlar (2015). On the other hand, thermal degradation of TPC was not affected by the 10°C temperature increment.

Figure 5. First-order kinetics of total phenolic content (A) and antioxidant capacity (B) of whole dried jujube fruits.

Figure 6. Arrhenius plots of total phenolic content and antioxidant capacity of whole dried jujube fruits.

AC thermal degradation could be described using the first-order kinetic model (Başlar et al., 2014; Oancea et al., 2017; Sarpong et al., 2018). In contrast to this, Orikasa et al. (2014) and Ertekin Filiz and Seydim (2018) have reported that zero-order kinetic model may also be used to describe AC thermal degradation. To the best of our knowledge, AC thermal degradation in whole dried jujube fruits was investigated for the first time. AC thermal degradation is given in Figure 5. In the current study, AC thermal degradation followed the first-order kinetic model. Oancea et al. (2017), Başlar et al. (2014), and Sarpong et al. (2018) have reported the first-order kinetic of AC in sour cherry extracts, oven-dried pomegranate, and convective dried banana during the thermal process. Kinetic parameters of AC thermal degradation are given in Table 7. Rate constants of AC thermal degradation increased with an increment in the drying temperature as expected. Rate constant of AC thermal degradation was reported to increase with process temperature by Oancea et al. (2017), Başlar et al. (2014), and Sarpong et al. (2018). Accordingly, the values of t1/2 and D were reduced. On the other hand, a 10°C temperature increment had no significant effect on AC thermal degradation. Figure 6 presents the Arrhenius equation of AC thermal degradation that was used for the calculation of the Ea. The Ea of AC thermal degradation was 44.68 kJ mol−1.

Conclusions

In the current study, drying characteristics and some quality parameters of the health-promising fruit, jujube (Zizyphus jujuba Mill.), in Turkey were investigated under different drying conditions.

  1. DR of jujube was highly influenced by the drying temperature. The longest drying time was found to be 48 h at 50°C, and the shortest was 18 h at 70°C.

  2. The best predicting models of experimental MR were determined as Parabolic model for 60°C and Page model for 50 and 70°C.

  3. Effective moisture diffusivity showed an increment with an increase in the drying temperature. The most effective moisture diffusivity was obtained at 70°C.

  4. When compared to dried jujube fruits regardless of the drying temperature, water-soluble vitamins, TPC, and AC of fresh jujube fruits were determined to be higher. While the highest loss of water-soluble vitamin occurred at 70°C because of a more rapid enzymatic and nonenzymatic degradation, TPC and AC were not significantly affected by the drying temperature.

  5. Vitamin C and B complexes, TPC, and AC thermal degradation were fitted to the first-order kinetic model.

  6. Vitamin C and niacin were very susceptible to temperature change. On the contrary, TPC and AC were the lowest sensitive compounds.

  7. Vitamin C and B complexes were strongly temperature dependent, while TPC and AC were not significantly affected by increment in temperature.

In further studies, the effect of different drying methods, such as microwave, vacuum, combinations of vacuum–microwave, microwave–hot air, should be investigated for dehydration of jujube fruits. Thus, the most suitable conditions and methods may be optimized by observing the loss of nutritional compounds during the process.

Acknowledgements

This study was supported by Pamukkale University (grant number: 2018FEBE024).

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