PAPER

Drying characteristics and degradation kinetics in some parameters of goji berry (Lycium Barbarum L.) fruit during hot air drying

Heysem Suat Batu1*, Çetin Kadakal2

1Department of Food Engineering, Institute of Science, Pamukkale University, Denizli, Turkey;

2Department of Food Engineering, Faculty of Engineering, Pamukkale University, Denizli, Turkey

Abstract

Drying kinetics, color properties, water-soluble vitamins, antioxidant capacity, total phenolic content, and thermal degradation kinetics of bioactive compounds in goji berries were investigated. Drying experiments were conducted at 50°C, 60°C, and 70°C. Page model was determined as the best model to predict experimental moisture ratio for all temperatures. Increment in drying temperature increased effective moisture diffusivity and drying rate values. Vitamins C and B6, antioxidant activity and total phenolic content were significantly reduced by drying. Thermal degradation of vitamins C and B6, antioxidant capacity and total phenolic content were found to fit the first-order kinetic model.

Key words: antioxidant capacity, degradation kinetics, drying kinetics, goji berry, total phenolic content, water-soluble vitamins

*Corresponding Author: H.S. Batu, Department of Food Engineering, Institute of Science, Pamukkale University, Denizli, Turkey. Tel: +90 5413181989. Email: h.s.batu@gmail.com

Received: 24 August 2020; Accepted: 07 October 2020; Published: 01 February 2021

DOI: 10.15586/ijfs.v33i1.1949

© 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

Goji berry (Lycium barbarum L.), which belongs to Solanaceous family, is grown in China, Tibet, and some regions of Asia. Goji berry is primarily found in East Asia and grown particularly in Japan, South Korea, and South China (Gao et al., 2017). Lycium barbarum L. is a deciduous woody shrub, mostly thorny, that grows 1–4 m in height (Griffiths and Huxley, 1992). Fruits are two-chambered, mostly orange-red in color, juicy, and fleshy (Chen et al., 2018). Ripe goji berries are 3–10 mm in diameter, 6–20-mm long, and have oblong or ellipsoid shape (Gao et al., 2017).

Goji berry contains a high amount of anthocyanin (Cui et al., 2011), carotenoids (lycopene, zeaxanthin dipalmitate, beta-carotene, zeaxanthin, and lutein), vitamins (tocopherol, glucopyranosyl ascorbic acid, and ascorbic acid), betaine, fatty acids, and peptidoglycans (Islam et al., 2017). Fatty acids commonly found in goji berry are linoleic, oleic, and palmitic acids (Cossignani et al., 2018). The fruit also includes organic acids such as malic, citric, shikimic, and fumaric acids (Mikulic-Petkovsek et al., 2012), monosaccharides, which are mannose, rhamnose, galactose, xylose, arabinose, and glucose, 18 amino acids, and galacturonic acid (Amagase and Farnsworth, 2011). Owing to bioactive substances and healthy functions, goji berries are popular in the western world (Bertoldi et al., 2019).

In traditional Chinese herbal medicine, goji berry has been used as a supplement for more than 2000 years (Burke et al., 2005). It is used to protect the liver, kidneys, and eyes, strengthen the eyesight (Shan et al., 2011), and reduce serum lipids and blood glucose levels. Also, Goji berry has other health-promoting effects such as anti-radiation, immune-enhancing, anti-fatigue, and anti-aging effects, stimulating hematopoiesis, and treating male infertility (Luo et al., 2004; Tian et al., 2013).

Drying is a protection technique that is widely applied to fresh products. Dehydration prolongs stability of fruits and vegetables by decreasing water content and minimizing physicochemical changes and microbial growth (Tepe and Tepe, 2020). Besides, the drying process protects valuable foods under effective conditions, prolongs shelf life, and reduces storage, transportation, and packaging costs because of decrement in the volume and weight of food products (Önal et al., 2019). One of the most important steps in the food processing industries is the dehydration process.

Sun drying is a preferred method because it is economical and does not require investment, but it is a disadvantageous method due to microbial reliability and loss in quality such as color and aroma (Göztok and Içier, 2017). Convective drying is one of the most used drying methods to protect agricultural products compared to other drying methods because it is simple and low costing. On the contrary, this method causes changes in sensory properties and nutritional values (Orikasa et al., 2014). The convective drying can be used as an alternative method instead of conventional (sun) drying. It has more advantages than conventional drying in terms of preventing microbial contamination, component protection, and involving lower drying time (Lewicki, 2006). Adiletta et al. (2015) and Fratianni et al., (2018) studied goji berry drying, but no data are available in literature about drying characteristics, degradation kinetics of water-soluble vitamins, antioxidant capacity (AC), and total phenolic content (TPC) of dried goji berry fruit.

Absence of data on drying characteristics and degradation of some bioactive compounds can be regarded as a gap in the drying process of goji berry fruit. In this context, determining drying characteristics and degradation of some bioactive compounds can be useful for the designing of drying process. Thus, the aims of this study are to: determine the drying characteristics of goji berry fruit at 50°C, 60°C, and 70°C, examine the influence of drying process on contents of vitamins C and B6, antioxidant capacity and total phenolic content of goji berries, and calculate the degradation kinetics of these bioactive compounds.

Materials and Methods

Materials and sample preparation

Goji berry fruits, of NQ1 variety, were obtained from Redlife in the Çivril district of Denizli province of Turkey. Geographical location of Denizli is between 28°30’–29°30’ east meridians and 37°12’–38°12’ north parallels and is located in the Aegean region of Turkey. The fruits were carefully collected in July 2019 from 10 randomly selected plants. Fresh fruits were washed to remove any foreign material and kept at –18°C before analysis.

Method

Drying process of samples

Goji berry samples were dried in a tray drying cabinet (Yücebaş Makine Tic. Ltd. Şti., İzmir, Turkey). Dryer comprised an electronic proportional controller (EUC442 model, ENDA, Turkey), an electric heater, and a centrifugal fan to provide airflow. The dryer’s internal size was 70 cm × 55 cm × 100 cm, the range of workable temperature was 40–120°C, and the range of workable relative humidity was 20%–95%. Three different drying air temperatures were used in the experiments: 50°C, 60°C, and 70°C. The cabinet was heated for 1 h before the start of drying process to reach a constant temperature; and 200 g of samples were uniformly placed on the drying tray. The drying process was performed up to the targeted dry matter content at a relative humidity of 20% and air velocity of 2 m/s. The drying experiments were performed in triplicate and weighed at certain time intervals with a 0.001 g precision digital scale.

Drying characteristics of goji berry fruit

Empirical models are more useful because theoretical drying models are complicated and the former models offer a direct relationship between drying time and moisture content (Moradi et al., 2020). Thin-layer drying models have great significance in designing the best drying conditions.

Moisture ratio (MR) must be calculated by Mi, Mt, and Me values to choose the best model.

MR=MtMeMiMe   (Eq. 1)

where

Mi: initial moisture content of samples (g water/g dry matter);

Mt: moisture content at any time (g water/g dry matter);

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

However, if the equilibrium moisture content (Me) is very low than Mt and Mi, it can be neglected and Equation 2 is used (Yousefi et al., 2013):

MR=MtMi   (Eq. 2)

Drying rate (DR) was determined using Equation 3:

DR=Mt+ΔtMtΔt   (Eq. 3)

where

Mt+Δt: moisture content at time difference;

Δt: time difference between two measuring points.

The relation between predicted and experimental data of goji berry fruits dried at different drying temperatures is explained with root mean square error (RMSE), reduced chi-square (χ2), and determination coefficient (R2). RMSE is a statistical parameter that expresses deviation between experimental and predicted values. The best equation predicting experimental data is determined accordingly with lower RMSE and χ2 and higher R2 values. The chi-square (Equation 4) and RMSE (Equation 5) values were calculated as follows:

x2=i=0N(MRpre,iMRexp,i)2Nn  (Eq. 4)
RMSE=1Ni=0NMRpre,iMRexp,i21/2   (Eq. 5)

where

MRpre,i: predicted MR;

MRexp,i: experimental MR;

N: number of observation data;

n: constants of thin layer drying models.

MATLAB software was used to calculate thin-layer modeling and for statistical analyses.

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

Fick’s diffusion equation described the drying characteristics of biomaterials. Crank (1975) suggested a solution to this equation to be used for spherical products. Equation 6 is recommended for spherical products, provided that there is no shrinkage and constant effective diffusivity (Doymaz, 2006):

MR=6π2n=11n2expn2π2Deffr2   (Eq. 6)

where

Deff: effective moisture diffusivity (m2/s)’

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

Equation 6 can be reduced (Saravacos and Raouzeos, 1986) and a new equation is provided below:

ln(MR)=ln6π2π2Deffr2t   (Eq. 7)

Equation 8 shows a straight line with a slope provided in the plot:

Slope=π2r2Deff   (Eq. 8)

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

Deff=D0expEaRT  (Eq. 9)

where

R: universal gas constant [8.314 J/mol (K) or 1.987 cal/mol (K)];

T: absolute temperature (K);

Ea: activation energy (kJ/mol or kcal/mol);

D0: pre-exponential constant (m2/s).

After regulation of natural logarithm in Equation 8, Equation 9 can be written as follows:

ln Deff=lnD0EaRT  (Eq. 10)

Natural logarithm of effective moisture diffusivity versus 1/T gives a straight line with a slope that represents activation energy.

Analysis of water-soluble vitamins

An extraction method proposed by Donmez (2015) was used for analysis of water-soluble vitamins. To determine water-soluble vitamins, a sample of 5 g of goji berries was taken, and after homogenization with distilled water (1:9, w:v), the homogenate was centrifuged at 4500 rpm for 10 min (Core NF 800R). The supernatant obtained from centrifugation was filtered using a 0.45-μm filter to be injected into a high-pressure liquid chromatography (HPLC) column.

Using a micro syringe, 20 μL of filtrate was injected into the HPLC column. Mobile phase consisted of 0.1 M HPLC grade KH2PO4 at pH 7.

The HPLC device (Shimadzu), in which analysis of water-soluble vitamins was performed, consisted of column oven (Shimadzu CTO-20A, Japan), pump (Shimadzu LC-20AD, Japan), degasser (Shimadzu DGU-20A3, Japan), photodiode array (PDA) detector, and HPLC software in a computer. The column used in the analysis was ACE C18 column (7.8 × 300 mm), column temperature was 25°C, and the flow rate of mobile phase was 0.8 mL/min (isocratic). Wavelengths used in analysis were 254 nm, 261 nm, 324 nm, and 234 nm for ascorbic acid, niacin, pyridoxine, and thiamine, respectively. Analysis was performed in triplicate.

A calibration curve of different concentrations of stock solutions (5, 10, 25, 50, 75, and 100 ppm) with high R2 (0.9999) was obtained. The content of water-soluble vitamins was calculated by the equation obtained from the calibration curve.

Analysis of antioxidant capacity and total phenolic content

The AC and TPC analysis was carried out using the methanolic extraction method proposed by Otağ (2015). A sample of 5 g of goji berries was mixed with 45 mL of 90% methanol and homogenized using a laboratory blender. The homogenate was centrifuged at a speed of 4500 rpm for 10 min. After centrifugation, supernatants were collected and filtered using a filter paper.

The TPC analysis was performed according to Singleton and Rossi (1965) with modifications. In this analysis, 1500 µL Folin–Ciocalteu solution (10% v/v) was added into 300 µL of extract and the mixture was kept in a dark place for 3 min. Then 1200 µL aqueous 7.5% Na2CO3 was added into the mixture. The final mixture was incubated for 2 h at room temperature in a dark place. After incubation, the absorbance measurement of the samples was carried out at a wavelength of 760 nm using spectrophotometer (T80, PG Ins., UK.). Analysis was carried out in triplicate, and TPC was expressed as mg/100 g gallic acid equivalent (GAE) dry weight (DW).

The AC analysis was carried out using the method suggested by Thaipong et al. (2006) with modifications. Here, 150 μL of extracts and 2850 μL of DPPH methanolic solution (absorbance value: 1.1 at a wavelength of 515 nm) were mixed. Absorbance of samples was measured at a wavelength of 515 nm using spectrometer after 60 min incubation in a dark place at room temperature. Each sample was analyzed in triplicate and AC was expressed as mmol Trolox equivalent (TE)/g DW.

Color measurement

Reflectance color value of goji berry skin was measured by using Hunter Lab Color Miniscan XE (45/0-L, USA). The samples were placed on a white background and the measurement was performed by covering with a transparent glass. The highest color difference (ΔE) was calculated using Equation 11 (Horuz et al., 2017):

ΔE=L0L2+a0a2+b0b2  (Eq. 11)

Calculation of kinetic parameters

The following equation (Equation 12) was used as a general equation to describe the reaction rate of the compounds that are degraded or formed by Labuza (1984):

dcdt=kCm  (Eq. 12)

For the zero-order kinetic model, equation can be written as follows:

C=C0Kt  (Eq. 13)

If Equation 13 is integrated and m = 1, then Equation 14 is written as follows:

lnC=lnC0kt  (Eq. 14)

where

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

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

k: rate constant (1/h);

t: time.

Temperature dependence of vitamins C and B complex, TPC, and AC can be calculated using Equation 15 (Labuza and Riboh, 1982):

k=k0xeEaRT   (Eq. 15)

When Equation 15 is regulated, Equation 16 is written as follows:

lnk=EaRx1τ+lnk0   (Eq. 16)

where

k0: frequency factor (1/h);

R: universal gas constant [8.314 × 10-3 kJ/mol (K) and 1.987 × 10-3 kcal/mol (K)];

T: absolute temperature (K);

Ea: activation energy (kcal/mol or kJ/mol).

Quotient indicator (Q10) expresses temperature-dependence of reaction rate and is calculated using Equation 17 (Labuza and Schimidl, 1985):

Q10=k2k110T2T1   (Eq. 17)

Half-life time, time required for half of concentration, for each temperature is calculated using Equation 18 for first-order kinetics (Labuza, 1984):

t1/2=ln0.5x1k=0.693x1k   (Eq. 18)

Time taken by the compound, or quality criterion, to lose 90% of its quality is expressed as D and is calculated for first-order kinetics as follows (Equation 19):

D=2.303x1k   (Eq. 19)

Statistical analysis

SPSS 22.0 software (IBM Corporation, Armonk, NY) was used for statistical analysis and expressed as mean ± standard deviation (SD). Analysis of variance (ANOVA) was used to evaluate differences between treatments with the significance level P = 0.05. Differences between groups were determined using the Duncan test.

Results and Discussion

Drying characteristics of goji berry fruits during hot air drying

The drying rate and moisture ratio values of goji berries during hot air drying are presented in Figure 1. Drying time and drying rate of goji berry fruits were significantly affected by drying temperature, and it is clearly seen that drying rate increases with the increment in drying temperature.

Figure 1. Moisture ratio and drying rate of goji berries during hot air drying.

Drying time decreased depending on the increment in temperature, so drying time was found to be 24 h at 50°C, 19 h at 60°C, and 9 h at 70°C. Adiletta et al. (2015) determined drying time as 21 h at 60°C for hot air-drying treatment of goji berries. Fratianni et al. (2018) dried goji berry fruits in a convective dryer and the drying process was finished in 45 h at 50°C, 21 h at 60°C, and 12 h at 70°C, and the velocity of air was 2.1 m/s. It could be that the increment in drying rate with increase in temperature might be due to the increase in heat transfer coefficient. The results of this study were similar to the results of the studies examined in literature.

Mathematical models used in modeling the drying process, constants, and the statistical data of mathematical models are listed in Table 1. Demiray et al. (2017) reported that the lower RMSE and χ2 and the higher R2 values are required for determining the best model. As seen in Table 1, the Page (1949) model is the best model predicting experimental moisture ratio of goji berry fruits for all drying temperatures (50°C, 60°C, and 70°C), with the lowest RMSE and χ2 and the highest R2 values.

Table 1. Thin-layer mathematical models, model constants, and statistical parameters of thin-layer drying curves.

Model Names and References Model Temperature Model Constants χ2 RMSE R2
Lewis / Lewis (1921) exp(-kt) 50°C k = 0.1186     0.001344007 0.03592 0.9793
60°C k = 0.1737     0.001277711 0.03484 0.9819
70°C k = 0.3450     0.000251669 0.01505 0.9977
Page / Page (1949) exp(-ktn) 50°C k = 0.1812 n = 0.8161   0.000264861 0.01561 0.9962
60°C k = 0.2508 n = 0.8107   0.000165378 0.01220 0.9979
70°C k = 0.3574 n = 0.9716   0.000290322 0.01524 0.9979
Henderson and Pabis / Henderson and Pabis (1961) aexp(-kt) 50°C k = 0.1075 a = 0.9150   0.000576501 0.02303 0.9918
60°C k = 0.1607 a = 0.9316   0.000866761 0.02793 0.9890
70°C k = 0.3438 a = 0.9964   0.000316013 0.01590 0.9977
Logaritmic / Doymaz (2011) aexp(-kt) + c 50°C k = 0.1341 a = 0.8742 c = 0.07044 0.000666050 0.02421 0.9910
60°C k = 0.2094 a = 0.8951 c = 0.07613 0.000296302 0.01587 0.9964
70°C k = 0.4267 a = 0.9425 c = 0.07490 0.000811922 0.02384 0.9948
Wang and Singh / Wang and Singh (1978) 1 + at + bt2 50°C a = -0.09269 b = 0.002374   0.003999604 0.06066 0.9433
60°C a = -0.13020 b = 0.004536   0.004329174 0.06242 0.9450
70°C a = -0.26180 b = 0.018240   0.001641672 0.03624 0.9880
Parabolic / Bi et al. (2015) a + bt + ct2 50°C a = 0.8701 b = -0.07149 c = 0.001653 0.001539746 0.03681 0.9800
60°C a = 0.8800 b = -0.10560 c = 0.003485 0.002166198 0.04291 0.9754
70°C a = 0.9556 b = -0.24320 c = 0.016610 0.001492261 0.03232 0.9917

RMSE, root mean square error.

Effective moisture diffusivity and activation energy of goji berry fruits during hot air drying

The effective moisture diffusivity (Deff) and activation energy (Ea) values of goji berry fruits are presented in Table 2, and the Deff values were calculated in the range of 2.98 × 10-8–1.04 × 10-8 m2/s. Effective moisture diffusivity is a useful indicator of dehydration effectiveness (Chen et al., 2016). When compared with other drying temperatures, the highest Deff value was determined in the drying process conducted at 70°C. Increase in the Deff value means the moisture content in goji berry samples is evaporated more easily. As understood from Equation 9 mentioned above, it is a known fact that the drying temperature is an important factor affecting the Deff value.

Table 2. Effective moisture diffusivity and activation energy of goji berry fruit.

Temperature Deff (m2/s) Ea (kJ/mol) Ea (kcal/mol)
50°C 1.04 × 10-8    
60°C 1.31 × 10-8 48.37 11.56
70°C 2.98 × 10-8    

No mention of Deff value during the drying of goji berry fruits with hot air was found in literature. Senadeera et al. (2014) found Deff values in the range of 1.32 × 10-6–1.34 × 10-6 m2/h because of the drying process executed on different types of grapes at 50°C, 0.5 m/s air velocity, and 20% moisture content. Chen et al. (2016) carried out hot air drying of wine grapes, grown in Canada, between 25°C and 80°C and determined the Deff value at 25°C and 80°C as 0.05 × 10-10 m2/s and 0.49 × 10-10 m2/s, respectively, at MR = 0.2. They observed that the Deff values increased 10 times with increase in temperature from 25°C to 80°C. Dong et al. (2013) studied the drying process of grapes at 30°C, 35°C, 40°C, and 45°C and found that the Deff value was higher at the highest temperature. In other words, the Deff value increases with increase in drying temperature, and the data examined in literature support our study.

The Arrhenius-type relation between Deff and 1/T is presented in Figure 2. The Ea values of goji berry fruits were found to be 48.37 kJ/mol and 11.56 kcal/mol. In literature, no activation energy data for hot air drying of goji berry fruits were found. When compared with similar berry fruits dried with hot air, Vega-Galvez et al. (2009) found Ea = 48.34 kJ/mol in blueberries. In another study done on hot air drying, Abdulla (2012) found Ea = 51.31 kJ/mol for golden fruits. López et al. (2010) and Shi et al. (2008) reported the Ea values of blueberry as 57.85 kJ/mol and 61.2 kJ/mol, respectively. Although the values found in some studies are similar to the values found in our study, others were higher. Differences between the results of the current study and other studies, in which other fruits were used, may be due to different factors such as different fruit structures, temperature, airflow rate, and relative humidity.

Figure 2. The Arrhenius-type relation between effective moisture diffusivity and 1/T.

Effect of drying process on water-soluble vitamins, total phenolic content, and antioxidant capacity

Effects of drying on water-soluble vitamins of goji berries are provided in Table 3. Carr and Frei (1999) indicated that vitamin C easily scavenges nitrogen species and reactive oxygen and thereby may prevent oxidative damage to nontrivial biological macromolecules such as proteins, lipids, and DNA. It is extremely important to maintain vitamin C during the drying process or to carry out this process with minimal loss; however, vitamin C is significantly affected by the drying process. In this study, value of vitamin C in fresh goji berries was found to be 112.75 ± 2.23 mg/100 g DW. Donno et al. (2015) found the concentration of vitamin C in goji berries to be 42 mg/100 g FW (fresh weight). The United States Department of Agriculture (USDA) has found the amount of vitamin C in dried goji fruit to be 48.4 mg/100 g (Koçyiğit and Sanlier, 2017). When compared with literature, it could be said that goji berries grown in Turkey are rich in vitamin C. There are statistical losses in values of vitamin C at all drying temperatures (P < 0.05). Vitamin C values at 50°C, 60°C and 70°C were determined as 39.45 ± 2.21, 26.48 ± 1.16 and 21.87 ± 0.971 mg/100 g DW, respectively. Since vitamin C has low stability against heat treatments, it is established as a quality index in foods during the processing (DiScala and Crapiste, 2008). Besides kinetic parameters, vitamin C may be a significant quality parameter in goji berries’ drying process. López et al. (2010) reported that there were significant loss in vitamin C values of blueberries at all drying temperatures, and the highest loss was 92% at 80°C. In a detailed review on ascorbic acid, Santos and Silva (2008) stated that a significant loss in ascorbic acid was seen due to hot air drying of fruits and vegetables. They even stated that no vitamin C was left in some drying processes applied on tomatoes over 100°C. Other studies (Araya-Farias et al., 2011; Kadakal et al., 2017) have demonstrated that the hot air-drying process significantly reduces the amount of vitamin C.

Table 3. Effect of drying process on vitamins C and B6, total phenolic content, and antioxidant capacity of goji berries.

Vitamin C* Loss
percentage
(%)
Pyridoxine
(B6)*
Loss
percentage
(%)
TPC** Loss
percentage
(%)
AC** Loss
percentage
(%)
Fresh 112.75 ± 2.23a 0 2.19 ± 0.046a 0 1838.43 ± 37.47a 0 0.077 ± 0.002a 0
50°C 39.45 ± 2.21b 65.03 0.937 ± 0.055b 56.56 491.00 ± 7.96b 73.29 0.017 ± 0.001b 77.92
60°C 26.48 ± 1.16c 76.84 0.681 ± 0.061c 69.04 450.17 ± 8.26b 75.51 0.014 ± 0.001bc 81.82
70°C 21.87 ± 0.971d 80.5 0.492 ± 0.034c 77.48 404.45 ± 6.89c 78 0.011 ± 0.001c 85.71

*Vitamins C and B6 was expressed as mg/100 g DW.

**TPC was expressed as mg GAE/100 g DW, AC was expressed mmol TE/g DW.

***Different letters in the same column are significantly different values (P < 0.05).

TPC, total phenolic content; GAE, gallic acid equivalent; TE, trolox equivalent; DW, dry weight.

In our study, the amount of vitamin B complex was analyzed in fresh goji berry fruits and the kinetic data were obtained during the drying process. The amount of pyridoxine (B6) in fresh goji berries was determined as 2.19 ± 0.046 mg/100 g DW but thiamine, riboflavin, and niacin were not detected. Right after hot air-drying process at different temperatures, amount of pyridoxine was determined as 0.937 ± 0.055, 0.681 ± 0.061, and 0.49 ± 0.034 mg/100 g DW at 50°C, 60°C, and 70°C, respectively. The highest loss appears to be in the drying process at 70°C. Ryley and Kajda (1994) stated that loss in the values of water-soluble vitamins was observed with the effect of heat treatment in various foods. In consequence of drying at different temperatures, an important decrease in the amount of pyridoxine was observed in goji berries. Decrease in the amount of water-soluble vitamin B6 increases with the increment in drying temperature.

Effects of hot air drying on total phenolic content and antioxidant capacity of goji fruits are presented in Table 3. The TPC and AC values of fresh goji berries were found as 1838.43 ± 37.47 mg/100 g DW and 0.077 ± 0.002 mmol TE/g DW, respectively. Islam et al. (2017) determined the TPC value of red goji berry fruits as 217–448 mg GAE/100 g. Ban et al. (2015) determined the TPC value of fresh goji berries in the range of 449.92–450.48 mg GAE/kg FW. Zhang et al. (2016) determined the TPC values of goji berry fruits in the range of 5840–7340 mg GAE/100 g FW. Pedro et al. (2018) investigated TPC by extraction of goji berry fruits in different concentrations of methanol and found it in the range of 1052.53–1736.36 mg GAE/100 g. The TPC of goji berry fruits because of drying processes at 50°C, 60°C, and 70°C was determined to be 491.00, 450.17, and 404.45 mg GAE/100 g DW, respectively. Islam et al. (2017) and Zhang et al. (2016) determined the AC value of red goji berry fruits as 16.07–17.47 mg µmol TE/g and 77.41–85.46 µMTE/g FW, respectively. Pedro et al. (2018) found the AC values of goji berry in the range of 0.94–1.51 mmol TE/100 g. Mikulic-Petkovsek et al. (2014) stated that a significant difference in the content of fruits is seen when grown at different locations. The compositional difference seen in the same varieties of fruits and vegetables is influenced by numerous factors such as environmental conditions of the region where the product is grown, especially soil quality, cultivation technique and cultural measures, maturity level, transportation and storage, and so on (Gökkür and Çelik, 2016). The reason why our results are different from those found in literature may be due to the reasons explained above.

Color properties of goji berry fruit during hot air drying

Color properties of fresh and dried goji berries were presented in Table 4. When compared with initial L*, a*, and b* values of goji fruits, values were significantly decreased due to drying process (P < 0.05) and the lowest L*, a*, and b* values were obtained at 70°C. ΔE indicates differences between colors of samples (Horuz et al., 2017). The ΔE value of dried goji fruits depends on drying conditions and ranges from 10.87 to 13.91. The highest ΔE was obtained from the goji berry fruits dried at 70°C.

Table 4. Color properties of goji berry fruits.

  L* a* b* ΔE
Fresh 25.97 ± 0.12a 25.16 ± 0.13a 17.30 ± 0.05a  
50°C 23.11 ± 0.09b 16.45 ± 0.07b 11.03 ± 0.11b 10.87
60°C 22.79 ± 0.05c 14.41 ± 0.09c 10.67 ± 0.07c 13.01
70°C 21.99 ± 0.06d 14.67 ± 0.08d 9.62 ± 0.05d 13.91

*Different letters in the same column are significantly different values (P < 0.05).

Kinetic parameters of vitamins C and B6

To the best of our knowledge, vitamin C degradation in the hot air-drying process of goji berries was investigated for the first time in this study. Thermal degradation of vitamin C in goji berries is shown in Figure 3; its content in fully dried goji berries is found to fit the first-order kinetic model. It is stated by Gamboa-Santos et al. (2014), Hiwilepo-van Hal et al. (2012), Kadakal et al. (2017), and Wang et al. (2017) that the thermal degradation of vitamin C fits the first-order kinetic model in different dried foods.

Figure 3. First-order kinetics of (A) vitamin C, (B) pyridoxine, (C) total phenolic content (TPC), and (D) antioxidant capacity (AC) of goji berries.

Air-drying may have a negative effect on the physical properties of products and cause degradation of aromatic compounds and nutrients (Araya-Farias et al., 2011). In other words, losses are observed in the compounds found in all foods, especially vitamin C, by heat treatment. Dağhan et al. (2018) studied the hot air drying of Isot at different temperatures and found that there was significant loss in vitamin C. They found the highest loss at 75°C and stated that vitamin C is highly sensitive to changes in temperature. Marfil et al. (2008) performed tomato hot air drying at different temperatures and reported that the loss of vitamin C in tomatoes increased with increase in drying temperature. Kinetic parameters of vitamin C in goji berries are presented in Table 5. Vitamin C degradation rate constants of goji fruits at 50°C, 60°C, and 70°C were found to be 0.047, 0.075, and 0.182 1/h, respectively. It is clearly observed that the rate constant increased but t1/2 and D values of vitamin C decreased due to the increment in temperature. Similarly, Demiray et al. (2013) stated that the k value increased with the increment in drying temperature. They also stated that the t1/2 value decreased with the increase of drying temperature in drying of tomatoes. Kadakal et al. (2017) stated that the degradation rate constant of vitamin C was increased due to the thermal increase in rosehip nectar while the t1/2 and D values were decreased. Our results are compatible with literature. Also, the Q10 value from 60°C to 70°C was found to be higher than from 50°C to 60°C. With this data obtained in our study, it is understood that the thermal degradation of vitamin C is more sensitive to the increment of temperature from 60°C to 70°C. The Q10 value of vitamin C thermal degradation increased with the decrement in drying temperature (Demiray et al., 2013; Kadakal et al., 2017). Kadakal et al. (2017) stated that high activation energy of reaction indicates that the reaction sensitivity of temperature is very high. The Arrhenius equation of vitamin C thermal degradation is given in Figure 4.

Table 5. First-order kinetic parameters of vitamins C and B6, total phenolic content, and antioxidant capacity of dried goji berries.

Compound Temperature k t1/2 D R2 Ea Ea Q10 Q10
(1/h) (h) (h) (kcal/mol) (kJ/mol) (50–60°C) (60–70°C)
Vitamin C 50°C 0.047 14.62 48.59 0.984 14.75 61.72 1.58 2.44
60°C 0.075 9.28 30.83 0.993
70°C 0.182 3.81 12.66 0.989
Pyridoxine (vitamin B6) 50°C 0.034 20.2 67.14 0.986 17.19 71.94 1.85 2.59
60°C 0.064 10.91 36.27 0.982
70°C 0.164 4.22 14.03 0.989
TPC 50°C 0.057 12.07 40.12 0.990 12.43 52.01 1.39 2.25
60°C 0.080 8.72 28.97 0.986
70°C 0.179 3.88 12.90 0.984
AC 50°C 0.060 11.49 38.19 0.989 13.12 54.90 1.53 2.17
60°C 0.092 7.53 25.03 0.976
70°C 0.199 3.48 11.55 0.954

TPC, total phenolic content; AC, antioxidant capacity.

Figure 4. Arrhenius plots of dried goji berries: (A) vitamin C, (B) pyridoxine, (C) total phenolic content (TPC), and (D) antioxidant capacity (AC).

To the best of our knowledge, the degradation of vitamin B6 in the hot air-drying process in goji berries has been investigated for the first time. Thermal degradation of vitamin B6 is shown in Figure 3, and the kinetic parameters of vitamin B6 thermal degradation are presented in Table 5. The thermal degradation of vitamin B6 content in fully dried goji berries is found to fit the first-order kinetic model. Vitamin B6 degradation rate constants of goji berries at 50°C, 60°C, and 70°C have been found to be 0.034, 0.064, and 0.164 1/h, respectively. Rate constant increased, but the t1/2 and D values of vitamin B6 decreased due to the increment in temperature. Kadakal et al. (2017) stated that the degradation rate constant in vitamin B complex increased due to thermal increase in rosehip nectar. Also, the Q10 value from 60°C to 70°C was found to be higher than that from 50°C to 60°C. When Q10 values vitamins C and B6 were compared, it was understood that vitamin B6 is more sensitive to increase in temperature. The Ea value of vitamin B6 was found to be 71.94 kJ/mol. When the Ea values of vitamins C and B6 were compared, the Ea value of vitamin B6 was higher than that of vitamin C, which means that vitamin B6 is more stable than vitamin C. At the same time, vitamin B6 is more sensitivity to changes in temperature than vitamin C.

Kinetic parameters of total phenolic content and antioxidant capacity

There are no data about the kinetic parameters of TPC in dried goji berries. The TPC thermal degradation is shown in Figure 3, and the kinetic parameters of TPC thermal degradation are listed in Table 5. The TPC thermal degradation rate constant increased and values of t1/2 and D decreased with the increment in drying temperature. Thermal degradation of TPC content in fully dried goji berries was found to fit the first-order kinetic model. The rate constant of TPC thermal degradation in goji fruits ranged from 0.057 to 0.179 1/h. TPC thermal degradation increases depending on the increment of temperature (Kadakal and Duman, 2018; Sarpong et al., 2018). López et al. (2010) reported that the TPC value decreased with increase in the temperature of drying air. The activation energy was calculated using the Arrhenius equation presented in Figure 4 and found to be 52.01 kJ/mol. The Q10 value from 60°C to 70°C was found to be higher than that from 50°C to 60°C. Thus, the thermal degradation of TPC is more sensitive to the increment of temperature from 60°C to 70°C.

To the best of our knowledge, AC thermal degradation in dried goji fruits was studied for the first time in the current study. The AC thermal degradation is shown in Figure 3, and the kinetic parameters of AC thermal degradation are given in Table 5. In the current study, thermal degradation of AC in fully dried goji berries was found to fit the first-order kinetic model. Oancea et al. (2017) used the first-order kinetic model on AC thermal degradation in sour cherry extract. Owing to temperature increment, the rate constant increased but t1/2 and D values of AC decreased. Oancea et al. (2017) and Sarpong et al. (2018) reported that the rate constant of AC increased with increase in temperature in sour cherry extract and banana slices, respectively. The Arrhenius equation of AC thermal degradation is presented in Figure 4 and Ea was found to be 54.90 kJ/mol. The Q10 values from 50°C to 60°C and from 60°C to 70°C were found as 1.53 and 2.17, respectively. In this context, the increment in Q10 value from 60°C to 70°C indicates that the thermal degradation of AC is more sensitive than that in the range of 50–60°C.

Conclusions

In this study, for the first time, drying characteristics and thermal degradation of some ingredients in goji berry (Lycium barbarum L.) grown in Turkey were investigated under different drying conditions. Page model was determined to be the best model to predict experimental moisture ratio at all drying temperatures (50°C, 60°C, and 70°C). Drying temperature affects the drying speed and drying time. Drying time ranged from 9 to 24 h at 50–70°C. With increase in drying temperature, effective moisture diffusivity increased and the highest effective moisture diffusivity was determined at 70°C. The drying process showed losses in vitamins C and B6, TPC, and AC, and the highest loss was observed at 70°C. The highest percentage loss was found in AC. The thermal degradation of Vitamins C and B6, TPC, and AC is found to fit the first-order kinetic model, and the drying rate values of all these in goji berries increased by drying temperature increment. Vitamins C and B6 were very susceptible to temperature increment, but TPC and AC were the lowest sensitive compounds in dried goji berries. The highest color difference (ΔE) was obtained in the goji berries dried at 70°C. The shortest drying time was observed in the goji berries dried at 70°C, and the drying process at 50°C provided the highest retention of bioactive compounds in goji berries. According to the data obtained and evaluated, the optimal drying temperature for goji berries is 50°C in hot air drying.

As additional studies, research should be conducted on obtaining dried goji berries with different and combined drying method, which could be a more efficient drying process with less component loss. Also, the content differences in goji berries grown at different locations should be investigated.

Acknowledgments

This study was supported by Pamukkale University with grant number 2018FEBE026.

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