Drying kinetics, color properties, watersoluble 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 B_{6}, antioxidant activity and total phenolic content were significantly reduced by drying. Thermal degradation of vitamins C and B_{6}, antioxidant capacity and total phenolic content were found to fit the firstorder kinetic model.
Goji berry (
Goji berry contains a high amount of anthocyanin (
In traditional Chinese herbal medicine, goji berry has been used as a supplement for more than 2000 years (
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 (
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 (
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 B_{6}, antioxidant capacity and total phenolic content of goji berries, and calculate the degradation kinetics of these bioactive compounds.
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.
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.
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 (
Moisture ratio (MR) must be calculated by M_{i,} M_{t}, and M_{e} values to choose the best model.
where
However, if the equilibrium moisture content (
Drying rate (DR) was determined using Equation 3:
where
Δ_{t}: 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 chisquare (
where
MATLAB software was used to calculate thinlayer modeling and for statistical analyses.
Fick’s diffusion equation described the drying characteristics of biomaterials.
where
Equation 6 can be reduced (
Equation 8 shows a straight line with a slope provided in the plot:
The Arrhenius equation of hot airdrying process was used for calculation of activation energy (
where
After regulation of natural logarithm in Equation 8, Equation 9 can be written as follows:
Natural logarithm of effective moisture diffusivity versus 1/T gives a straight line with a slope that represents activation energy.
An extraction method proposed by Donmez (2015) was used for analysis of watersoluble vitamins. To determine watersoluble 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 0.45μm filter to be injected into a highpressure 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 KH_{2}PO_{4} at pH 7.
The HPLC device (Shimadzu), in which analysis of watersoluble vitamins was performed, consisted of column oven (Shimadzu CTO20A, Japan), pump (Shimadzu LC20AD, Japan), degasser (Shimadzu DGU20A3, 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
The AC and TPC analysis was carried out using the methanolic extraction method proposed by
The TPC analysis was performed according to
The AC analysis was carried out using the method suggested by
Reflectance color value of goji berry skin was measured by using Hunter Lab Color Miniscan XE (45/0L, USA). The samples were placed on a white background and the measurement was performed by covering with a transparent glass. The highest color difference (Δ
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
For the zeroorder kinetic model, equation can be written as follows:
If Equation 13 is integrated and
where
ln
ln
Temperature dependence of vitamins C and B complex, TPC, and AC can be calculated using Equation 15 (
When Equation 15 is regulated, Equation 16 is written as follows:
where
Quotient indicator (
Halflife time, time required for half of concentration, for each temperature is calculated using Equation 18 for firstorder kinetics (
Time taken by the compound, or quality criterion, to lose 90% of its quality is expressed as
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.
The drying rate and moisture ratio values of goji berries during hot air drying are presented in
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.
Mathematical models used in modeling the drying process, constants, and the statistical data of mathematical models are listed in
Thinlayer mathematical models, model constants, and statistical parameters of thinlayer drying curves.
Model Names and References  Model  Temperature  Model Constants  χ^{2}  RMSE  R^{2}  

Lewis / 
exp(kt)  50°C  0.001344007  0.03592  0.9793  
60°C  0.001277711  0.03484  0.9819  
70°C  0.000251669  0.01505  0.9977  
Page / 
exp(kt^{n})  50°C  0.000264861  0.01561  0.9962  
60°C  0.000165378  0.01220  0.9979  
70°C  0.000290322  0.01524  0.9979  
Henderson and Pabis / 
aexp(kt)  50°C  0.000576501  0.02303  0.9918  
60°C  0.000866761  0.02793  0.9890  
70°C  0.000316013  0.01590  0.9977  
Logaritmic / 
aexp(kt) + c  50°C  0.000666050  0.02421  0.9910  
60°C  0.000296302  0.01587  0.9964  
70°C  0.000811922  0.02384  0.9948  
Wang and Singh / 
1 + at + bt^{2}  50°C  0.003999604  0.06066  0.9433  
60°C  0.004329174  0.06242  0.9450  
70°C  0.001641672  0.03624  0.9880  
Parabolic / 
a + bt + ct^{2}  50°C  0.001539746  0.03681  0.9800  
60°C  0.002166198  0.04291  0.9754  
70°C  0.001492261  0.03232  0.9917 
RMSE, root mean square error.
The effective moisture diffusivity (
Effective moisture diffusivity and activation energy of goji berry fruit.
Temperature  

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
The Arrheniustype relation between
The Arrheniustype relation between effective moisture diffusivity and 1/T.
Effects of drying on watersoluble vitamins of goji berries are provided in
Effect of drying process on vitamins C and B_{6}, total phenolic content, and antioxidant capacity of goji berries.
Vitamin C^{*}  Loss 
Pyridoxine 
Loss 
TPC^{**}  Loss 
AC^{**}  Loss 


Fresh  112.75 ± 2.23^{a}  0  2.19 ± 0.046^{a}  0  1838.43 ± 37.47^{a}  0  0.077 ± 0.002^{a}  0 
50°C  39.45 ± 2.21^{b}  65.03  0.937 ± 0.055^{b}  56.56  491.00 ± 7.96^{b}  73.29  0.017 ± 0.001^{b}  77.92 
60°C  26.48 ± 1.16^{c}  76.84  0.681 ± 0.061^{c}  69.04  450.17 ± 8.26^{b}  75.51  0.014 ± 0.001^{bc}  81.82 
70°C  21.87 ± 0.971^{d}  80.5  0.492 ± 0.034^{c}  77.48  404.45 ± 6.89^{c}  78  0.011 ± 0.001^{c}  85.71 
Vitamins C and B_{6} 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 (B_{6}) 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 airdrying 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.
Effects of hot air drying on total phenolic content and antioxidant capacity of goji fruits are presented in
Color properties of fresh and dried goji berries were presented in
Color properties of goji berry fruits.
L^{*}  a^{*}  b^{*}  Δ 


Fresh  25.97 ± 0.12^{a}  25.16 ± 0.13^{a}  17.30 ± 0.05^{a}  
50°C  23.11 ± 0.09^{b}  16.45 ± 0.07^{b}  11.03 ± 0.11^{b}  10.87 
60°C  22.79 ± 0.05^{c}  14.41 ± 0.09^{c}  10.67 ± 0.07^{c}  13.01 
70°C  21.99 ± 0.06^{d}  14.67 ± 0.08^{d}  9.62 ± 0.05^{d}  13.91 
Different letters in the same column are significantly different values (P < 0.05).
To the best of our knowledge, vitamin C degradation in the hot airdrying process of goji berries was investigated for the first time in this study. Thermal degradation of vitamin C in goji berries is shown in
Firstorder kinetics of (A) vitamin C, (B) pyridoxine, (C) total phenolic content (TPC), and (D) antioxidant capacity (AC) of goji berries.
Airdrying may have a negative effect on the physical properties of products and cause degradation of aromatic compounds and nutrients (
Firstorder kinetic parameters of vitamins C and B_{6}, total phenolic content, and antioxidant capacity of dried goji berries.
Compound  Temperature  

(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 B_{6})  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.
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 B_{6} in the hot airdrying process in goji berries has been investigated for the first time. Thermal degradation of vitamin B_{6} is shown in
There are no data about the kinetic parameters of TPC in dried goji berries. The TPC thermal degradation is shown in
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
In this study, for the first time, drying characteristics and thermal degradation of some ingredients in goji berry (
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.
This study was supported by Pamukkale University with grant number 2018FEBE026.