Abstract-- Jerusalem artichoke (Helianthus tuberosus L.) is primarily used as a biomass for ethanol production. It is also utilized for human consumption as a healthy snack by having good nutritional value and inulin content. In this study, ultrasound pre-treatment (USPT) was selected to minimize drying defects and 5, 10, and 15 min. treatments were applied before hot air drying at 60 °C and 80 °C. The effect of pre-treatment on drying performances and properties including color, total phenolic content and total antioxidant were investigated. Effective diffusion coefficient values ranged from 1.72 × 10-8 m2/min to 2.94 × 10-8 to m2/min, and it was observed that the effective moisture diffusion coefficient was higher in the samples where ultrasound was applied compared to the control sample. L * value 72.72 ± 2.71 was found highest at samples that were 5 minutes ultrasound treated and dried at 60 °C for 60 minutes. The highest antioxidant and phenolic content in the drying process at 60 °C was determined in the samples where ultrasound was applied for 15 minutes. As a result, the application of USPT can result in a better conserved quality in a shorter drying time with the required final product moisture content.
Keywords-- Jerusalem artichoke, ultrasound, drying kinetics, antioxidant activity, color change
I. INTRODUCTION
Jerusalem artichoke (Helianthus tuberosus L.) is an annual and root or tuber loading plant that originates in Central North America and has been cultivated in Europe since the 17th century. Besides industrial applications as paper production, a being a source of ethanol, acetone, butanol from its’ tuber part or whole plant, Jerusalem artichoke is greatly beneficial for health, containing 77.2-86.7% water, 10.5-17.6% carbohydrate, 2-3.6% protein, 7.4-12.0% inulin, 0.6-4.0% dietary fiber (excluding inulin), 0.1-0.6% fat, potassium and an important food rich in calcium in root or tuber and high-quality protein is available in leaves. Fructans, on the other hand, are the largest parts of the total biomass of Jerusalem artichoke (Bach et al., 2015). The “inulin” substance, which is high in Jerusalem artichoke, is a powerful prebiotic and has been found to have important functions on the intestines and it can be used as a stabilizer, thickener, fat substitute, and sweetener (Chen et al., 2016). As a result of the chemical hydrolysis of inulin found in Jerusalem artichoke, fructose can be obtained, which is the sweetest of natural sugars and has a low glycemic index compared to sucrose, and studies on this show that Jerusalem artichoke can find more use in the food industry (Prangviset et al., 2018). All these combinations make Jerusalem artichoke a good and versatile raw material for the food industry and a valuable alternative for healthy snacks.
Drying is the most common and widely applied process in the food industry to store food products with longer shelf lives by reducing moisture content and water activity to limit biological and chemical activities. Removing water from food prevents microbial growth, as well as inhibiting enzymatic and some chemical reactions. It also has benefits such as lowering shipping and storage costs and longer shelf life foods in a cheap way (Da Silva et al., 2014). However, the drying process may cause quality losses of the final product such as vitamin, mineral, antioxidant content, and color. For this reason, pre-treatments such as microwave, vacuum, pulsed electricity, ultrasound, which can preserve the quality characteristics of foods, have been applied before drying in recent years (Izli et al., 2019; Aydar, 2021). The ultrasound process, which reduces the internal resistance and facilitates the removal of water during drying by opening microscopic channels in food, is a technology that has been used frequently in food in recent years (Ricce et al., 2016; Sunil et al., 2017). There are studies in which ultrasound is used as a pre-treatment in many foods such as apple, carrot, garlic, okra, kiwi, and melon before drying. In many of these studies, it was found that ultrasound caused significant improvements in product quality characteristics such as color, phenolic content, and antioxidant activity by shortening the drying time (Fijalkowska et al., 2015; Ricce et al., 2016; Sunil et al., 2017). It is important to perform the drying process in a controlled manner to preserve the present phenolic and vitamin content of the Jerusalem artichoke used in bread and pastry products (Inchuen et al., 2014; Porniammongkol et al., 2014; Antal et al., 2017). In literature, various drying processes have been applied to use Jerusalem artichoke with high nutritional value for a longer shelf life and serving for different application areas (Inchuen et al., 2014; Doymaz, 2018a), however, there is no study in the literature available about drying of Jerusalem artichoke after being subjected to ultrasound pre-treatment. In this study, the effects of drying and ultrasound parameters on the color, total phenolic content, total antioxidant activity of Jerusalem artichoke were studied and where the quality properties were best preserved was evaluated.
II. METHODS
A. Material
Jerusalem artichoke was obtained from a local market in Izmir. All chemicals used in this study (Methanol, Gallic Acid, Sodium Carbonate, Folin Ciocalteu, Trolox, and DPPH solutions) were obtained from Sigma-Aldrich (Taufkirchen, Germany). Jerusalem artichokes were washed, peeled and sliced in 5 mm thickness (2L in drying model) before treatments.
B. Experimental
Jerusalem artichoke slices in 115±2 grams were weighed and transferred into an ultrasonic bath (Alex Machine, PR-6711, 150kW, and 25 kHz ultrasound frequency of 4.5 L; Tank volume, Istanbul, Turkey) with 2.3 L of pure water at room temperature. Ultrasonication was applied for 5, 10, and 15 min as pre-treatment before the drying procedure. The temperature during sonication was measured by using an immersion type thermometer (Model TP101, Isolab, Izmir, Turkey). Both control and USPT samples were placed into a tray dryer (Eksis machine TK10 Model, Izmir, Turkey). Up to 210 and 180 min. air drying was applied to sliced samples at 60 °C and 80 °C respectively to obtain the desired amount of moisture content at the dried products. Samples were taken out of the dryer every 30 minutes and weighed, and their moisture rates are expressed in % dry base
C. Physicochemical Characterization
Total phenolic content for both in sonicated and control samples of the Jerusalem artichoke was evaluated with Folin-Ciocalteu reagent and UV spectrophotometer by reading the absorbance values at 765 nm wavelength and determined as mg GAE / 100 g sample (Nizio et al., 2018). Antioxidant capacity was determined by using DPPH free radicals’ method. Methanol was used as a solvent, the amount of antioxidant value was measured in a spectrophotometer at a wavelength of 515 nm and the results were given as µmol TE / 100 g. (Al-rimawi et al., 2016). Color parameters L*, a*, b* (CIE) were measured with a colorimeter (Konica Minolta, CR 300 Model, Sensing, Inc., Osaka, Japan). Measured L*,a*, and b* values were representing lightness, red-green and yellow-blue, respectively (Arias et al., 2000). After the colorimeter was calibrated against a white surface and a black surface, six replicate measurements were made for each sample. Results were given as mean and standard deviation.
D. Mathematical Models of Drying Kinetics
Physical modeling of drying was evaluated by calculating moisture
ratio (
) by using Eq.
1 in where 
is moisture
content at a given time (w/w) and 
is the initial
moisture content of the samples (w/w) (Singh and Heldman 2013).
. (1)
Since drying is an unsteady mass transfer, in terms of the diffusion model, the effective diffusion coefficient of Jerusalem artichoke was calculated according to Fick's second diffusion law as given Eq. 2. Assumptions as; moisture uniformity, negligible shrinkage, only diffusion stage mass transfer, Jerusalem artichoke were isotropic and homogeneous, heat transfer coefficients were constant and drying was isothermal were considered (Aydar 2021).
(2)
where the effective diffusion coefficient Deff (m2/min), the moisture content in dry base 
, t is the processing
duration in minutes, n is a positive constant and 
is the finite
thickness of slabs in m (
= 5±0.2 mm)
in Eq 3.
(3)
For long drying periods, an equation can be modified to be as the first set of terms as given in Fq. 4. Slope of a logarithmic graph of experiments yields to evaluate Deff for each condition.
(4)
Besides the diffusion model, mostly used semi-empiric
models as Newton (Eq.5 ), Page (Eq. 6) and Midilli (Eq. 7) models were applied
by considering values versus drying time. The coefficient
of determination (R2), Chi-square (X2), root mean square
error (RMSE), mean relative percent error (P) were used to compare fitness.
Coefficients were evaluated for best fit models with the highest R2
and the least P and the least RMSE and the lower values of the reduced X2
(Midilli et al., 2002; Doymaz 2018a).
(6)
(7)
E. Statistical Methods
One-way ANOVA analysis was performed to determine the effect of ultrasound application time on the quality characteristics of the product (SAS 9.2 Version Institute Inc., Cary, NC, USA). Tukey's HSD was used in multiple comparison analyses for α = 0.05.
III. RESULTS
A. Physicochemical characterization
Color is one of the most important quality features and an indicator of quality that can give an idea about the spoilage of food, as a result, its importance in the consumer's food preferences is acknowledged. During the drying of foods, many factors such as thermal and oxidative reactions, exposure of phenolic compounds to enzymatic or non-enzymatic browning reactions cause color change (Wu et al., 2014). The color changes of Jerusalem artichoke slices that were USPT and dried and control samples without pre-treatment were given in Table 1 and Table 2. In a study investigating the infrared heating technique in the drying of carrot slices, it was stated that the decrease in the surface moisture content of the samples increased the reflectivity of the carrots and caused an increase in the L* value (Wu et al., 2014).
Similarly, in this study, the highest L* value was observed at 60 minutes drying at 60 °C which was USPT for 5 minutes. L* value increased due to the increase in reflectivity of samples during the first 60 minutes when
Table 1. Color change of Jerusalem artichoke at 60℃ air drying
Different letters within the same column represent significant differences (P <0.05).
the product moisture rate decreases by more than 50%, then by the decrease in the amount of moisture, reflectivity was decreased due to exposing higher temperatures in the later stages of drying, with the occurrence of enzymatic browning reactions. The fact that the color of Jerusalem artichoke dried at 80 ° C was browner than those dried at 60 ° C was due to the higher drying temperature which leads to a non-enzymatic browning as well (Yang et al., 2010). In a study investigating the infrared heating technique in the drying of carrot slices, it was stated that the decrease in surface moisture content of the samples increased the reflectivity of the carrots and caused an increase in the L* value (Wu et al., 2014). On the other hand, the samples dried at 80 °C, the difference between the L* values obtained 0 and 180 min. drying was not found statistically significant (P> 0.01). Among all applications, the highest L* value was detected with 72.72 ± 2.71 for 5 minutes of USPT at the 60th minute of drying of the samples dried at 60 °C while the lowest L * value with 23.05 ± 1.01 was observed at the 0th minute of drying at 80°C for the samples pre-treated for 15 minutes. In another study, in which okra was dried in the microwave after 30 and 60 min of USPT, it was found that the least change in L* values of okra was observed in 60 minutes
Table 2. Color change of Jerusalem artichoke at 80℃ air drying
Different letters within the same column represent significant differences (P <0.05).
of ultrasound treated samples (Sunil et al., 2017). For seaweed drying ultrasonic probe was used as a pre-treatment and authors found that L* value was negatively affected by the intensity of pre-treatment however the higher L* values were observed compared to control samples (Kadam et al., 2015). The increase in the temperature of the ultrasonic water bath up to 35-40ºC during the ultrasound application process caused a decrease in the L* value of the products. It was observed that as the ultrasound duration increased, the L* value decreased linearly, but the highest increase in the L* value at the end of drying was observed in the ultrasound applied samples. In a study where 10 minutes of ultrasound was applied before drying the melon, it was found that the biggest increase in L* value after drying was in the samples that were applied ultrasound (Dias da Silva et al., 2016). In a study, a * values of the dried apples were measured which were pre-treated with 21 and 35 kHz frequencies and it was found that a* values were least affected by drying for the samples pre-treated 10 minutes (Fijalkowska et al., 2015). In this study, the highest a* value was determined as 0.37 ± 0.04 for dried Jerusalem artichoke with 15 min pre-treatment and this value was higher than all other control samples. Similar trends were observed for b* values. Consequently, ultrasonic pre-treatment was found important to preserve color quality compared to non-treated samples while drying.
Phenolic and antioxidant substances are also directly related to the quality characteristics of a foodstuff besides such attributes as color, flavor, and texture (Yildiz and İzli, 2019). The total phenolic content and antioxidant activity amounts determined under different drying conditions were given in Table 3. It was observed that the antioxidant activity of Jerusalem artichoke increased as the USPT duration increased for any drying temperatures. It was thought that antioxidant activity was better preserved in the samples that have been subjected to ultrasound. Since the Jerusalem artichokes were exposed to drying temperature for a shorter time as ultrasound reduces the drying time which was found in previous studies as well (Aydar et al., 2021; Llavata et al., 2020). It has been stated that the higher amount of ferulic acid, gallic acid and catechin in the samples treated with ultrasound pre-treatment in drying of kiwi was because the non-chemically produced hydroxyl radicals binding to the aromatic ring of phenolic compounds in the ultrasound application increased the antioxidant activity (Horuz et al., 2017). Similar to antioxidant activity, it was stated that as the drying temperature increased, the total amount of phenolic content also increased (Table 3). In samples dried at any temperatures, USPT had positive effects on total phenolic and antioxidant materials compared to control samples dried without ultrasound. While the total antioxidant activity was 81.41 ± 0.19 µmol TE / 100 grams in Jerusalem artichokes dried at 60 ° C, it was found as 89.90 ± 0.90 µmol TE / 100 grams in samples applied for 15 minutes of ultrasound. The difference between the total antioxidant activity of Jerusalem artichokes dried at 80 °C and the total antioxidant activity of Jerusalem artichoke dried at 80 °C after 10 and 15 minutes of ultrasound was found to be statistically significant (P <0.01). It was thought that the higher total phenolic substance and antioxidant activity in the samples that were applied ultrasound was due to the shorter drying time compared to the samples without ultrasound as discussed in previous drying studies in onion, banana and garlic (Aydar et al., 2021).
B. Mathematical models of drying kinetics
Factors such as the variety of dried sample, maturity index, initial moisture content, pre-treatments, drying conditions, and drying methods have important effects on drying kinetics (Porniammongkol et al., 2014; Llavata et al., 2020). The initial moisture content of Jerusalem artichoke was found to be 80.33 ± 1.31%. In Fig 1a and b, time-dependent humidity changes of Jerusalem artichokes dried at 60 °C and 80 °C were shown. Accordingly, it was observed that as the drying temperature, drying time, and ultrasound duration increased, the moisture rate decreased. It has been determined that the samples dried at 80 °C with ultrasound treatment resulting in a higher effective diffusion coefficient than the Jerusalem artichokes that were subjected to ultrasound and dried at 60 °C. This can be explained by the fact that the increase in drying temperature caused the decrease in relative humidity in air easing mass transfer by drying force coming out by concentration difference and thus the drying air had a higher drying capacity (Doymaz, 2018b).
Table 3. Total phenolic content and antioxidant activity of dried samples
|
Experiment |
Antioxidant activity (µmol TE/100 g) |
Total phenolic content (mg gallic acid /100 g) |
|
60℃ |
81.41±0.19d |
326.54±16.01c |
|
60℃5U |
89.37±1.15bcd |
355.49±18.75bc |
|
60℃10U |
85.13±1.80cd |
353.50±4.39bc |
|
60℃15U |
89.90±0.90bcd |
364.53±32.54abc |
|
80 ℃ |
88.31±4.07cd |
362.08±8.42abc |
|
80℃5U |
92.02±1.77abc |
374.36±10.53ab |
|
80℃10U |
100.51±5.95a |
401.01±3.36a |
|
80℃15U |
97.86±3.50ab |
395.14±5.64ab |
The difference between the averages shown by different letters in the same column is significant. (P <0.01).
Table 4. Coefficients of Midilli et al. model
|
|
Model Coefficients |
|||
|
Experiment |
a |
b |
k |
n |
|
60℃ |
0.9809 |
0.0000 |
0.0412 |
0.1718 |
|
60℃5U |
-11.4579 |
0.0088 |
0.0026 |
0.0019 |
|
60℃10U |
-11.0785 |
0.0089 |
0.0004 |
0.0181 |
|
60℃15U |
-10.7123 |
0.0086 |
0.1194 |
0.0000 |
|
80 ℃ |
0.9745 |
0.0000 |
0.0666 |
0.2126 |
|
80℃5U |
-10.5800 |
0.0087 |
0.0000 |
-0.1634 |
|
80℃10U |
-11.0685 |
0.0087 |
0.0023 |
0.0000 |
|
80℃15U |
-11.2751 |
0.0087 |
0.0001 |
0.0009 |
Table 5. Effective moisture diffusivity of experiments
|
Experiment |
Effective Moisture Diffisuvity |
R2 |
Chi Square |
RMSE |
|
60℃ |
1.72x10-08 |
0.9996 |
0.00668 |
0.00005 |
|
60℃5U |
1.77x10-08 |
0.9999 |
0.00186 |
0.00000 |
|
60℃10U |
1.79 x10-08 |
0.9999 |
0.0008 |
0.00000 |
|
60℃15U |
1.97 x10-08 |
0.9999 |
0.00053 |
0.00000 |
|
80 ℃ |
2.05 x10-08 |
0.9893 |
0.02751 |
0.00088 |
|
80℃5U |
2.61 x10-08 |
0.9999 |
0.02485 |
0.00072 |
|
80℃10U |
2.87 x10-08 |
0.9998 |
0.00001 |
0.00000 |
|
80℃15U |
2.94x10-08 |
0.9999 |
0.37795 |
0.16665 |
(a)
(b)
Fig 1. Change of moisture ratios of Jerusalem artichokes during drying at 60 °C (a), at 80 °C (b)
In terms of empiric model fitness, by comparing at R2, chi-square, and RMSE values, it was decided that the model that best describes the drying behavior of Jerusalem artichoke dried after USPT was the Midilli Model (Table 4). Porniammongkol et al., (2014) in their studies, determined the hot air, sun drying, and microwave drying kinetics of Jerusalem artichoke and found that the Midilli Model was the best model for hot air drying with R2, χ2, and RMSE with the values as 0.983, 0.041 and 0.0024 respectively (Inchuen et al., 2014; Porniammongkol et al., 2014).
Although the lowest R2 value for the Midilli Model was determined at 0.989 and 80 °C in samples without ultrasound, it was found that it was a model that can be used with low chi-square and RMSE values in all drying applications. In 60 °C and 80 °C drying processes, it has been observed that the effective moisture diffusion coefficient increased as the ultrasound duration increased (Table 5). Similarly, in studies with other drying fruits and vegetables by applying USPT, it was found that ultrasound facilitates the release of water from the sample by creating microchannels, and at the same time, acoustic cavitation increases mass transfer and drying speed by causing cell disruption yielding mixing as well.
Cavitation phenomena were the main reason for both sponge effect and reduction of resistance to mass transfer for any unit operations including drying and extraction (Fijalkowska et al., 2015; Zhang et al., 2016; Yılmaz and Tavman, 2017; Wang et al., 2019). The increase in temperature was observed with prolonged sonication which yielded an increase in effective moisture diffusion in the drying stage. The effective diffusion coefficient values varied between 1.72 × 10-8 - 2.94 × 10-8 m2 / min, and the lowest value was calculated for the samples that were dried at 60 °C with no pretreatment while the highest diffusion coefficient was calculated for 15 min USPT Jerusalem artichoke dried at 80 °C. Longer USPT with higher drying temperature yielded to a better and faster dryer as observed by the help of mixing and cavitation effect of sonication as discussed in previous studies for fruit and vegetable drying (Fijalkowska et al., 2015; Ricce et al., 2016).
IV. CONCLUSION
In this study, the effects of USPT as a novel pre-treatment at different times before drying in traditional hot air on drying characteristics and quality properties of Jerusalem artichoke such as total phenolic content, antioxidant activity and color were investigated. As the drying temperature and ultrasound duration increased, the effective moisture diffusion coefficient increased, and the product moisture content decreased in the final product. The highest L* value was determined with 72.72 ± 2.71 in samples that were pre-treated for 5 minutes with ultrasound and dried at 60 °C for 60 minutes. The highest antioxidant activity was found in 100.51 ± 5.95 µmol TE / 100 g and total phenolic content 401.01 ± 3.36 mg gallic acid / 100 g in Jerusalem artichoke dried at 80°C for 10 minutes with ultrasound. As a result, it was determined that USPT has a significant effect on color, total phenol, and total antioxidant activity and decreases the drying time by increasing the drying speed. Sonication can be used as a pre-treatment that saves energy during the drying process.
ACKNOWLEDGEMENT
This project is supported by TUBITAK 2209 – A programme.
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Received: March 6, 2021
Sent to Subject Editor: March 18, 2021
Accepted: October 26, 2021
Recommended by Subject Editor Mariano Martin Martin