Effects of ultrasonic pretreatment and drying approaches on the drying kinetics and rehydration of sprouted mung beans

Sprouting is one of the most traditional methods used to decrease most of the antinutritional elements in legumes. The assistance of ultrasound appears to enhance the drying step of the sprouted legumes. The aim of this study was to examine the influence of ultrasound pretreatment and drying methods (hot‐air and infrared) on the drying time, mass transfer kinetic, effective moisture diffusivity (Deff), and rehydration ratio of sprouted mung beans. The ultrasound process (40 kHz and 150 W) was performed in an ultrasonic bath for 0, 5, 10, 15, and 20 min. The results showed that the ultrasonic pretreatment increased the moisture diffusion capacity (higher moisture loss) and reduced the drying time of sprouted mung beans. Also, the drying time of samples in the infrared dryer was significantly less than that in the hot‐air dryer (p < .05). The Deff determined by Fick's second law was varied from 1.36 × 10−10 to 1.88 × 10−10 m2 s−1, and from 1.18 × 10−9 to 1.85 × 10−9 m2 s−1, for samples dried in hot‐air and infrared dryers, respectively. Comparing the coefficient of determination (r), sum of squared error (SSE), and root mean squared error (RMSE) values of 10 models, it was concluded that the Midilli model represents the drying characteristics of sprouted mung beans better than the others. The rehydration ratio of dried samples in hot‐air and infrared dryers increased when the ultrasound pretreatment time was increased. In general, the use of ultrasound pretreatment (about 20 min) and an infrared dryer is a promising drying technique for sprouted mung beans with higher mass transfer and a shorter drying time.


| INTRODUCTION
Legume crops are widely grown around the world and are a major source of protein in many least developed countries.These food crops are produced in environmentally sustainable ways and are also an economical source of plant protein compared with animal protein (Affrifah et al., 2023).Mung bean (Vigna radiata L.) is a green legume rich in protein (22%-24%), oligosaccharides and fibers, high levels of polyphenols, bioactive phytochemicals, flavonoids (vitexin and isovitexin), amino acids and antioxidants (regulating cholesterol levels and scavenging free radicals), and phytonutrients (Affrifah et al., 2023;Tang et al., 2014;Bai et al., 2016;Liyanage et al., 2018;Kaur et al., 2023).Eating mung bean does not cause flatulence as much as other legumes due to its easy protein and carbohydrate digestibility (Elobuike et al., 2021).Also, it is an excellent source of protein, and its proteins are rich in lysine (which cereals are mostly lacking), leucine, and threonine (Meng et al., 2019).make the snack.Their results showed that the sorghum-mung bean snack was highly rated by the sensory panel, proving its marketing properties.The purpose of Li et al.'s (2011) research was to examine the starch characteristics and processing conditions of 10 types of mung bean.Their results showed that the starches separated from various mung bean cultivars possess various physicochemical properties, and the different mung bean cultivars demonstrated various characteristics.In addition, boiled and sprouted mung beans are often used to make sprout salad (Liyanage et al., 2018).
Sprouting, as a basic processing operation, improves the edibility, nutritional value, and functional properties of legumes (Elobuike et al., 2021).Also, the sprouting process is considered to enhance the nutritional characteristics and quality properties of mung beans (El-Adawy et al., 2003;Sangsukiam & Duangmal, 2017).According to Elobuike et al. (2021), the protein, crude fiber, ash, moisture, vitamins A, C, B 1 , and B 2 , calcium, magnesium, iron, zinc, phosphorus, and potassium of mung bean flour increased while the crude fat, carbohydrate, energy, phytate, oxalate, trypsin inhibitor, tannin, raffinose, and starchyose contents reduced throughout the sprouting procedure (28 C, 26% R.H, and 24-120 h).The results of Liyanage et al. (2018) research confirmed that sprouted mung beans have a higher content of soluble fiber and hypocholesterolemic capacity.
Powder of the dried sprouted grains is known as a good source of nutrients for the enrichment and improving the quality and physicochemical properties of food products (including noodles, pasta, chiffon cake, bread, and sausage) that can be available during the year (El-Adawy et al., 2003;Jokar et al., 2019;Liu et al., 2018;Ruan et al., 2019;Sedani et al., 2021;Shingare & Thorat, 2013).Jokar et al. (2019) used sprouted wheat powder in the production of sausage.
Their results showed that the amount of water-holding capacity of sausage samples was increased when sprouted wheat powder was added to the sausage formulation.In another study, the impacts of adding sprouted mung bean powder at different concentrations (0%, 10%, 20%, and 30%) on noodle production characteristics were examined by Liu et al. (2018).The results demonstrated that the protein content of samples increased during sprouting time.
Ultrasound pre-treatment as a non-thermal food processing technology could be a better pre-treatment technique for food processing due to its benefits, which comprise energy savings, preservation of original freshness and nutritional contents, keeping bioactive compounds, a decline in processing duration, and cost.Ultrasound pretreatment accelerates the mass transfer in the dehydration and drying of fruit and vegetable slices, mostly due to the breakdown of cells and the creation of microchannels (Allahdad et al., 2019;Mukhtar et al., 2022;Salehi, 2023a;Tayyab Rashid et al., 2020).Wang et al. (2019) used ultrasound pre-treatment (20 kHz frequency and 400 W) to improve the drying kinetics of kiwifruit slices.Their results confirmed that this pre-treatment method can improve the drying process and preserve a high amount of phenols.In addition, the results of Liu et al. (2020) confirmed that the ultrasound pre-treatment can improve the color (higher lightness) of dried cranberries.Yildiz and Aadil (2022) and Yıldız et al. (2022) confirm that ultrasonic processing is a promising alternative to replace chemical and/or physical approaches to extend shelf-life and maintain quality characteristics of mango and fresh-cut kiwifruit during cold storage, respectively.Sprouted mung bean seeds are the treasure of bioactive compounds (Sarjerao et al., 2022).Sangsukiam and Duangmal (2017) and Sarjerao et al. (2022) confirmed that the physicochemical properties and quality of dried sprouted mung beans were influenced by the germination period and drying conditions.Among the different techniques used for food product drying, infrared is one of the suitable techniques used to reduce moisture.Higher heat transfer rate, lesser drying time, a higher quality of dehydrated fruit and vegetable product, high performance, and energy savings in the procedure are revealed as the main advantages of the infrared drying method over hot-air drying (Salehi, 2020).Pretreatment and drying conditions can greatly influence the moisture diffusivity of sprouted grains (Sarjerao et al., 2022).
The impact of ultrasound enhancement is different for specific processes and depends on the stage of application and the duration of ultrasound exposure.I found no report on the effects of ultrasound pretreatment on the hot-air and infrared drying kinetics of sprouted mung beans in the literature.Hence, the purpose of this study was to estimate the impacts of ultrasound pretreatment and drying approaches on the drying time, mass transfer kinetic, effective moisture diffusivity (D eff ), and rehydration of sprouted mung beans.In addition, the moisture ratio (MR) changes of sprouted mung beans during drying were modeled.

| Sprouting process
Mung bean seeds were purchased from the market in Arak, Markazi Province, Iran.The seeds were hand-cleaned to render them free of dust and then stored in a dry and cool place under ambient conditions until used.The samples were washed and soaked in tap water for 24 h at room temperature (25 ± 1 C) (seeds to water ratio 1:4, w/v) (El-Adawy et al., 2003).Soaked seeds were kept inside a polyethylene container covered with a clean kitchen towel and allowed to germinate for 72 h in the dark at room temperature (25 ± 1 C) (El-Adawy et al., 2003).In this study, the moisture content (MC) of the raw, soaked, sprouted, and dried mung beans was determined using an oven (55 L, Shimaz, Iran) at 105 C for 5 h (Salehi & Inanloodoghouz, 2023).

| Ultrasound pretreatment
To apply the sonication treatments to the germinated seeds, an ultrasonic bath (vCLEAN1-L2, Backer, Iran) was employed with a frequency of 40 kHz and a power of 100 W. The tank of the device was filled with 2 L of distilled water, and then, after the temperature of the water reached 25 C, the germinated seeds were placed directly in the bath.In this study, the effect of the ultrasound pretreatment at five intervals time of 0, 5, 10, 15, and 20 min on the sprouted mung bean seeds was investigated.

| Hot-air drying
After each treatment, the sprouted mung beans were dried in an oven (70 ± 2 C, 55 L, Shimaz, Iran) until reaching a constant weight (Figure 1).

| Infrared drying
In this study, an infrared dryer (length 440 mm, width 200 mm, and height 400 mm) with an infrared radiation source (250 W, nearinfrared [NIR], Noor Lamp Company, Iran) was used for drying germinated mung beans.In this dryer, the distance of germinated seeds from the radiation lamp was 5 cm.The mass changes of samples were measured using a Lutron GM-300p digital balance (Taipei, Taiwan) with a sensitivity of ±0.01 g.

| Drying kinetics
The dehydration kinetics of sprouted mung beans has been explained using 10 simplified drying equations (Approximation of diffusion, Henderson and Pabis, Logarithmic, Midilli, Newton, Page, Quadratic, Two-term, Verma, and Wang and Singh) (Manikantan et al., 2022;Salehi & Satorabi, 2021).In this model, the MR was employed to represent dehydration data, and for every experiment condition, MR values were plotted against drying time, and model parameters were calculated and compared.Equations 1 and 2 calculate the drying rate (DR) and MR of sprouted mung beans during drying, respectively: where M t2 and M t1 are the MCs of the sprouts at times t 2 and t 1.
Also, where M t , M e , and M o are the MC of the sprouts at time t, equilibrium, and initial MC of the sprouts on a dry basis (g water/g dry matter).
Regression analysis was carried out using Matlab software (version R2012a) to estimate model parameters.A coefficient of determination (the correlation between the response values and the expected response values, r), a sum of squares due to error (SSE), and a root mean squared error (RMSE) were the three criteria used to estimate the experimental data adjustment.When the highest R 2 value is combined with the lowest SSE and RMSE values, a satisfactory fit between the actual data and the correlations is established.

| Calculation of moisture diffusivity (D eff )
Fick's second law of diffusion using spherical coordinates (Equation 3) was used to calculate the moisture diffusivity of germinated mung beans at various hot-air and infrared drying conditions.
where D eff is the effective moisture diffusivity (m 2 s À1 ), r is the average radius of the sprouted mung beans, which is equal to 0.00225 m in this study, n is a positive integer, and t is the drying time (s).For food drying process modeling, Equation 3 can be rewritten as Equation 4: In this study, Equation 4 is employed for the calculation of D eff values from the slope of lnMR and t (experimental drying duration, s) plot, which is calculated by Equation 5 (Salehi, Inanloodoghouz, & Ghazvineh, 2023): F I G U R E 1 Schematic of sprouting, ultrasound pretreatment, and drying process of mung beans.

| Rehydration
The

| Statistical analysis
The dehydration tests of sprouted mung beans were conducted in a 5 Â 2 Â 3 (ultrasound treatment time, dryer type, and three replicates) factorial design.All measurements were conducted in triplicate, and a completely randomized design was used for statistical analysis (Salehi et al., 2015).Also, the means were compared using Duncan's multiple range test at p-value < .05(using SAS 9.1, USA).
Statistical analysis of empirical results (data) confirmed that the dryer type (hot-air and infrared), sonication time, and its interactions have a significant effect on the drying time of sprouted mung beans (p < .01) (Table 1).The impacts of dryer type and ultrasound pretreatment time on the drying time of sprouted mung beans are shown in Figure 2. Drying is a traditional method to increase the shelf-life of food products (Salehi, 2023b).The effects of sonication time on the MC changes (dry basis) of sprouted mung beans during drying in the hot-air and infrared dryers are shown in Figures 3 and 4, respectively.
As seen in these figures, the application of ultrasound has increased the rate of moisture removal from sprouted mung beans and, as a result, it increased the DR of the samples.F I G U R E 2 Effect of ultrasound pretreatment on the drying time of sprouted mung beans (HA = hot-air dryer; IR = infrared dryer).Different letters above the columns indicate a significant differences (p < .05).

| Effective moisture diffusivity coefficient (D eff )
The use of ultrasonic pretreatment before the drying process is aimed at changing the structure of the raw material (increasing porosity and

| Kinetics modeling
The drying behavior of sprouted mung beans in hot-air and infrared dryers was fitted with the Midilli model (Equation 6).This model showed a good fit with the maximum r-value (greater than 0.9945) and the minimum SSE and RMSE values (lower than 0.0023 and 0.0144, respectively) for all conditions compared with those of the other models.The calculated constant coefficients of the Midilli equation, including a, k, n, and b, are reported in Table 2 along with matching statistical error values (SSE, RMSE, and r) for all dehydration F I G U R E 5 Effect of ultrasound pretreatment on the effective moisture diffusivity coefficient of sprouted mung beans (HA = hot-air dryer; IR = infrared dryer).Different letters above the columns indicate a significant differences (p < .05).

T A B L E 2
The constants and coefficients of the approved model (Midilli).conditions.The values of SSE, RMSE, and r for all conditions were in the ranges of 0.0003-0.0023,0.0038-0.0144,and 0.9945-0.9999,respectively.

| Rehydration
The most important advantages of ultrasonic technology are low food production costs, low energy consumption, and flexibility compared with other techniques, keeping in mind its suitability for the handling of solid and liquid foods (Ranjha et al., 2021).Statistical analysis of experimental results (data) confirmed that the dryer type (hot-air and infrared) and sonication time have a significant effect on the rehydration ratio of dried sprouted mung beans ( p < .01)(Table 3).The impacts of dryer type and ultrasound pretreatment time on the rehydration ratio of dried sprouted mung beans are shown in Figure 7.The rehydration ratio of pretreatment samples by 20-min ultrasound and dried in the hot-air dryer was significantly higher than the samples dried in the infrared dryer (p < .05).This issue can be due to the higher volume and porous structure with lower shrinkage in hotair-dried sprouted samples, which give a higher diffusion of moisture Comparison of fitted data by Midilli model with experimental results of moisture ratio (sonication time = 10 min and at infrared dryer).
T A B L E 3 Results of analysis of variance for rehydration of dried sprouted mung beans in hot-air and infrared dryers.
rehydration tests were conducted with a water bath (R.J42, Pars Azma Co., Iran).Dried sprouted mung beans were weighed and immersed for 30 min in distilled water in a 250-mL glass beaker at 50 C.Then, the extra moisture was drained for 30 s, and the samples were weighed again.The rehydration ratio values (%) of dried sprouted mung beans were determined as the ratio of the final mass of rehydrated samples over the dried sample mass Â 100(Salehi, Razavi Kamran, & Goharpour, 2023).
The drying time of samples dried in the infrared dryer was much shorter than the samples dried in the hot-air dryer at all conditions (p < .05).The average drying time of sprouted mung beans dried in the hot-air and infrared dryers were 203.0 and 25.6 min, respectively.The drying time of sprouted mung beans decreased by increasing the ultrasound pretreatment time.With increasing the ultrasound pretreatment time from 0 to 20 min, the drying time of samples inside the hotair dryer decreased from 220 to 170 min (p < .05).Also, by increasing the ultrasound pretreatment time from 0 to 20 min, the drying time of samples inside the infrared dryer decreased from 29.3 to 21.7 min (p > .05).The results of the Wang et al. (2019) study showed that ultrasound pre-treatment decreased the drying time of treated samples by 16.7%-25.0%compared with the untreated kiwifruit slices.
loosening the tissues).The impacts of dryer type and ultrasound pretreatment time on the D eff values of sprouted mung beans are shown in Figure5.When applying ultrasonic technology before drying, the drying kinetics was increased (higher D eff values).Also, the D eff values of samples dried in an infrared dryer were significantly higher than the samples dried in a hot-air dryer ( p < .05).The average D eff values of sprouted mung bean during drying in a hot-air dryer increased from 1.36 Â 10 À10 to 1.88 Â 10 À10 m 2 s À1 when the ultrasound pretreatment time was increased from 0 to 20 min (p < .05).In addition, theF I G U R E 3 Effect of ultrasound pretreatment (0-20 min) on the moisture content changes of sprouted mung beans during drying in the hotair dryer.F I G U R E 4 Effect of ultrasound pretreatment (0-20 min) on the moisture content changes of sprouted mung beans during drying in the infrared dryer.average D eff values of sprouted mung bean during drying in an infrared dryer increased from 1.18 Â 10 À9 to 1.85 Â 10 À9 m 2 s À1 when the ultrasound pretreatment time was increased from 0 to 20 min ( p > .05).These values lie within the common range of 10 À11 -10 À9 m 2 s À1 for other grains.Rafiee et al. (2008) used a hot-air dryer (at 35-70 C) for drying wheat (Tajan).Their results showed that the D eff values for the whole wheat ranged from 2.3 Â 10 À11 to 1.1 Â 10 À10 m 2 s À1 , and it was increased with the increase of the drying air temperatures.Results of the Sarjerao et al. (2022) studyshowed that the D eff value for vacuum-dried sprouted mung beans was more than tray dryer at all conditions.Also, the D eff value for tray and vacuum dryers was reported between 3.98 Â 10 À11 and 7.35 Â 10 À11 m 2 s À1 , and 4.50 Â 10 À11 and 7.892 Â 10 À11 m 2 s À1 , respectively.Manikantan et al. (2022) used a tray dryer (at 50-80 C for 24-48 h) for drying sprouted wheat grains.Their results showed that the D eff values of sprouted wheat grains (at a 48-h drying period) increased from 1.9 Â 10 À9 to 2.6 Â 10 À9 m 2 s À1 with an increase in dryer temperature from 50 to 80 C. In another study, Shingare and Thorat (2013) used a fluidized bed dryer for drying sprouted wheat grains.They reported that the D eff values of sprouted wheat grains during drying in the fluidized bed dryer were between the range of 7.3 Â 10 À10 and 30.4 Â 10 À10 m 2 s À1 .

Figure 6
Figure 6 demonstrates the comparison of fitted MR data using the Midilli equation with experimental results (sonication

F
I G U R E 7 Effect of ultrasound pretreatment on the rehydration of dried sprouted mung beans (HA = hot-air dryer; IR = infrared dryer).Different letters above the columns indicate a significant differences (p < .05).inside the dried mung bean cells and therefore a higher rehydration ratio.The average rehydration ratio of dried sprouted mung bean in the hot-air dryer increased from 255.20% to 287.72% when the ultrasound pretreatment time was increased from 0 to 20 min ( p > .05).In addition, the average rehydration ratio of dried sprouted mung bean in the infrared dryer increased from 188.66% to 238.54% when the ultrasound pretreatment time was increased from 0 to 20 min ( p < .05).Results of theSarjerao et al. (2022) study showed that the rehydration ratio values for vacuum-dried sprouted mung beans were much higher than tray dryer at all conditions.They reported that the rehydration ratio values (after 30 min of soaking) for tray and vacuum-dried samples were 193% and 221%, respectively.Sedani et al. (2021) reported that the rehydration ratio values (after 10 min of soaking at 93 C) for stepwise decreasing microwave power-dried moth bean sprouts were 196%.4 | CONCLUSIONSIn this study, the influence of ultrasound pretreatment and dryer type on the thin-layer drying characteristics of sprouted mung beans was studied.First, the mung bean seeds were sprouted for 72 h at room temperature (25 ± 1 C).The results clearly indicate that the ultrasound pretreatment has great potential for use as a mass transfer accelerator in the drying procedure of sprouted mung beans.The drying time of samples dried in the infrared dryer was significantly shorter than the samples dehydrated in the hot-air dryer ( p < .05).The average D eff values of sprouted mung beans during drying in the hot-air and infrared dryers increased when the ultrasound pretreatment duration was increased from 0 to 20 min.The experimental data of the drying curve was fitted to different thin-layer equations, and the Midilli equation with empirical constants (a, k, n, and b) was the best to describe the drying kinetics of sprouted mung beans.This equation showed a good fit with the maximum r-value (greater than 0.9945) and the minimum SSE and RMSE values (lower than 0.0023 and 0.0144, respectively) for all conditions compared with those of the other models.The rehydration ratio of pretreatment samples by 20-min ultrasound and dried in the hot-air dryer was significantly higher than the samples dried in the infrared dryer (p < .05).The results of this study confirmed that ultrasound pretreatment improved the mass transfer rate of sprouted mung beans during drying in the hot-air and infrared dryers.
Where the a, k, n, and b are coefficients of the Midilli model (dimensionless).