Hot air assisted Microwave Drying on Moringa leaves, Spinach leaves, Lemongrass, Banana, and Ginger: Drying Characteristics and Antioxidant Activity

Moringa leaves, spinach, lemongrass, banana, and ginger have various bioactive phytochemicals, micronutrients, and health-improving pharmacological effects. The main objective of the present study is to dry moringa leaves, spinach, lemongrass, banana, and ginger using a hot air-assisted microwave drier and the effect of microwave power on activation energy, effective moisture diffusivity, drying rate, and antioxidant activity. The hot air temperature is maintained at 50°C with airflow rate is 2 m/s. For moringa leaves, spinach, lemongrass, and ginger, 0.2, 0.4, 0.6 kW microwave power was used. But for banana, 0.35, 0.45, 0.55 kW microwave power was used. Nine drying models were used to validate the experimental data obtained during drying. To get the best fit model, MATLAB software version 11 was used. At 0.6 kW microwave power, effective moisture diffusivity, drying rate, and antioxidant activity increased while activation energy and drying time decreased. The study result revealed that the samples dried at 0.6 kW for moringa leaves, spinach, ginger, lemongrass and 0.35kW for banana showed the highest antioxidant activity. From the result, the study concluded that hot air-assisted microwave drying can be used for drying moringa leaves, spinach, lemongrass, banana, and ginger.


Introduction:
Moringa oleifera is commonly called moringa or drumstick.The main parts of moringa used for consumption are leaves, flowers, and fruit.Kaempferol and quercetin flavonoids were abundant in Moringa leaves [1], anthocyanins, and alkaloids such as moringine and moringinine [2,3].Moringa leaves also have magnesium, calcium, phosphorus, and iron [3].The moringa leaves have plenty of medicinal uses, such as used to cure conjunctivitis, bronchitis, tuberculosis, asthma, gastric ulcers, diabetes, and fatigue [1,2].Spinach also called Spinacia oleracea grows all around the year and one of the popular green vegetables grown in the cold 1258 (2023) 012007 IOP Publishing doi:10.1088/1755-1315/1258/1/012007 2 season [4].Spinach is rich in fiber and protein.It also has many micronutrients such as manganese, iron, magnesium, and zinc.It contains folate, pyridoxine, riboflavin, and thiamine [5].Anti-carcinogenic, antimicrobial, and antioxidant properties are the some of the pharmacological properties of spinach [4].Banana is one of the main fruit consumed commonly around the world.Besides its starchy nature and sweet taste, banana also a storehouse for many nutritional components such as sterols, linoleic acid, vitamin B, tannins, vitamin C, polyphenols, triterpenes, and glycosides.These compounds are mainly responsible for controlling biological activities and functions for day-to-day life [6,7,8,9].Cymbopogon citratus commonly known as lemongrass and it is a grass plant.The name itself suggests that the lemongrass gives a lemon-like odor which is unique to the lemongrass along with having health benefitting bioactive compounds.Citral, a cyclic monoterpene is responsible for giving lemon scent to the lemongrass.Lemongrass has phytate, flavanoids, minerals, phenolic compounds, and essential oils.The lemongrass has the following pharmacological properties, such as anti-bacterial, anti-fungal, anti-obesity, antioxidant, anti-nociceptive, and antiinflammatory properties [10].Ginger biologically known as Zinger officinale is a commonly consumed spice in the culinary.It has many functions compounds such as sesquiterpenes, zingerone, gingerols, shogaols, sesquiterpenoids, and monoterpenoids [11,12,13].Ginger is used to lowering nausea and vomiting sensations during pregnancy.It also used to treat stomach-related minor illnesses such as indigestion, stomach pain, and bloating [13].Ginger is one of the best home medicines for dry coughs in Bangladesh [14].
Most fruit and vegetables are highly perishables; to prevent them from degrading conventional hot air-drying method was used.But conventional drying methods use high temperature for drying which could lower the retention of bioactive compounds and micronutrients which are high heat-sensitive in nature.High-temperature treatments also have an impact on the sensory parameters of the dried food [15].One of the non-thermal drying methods is microwave drying.It utilizes electromagnetic radiation to produce heat in the food stuffs and provides uniform heating and high thermal potential to the centre of the edibles.Microwave drying increases the final quality of the end product by minimum time [16].By combining hot air with microwave drying reduce the drying time of food materials as hot air stimulate the water molecules in it and increase the end product quality [17].Some of the previous studies of hot air assisted microwave drying on food products to include okra [18], kiwi [19], garlic [17], persimmon [20], and mushroom [21].However, no work was reported on the drying of moringa leaves, spinach, lemongrass, and ginger.The main objectives of the present work is to analyze the effect of effective moisture diffusivity, drying rate, activation energy, thin layer drying models, and its antioxidant activity in the microwave dried moringa leaves, spinach, lemongrass, banana, and ginger.

Sample preparation
Fresh moringa leaves, spinach, lemongrass, banana, and ginger were bought from the local market in Erode, Tamil Nadu.The fresh ingredients were washed and drained.The samples of 10 g were taken for uniform size was taken.For microwave drying, whole moringa and spinach leaves were taken.The lemongrass is cut into 2cm of length equally.Banana is peeled and the pulp is cut into 4 mm thickness uniformly and ginger is cut into 2 mm thickness.

Drying Equipment
Hot air-assisted microwave dryer of model MT-812-410-V1 was used for drying.The dryer voltage is 230 volts with current 20 A. The dryer microwave frequency is 2450 MHz with microwave power 0.2 -0.8 kW.An air blower is connected to the bottom of the microwave cavity.Three turning screws for adjusting and minimizing the reflection power and a fiber optic temperature sensor for monitoring the sample temperature.The length, breadth and height of the radio frequency cavity is about 473 * 473 * 27 cm, respectively.A Data logging weighing system is attached to the equipment.The temperature for hot air is maintained at 50 0 C and the airflow rate is 2 m/s.For moringa leaves, spinach, lemongrass, and ginger, 0.2, 0.4, 0.6 kW microwave power was used [22].But for banana, 0.35, 0.45, 0.55 kW microwave power was used because higher microwave power led to charring in banana [23].Using digital balance, moisture loss was recorded at 2 min interval time during drying.The samples were dried until equilibrium was reached.All the drying experiments were replicated three times for all samples and all drying conditions.Each of the samples was collected separately and stored in an airtight container for further analysis.

Drying rate
Removal of water from the sample at a particular time is known as the drying rate.The overall drying rate (g H2O/g solids/min) was calculated using equation (1) Drying rate = (Amount of water removed, g) / ((Dry solids, g) * (time, min)) (1)

Effective moisture diffusivity
The effective moisture diffusivity (Deff) was calculated at each corresponding moisture content and time in this study.The average effective moisture diffusivity Deff (Avg) was calculated by using equation (

Mathematical models
In this study, the experimental drying data of moringa leaves, spinach, lemongrass, banana, and ginger at different microwave powers were fitted into commonly used thin-layer drying model equations, listed in Table 1.For mathematical modeling MATLAB software version, 11 was used.

Fitting the mathematical models
The goodness of fit was assessed to the experimental moisture ratio and time data from the coefficient of determination (R 2 ), and root mean square error (RMSE).For the mathematical models, The higher the R 2 values, and the lower the RMSE values gives the better goodness of fit [25].

Determination of DPPH radical scavenging activity
The antioxidant activity of the extract was measured by DPPH assay.The methanol extracts of 0.1 ml were mixed for 30 s with a DPPH solution of 3.9 ml (6 x 10 -5 M).The solution was incubated at room temperature for 30 min.At the end of the incubation period, the decrease in absorbance was measured at 517 nm with a spectrophotometer.Clear DPPH solution was used as control.The antioxidant activity was calculated as following equation ( 4) [26].
Where A is the absorbance at 517 nm.

Statistical Analysis
All experiments results were performed in triplicate and the results were expressed as means ± SD (standard deviation).Details were exposed to the analysis of variance (ANOVA) and mean comparisons were done by Duncan's multiple range test (p<0.05)[27].Duncan's test was performed using SPSS software version 22.0 (SPSS Inc, USA).For mathematical thin layer drying modeling MATLAB software version, 11 was used.

Effect of hot air assisted microwave drying on Moringa leaves
Moringa leave were dried at 0.2, 0.4, and 0.6 kW microwave power.The different microwave power levels used to infer drying rates of moringa leaves.These microwave power have significant impact in the drying characteristics of moringa leaves.From figure 1, higher microwave power, lower the drying time and higher the drying rate.At 0.2kW, time taken to dry moringa leaves is 20 mins followed by 0.4 kW is 16 mins and 0.6 kW is 14 mins.From the data, we infer that time taken for drying of moringa leaves at 0.2 and 0.4 kW is more than 0.6 kW.The drying rate at 0.6kW is 0.2 g water/g solids/min which is higher than drying rate from 0.2 and 0.4kW.Similar findings have been reported for moringa leaves microwave dried at 150 -900 W [22].The drying rate increased with a decrease in drying time.This might be attributed to greater removal of moisture content at higher microwave power that resulted in enhanced drying potential.

Effect of hot air assisted microwave drying on Spinach leaves
From figure 2, the drying time for spinach is low and drying rate is high in 0.6kW.The time taken to dry spinach in 0.6kW is 34 mins followed by 38 mins in 0.4kW and 46 mins in 0.2kW.Drying rate of spinach at 0.6kW is slightly higher than at 0.2kW.The drying rate difference between 0.4kW and 0.6 KW is steep because of the internal temperature in the microwave dryer [28].Similar findings have been reported for spinach leaves microwave dried at 180 -900 W. Total drying time is from 7.7 min to 25 min [29].

Effect of hot air assisted microwave drying on Lemongrass
The drying time at 0.2, 0.4, 0.6 kW are 20, 16, 14 mins, respectively as shown in fig 3. The slope curve is not very smooth as temperature is constantly maintained at 50 0 C [30].The less drying time in lemongrass is due to high microwave power and hot air passed to it.Because moisture removed from centre to surface area by moisture diffusion.The higher microwave power and additional heat can make the moisture diffusion faster [30].

Effect of hot air assisted microwave drying on Ginger
From the figure 4, the least drying time is 20 mins observed at 0.6kW and the highest is 34 mins in 0.2kW.Regardless of microwave power, the drying of ginger is in falling rate period [31].The drying rate at 0.2 and 0.6 kW have slight variation with high drying rate in 0.6kW.The acceleration in the water molecules followed by the collapsing of binding force in the molecule, then drop in it due to heat removal [32].Similar results were obtained by [33] for microwave drying of ginger.The study reported that reduction in drying time had been observed by 3.1 times for ginger slices dried at 900 W instead of 300 W [33].

Effect of hot air assisted microwave drying on Banana
The microwave power used for banana is 0.35, 0.45, 0.55 kW which is differ from other products like moringa, spinach, lemongrass and ginger as it uses 0.2, 0.4, and 0.6kW.Because in higher microwave power, banana become charred.The Charring of banana at 0.55kW implies the poor nutritional quality of banana.As most nutritional compounds are sensitive in nature [34].Time taken to dry banana at 0.35 kW is 38 min, at 0.45kW is 36 mins and at 0.55 kW, 34 mins.From the Fig 5, there is a gradual decrease in drying time followed by high drying rate with increase in microwave power.Moreover, bananas dried at 0.35 kW had shown high nutritional content than 0.45 and 0.55 kW.A similar result in banana samples (4.3±0:177 mm thick) dried using the following drying regimes and microwave (at 350 W, 4.3 mm thick sample) drying.The drying of banana slices took place in the falling rate drying period with convection drying taking the longest time [23].The higher the drying rates, the higher the power level in hot airmicrowave dried kiwi fruits [19].

Effect of microwave power on Effective moisture diffusivity
Effective moisture diffusivity (Deff) is the rate of moisture movement.From table 2, the higher Deff, the higher the microwave output power [22].High Deff for moringa leaves, spinach leaves, ginger, and lemongrass was observed at 0.6 kW, except banana.For banana, high Deff was observed at 0.55 kW.For optimizing the best drying conditions, the Deff value should be higher.The effective moisture diffusivity values (Deff), obtained from this study were within the general acceptable range of 10 -11 to 10 -9 m 2 .s - for food products [35].

Effect of microwave power on Activation energy
Activation Energy (Ea) is the minimum amount of energy required to start a reaction.From table 2, Ea decreases with an increase in microwave output power.Lowest Ea for moringa leaves, spinach leaves, ginger, and lemongrass was observed at 0.6 kW, except banana.For banana, low Ea was observed at 0.55 kW.For optimizing best drying conditions, the Ea value should be lower [23,32] (Maskan 2000, Ganesapillai, Murugan et al. 2012).An increase in effective moisture diffusivity and decrease in activation energy related to increasing in microwave output power, increase the water diffusivity and moisture transfer rate which in terms decrease the drying time [22,25]

Thin-layer drying models
The moisture ratio is proportionate to equilibrium moisture content and the moisture content difference of the dried material.In general, the equilibrium moisture content was taken as zero for microwave drying.The mathematical drying model for thin-layer drying was fitted using moisture ratio (MR) and drying time (t) (Fig 6).Using MATLAB, co-efficient of determination, root mean square error, and constant values are determined for 0.2, 0.4, and 0.6 kW microwave power.The obtained moisture ratio and drying time of all dried microwave samples were analyzed using different empirical and semi-empirical equations (table 1).Co-efficient of determination (R 2 ) was one the important bases in the selection of the best equation and root mean square (RMSE) was used to verify the fit consistency.For the best fit, the R 2 values should be the highest and the RMSE values should be the lowest [22] (Potisate and Phoungchandang 2015).From table 2, the best fit model for moringa leaves at 0.4 and 0.6 kW is the page model.At 0.4 and 0.6 kW, highest R 2 (0.9142 and 0.991, respectively) value and lowest RMSE (0.06328 and 0.02744, respectively) value was observed.At 0.2 kW, the highest coefficient of determination (0.9273) and lowest root mean square (0.08274) were obtained with Henderson and Pabis model.Similar findings have been reported for moringa leaves microwave dried at 150 -900 W. It was found that the three-parameter model gave the best fit for samples dried at 900 W [22] (Potisate and Phoungchandang 2015).For spinach leaves at 0.2 and 0.4 kW, the highest coefficient of determination (0.9961 and 0.9919, respectively) and lowest root mean square (0.01928 and 0.03217, respectively) were obtained with the page model.At 0.6 kW, the highest coefficient of determination (0.9992) and lowest root mean square (0.0288) were obtained with Midilli-Kucuk model.Similar findings have been reported for spinach leaves microwave dried at 180 -900 W [28] (Dadali, Demirhan et al. 2007).For lemongrass at 0.2, 0.4 and 0.6 kW, the highest coefficient of determination (0.9986, 0.9969, and 0.9863, respectively) and lowest root mean square (0.01157, 0.01804, and 0.04501, respectively) were obtained with two-term model.Similar findings have been studied in the microwave-assisted extraction of bioactive compounds from lemongrass [35] (Simha, Mathew et al. 2016).For ginger at 0.2 and 0.6 kW, the highest coefficient of determination (0.9988 and 0.9951, respectively) and lowest root mean square (0.01245 and 0.02665, respectively) were obtained with the page model.At 0.6 kW, the highest coefficient of determination (0.9977) and lowest root mean square (0.01617) were obtained with the diffusion model.Similar results were obtained by [32] for microwave drying ginger at 100 -900 kW.Table 3 shows the co-efficient of parameters (constant) values of models used for hot air-assisted microwave dried samples.The value of the coefficient of parameter k value was higher when the microwave powder increased.The same condition resulted in microwave drier spinach for microwave power ranging from 90 W to 350 W [37].

Effect of microwave power on antioxidant activity
Microwave dried moringa leaves, spinach, ginger, lemongrass, and banana were investigated for their antioxidant activity using DPPH assay (table 2.).The results inferred that moringa leaves, spinach, ginger, lemongrass dried at 0.6 kW and banana dried at 0.35kW showed the highest antioxidant activity.The increase in microwave power for drying of the samples, increase the antioxidant activity [38].The same condition has been found in similar work is done [22] in microwave drying of moringa leaves.In bananas, the increase in microwave power for drying the samples leads to degradation of its antioxidant activity [23].The highest antioxidant activity of moringa leaves, spinach, lemongrass, ginger, and banana was 79.31 ±0.02, 82.13 ±0.068, 89.74±0.035,91.95±0.534,and 82.28±0.0529,respectively.

Conclusion
The moringa leaves, spinach, lemongrass, ginger, and banana were dried under various conditions in a Hot air assisted microwave dryer.Microwave power, drying rate, effective moisture diffusivity, and antioxidant activity increased at 0.6kW in moringa, spinach, lemongrass and ginger while drying time and activation energy decreased.Compared to banana dried at 0.55 kW, the banana dried at 0.35 kW exhibited superior quality.The increased effective moisture diffusivity and decrease in activation energy decrease the drying time followed by an increase in microwave power.The lower the drying time, the higher the quality of the end product.The best fit model was evaluated by using an increased co-efficient of determination and reduced root mean square error.Page model gave the best fit for moringa leaves and ginger.For spinach, the Midilli-Kucuk model gave the best fit.Similarly for lemongrass, the best fit was obtained using two-term model.For banana, Henderson and Pabis's model was the best fit.Based on the result of the work, higher microwave power has higher antioxidant activity in moringa, spinach, lemongrass, and ginger microwave dried samples.For bananas, lower microwave power has better retention of antioxidant activities.An increase in microwave output power decreases the drying time.

Figure 1 .
Figure 1.Drying rate Vs time of moringa leaves at the different power level

Figure 2 .
Figure 2. Drying rate Vs time of spinach leaves at the different power level

Figure 3 .
Figure 3. Drying rate Vs time of lemongrass at the different power level

Figure 4 .
Figure 4. Drying rate Vs time of ginger at the different power level

Figure 5 .
Figure 5. Drying rate Vs time of banana at the different power level (Potisate and Phoungchandang 2015, Taha 2015).

Table 2 .
Effective moisture diffusivity, Activation energy, and Antioxidant activity of the means in the same column followed by different superscript letters are significantly different (P <0.05).Data represent the mean ± standard deviation of each sample (n = 3).* Values marked in bold show the highest Deff and lowest Ea values among the hot air assisted microwave dried samples.

Table 3 .
Coefficient of determination and root mean square error of different empirical and semi-empirical equations for all microwave dried samples

Table 4 .
Coefficient of parameters of thin-layer models for all dried samples