Kinetics of a thin‐layer microwave‐assisted infrared drying of lentil seeds

Herein, we report the drying kinetics behavior of tempered lentil seeds (CDC Maxim variety) by utilizing a microwave‐assisted infrared thermal method and thereby presenting a successful mathematical model for it. The drying characteristics of lentils using thin‐layer microwave drying with and without hot air predrying were evaluated in a laboratory scale microwave dryer. The drying experiments were carried out at 300 and 750 W, and the predrying experiment was performed at room temperature (23°C). Out of several thin‐layer mathematical models evaluated with the experimental data, Page model has been found the most appropriate model to predict drying process of lentils with high value of coefficient of determination (0.995), low values of chi‐square (0.0012), root mean square error (0.0343), and mean relative percentage error (4.9997). Further, the influence of bioyield force and changes in the particle density of processed seeds have also been evaluated in the present study. The results showed that combination of low infrared power (0.375 kW) to the different microwave power levels led to a significant reduction of drying time. The results also showed that processing of lentil seeds significantly reduces the bioyield force of raw seeds, providing less firmness to the product and thereby shortening the cooking time. The above findings can facilitate the design and operation of infrared‐assisted microwave drying of other legumes.

From a nutritional perspective, lentil is a pulse crop low in fat and rich in carbohydrates, proteins, dietary fiber, vitamins, and minerals, so it is an excellent choice component in a healthy diet (Wang, Hatcher, Toews, & Gawalko, 2009). Pulses, in general, have been found to reduce the risk of cardiovascular disease, cancer, diabetes, osteoporosis, hypertension, gastrointestinal disorders, adrenal disease, reduction of LDL cholesterol, and obesity (Ahmed, Mulla, Al-Ruwaih, & Arfat, 2019;Subhani, Shand, Pickardb, & Wanasundara, 2015). The high amounts of protein in lentil provide an excellent source of essential amino acids, decreasing concerns about food security and protein malnutrition in the world (Boye, Zare, & Pletch, 2010). The legumederived protein components can also be added into new innovative food products due to their wide range of functionalities like substitutes for meat and baked products.
Although these nutritional benefits of pulses/legumes have been well-documented, however, their use is very low than recommended intake levels (Boye et al., 2010). The reasons for these small intake levels include the following: pulses have some antinutritional components such as trypsin inhibitors, phytic acid, tannins, and oligosaccharides that reduce their digestibility (Wang et al., 2009); the existence of beany flavors in legumes could reduce their acceptance by people (Walker & Kochhar, 2007), and pulses need lengthy preparation or cooking time (Wang et al., 2009). To address these issues and increase the consumption of pulses, pretreatment with heat has been recommended. The effect of thermal heating, such as microwave, roasting, hydrothermal (boiling), and micronization (infrared radiation), on the structure of lentil has been studied extensively by different researchers (Gonza´lez & Pe´rez, 2002;Ma et al., 2011;Nongmaithem & Meda, 2017;Scanlon, Cenkowski, Segall, & Arntfield, 2005;Subhani et al., 2015). It has been reported that the presence of high moisture content in lentil seeds before thermal processing would play an essential role in determining the physicochemical properties of lentil (Ma et al., 2011;Subhani et al., 2015).
In the present study, the use of microwave thermal heating method in combination with the infrared process is the other novel thermal treatment, which can be used in the processing of legumes like red lentils. The combination of infrared with microwave would help in reducing the drying time of the lentils. Microwave thermal process leads to the accumulation of moisture at the food surface, so infrared heating can be added to remove the surface moisture of materials (Ozturk, Şakıyan, & Ozlem Alifakı, 2017). This method of dual processing will increase the porosity inside the seeds, resulting in the reduction of lentils cooking time by making lentils softer. The amount of this rise is highly dependent on thermal methods and its condition. For example, time of drying, temperature, and the amount of tempering moisture before drying play vital role for this aim in micronization thermal processing (Scanlon, Segall, & Cenkowski, 1999;Subhani et al., 2015;Zheng, Fasina, Sosulski, & Tyler, 1998).
The lengthy cooking time of legumes like lentils is one of the reasons for their reduced acceptability among consumers. Therefore, the pretreatment of these nutritionally beneficial legumes with combination of microwave and infrared method can be used to address this issue. Hence, the present study was aimed to investigate the drying kinetics of tempered lentil seeds by using new microwaveinfrared dual thermal processing technique and providing a mathematical model for the process. Further, to investigate the effect of this developed process on the reduction in cooking time of lentil, the particle density and bioyield force of processed seeds have been measured and were compared with raw lentil seeds.

| Lentil seeds
In the present research, the commercially available red lentil seeds (CDC Maxim variety) were procured from a local producer in Saskatchewan, Canada.

| Preparing seeds for thermal process
Thermal processing of seeds was performed and tempered to three higher levels of moisture contents (19%, 31.6%, and 47.1% dry basis).
To increase the moisture content of the sample, predetermined amounts of deionized water was added to lentil seeds in sealed jars as per standard protocols.
To calculate the required amount of water, the following formula was used: where W is the weight of water required (grams), L is the weight of lentil seeds (grams), Moisture t is the moisture content required at tempering, and Moisture 0 is the moisture content of seeds before tempering. The moisture content of lentil seeds before and after tempering will be determined using the method. The initial moisture content of lentil seed was found to be 8.5% dry basis (7.8% wet basis).

| Thermal process procedure
Advantium™ oven (GE Company, Louisville, USA) was used for infrared-microwave heating. The maximum powers of microwave and the upper halogen infrared lamps in this device are 0.70 and 0.75 kW, respectively. Two levels of microwave power, 0.35 and 0.7 kW, and three levels of infrared power 0, 0.375 and 0.75 kW were selected to perform drying of tempered lentil seeds until they return to their initial moisture content. During this process, the loss in weight of samples were recorded at varied time intervals from 15 s to 1 min, and drying experiments were performed in triplicates.

| Thin-layer mathematical modeling
Four semiempirical thin-layer models as shown in Table 1 were selected to fit the data obtained by the drying process of lentil seeds.
Moisture ratio in the models was calculated by the following equation:  Table 2.

| Particle density
The true volume of lentil samples was determined using a gas pycnometer (Micrometrics AccuPyc 1340). Based on the weight of the sample, the true density was calculated.

| Mechanical properties
The mechanical properties of raw and tempered lentil seeds were measured by using Instron machine (Model Instron 3366, Massachusetts, USA) equipped with a 50-kN load cell and integrator. The seeds were compressed between the plates of the machine until the first fracture occurred. This point in the force-deformation curve is known as bioyield point. In this study, the seeds were placed horizontally between the plates, and tests were carried out at a loading rate of 10 mmÁmin −1 for loading directions.

| Data analysis
The analysis of variance was performed using SPSS software and the general linear model, whereas the Tukey's Multiple Mean Comparison tests were carried out to determine the differences between various treatment.

| Drying behavior and evaluation of thin-layer drying models
The variations in the moisture content with drying time for different tempered lentil seeds at different microwave powers and infra- Thereafter, the drying data obtained for the different tempered lentil seeds were fitted to the different drying models, and the performance indicators of these models have been given in Table 3. It

| Thermal treatment and changes in particle density
The changes in the particle density of lentil seeds before and after tempering and drying processes have been shown in Figure 3. It is clearly seen from the figure that there is a significant reduction in particle density values of the raw lentil seeds when their moisture contents increase ranging from 1.441 (gr/cm 3 ) for the grain with 8.5 (% d. b) moisture content to 1.368 (gr/cm 3 ) for the grain with 47.1 (% d.b) moisture content. This reduction in particle density with increase in moisture content indicated that the rise in mass due to the gaining water to increase the moisture of grain is lower than the accompanying volumetric expansion of the seed. The reported results are in congruence with the observations of other researchers (Bhattacharya, Hampapura, & Suvendu, 2005;Gharibzahedi, Ghasemlou, Razavi, Jafarii, & Faraji, 2011;Irvine, Jayas, White, & Brixton, 1992;Maneesh Kumar, Sarat Chandra, & Debnath, 2018) Furthermore, it is significant if the reduction in particle density of lentil, which is created by increasing its moisture content, can be maintained during thermal processing. Therefore, the porosity will be made inside the grain by removing the moisture. Because the moisture diffusivity in the air space is considerably higher than in solids,

(a) (c) (b)
F I G U R E 1 Variation of moisture ratio against drying time at 0.35 kW microwave powers and different infrared powers for lentil seeds tempered to various moisture contents (a) tempered to 19%; (b) tempered to 31.6%; (c) tempered to 47.1% the seeds with higher porosity will have higher water uptake rate leading to reduction of cooking time. Scanlon et al. (2005) also evaluated the effects of micronization thermal treatment on the water uptake rate and rewetting coefficient of lentil and showed that micronization would increase such properties. They also reported a significant rise in moisture uptake rate for micronized lentils tempered to 17% moisture content from 13% initial moisture content, whereas for the higher tempered micronized lentils (25 to 45% moisture content), the rise was incremental and steady.
The particle densities of thermally processed lentil seeds have also been shown in Figure 3. The results clearly reveal that the reduction in particle densities of the processed grains are mainly dependent on the moisture contents prior to different thermal treatments, in a way that the drop in the particle densities of dried samples was just observed for those tempered to 19 and 31.6 (% d.b) moisture content.
The different powers of microwave and also the addition of different powers of infrared did not show any significant effect in this reduction.
However, the particle densities of dried lentil seeds tempered to the highest moisture content 47.1 (% d.b) and under different thermal treatments were not statistically different from the raw lentil seeds. This trend is not compatible with our expectation of reducing this property with drying treatment. This can be due to the the fact that such behavior can be found by looking at the final shape of these processed seeds as presented in Figure 4. As shown in Figure 4, it is clear that the rupture of seed coat and puffing of seeds occurred for the dried lentil grains tempered to 47.1% d.b (Figure 4d) and with lesser intensity for those tempered to 31.6% (Figure 4c) as compared with raw untreated lentils ( Figure 4a). However, the seeds tempered to 18% d.b were not puffed as presented in Figure 4b. The studies show that when samples were placed in the pycnometer device, its helium gas can penetrate inside the seed between the grain's cotyledons, which (a) (c)

(b)
F I G U R E 2 Variation of moisture ratio against drying time at 0.7 kW microwave powers and different infrared powers for lentil seeds tempered to various moisture contents (a) tempered to 19%; (b) tempered to 31.6%, (c) tempered to 47.1% T A B L E 3 Results of performance indicators of different thin-layer drying models of tempered lentil seeds results in measuring the lower total volume of seeds and presents higher density volume. It must be considered that even this phenomenon happened, especially for seeds tempered to the highest moisture content, their particle density did not exceed than the raw samples.

| Thermal treatment and changes in bio-yield point
To obtain a general idea of the compression curve shapes of samples, force-deformation curves for raw lentil seed and dried seeds tempered to various moisture contents under the thermal processing condition of 0.35 and 0.375 kW of microwave and infrared powers were presented in Figure 5. The first fracture point was considered as bioyield point presenting the crack creation in the seed. The multiple fractures followed by this point also shows the expansion of cracks inside the grain. Bhattacharya et al. (2005) also found a similar trend in the compression curve of raw and dehulled lentil seed.
The quantities of forces associated with the bioyield point for dried grains are appropriate values to compare the amount of seeds softness. The samples with lower bioyield forces are less firm; hence, they will be cooked faster. The multiple mean comparisons of these values for raw and processed lentil seeds has been demonstrated in Figure 6. It is clear that doing thermal processing will significantly reduce the bioyield force of raw lentil seeds. In addition, the results revealed that the amount of moisture content before drying is the key determining factor to this reduction, in a way that in the same condition of grains thermal processing, those tempered to higher moisture contents represented lower bioyield points.
It is also expected that by increasing the power of the microwave, a lower bioyield force will be achieved, due to the rise in the rate of drying and water movement in the seed, although the F I G U R E 3 The particle densities of raw, tempered and thermally processed lentil seeds

| CONCLUSIONS
The analysis of drying time of lentils reveals that increasing the microwave power from 0.35 to 0.7 kW will significantly reduce the drying time. Combining low infrared power of 0.375 kW to the different microwave power levels results in the reduction of drying time, especially when low microwave power is used. However, adding higher infrared power in various thermal processes did not noticeably change the drying time compared with the conditions using lower infrared power. Among different thin layer models, Page model has been found the most significant model to describe F I G U R E 6 The bioyield force of raw and thermally processed lentil seeds F I G U R E 5 Force-deformation curves for raw and dried lentil seeds tempered to various moisture contents under the thermal processing condition of 0.35 and 0.375 kW of microwave and infrared powers drying process of lentil in this study by having the high value of the coefficient of determination and low amounts of reduced chi-square, RMSE, and MRPE. Thermal process of lentil seeds tempered to 19% and 32% (d.b) decreases the particle density of lentil grains compared with raw lentil seeds, proving the expansion happened in the seeds, which leads to the reduction of cooking time. The rupturing of seed coat and puffing of seeds was observed for the thermal process of lentil grains tempered to 47% (d.b) and with lesser intensity for those tempered to 31.6% (d.b), affecting particle densities measurement. Hence, the use of particle density property is not enough to evaluate the effect of thermal processing on cooking time. Thermal processing of lentil seeds with microwave-infrared combination significantly decreases the bioyield force of raw seeds, providing the less firmness to the product, and hence, they need shorter cooking time.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study can be made available by the corresponding author upon reasonable request.