Effect of drying on kinetics, physiochemical, and antioxidant properties of black gram nuggets

Black gram (Vigna mungo) nuggets are locally known as “bori” and are an indigenous food and are consumed widely in the Indian subcontinent. The objective of the work was to compare the mode of selected drying techniques (hot‐air, freeze, and microwave drying) on drying kinetics followed by the development of suitable mathematical modeling for the process. Additionally, the antioxidant, color, and textural properties of the dried nuggets were evaluated for their acceptability and associated health benefits. Based on regression parameters, it was found that the Page model fitted well (R2 = 0.99) with the experimental data when compared with other models. The effective moisture diffusivity exhibited an inverse relation to drying time. Among tested drying techniques, it was found that TB in microwave drying at 450 W had the highest amount of phenolic content (5.27 mg/g), flavonoid content (1.72 mg/100 g), ferric reducing antioxidant power assay (45.71 μmol/g), 61.42 μmol/g of ABTS assay, and 67.81 μmol/g of DPPH assay values. The freeze‐drying products were better for physicochemical parameters than other drying process products. The presence of phytochemicals was responsible for the high bioactivity of microwave‐dried nuggets.


| INTRODUCTION
Most people worldwide use pulses as a valuable source of protein and other nutrients. It has also been reported that certain phytochemicals such as polyphenols, flavonoids, and phytosterols are found in legumes with health benefits (Sreerama et al., 2010). Protein-rich food consumption per capita in industrialized countries is high. However, the composition varies by geographical location. Typically, Europeans consume 70% more animal-based protein than the recommended daily allowance. It causes health risks because of the presence of higher cholesterol and other harmful fats, offering a threat to the development of a variety of diet-related diseases such as cardiovascular disease, cancer, and diabetes (Boada et al., 2016;Nadathur et al., 2017). To improve the nutritional content of meals, plant proteins could be a good choice as healthy enhancers or as a substitute for animal proteins. Plant protein boosts energy, supports the immune system, lowers the risk of cardiovascular disease, and aids weight loss (Nadathur et al., 2017). However, a mix of plant-based foods such as grains and legumes, which are commonly used in traditional diets such as the Mediterranean diet (Martínez-González et al., 2017), can offer vital amino acids and improve vascular health (Agnoli et al., 2017;McCarty, 2016).
Among plant proteins, black gram (Vigna mungo), a member of the Leguminosae family, plays a vital role in the human diet with proven health benefits. The black gram can be used with or without the husk.
The de-husked cotyledon of black gram, known as dal, is used to make dishes like idli, dosa, papad, and nuggets. On the other hand, the portions with husk, known as "Kaloi," contain a high amount of protein, fiber, vitamins, and minerals (e.g., calcium and iron) and are used to prepare some traditional foods. Such products have shown potential industrial applications in the quality improvement of various meat products. The husk (seed coat, aleurone layer, and plumule)-waste generated by the black gram processing industry-contains various nutrients, including dietary fiber and bioactive compounds and, therefore, has potential in functional foods.
Pulse nuggets are popular traditional food items in the Indian subcontinent. Overnight soaked, pulses are first ground to a smooth paste. It is then aerated to attain the proper consistency and density, and this form is known as a batter. Small aliquots of batter (≈10 g) are manually poured on mustard oil to rub over the tray, clean cloth, or wooden stage in arrays and sun-dried for 2 to 3 days until the nuggets become crispy with a hollow inner core. The batter must be dried using a mechanical drying system to produce the nuggets commercially. Hot-air, freeze-drying, and microwave drying are the most common available drying systems. The most popular and effective methods of drying food are hot air drying or tray drying. However, it destroys the squishy microstructure and reduces the amount of bioactive thermolabile compounds in plant products (Salim et al., 2017). Microwave drying, on the other hand, has more advantages, such as higher drying rate, lower drying temperature, homogeneous energy delivery on the material, formation of suitable final product characteristics, better space utilization, destructive effect on food, and providing better process control (Demiray et al., 2017). Mathematical models on different drying techniques play a significant role in designing, and optimizing the drying process and evaluating the nutritional status to define the properties of cereal, pulses, and lignocellulose plant materials (Saha et al., 2019;Sinir et al., 2019).
Several studies have been conducted on various aspects of nuggets, including the characteristics of the batter, the preparation of nuggets, and the drying behavior (Swami et al., 2006;Swami & Tovée, 2005, 2007a, 2007b. Those studies were focused on the preparation of nuggets from the only cotyledon part of a black gram by sunlight, convective air dryers, and textural profile analysis. However, a limited number of studies have focused on drying kinetics using various types of driers and nuggets' physicochemical and antioxidant properties. Therefore, the work's objectives were to compare drying techniques, namely, hot-air, microwave, and freeze-drying of black gram nuggets, followed by drying kinetics and the development of mathematical models based on those drying data. Additionally, the antioxidant (total phenolic content, entire flavonoid content, FRAP assay, DPPH assay, and ABTS assay), color, and textural properties of dried products were evaluated for their acceptability and health benefits. Various drying models were employed for nugget preparation and compared to understand the drying behavior.

| Preparation of nuggets
The initial moisture content of collected cotyledon, whole black gram, and husk was about 12% (dry basis, db). They were soaked in tap water at 23 ± 2 C for 4 h. The soaked product moisture content was 54% (db). All sample moisture content was analyzed by a moisture analyzer (Wensar Weighing Scales Limited, PGB1MB, India). It was ground to a smooth paste using a wet grinder (Panasonic, MK-SW200BLK, India) with the addition of drinking water. Then the batter was filtrated through the 60-mesh sieve, where 90% of the batter passed the sieve. The moisture content of the prepared batter was 68.11 ± 0.83% (db). The batter was whipped or beated for 90 seconds and operated at 1350 min À1 (approximately) to achieve air incorporation. Air incorporation level was 22.07 ± 1.44% (v/v) for cotyledon and whole gram but 5.16% (v/v) for tush samples. The reduced air intake in bran was the presence of dietary fibers in large quantities and carbohydrates and protein in small amounts. Nuggets were prepared by placing approximately 5 ± 1 ml of batter on fine cotton cloths and then in a different dryer. The nuggets were ready in cylindrical shape of 28 mm in diameter (D) and 23 mm in width (L). At least twenty (20) thicknesses were measured from different points, and only five (5) fell within this thickness. To maintain all drying processes, all samples' initial and final weight is 9.04 and 0.452 g, respectively.
After preparing the nuggets, the cotyledon nuggets or cotyledon bori or dal bori were marked as CB, whole black gram or kolai nuggets or kolai bori mark was marked as KB, and tush or husk nuggets or tush bori or khosa bori was marked as TB. After drying, the final moisture content of nugget samples was 2.877 ± 1.387% (db.).

| The drying of nuggets
Three types of dryers were used in this study to dry the sample and repeated triplicate of every kind of drying.

| Hot-air drying
Hot-air drying was performed in a tray dryer (Mac Pharma Tech, SS-24, India), and the dimensions of each tray were 0.75 Â 0.35 m. The relative humidity of the drying air was not regulated, and it varied from 15% to 40%. During the drying process, the dryer was operated at 60 C for 6 h of CB, 6.5 h of KB, and 5.5 h of TB, and the air velocity was maintained at 1.5 m/s. Before placing the sample on the stainless-steel tray, the dryer was left on for at least 20 min to reach steady-state conditions. Then 200-g samples of nuggets were placed onto the perforated stainless-steel tray coated with a muslin cloth in a single layer. Each piece (approximately 9.04 g) was withdrawn at 40-, 45-, and 50-min intervals of TB, CB, and KB, respectively, to determine the moisture content at different drying times.

| Freeze-drying
Here 200 g of nuggets prepared from batter was frozen at À40 C for 4 h in an ultra-low temperature freezer (New Brunswick Scientific, C340-86, Cambridge). Then freeze-drying was performed in a freeze dryer (Eyela, FDU1200, Japan) at the pressure of 14 Pa and the condenser temperature of À45 C for 8 h.

| Microwave drying
A microwave oven (Samsung, CE1041DFB/XTL, India) was used to perform the microwave drying, and the working frequency was maintained at 2450 MHz and adjustable irradiation time and power levels.

| Drying kinetics
The drying kinetics of nuggets were tested using six selected models, as listed below. The experiential models were obtained directly from the general solution of Fick's law and are used for designing new or better drying systems (Nadi & Tzempelikos, 2018). The WPS Workbench Ink. (World Programming, WPS Analytics, UK) was used to perform the regression analysis. The better suitable fitted model was ascertained by the equivalence of the less sum of square error (SS error ), the root-means-square error (RMSE), and more correlation coefficient where ŷ is the mean response and y i is the ith observed response value. MR exp,i is the experimental moisture ratio, MR pre,i is the predicted moisture ratio, N is the number of observations, and n is the number of constants in the drying model.
M t is the moisture content at time t and M 0 is the initial moisture content. All moisture contents were expressed in g water/g dm; a, b, c, g-dimensionless drying constant; k-constant of drying velocity (h À1 ); n-dimensionless drying constant; and t-time (h).
Furthermore, effective moisture diffusivity (D eff ) for cylindrical nuggets was calculated using the following equation: L was the Sample half-thickness (L = 11.5 mm). Temperature and moisture diffusivity were constant and shrinkage was negligible throughout the drying process.

| Measurement of bulk density and porosity
The bulk density of the black gram nuggets was measured by the procedure described by Gursoy et al. (2013). The following formula calculated the percentage of porosity of black gram nuggets.
BD was the bulk density, and PD was the particle density of the black gram nuggets.
The following formula calculated the volume of black gram nuggets.
2.5.2 | Texture profile analysis (TPA) TPA tests were carried out using a TA.XT texture analyzer (Stable Micro System Ltd, TA. XT Express, UK) (Swami et al., 2007). A 36-mm diameter (P/36R) aluminum cylindrical probe was used to measure the TPA of the bori samples.

| Color measurement
The color measurements were conducted in a Hunter Lab colorimeter (Hunter Associates Laboratory Inc, 45/0 of color flex, USA) as described by Nahar et al. (2022). The color change of batter and nuggets were calculated by color difference value (ΔE*) during the drying process.
where 0 refers to the color of the batter of black gram nuggets and ΔE* denotes the color changes of batter and nuggets of black grammention replications.

| Antioxidant properties
The 1 HPLC study was performed for three types of nuggets (CB, KB, and TB) that were prepared by drying in a microwave at the power of 450 W because the nuggets designed in microwave drying had high TPC and TFC content.

| Statistical analysis
All mathematical model was conducted on WPS Workbench Ink.

| Drying of black gram nuggets
Moisture ratio (MR) was plotted against dehydration time (Figure 1).
The figure showed that the MR value decreased with time in an asymptotic manner. The probable cause might be the removal of free water.
The rate of moisture loss was rapid in the beginning but it slowed down as the vapor diffusion increased as explained in the previous study Sung et al., 2020). The effective moisture diffusivity was calculated using Equation 5, and the values for dried CB, KB, and TB varied from 84.6717 Â 10 À8 to 0.7925 Â 10 À8 , 118.9607 Â 10 À8 to 0.7605 Â 10 À8 , and 48.676 Â 10 À8 to 0.7305 Â 10 À8 m 2 /s, respectively ( Table 2). The highest value of moisture diffusivity was obtained in microwave drying at 600 W, and the least was brought in the freeze-drying of nuggets. Similar variations were reported for spinach and corn cob (Bualuang et al., 2017;Saha et al., 2019). freeze-drying comes in second place after tray drying due to poor capacity and high operational costs The benefits of microwave drying as compared with convective drying which come from heating the product from the inside by using electromagnetic radiation, which causes the development of internal vapor pressure, which drives moisture out of the product with short drying times and an increase in drying rate (Dronachari & Yadav, 2015;Nahar et al., 2022).
For dried black gram nuggets, six mathematical models (Table 1, models 6-11) were applied to MR data, resulting in better instruction for all models except the logarithmic model (Table 2)  in earlier studies (Heydari et al., 2020). Among freeze and tray drying, freeze drying was least preferred due to its low capacity and high operational cost. When comparing all drying methods, microwave drying is the most efficient in terms of price and sample drying time (Nahar et al., 2022).
A graph is created based on the Page model (see Figure 1).
According to the study, Deff with moisture content decreased dramatically in the initial part of the dropping rate period but reduced more gradually in the subsequent phase. Graph d displays the linear relationship between the expected and observed values. Based on the fact that the curve is linear and slopes 45 from the origin, we can say that the projected model fits the actual drying data well.
Photographs of batter and dried nuggets are shown in Figure 2.

| Structural analysis
The structural properties of black gram nuggets were assessed using various conditions by comparing the products' hardness, resilience, and porosity. Hardness and resilience for different drying procedure were increased in the following order: FD < TD < MD-600 W < MD-450 W < MD-300 W < MD-180 W (Table 3) In the sample study of black gram nuggets, the hardness was increased with the decrease of the porosity and moisture value, and a similar correlation study was observed in date flesh (Rahman & Al-Farsi, 2005). In the microwave study, MD 450 W process was best for low hardness, low resilience value and high porosity value. Concerning the increased matter of porosity, decreased hardness, and resilience value, the KB sample was better than the other samples.

| Color
Three chromatic coordinates, L, a, and b, which stand for brightness/ darkness, redness/greenness, and yellowness/blueness, are used to assess color variations in food products. This is dependent on the kinds and numbers of particular constituents. The browning process and pigment oxidation are the major causes of color alterations following a heat treatment (Youssef & Mokhtar, 2014). The high ΔE and low L*, a*, and b* values indicated that the whole microwave drying method had significantly blackened the nuggets. All FD and only TD-KB nuggets had significantly lightened, as could be supported by the    (Table 4). Similar experimental results were reported in previous studies by Apinyavisit et al. (2017). When the samples were dried in the microwave at different powers, the cellular structure's nonenzymatic browning and superior degree of degradation at high temperatures were responsible for the color change.

| Antioxidant properties
The radical scavenging activity of plants can be measured, a quick and efficient technique to gauge their antioxidant capacity. Here, the FRAP, DPPH, and ABTS assays assess antioxidant activity. The total polyphenol (5.271 ± 0.0881 mg GAE/g dm) and flavonoid (1.7214 ± 0.1702 mg QE/100 g dm) contents were highest in all TB than KB and CB (Figure 3a) samples. The seed coat of Pulses was a significant contributor to the antioxidant content of whole seeds (Dueñas et al., 2006;Luo et al., 2016). The total polyphenol and flavonoid F I G U R E 2 Picture of different nuggets at different drying process content was observed in chickpea (Sreerama et al., 2010), mung beans, and faba beans (Boudjou et al., 2013;Luo et al., 2016;Tajoddin et al., 2010). Dark-colored pulses had been reported to have higher levels of polyphenols than pale ones and, at the same time, had higher antioxidant activity (Xu et al., 2007).
Microwave drying methods significantly enhanced the accessibility of the phenolic and flavonoid components because of the unblocking of phenolic and flavonoid content in microwave samples ( Figure 3a). Similar work was observed in a previous study (Bualuang et al., 2017;Ragaee et al., 2014). In short, the observed trend for the variety of antioxidant content was: MD-450 W > MD-600 W > MD-300 W > FD > MD-180 W > TD (Figure 3a). High-temperature inactivated polyphenol oxidase during microwave drying is the key enzyme responsible for enzymatic oxidation, resulting in the best perception of the unbound antioxidant compounds. The polyphenol oxidase activity was high at temperatures close to 55 C, but at 75 C, it was reduced rapidly and was observed in different drying processes (Goula et al., 2016;Mphahlele et al., 2016). were mentioned in (Figure 3c).
An easy and effective way to rank plants for antioxidant activity is to determine radical scavenging activity. It has been reported that the effect of various antioxidants depends on the presence of hydroxyl groups as free radical scavengers, quenchers of singlet oxygen, reducing oxidants, or chelators of metal ions as phenolic compounds (Vashisth et al., 2011). In this study, the antioxidant activity of all nugget extraction was assessed by DPPH, ABTS, and FRAP assays.
Significantly higher (p < 0.05) radical scavenging activity and reducing power were shown in microwave nuggets than in TD and FD samples.

| CONCLUSION
An effective processing method for nuggets was found for the microwave drying method, and the potential application of the method has been highlighted in this study. Mathematical modeling, the essential part of the drying system, is vital for designing and building a fully operational drying system. It might help to generate a product with high bioactivity and quality. Among the mathematical models, the Page model has been considered the best model to describe the drying properties of nuggets. The freeze-drying method has been defined F I G U R E 3 Variation of (a) antioxidant content, (b) antioxidant activity of different black gram nuggets with the drying process, and (c) HPLC chromatogram of nuggets (TB): gallic acid (4.266 min), protocatechuic acid (4.665 min), dihydroxy benzoic acid (4.815 min), catechin (5.517 min), caffeic acid (6.272 min), chlorogenic acid (7.202 min), vanillic acid (

CONFLICT OF INTEREST
The authors stated that they have no conflict of interest.

DATA AVAILABILITY STATEMENT
All data are present in the paper.