Comparative examination of leaching kinetics of soluble solids and effluent characteristics in different soaking processes of paddy parboiling

Rice (Oryza sativa L.) is one of the most important edible cereal crops, and it plays a major role in the world's economy (Kumar et al. 2016). Nearly two-thirds of the global population consumes rice to fulfil its daily energy requirement, and in Asian countries, approximately 75% of the calorie requirement is derived from rice (Roy et al. 2011). However, the increase in population causes a reduction in the stock of stored rice and increases the demand for paddy cultivation. Based on the Food and Agriculture Organization statistics in 2022, a considerable increase in global rice production is expected to reach around 523.9 million tons in 2023/24. However, rice processing is a combination of several steps to convert the paddy grains into milled rice. The majority of the population consumes milled rice, and people in Japan prefer well-milled sticky rice (Roy et al. 2011). Furthermore, paddy is processed into rice and consumed either in raw or parboiled form, and approximately 50% of the world's population consumes parboiled rice (Miah et al. 2002; Sayanthan & Thusyanthy 2018).

Parboiling is the hydrothermal treatment given to the paddy grains so as to gelatinize the starch and improve the milling yield (Behera & Sutar 2018). Although parboiling induces physical and chemical changes in paddy grains, it reduces insect attack and retains a higher amount of nutrients (Behera & Sutar 2018). In addition, the protein content of parboiled rice (11.34%) was greater than that of rough rice (8.38%), and the calcium level of parboiled rice is six times greater than that of raw rice (Muchlisyiyah et al. 2023). From this literature, it is clear that the parboiling process directly affects the nutritional content of the rice grains and improves the quality of the final products. Moreover, Pathiraje et al. (2011) reported a maximum reduction in glycemic index (GI) of 10% in parboiled rice compared to raw rice, and according to this reported work, parboiling reduced the GI in rice because of the retrogradation of starch during the parboiling process (Pathiraje et al. 2011). Furthermore, the parboiling process consists of the following three major steps: soaking, steaming, and drying (Behera & Sutar 2018). Paddy grains hydrate during the soaking; starch gelatinization occurs during the steaming process; and restoring of hardness occurs during the drying process (Roy et al. 2019).

Soaking is a more time-consuming process, and the main objective of soaking is to obtain uniform water absorption up to 30% moisture content in wet basis (wb) (Miah et al. 2002; Leethanapanich et al. 2016). Additionally, soaking changes the quantitative and qualitative characteristics of grains including their colour, smell, and nutritional content (Thakur & Gupta 2006). Furthermore, different methods were used to soak the grains, such as cold soaking (CS), hot water soaking, and vacuum soaking (Behera & Sutar 2018). CS is the most common practice in the paddy soaking process, and it takes 36–72 h at room temperature (Miah et al. 2002, Neshankine & Kannan 2021). Extended soaking duration enhances microbial activities and increases the biological oxygen demand (BOD) (1,350–1,800 ppm) in soaking water with a pungent odour (Sayanthan & Thusyanthy 2018). To avoid these difficulties, hot water soaking was introduced, and it depends on the temperature of the soaking water (Champathi Gunathilake 2018).

Soaking in hot water accelerates the hydration rate considerably quicker than in cold water. Behera & Sutar (2018) reported that the moisture diffusivity values for 25 and 75 °C were 1.78 × 10⁻¹¹ m²/s and 7.17 × 10⁻¹¹ m²/s, respectively (Behera & Sutar 2018). According to this study, temperature and moisture diffusivity showed a positive correlation, and higher temperature adversely affects the head rice yield. Moreover, increasing temperature reduces the soaking time, and insufficient soaking time increases the percentage of grain breakage during the milling process (Bhattacharya & Subba 1966; Zhu et al. 2019). However, Leethanapanich et al. (2016) reported an increase in head rice yield of parboiled rice with enhancing soaking temperatures of 50, 60, or 70 °C for 5, 4, or 3.5 h, respectively (Leethanapanich et al. 2016). Moreover, according to Taghinezhad et al. (2015), the highest head rice yield (67.05%) was observed at 65 °C and the minimum head rice yield (62.16%) was found at 75 °C (Taghinezhad et al. 2015). Based on this literature, investigation of the optimum soaking duration and temperature in hot water soaking are essential to improve the milling yield of rice. To avoid these obstacles, Neshankine & Kannan (2021) suggested a submerged aerated soaking (SAS) process with the recirculation of water.

SAS is a novel investigation in the paddy parboiling industry with the aim of reducing the effluent strength. However, commercialization of the aerated soaking method is still in the developmental stage due to the higher energy consumption and final quality of the effluent. While using CS, the BOD of the soaked water is greater than in the SAS and hot water soaking processes. According to the previous works, the BOD value of the CS process was 1,039 mg/L and it was higher than the recommended level (80 mg/L) set by the World Health Organization (WHO) (Roy et al. 2011). Obviously, effluent with elevated BOD level reduced the overall quality of water. In addition, the BOD level of the SAS process was 600 mg/L (Neshankine & Kannan 2021). According to these investigations, it is obvious that the effluent in the CS contains minimum DO level with higher level of microorganisms, and these aerobic and anaerobic microorganisms consume solid materials: phenolic compounds, sugar, and amino acids in the soaking water.

However, leaching of total phenol (62 mg/L) and total sugar (641 mg/L) were higher in the hot water soaking process than in the CS (Roy et al. 2011). Panda & Shrivastava (2019) investigated higher leaching loss for hot water soaking (1.23 ± 0.078 g/kg) than for microwave soaking (0.321 ± 0.043 g/kg). According to this literature, the leaching of solids was higher in hot water soaking than other soaking methods, and it is essential to reduce nutrient leaching in the soaking process to maintain the final quality of grains. Furthermore, effluent coming from the parboiling process contains higher amounts of TS (998.1–1459.1 mg/L) and TDS (670 mg/L) content, and these values were greater than the recommended standards (TDS: 300–600 ppm) mentioned by the WHO (Sayanthan & Thusyanthy 2018). Moreover, direct discharge of this nutrient rich effluent causes significant environmental pollution including the eutrophication process (Sayanthan & Thusyanthy 2018). Considering these obstacles, it is necessary to reduce solid leaching in the parboiling industry using sophisticated strategies. Anyhow, based on the previous studies, evaluation of the leaching of soluble solids in the hot water soaking process and the comparison of three soaking methods, namely, CS, hot water soaking, and SAS processes, are still at the embryonic stage.

Therefore, in this study effluent characteristics and paddy grain parameters were measured during hot water soaking at six temperatures: 35, 40, 45, 50, 55, and 60 °C. Additionally, the CS and SAS processes were evaluated until the paddy grains reached 30% moisture content (wb). The TS content of the soak water was used to compare the soluble solids leaching during hot water soaking against CS and SAS. Notably, the aeration process demonstrated higher efficiency than hot water soaking in the parboiling process. The study also encompasses the kinetic analysis of soluble solids leaching during the soaking process. The findings offer significant insights into optimizing the final quality of paddy grains by identifying the most effective soaking method. By investigating the scientific aspects of three different soaking methods commonly used in the parboiling process, the study aims to develop an energy-efficient system that minimizes soluble solids leaching in commercial scale paddy parboiling. The novelty of this research lies in its comprehensive analysis of soaking methods using a novel SAS approach and their impact on both effluent characteristics and paddy grain quality, providing a foundation for enhancing the paddy parboiling process.

The experiment was conducted at the environmental and hydroresearch laboratory, Department of Agricultural Engineering, Faculty of Agriculture, University of Jaffna, Ariviyal Nagar, Kilinochchi, Sri Lanka.

The intermediate bold shape paddy variety Bg 352 was purchased from Mahaweli Authority, System-L, Sampathnuwara, Mullaitivu, Sri Lanka. Impurities in the paddy sample were removed using the winnowing process. Lighter materials (unfilled grains, weed seeds, and straw) were removed, and heavier components (heavier grains and stones) were recovered through this process (IRRI 2013). Moreover, these heavier grains and stones were separated using visual observation to improve the efficiency of the soaking process. Reverse osmosis (RO) water was used for the soaking process and high-purity toluene (C₇H₈) was used for the true density measurement.

Size, aspect ratio, moisture content, bulk density, particle density, porosity, and percentage of husk in the paddy sample were measured using the following standard methods to determine the initial physical properties of paddy grains.

Randomly selected clean paddy grains were taken for analyzing the length and width of paddy grains. The average length and width of the paddy grains were measured using a vernier calliper (E – PITA15, Nakamura Mfg. Co. Ltd, Japan) with a 0.01 mm reading accuracy. The length of grains was determined by measuring the length between two tips, and the longest perpendicular length of grain was considered as the width of the paddy grain (IRRI 2013).

Here, W_i is the initial weight of paddy grains (g); W_f is the final weight of paddy grains (g).

The SAS process was carried out two times using an experimental setup as shown in Figure 3. It was continued for 24 h to obtain uniform moisture content up to 30% (wb) in paddy grains. Water circulation was started after 12 h of soaking process. At the end of the initial 12 h of soaking, the effluent reached the minimum DO profile, and water circulation was started to maintain the DO content inside the soaking tank. All the parameters, namely, pH, TDS, TS, EC, DO, BOD, turbidity, moisture content, elongation percentage of paddy grains, and microbial cells, were quantified.

The hot water soaking process was conducted at 35, 40, 45, 50, 55, and 60 °C using an oil bath (BOA200/310, YAMATO SCIENTIFIC Co. Ltd, Japan) until paddy grains had reached the 30% (wb) moisture content. The temperature of the water within the beakers was measured using a thermometer (SK-250WP-N, SATO KECRYOKI, Japan). All tests were continued with two replicants. The pH, TDS, TS, EC, DO, BOD, turbidity, elongation percentage, and moisture content of the paddy grains were measured using standard methods. Finally, microbial cells were counted using a haemocytometer.

Hydration rate and elongation percentage were measured using the following standard methods.

Moisture content of paddy grains in wb was estimated per Equation (2.3.2) at 15-min intervals in the hot water soaking process and first 2 h of CS and SAS processes. After that, hydration rate was measured at 2 h interval in CS (up to 48 h) and first 12 h in SAS. After starting the aeration process, measurements were taken at 15-min intervals during the initial 2 h in the SAS process due to the changes in the DO profile in soaked water. Finally, measurements were continued with 2 h intervals until paddy grains had reached 30% (wb) moisture content.

Cleaned and sterilized sample bottles were used to collect the effluent without external contamination. Effluent measurements were conducted at the Central Environmental Authority, Northern Provincial office, Ponnagar, Ariviyal Nagar, Kilinochchi, Sri Lanka.

The pH of the soaking water was measured using a pH meter (HM-42X, DKK-TOA CORPORATION, Japan) at 15-min intervals in the hot water soaking and the first 2 h in CS and SAS. After that, 2 h duration was followed for the CS process and initial 12 h in the aeration process. Due to the changes in DO profile in soaked water, pH was measured at 15-min interval during the initial 2 h of the aeration process, and after that, measurements were taken at 2 h intervals during the SAS process.

TDS content of soaked water in the three soaking processes, hot water soaking, CS, and SAS, was measured using a conductivity meter (STARTER 300C, Japan) at a 15-min interval for initial 2 h in CS and SAS. TDS was measured at a 15-min interval in the hot water soaking process. After the initial 2 h of soaking, the TDS profile was observed at 2 h intervals during the CS and SAS processes. Due to the aeration process, measurements were taken at 15-min intervals during the first 2 h of the SAS process. Finally, 2 h interval was followed to collect the samples.

EC was measured using the conductivity meter (HI98331, Hanna Instruments, Romania) and measurements were taken at a 15-min interval in the hot water soaking and initial 2 h of CS and SAS processes. Subsequently, measurements were taken at a 2 h interval in the CS process and the initial 12 h of SAS process. Finally, EC was measured at a 15-min interval during the initial 2 h in the SAS process due to the aeration process, and 2 h interval was followed from 14 to 24 h during the SAS process.

A DO meter (DO 31P, DKK-TOA CORPORATION, Japan) was used to determine the DO level in the soaked water, and measurements were taken at 15-min intervals for the hot water soaking and initial 2 h of CS and SAS. The measurements were taken at 2 h intervals in the CS and SAS until paddy grains attained 30% moisture content (wb). Moreover, DO level was measured during the aeration process at 15-min intervals during the initial 2 h to determine the oxygen dynamics in soaked water, and the measurements were continued with 2 h intervals up to 24 h.

A turbidity meter (DR/890, HACH Company, USA) was used to determine the turbidity of the soaked water. The measurements were taken at 15-min intervals during the initial 2 h of CS and SAS processes and turbidity was determined at 15-min intervals in the hot water soaking process. Paddy grains were soaked until their moisture level reached 30% (wb). After the initial 2 h of soaking, turbidity was measured at 2 h intervals in CS and SAS (initial 12 h). Measurements were taken at 15-min intervals from 12 to 14 h in SAS process and continued with 2 h intervals due to the aeration process.

The DO content of the effluent was measured at the beginning and after 5 days by Winkler's titration method. The difference between these two DO values was correlated with the BOD₅ value, and measurements were taken at the end of the soaking processes.

Pseudo-second-order model was used to determine the leaching rate of the paddy grains, and the Peleg model was used to explain the sorption behaviour of the paddy grains during the soaking process. Furthermore, analysis of the leaching rate is essential for determining the efficacy of the soaking process.

According to Equation (2.9.7), ‘ + ’ was used in absorption or adsorption processes and ‘–’ was used during the desorption or drying processes.

Table 1 describes the physical properties of the paddy sample, namely, size, aspect ratio, moisture content, bulk density, particle density, porosity, and percentage of husk. These physical properties are used to determine the qualities of the final product (Varnamkhasti et al. 2008). The length, width, aspect ratio, moisture content, bulk density, particle density, porosity, and percentage of husk were 7.85 ± 0.014 mm, 3.08 ± 0.021 mm, 2.55, 13.84 ± 0.01%, 536 ± 0.03 kg/m³, 1,410 ± 0.0018 kg/m³, 62.07 ± 0.15%, and 36.92 ± 0.24%, respectively. According to that, the Rice Research and Development Institute (RRDI) in Sri Lanka classified Bg 352 as intermediate bold grains. Furthermore, these length and width measurements significantly affect the water absorption rate, and longer grains with a large surface area accelerate the water absorption process.

Furthermore, newly harvested dried paddy contained moisture content of less than 14% (wb) (Setyaningsih et al. 2016). This experiment showed that the initial moisture content of Bg 352 (13.84 ± 0.01%) was appropriate for the soaking process. Moreover, paddy with lower moisture content enhanced the moisture gradient between paddy grains and soaked water. Furthermore, Varnamkhasti et al. (2008) reported that bulk density, particle density, and porosity affected the rate of mass and heat transfer of moisture during paddy processing, and the bulk density of grains was utilized to design the soaking tanks with proper dimensions (Varnamkhasti et al. 2008). Moreover, grains with lower bulk densities contained more pore space and induced the water absorption process. Reddy & Chakraverty (2004) reported the bulk density of raw paddy (IR-36) as ranging from 522 to 566 kg/m³ with increasing moisture content, and this escalation was observed as a result of the increase in mass of the paddy grains during the soaking process (Reddy & Chakraverty 2004).

Furthermore, the particle density of selected paddy grains was 1,410 ± 0.0018 kg/m³, and Reddy & Chakraverty (2004) evidenced the particle density of raw paddy (IR-36) decreased from 1.405 to 1.348 g/cm³ with the increasing moisture content of paddy grains (Reddy & Chakraverty 2004). It has been evidenced from this literature that the higher particle density enhanced the compact grain structure with less pore space and decelerated the process of water absorption. The porosity value found for Bg 352 in this study was close to the value by Varnamkhasti et al. (2008), which was approximately 60% (Varnamkhasti et al. 2008). For raw paddy, porosity decreased from 62.84 to 58.01% with an increase in the moisture content of paddy grains, and high-porosity grains allow more water penetration during the soaking process (Reddy & Chakraverty 2004). Furthermore, husk acts as a protective barrier for grains, and higher percentage of husk directly affects the water absorption process in paddy soaking (Ejebe et al. 2019). According to these studies, it is clear that the physical properties of paddy grains: size, aspect ratio, moisture content, bulk density, particle density, porosity, and percentage of husk emphasize the final qualities of the product during the paddy soaking process.

After the initial 2 h of the CS process, the moisture content of paddy grains was increased by up to 30.03 ± 0.01% during 48 h with a decreasing rate. The reason behind this scenario is the reduction of the vapour pressure gradient between paddy grains and soaked water. In addition, the samba paddy variety required a prolonged soaking duration (24–48 h) compared to the nadu variety (24–30 h) due to the varietal differences of paddy (Kannan 2015). However, after the initial 12 h of soaking in SAS, the moisture content of paddy grains increased rapidly due to the water circulation and aeration process. As shown in Figure 4(a), after 14 h of soaking, the moisture content values for CS and SAS were 22.94 ± 0.665% and 25.54 ± 0.233%, respectively. However, the SAS process was ceased after 24 h due to the moisture content of 30.04 ± 0.106% (wb).

Figure 4(b) describes the relationship between moisture content and time in hot water soaking at six temperatures: 35, 40, 45, 50, 55, and 60 °C. As shown in the Figure 4(b), the rate of moisture absorption increased with increasing temperature. It was evidenced by Champathi Gunathilake (2018) that the hot water soaking at 70 and 80 °C took 2 and 1 h, respectively (Champathi Gunathilake 2018). In this study, the moisture content of paddy grains gradually increased up to 30% (wb) in the hot water soaking process. However, moisture content was increased linearly during the initial 15-min duration. Moreover, this was due to the permeability of the outermost layers, which rapidly obtained equilibrium with the hydration medium by capillary imbibition (Bello et al. 2004). Anyhow, the rapid increase in moisture content, which was similar to the Figure 4(b), was noticed when soaking paddy in hot water at different temperature conditions (60, 90 °C) (Oli et al. 2014).

According to these soaking methods, namely, CS, SAS, and hot water soaking, CS required longer duration (48 h) to reach 30% moisture content (wb) than the SAS process (24 h). This was due to the lower temperature (28 °C) of soaking water, and ultimately it reduced the penetration of water through the outer husk and slowed down the enzymatic activities responsible for breaking down the starch in paddy grains. According to this study, the time duration to obtain 30% moisture content in CS and SAS was 48 and 24 h, respectively. Therefore, it is obvious that the CS process in the parboiling process required an extended soaking duration, which affected the efficiency of the soaking process on a commercial scale. Moreover, as the temperature increased from 35 to 60 °C, the soaking time was reduced from 11 to 2 h. However, if the temperature increased more than the gelatinization temperature (70 °C), it caused the undesirable effects of splitting and leaching of solids in paddy grains (Bhattacharya & Subba 1966; Aghinezhad et al. 2016). Comparing all these obstacles, SAS was more suitable to maintain the optimum moisture content with minimum resource utilization.

After that, significant change was observed in the SAS approach, and the hydration rate was higher in the SAS process than the CS after the initial 12 h of the soaking due to the water recirculation process. However, paddy grains attained 10.02 ± 0.04 of elongation percentage after the 24 h of the SAS process due to the moisture diffusion between paddy grains and the soaked water. Moreover, similar results were reported by Neshankine & Kannan (2021) who observed 10.36% elongation. Compared to the CS process, the SAS method showed the greater swelling percentage because of the higher hydration rate than the CS process.

Furthermore, in this study, it has been proven that there is a positive relationship between the temperature and the moisture absorption process. According to that, obviously, elongation percentage was changed with the temperature (Perez et al. 2011). Paddy grains soaked at elevated temperatures were set to achieve 30% moisture content within a shorter period than paddy soaked at ambient temperature. Hence, a higher elongation percentage was obtained in the hot water soaking process compared to the other soaking methods. According to Figure 6, the elongation percentage obtained by paddy grains at 35, 40, 45, 50, 55, and 60 °C were 5.79 ± 0.07%, 6.80 ± 0.06%, 7.14 ± 0.08%, 8.18 ± 0.06%, 9.42 ± 0.05%, and 10.06 ± 0.01%, respectively. According to Bayram et al. (2004a), a sharp rise was observed in the percentage change in length of soybean during the soaking process, and the elongation percentage was higher during 30 min for the 50 and 70 °C soaking processes (Bayram et al. 2004a). This literature emphasized the significance of the temperature influence in the hydration and elongation of grains. According to that, elevated temperatures enhanced the elongation of grains more effectively than lower temperatures (Kale et al. 2017).

Furthermore, starch granules tend to reduce the absorption and swelling capacity in the CS because of hydrogen bonds between the amylose and amylopectin (Rocha-Villarreal et al. 2018). Anyhow, during the hot water soaking, these hydrogen bonds disrupted and facilitated the absorption by starch granules, resulting in greater elongation percentage than the CS process (Rocha-Villarreal et al. 2018). However, elevated temperature enhanced the subsequent leaching of solids during the soaking process (Bayram et al. 2004b). Furthermore, prolonged soaking duration in the CS process and subsequent leaching losses in the hot water soaking directly affected the final quality of the product, and considering these factors, the efficiency of the SAS process is greater than other soaking methods. Overall, the performance of the SAS approach is more appropriate during the commercial scale paddy parboiling process.

The initial pH in the CS and SAS was reduced up to 6.73 ± 0.01 and 6.65 ± 0.06, respectively, during the initial 12 h of the soaking. This gradual decrease was observed because of the microbial respiration, and degradation of the phenolic compounds, namely, phenolic acids and their aldehydes, which caused a reduction in pH in the soaked water (Kannan 2015; Setyaningsih et al. 2016). Moreover, organic matter was decomposed by the aerobic bacteria in soaked water, resulting in carbon dioxide, which was dissolved in the water and produced carbonic acid, reducing the pH of the soaked water (Waters et al. 2011; Ali & Mishra 2022). However, as shown in Figure 7(a), minimum pH value of 6.11 ± 0.04 was observed at the end of the CS process due to the prolonged soaking duration and microbial activities that caused the release of amino nitrogen and phenolic contents into the soaked water (Ramalingam & Raj 1996; Roy et al. 2011).

In addition, after 12 h of soaking, the aeration process was started, which directly affected the pH of the effluent. This effect was illustrated by Figure 7(a), and the pH of the solution was increased from 6.65 ± 0.06 to 7.35 ± 0.01. However, the circulation of water and the sprinkling of water bubbles through the showerhead caused the exchange of gases between the water and the surrounding air (Kannan 2015; Ali & Mishra 2022). Ultimately, it reduced the dissolved carbon dioxide level by increasing the pH in the effluent (Ali & Mishra 2022). Moreover, a previous study, related to the aerated soaking of paddy, also reported a higher pH level of 6.62 ± 0.08 than the CS process (Kannan 2015).

Furthermore, Figure 7(b) represents the findings of the current study about the impact of temperature on the changes in pH in the paddy effluent. According to the observed trend, the pH was reduced with an increase in temperature during the soaking process. As shown in Figure 7(b), a progressive reduction in the pH (5.79 ± 0.06) was observed at 60 °C. According to these results, elevated temperature increased the leaching of soluble compounds from the paddy grains and reduced the pH of the soaked water. Moreover, a prior study using hot water to soak soybeans also reported a lower pH level at elevated temperatures (Bayram et al. 2004b). Moreover, during the hot water soaking, starch in paddy grains is hydrolyzed into sugar, and phenolic compounds, which are present in paddy husk, are released into the soaked water (Ramalingam & Raj 1996). In addition, these phenolic compounds are noxious even at relatively low concentrations, and the amount of phenolic compound in rice mill effluent was 16.21 mg/L (pH 4.8) (Kumar et al. 2016). According to this literature, it is clear that higher phenolic concentration resulted in a reduction in the pH level of the soaked water.

In addition, the dissociation of water molecules into ionic constituents, namely, hydrogen ions (H⁺) and hydroxide ions (OH⁻), was dependent on temperature, and higher temperatures caused a higher concentration of H⁺ ions, resulting in an acidic condition (Waters et al. 2011). Remarkably, according to this study, effluent which was quantified during the hot water soaking process showed the minimum pH level (5.79 ± 0.06 at 60 °C). Accordingly, the effluent in hot water soaking required extensive pretreatment methods compared to that of the CS and SAS processes before release into the environment (Kumar et al. 2016). Furthermore, effluent coming from CS was composed of a huge amount of microbial colonies with a low pH level (Ramalingam & Raj 1996; Kumar et al. 2016). Moreover, the effluent released from the SAS process showed the pH value of 7.35 ± 0.01, close to the neutral condition and less hazardous to the environment. However, there is limited utilization of the SAS process on the commercial scale due to the lack of knowledge. Therefore, sophisticated strategies are needed to enhance the SAS process in the commercial sector with minimum resource utilization.

Furthermore, after the initial 2 h, TDS concentration was increased at a decreasing rate due to the reduction of solids gradient between paddy grains and the effluent. However, at the end of the 12 h of soaking, the TDS level in the CS and SAS processes was gradually increased up to 0.7215 ± 0.019 g/L and 0.692 ± 0.023 g/L, respectively. After the 12 h soaking process, the TDS value rapidly reached up to 1.631 ± 0.027 g/L in the SAS due to the aeration process. Moreover, due to the aeration process, water bubbles were agitated, which improved the dissolved solids in the effluent rather than have them settle down at the bottom of the system. Moreover, due to the increase of aerobic microbial activities, complex solids were degraded into simple molecules, which affected the TDS concentration (Neshankine & Kannan 2021). After 24 h, SAS process was stopped as the paddy grains obtained the 30% (wb) moisture content.

Furthermore, the CS process was continued for up to 48 h, and the maximum value of TDS in CS was 0.945 ± 0.004 g/L. This value was greater than the reported value (522.1–833.1 mg/L) because of varietal differences of paddy (Sayanthan & Thusyanthy 2018). Although, Figure 8(b) represents the changes in TDS value in the hot water soaking process, ultimately, with the increasing temperature, TDS was increased. Moreover, TDS concentration in soaked water was greatly influenced by the temperature, and it showed a positive correlation with temperature (Kumar et al. 2023). As shown in Figure 8(b), TDS values at 35, 40, 45, 50, 55, and 60 °C were 1.166 ± 0.025 g/L, 1.245 ± 0.007 g/L, 1.367 ± 0.029 g/L, 1.567 ± 0.004 g/L, 1.614 ± 0.010 g/L, and 1.724 ± 0.013 g/L, respectively. According to these measurements, it is clear that TDS in soaked water depended on temperature. However, this increase was due to the greater mobility of ions with temperature, and ultimately it increased the solubility of solids (Kumar et al. 2023). Moreover, during the initial 15 min, the TDS concentration of effluent was rapidly increased at each temperature due to the higher concentration gradient between paddy grains and soaked water. This higher concentration gradient induced the leaching of soluble solids from the paddy grains.

Based on Figure 8, the final TDS values of soaked water in CS, SAS, and hot water soaking at 60 °C were 0.945 ± 0.004 g/L, 1.631 ± 0.027 g/L, and 1.724 ± 0.013 g/L, respectively. Therefore, paddy soaked in hot water exhibited the higher leaching of soluble solids. It has been reported in previous studies that the increase in temperature above the gelatinization point resulted in the leaching out of solids and reduced the quality of the head rice yield (Bhattacharya & Subba 1966). Anyhow, comparing these three methods, it is clear that hot water soaking of paddy enhanced the leaching of solids in paddy grains compared to the CS and SAS processes.

After the rapid increase, the EC was increased with a decreasing rate up to 12 h for CS (1.119 ± 0.017 mS/cm) and SAS (1.080 ± 0.0365 mS/cm) due to the reduction of the dissolved solid gradient between the paddy grains and soaked water. After that, as shown in Figure 9(a), the EC gradually increased up to 1.465 ± 0.023 mS/cm at the end of the CS process. However, Gerber et al. (2016) reported that the EC of the raw effluent was 1.588 mS/cm, which was greater than the measured value because of the dissolved ions or solids content of paddy varieties (Gerber et al. 2016; Kumar et al. 2023). However, after the 12 h of soaking period in SAS, the EC was rapidly increased owing to the suction force that occurred due to the aeration process (Neshankine & Kannan 2021). It is clear that the observed EC values after 14 h in CS and SAS were 1.147 ± 0.009 mS/cm and 1.974 ± 0.0144 mS/cm, respectively. Moreover, this suction force was able to alter the ion mobility and improve the ion concentration by increasing the EC in soaked water (Kannan 2015). However, as shown in Figure 9(a), the EC value of the SAS process (2.548 ± 0.042 mS/cm) was greater than the reported value (1.55 ± 0.29 mS/cm) due to the initial ion concentration and the microbial activities in the soaked water (Kannan 2015).

Furthermore, Figure 9(b) displays the relationship between EC and temperature. The EC was increased with increasing temperature because of the higher mobility of ions (Kumar et al. 2023). Moreover, warm temperatures improved the kinetic energy of water molecules and dissociated the dissolved solids into constituent ions, which led to more ions with greater conductivity (Kumar et al. 2023). EC values at 35, 40, 45, 50, 55, and 60 °C were 1.809 ± 0.007 mS/cm, 1.945 ± 0.011 mS/cm, 2.119 ± 0.021 mS/cm, 2.448 ± 0.007 mS/cm, 2.471 ± 0.020 mS/cm, and 2.651 ± 0.039 mS/cm, respectively. According to the present study, it is clear that elevated temperature modified the charged ions in the soaked water, allowing it to conduct electricity more easily. Furthermore, the EC value (3.6 mS/cm) in soaked water at 70 °C was greater than the EC at 50 °C (Bayram et al. 2004b). With that, it has been proven that the increasing temperature improved the solid content and EC in soaked water.

However, these measured values resembled the variations in TDS content in these soaking processes. It has been evidenced by Rusydi (2018), who reported the linear relationship between TDS and EC. According to that, TDS (mg/L) = K × EC (μS/cm), where K is the correction factor, and an average range from 0.5 to 0.9 is acceptable (Rusydi 2018). Overall, these findings were matched with the reported relationship. Furthermore, according to Figure 9, higher EC (2.651 ± 0.039 mS/cm) was demonstrated at high temperatures due to the higher amount of charged ions. Therefore, it is clear that the leaching of soluble solids in hot water soaking was greater than CS and SAS. Moreover, the inorganic dissolved solids (chloride, sulphate, nitrate, magnesium) were leached into the soaked water with increasing temperature (Bayram et al. 2004b). Ultimately, it resulted in reducing the mineral and nutrient content of grains by increasing the EC in the soaked water. Moreover, soaking at low temperatures is essential to minimize the solid loss in the soaking process. Comparing all these methods, SAS was introduced as an eco-friendly method to retain the dissolved solids in paddy grains with optimum EC.

Furthermore, as shown in Figure 10(a), at the end of the 12 h, the TS values in the CS and SAS processes were 0.742 ± 0.060 g/L and 0.737 ± 0.067 g/L, respectively. Moreover, the final TS value in CS was 1.236 ± 0.007 g/L, and this value was close to the reported value, which was 0.9981–1.4591 g/L in the rice mill effluent (Sayanthan & Thusyanthy 2018). However, this increasing pattern in the TS level was dominant in the soaking process due to the diffusion of water into the paddy grains. Moreover, internal materials in grains became more soluble owing to the dispersed water in the paddy grains (Thakur & Gupta 2006).

After the 12 h of soaking, the aeration process was carried out in SAS, and the TS level in the soaked water was increased rapidly up to 1.717 ± 0.027 g/L at the end of the 24 h. This significant increase was demonstrated due to the aeration process. However, this aeration enhanced organic matter degradation and chemical reactions, which caused the solubilization of solid materials. As a matter of fact, the total amount of solids degraded due to the aerobic microbial activities, and insoluble compounds were transformed into soluble materials using the enzymatic reactions of microbes (Zhang et al. 2004). Anyhow, SAS was responsible for the higher TS level compared to CS, and the SAS process was stopped after 24 h because the paddy grains reached 30.04 ± 0.106% moisture content (wb). According to the literature, the optimum moisture content for paddy grains to attain during an efficient soaking process was 30% (Leethanapanich et al. 2016). Moreover, excessive moisture absorption caused the splitting of the paddy husk and produced the lower milling yield during the paddy processing (Igathinathane et al. 2005).

Furthermore, Figure 10(b) displays the changes in TS values in the hot water soaking process. It is clear from Figure 10(b) that high temperatures produced more TS content than low temperatures. With an increase in temperature (35 to 60 °C), the amount of TS in the soaked water was increased from 1.316 ± 0.008, to 1.946 ± 0.045 g/L, respectively. This was referred to in prior work that used to explore the soaking process of grains, and according to that investigation, the TS level in soaked water increased because of the higher temperature and the soluble solids content in soaked water at 30, 50, and 70 °C were 0.5 g/g, 1.2 g/g, and 2.3 g/g, respectively (Bayram et al. 2004b). However, in this current study, leaching of solids in paddy grains was higher at elevated temperatures, and this was proven with qualitative parameters (colour).

However, based on these findings, hot water soaking performed the highest TS value (1.946 ± 0.045 g/L) compared to the CS and SAS processes. Ramalingam & Raj (1996) reported that the TS level was higher in hot water soaking than in the CS process. According to that study, the total sugar values in hot water soaking and CS processes were 27.9 mg/100 mL and 4.7 mg/100 mL, respectively. Furthermore, comparing the leaching rate of CS and hot water soaking processes, the SAS process was introduced to minimize the TS content. In conclusion, it is critical to develop sophisticated strategies to minimize the loss of solids in the paddy soaking process by adjusting the soaking time and temperature and implementing the pretreatment methods, including the steaming process.

Furthermore, DO values in CS and SAS processes were depleted to 2.78 ± 0.12 mg/L and 2.64 ± 0.07 mg/L, during the initial 12 h of the soaking, and caused the hypoxia condition in the soaked water. The reason behind this scenario was the prolonged soaking duration and the microbial activities in soaked water (Ali & Mishra 2022). Additionally, these microbial colonies consumed a considerable amount of DO to decompose the organic matter and ultimately reduced the amount of oxygen dissolved in water (Ali & Mishra 2022). Furthermore, after the initial 12 h of soaking, the DO was gradually decreased up to 1.18 ± 0.08 mg/L in the CS process due to the overpopulation of anaerobic microorganisms and the reduction of atmospheric exchange of oxygen into water due to the 1.5 m height soaking tank (Ali & Mishra 2022). Moreover, this is a deviation from past studies where a minimum DO level (0.9 mg/L) in paddy effluent was observed due to the inorganic reductants and microbial activities (Pradhan & Sahu 2004).

As shown in Figure 11(a), after the initial 12 h of the SAS process, a notable rise was observed in the DO profile due to the water circulation. However, the sprinkling of water through the fine holes increased the surface area of the water bubbles and ultimately improved the DO content in soaked water owing to the diffusion of oxygen from the surrounding air (Kamarudin et al. 2020). According to the illustration in Figure 11(a), two saturation–depletion cycles were observed because of the water circulation. After the 12 h of soaking, the DO level rose to the saturation level (7.89 ± 0.07 mg/L) with 70-min aeration process. According to these observations, it is clear that aeration enhanced the DO content because it caused the physical agitation in water bubbles by increasing the surface area and diffusion process (Ali & Mishra 2022). After that, the DO profile was depleted up to 3.21 ± 0.01 mg/L due to the aerobic microbial activities. Additionally, between 16 and 24 h, another oxygen saturation–depletion cycle was observed, and it took more time (2 h) to reach the saturation point (7.42 ± 0.08 mg/L). However, at the end of the SAS process, a rapid reduction in DO content (2.74 ± 0.04 mg/L) was observed due to the excessive proliferation of microbial colonies (Kannan 2015). A similar study on the DO profile the effluent in the SAS process reported a minimum DO content of 1.1 ± 0.4 mg/L (Kannan 2015).

Furthermore, the impact of temperature on the DO profile in paddy (Bg 352) soaking is shown in Figure 11(b). The DO contents at 35, 40, 45, 50, 55, and 60 °C were 5.88 ± 0.05 mg/L, 6.06 ± 0.05 mg/L, 5.4 ± 0.02 mg/L, 5.04 ± 0.07 mg/L, 4.21 ± 0.03 mg/L, and 3.72 ± 0.04 mg/L, respectively. According to that, the availability of DO in soaked water was reduced with an increase in temperature (Tai et al. 2012). Moreover, it has been reasoned that the higher temperature facilitated the molecular vibrations, which caused a decrease in the intermolecular spaces between water molecules (Ali & Mishra 2022). However, this reduction in DO levels with increasing temperature was further explained by Henry's law, and according to that, the solubility of the gases decreased as the temperature increased (Blath et al. 2011; Sander 2015). According to the previous literature, increasing temperature from 30 to 120 °C, the CO₂ solubility decreased and increased Henry's law constant (Blath et al. 2011). Ultimately, it reduced the ability of water to retain gases. Considering all this information, it is clear that as the temperature rises, the ability of water to retain oxygen diminishes, which aligns with this investigation.

Furthermore, effluent with a low DO profile was released from modern rice mills, which followed the hot water soaking process, and had drastic impacts on the environment with its high chemical oxygen demand (COD) (Ramalingam & Raj 1996; Kumar & Deswal 2021). Additionally, the reported COD value in the hot water soaking process was 2,491 mg/L, and higher temperatures accelerated the decomposition of complex organic materials, resulting in elevated COD (Ramalingam & Raj 1996). Based on the DO dynamics in the soaked water, the minimum DO value (1.18 ± 0.08 mg/L) was observed in the CS process due to the prolonged soaking duration (Ramalingam & Raj 1996). Furthermore, hot water soaking caused significant reduction in DO with higher energy consumption. Therefore, considering all these difficulties, the SAS process was investigated to minimize the effluent strength in an eco-friendly manner (Kannan 2015). In conclusion, the SAS process is more favourable than the CS and hot water soaking processes.

After that, the turbidity of the soaked water was increased at a decreasing rate up to 12 h. The cause of this was the compounds (sugars, soluble proteins, vitamins, and non-starch bound lipids) that were leached into the effluent during the soaking process (Bayram et al. 2004b; Yu et al. 2017). Moreover, the turbidity of the soaked water was significantly affected by the soaking duration and the temperature (Bayram et al. 2004b). However, based on Figure 12(a), turbidity was increased during 48 h, and the maximum turbidity (501.0 ± 1.4 NTU) was achieved at the end of the CS process. In addition, after the 12 h of the soaking, a significant increase was observed in the SAS process. This rapid escalation was noticed because of the circulation of water. However, TS content in the soaked water was the primary cause of the greater turbidity that was observed during the SAS process. According to Section 3.3.4, the TS content of the soaked water was increased up to 1.717 ± 0.027 g/L, comparatively considerable increase in the turbidity level (510.0 ± 1.4 NTU) was observed at the end of the SAS process. It is important to note that the turbidity increase was noticed owing to the increase in suspended solids (Packman et al. 1999).

Figure 12(b) represents the turbidity level at different soaking temperatures, and the increase in temperature modified the turbidity of the soaked water. The measured turbidity levels at 35, 40, 45, 50, 55, and 60 °C were 334.0 ± 7.07 NTU, 365.0 ± 5.66 NTU, 398.0 ± 5.66 NTU, 436.0 ± 7.07 NTU, 462.5 ± 6.36 NTU, and 521.0 ± 4.24 NTU, respectively. The change in turbidity at low temperature (35 °C) was less than at 60 °C due to the effect of temperature on the leaching of solid constituents (Bayram et al. 2004b). In case of higher temperatures, random movement of suspended particles was accelerated, resulting in a higher turbidity condition (Shi et al. 2022). However, this irregular physical motion of tiny particles in the fluid was referred to as the Brownian motion (Shi et al. 2022). Furthermore, a related study was reported by Bayram et al. (2004b), and in that literature, the turbidity in the effluent showed 46.09% increase from 30 to 50 °C due to the increase in temperature. Based on these results, it is obvious that an elevated temperature tends to increase the turbidity in the soaked water.

Furthermore, the measured turbidity values at the end of CS, SAS, and hot water soaking processes were 501.0 ± 1.4 NTU, 510.0 ± 1.4 NTU, and 521.0 ± 4.24 NTU, respectively. Remarkably, these turbidity values were observed as a result of leached solids from the paddy grains, which interfered with the penetration of light, resulting in environmental pollution (Kumar Karnena & Saritha 2022). Moreover, prolonged attenuation in light declined the photosynthesis process (Bessell-Browne et al. 2017). Hot water soaking has a more pronounced effect on the environment than the CS and SAS treatments in terms of turbidity. Moreover, the CS encompasses a lower turbidity level (501.0 ± 1.4 NTU) than the SAS process (510.0 ± 1.4 NTU). Anyhow, it required a prolonged duration (48 h) with larger quantity of water for the soaking process. Considering all these insights, the SAS process is more favourable than the other soaking methods, and further innovations are needed to show better efficiency in the parboiling process.

During the soaking process, starch, proteins, and fat, along with other nutrients, undergo considerable changes, simultaneously leaching certain vitamins and minerals from the grains (Rocha-Villarreal et al. 2018). For instance, Roy et al. (2011) reported that the total sugar content in paddy effluent was 47 mg/L, which provided the substrate for microbial growth. It is obvious that these reducing sugars (glucose and fructose) act as an energy source for the microorganisms and enhance their growth and metabolism (Gomes et al. 2010; Jufri 2020). Furthermore, as shown in Figure 13, the microbial count in the SAS process was reduced by 49.32% from the CS process because of the water circulation process. Moreover, water circulation created an artificial aerobic environment that diminished the anaerobic microbial proliferation inside the soaking column. Accordingly, it has been evidenced using changes in the DO profile in Section 3.3.5, and according to the DO level in the CS and SAS processes, the obtained DO values were 1.18 ± 0.08 mg/L and 2.74 ± 0.04 mg/L, respectively.

According to the illustration in Figure 13, the lowest microbial count (22.5 × 10⁴/mL) was noticed at 35 °C, while the higher microbial proliferation (77.5 × 10⁴/mL) was detected at 60 °C during the hot water soaking process. Even though the effluent included a considerable amount of nutrients, the hot water soaking procedure promoted fewer microbial colonies than the other soaking techniques. It is obvious that due to the acceleration of water molecules’ vibrations with temperature, the available oxygen content was reduced to 4.24 ± 0.07 mg/L (60 °C) as mentioned in Section 3.3.5. Ultimately, it improved the competition between microbes for the available resources and reduced the aerobic microbial population in soaked water with a shorter time period.

Furthermore, direct discharge of this soaked water with a higher population of microbes and leached nutrients, namely, organic and inorganic matter, caused the eutrophication effect, which led to undesirable growth of vegetative structures and algal bloom in the surface water bodies (Kumar & Deswal 2021). For instance, direct disposal of soaked water in the CS process with a higher microbial count and lower oxygen level caused significant damage to the environment, resulting in the mortality of aquatic animals (Kumar & Deswal 2021). Moreover, stagnant soaked water with higher microbial activities acts as a major source of health hazards (Karunaratne & Gunasekera 2009). In addition, due to the aeration process, anaerobic microbes tend to minimize their proliferation while reducing the pungent odour (Kannan 2015). Comparing all these reported works and findings, the SAS process was suggested as an eco-friendly method for the paddy parboiling industry.

In addition, Roy et al. (2011) discovered that the BOD level of CS was 1,039 mg/L, which deviated from the measured BOD level during the CS process (1411.5 ± 17.68 mg/L) due to external factors: temperature, soaking duration, and initial DO concentration (Roy et al. 2011; Kamarudin et al. 2020). Furthermore, the extended soaking duration facilitated the growth of microorganisms, compromising the various types of anaerobic bacteria, lactic acid bacteria, Staphylococci, and yeast (Miah et al. 2002). As shown in Figure 14, a considerable reduction in the BOD value (58.4%) was observed during the end of the SAS process compared to the CS. This reduction was primarily due to the water circulation process, and it created the aerobic environment inside the soaking tank. This resulted in the addition of oxygen to the soaking column, which improved the oxygen level in the soaked water (Kannan 2015). Moreover, this oxygen oxidized the organic material and reduced the demand for oxygen. Based on the previous findings, the reported BOD value for the SAS process was 750 mg/L, which was substantially greater than the value observed in this study (586.5 ± 12.02 mg/L). This was due to the initial DO level (8.12 mg/L) and organic matter content of the soaked water (Kannan 2015). However, based on these quantified BOD values, the BOD level in the CS process was obviously greater than the SAS process.

Furthermore, as shown in Figure 14, the BOD values were increased from 60 ± 5.66 mg/L (35 °C) to 379.5 ± 6.36 mg/L (60 °C) with increasing temperature. It is obvious that the temperature caused a significant impact on the BOD values in the paddy soaking process. However, when the soaking temperature escalated up to 60 °C, a considerable increase in the BOD value (379.5 ± 6.36 mg/L) was noticed during the soaking process. The reason behind this phenomenon is the effect of the DO level in soaked water, and the reduction of DO content caused the elevated BOD level (Kamarudin et al. 2020). The higher temperature reduced the solubility of the DO level and exacerbated the BOD level in soak water because of the acceleration of microbial activities (Kamarudin et al. 2020). Moreover, Roy et al. (2011) investigated that the BOD level in the hot water soaking process (30 mg/L–129 mg/L) was significantly lower than the current study value because of the variation in the microbial population and organic matter content of the soaked water (Roy et al. 2011).

Furthermore, continuous discharge of soaked water with higher BOD values caused the eutrophication effect in water bodies, resulting in oxygen depletion conditions with mortality of aquatic animals (Kumar & Deswal 2021). Lower BOD values were obtained during the hot water soaking process compared to the CS. The direct discharge of effluent with a higher BOD level and leached nutrients performed the greater adverse impact on the environment. Therefore, pretreatment methods for the soaked water were required before release into the environment. Overall, these findings highlighted the effluent strength in terms of BOD in different soaking methods, and further systematic modifications are required for the SAS approach to obtain maximum efficiency.

Furthermore, the pseudo-second-order parameters, along with the regression values (R²) and leaching rates (V_d), are listed in Table 2. As presented in Table 2, the experimental c_s values approximately coincided with the theoretical c_s values quantified by the pseudo-second-order model. The obtained experimental and theoretical c_s values during the CS process were 1.236 and 0.809 g/L, respectively. However, TS content, which was leached during the CS process, was fitted with the pseudo-second-order model at the end of the complete soaking process, as shown in Figure 15. In addition, the linearized second-order model was reasonably fitted with the SAS process with experimental c_s (1.717 g/L) and theoretical c_s values (1.444 g/L). Additionally, the calculated c_s value in SAS process was increased by 52.78% compared to the CS process. This increase was observed in the SAS process owing to the water circulation process, due to which the leaching process was accelerated and the solid gradient was diminished between the paddy grains and soaked water (Neshankine & Kannan 2021). It has been evidenced in Section 3.3.4, and the obtained concentration of TS (1.717 ± 0.027 g/L) was greater than the other soaking processes.

Furthermore, the second-order kinetic model was used at six different temperatures. In the hot water soaking process, total soaking duration was reduced with increasing temperature, as mentioned in Section 3.3.4. Due to that, the hot water soaking process was fragmented into four phases using soaking duration for clear understanding. According to the overall R² values in the hot water soaking process, this model was reasonably fitted with experimental data. The experimental c_s and theoretical c_s values at 35 °C were 1.316 and 1.269 g/L, and the model was fitted for kinetic studies of soluble solids leaching. Obviously, the tabulated k values were gradually reduced during the hot water soaking process. For instance, a gradual reduction in k value was observed during the hot water soaking at 45 °C, and the calculated V_d values were 0.021, 13.0 × 10⁻³, 4.51 × 10⁻³ g/L min. Accordingly, it is clear that the concentration gradient was reduced with soaking duration in the hot water soaking process.

Furthermore, due to the increase in temperature from 35 to 60 °C, the leached solid concentration in the soaked water was increased, and according to that, an excess amount of solid (0.648 g/L) was identified in the final effluent. Consequently, the leaching rate was increased from 3.43 × 10⁻³ to 0.081 g/L min with increasing temperature. However, elevated temperatures enhanced the molecular motion and improved the leaching of solid constituents from the grains (Sobouti et al. 2019). Similar studies were reported by Kumar et al. (2010) using the dissolution kinetic profile of solid materials, and the quantified dissolution rate values were increased from 1874.26 to 5565.25 kg/m³ min due to the effect of temperature (Kumar et al. 2010).

Furthermore, higher temperatures enhanced the moisture absorption process during the initial period of the soaking process, and it absorbed the moisture from the solution due to the hydrophilic nature of the paddy grains (Perez et al. 2012). However, the paddy grains swelled up due to the moisture diffusion process, and excessive swelling induced the subsequent internal stress in paddy grains (Perez et al. 2012). These water molecules provided a medium for the dissolution of solid constituents, and it eliminated valuable nutrients from grains into the soaked water (Costa et al. 2018). According to this literature, it has been clear that hot water soaking caused the higher leaching rate with minimum nutrient retention in grains. With that undesirable impact, the hot water soaking process is considered a less effective approach in the parboiling process. Furthermore, prolonged soaking duration and excessive water requirement minimized the efficiency of the CS process, and considering all these drawbacks, the SAS approach was more suitable for the soaking process in terms of efficiency with retaining the maximum nutrient content.

However, the coefficient of determinations for predicted TS extraction content during the SAS process showed a poor correlation with the experimental data (R² value 0.22). Therefore, Peleg's extraction model was used as an alternative model to evaluate the leaching of solids during the SAS process. The Peleg model is an empirical model and was initially used to describe the moisture sorption behaviours (Milićević et al. 2021). Moreover, this model has been applied to evaluate the solid–liquid extraction and leaching kinetics of various plant metabolites (phenolics) (N. Milićević et al. 2021). Recently, Peleg's extraction model was used to analyze the leaching of phytochemicals from beans during the hydration process (Kumar et al. 2024). According to that, the graph was plotted using 1/C_t and 1/t as shown in Figure S1, Supplementary information. As presented in Table S1, Supplementary information, the R² value of the SAS process was 0.56. That value was not accurately matched with the experimental data. Therefore, more extensive research is needed to validate this concept.

Furthermore, K₁ is the constant associated with mass transfer rate, and higher water absorption resulted in lower K₁ values (Turhan et al. 2002). As presented in Table 3, the K₁ values of CS and SAS were 25.396 min/% and 23.307 min/%, respectively, and the model was fitted with R² 0.91 in CS and R² 0.68 in the SAS processes. As represented in Table 3, K₁ values reduced from 14.308 min/% (35 °C) to 0.7499 min/% (60 °C) with increasing temperature. According to this condition, the water absorption was increased in the paddy grains and reduced the soaking duration, and it was proven in the Section 3.2.1. Adesina et al. (2017) investigated Peleg constant for paddy grains, and found that K₁ values for paddy at 50, 60, and 70 °C were 0.75 h/%, 3.79 h/%, and 2.03 h/%, respectively (Adesina et al. 2017). According to that, the magnitude of K₁ values in this work was different from prior literature values due to the varietal differences in paddy grains (Turhan et al. 2002).

Furthermore, K₂ is a constant that determined the maximum water absorption capacity, and lower K₂ values were associated with the higher water absorption capacities (Turhan et al. 2002). Moreover, the K₂ values in CS and SAS processes were 0.0293%⁻¹ and 0.0267%⁻¹, respectively. According to these K₂ values, water absorption capacity was higher in the SAS process than in the CS process. Furthermore, as presented in Table 3, the K₂ values increased from 0.0203%⁻¹ to 0.0346%⁻¹ with increasing temperature. Moreover, water penetration into the grains was induced due to the increase in temperature and reduced the soaking duration (Turhan et al. 2002).

Table 4 evaluates the environmental impact of paddy effluent and compares the effluent parameters with relevant regulatory standards. The pH value in the SAS process fell within the WHO recommended range (6.5–9.2) (Alhaj et al. 2020). Moreover, a pH of 7.35 ± 0.01 is considered neutral or slightly alkaline, which reduces the risk of harm to aquatic organisms. The TDS level in the CS process ranged from 0.5 to 1.0 g/L, while both SAS and hot water soaking processes exhibited elevated TDS content. This higher TDS level reduced the solubility of gases in soaked water and contributed to negative environmental impacts. For instance, water with elevated TDS level is less suitable for common uses (irrigation), and the higher the TDS, the greater the TS content in the soaked water (Afrad et al. 2020).

Due to the mathematical relationship between TDS and EC, the highest EC (1.724 ± 0.013 g/L) was observed during the hot water soaking process (60 °C) at 2 h duration. Compared to the CS and SAS processes, hot water soaking required less time but resulted in the highest TDS and TS profiles. Furthermore, BOD is a key factor influencing the effluent strength. Continuous discharge of effluent into the environment can cause hypoxia conditions in surface water bodies (Abdullahi et al. 2021). In the current study data, effluent from the CS process exhibited the highest BOD value (1411.5 ± 17.68 mg/L), and obviously it was greater than the recommended value by WHO (Alhaj et al. 2020). This was primarily due to the DO profile of the effluent, attributed to the prolonged soaking duration.

In summary, both the CS and hot water soaking processes resulted in greater changes in effluents, particularly in terms of TDS, EC, and BOD levels. However, the SAS process produced effluent with the least environmental impact compared to the CS and hot water soaking processes. Therefore, the SAS process is recommended as an alternative method to mitigate these obstacles and improve the environmental quality around the parboiling industry.

Table 5 describes the cost-effectiveness of the different soaking methods in this lab-scale experiment. As presented in Table 5, Rs 344.00 was spent to soak the 1 kg of paddy in the CS process. But it took 48 h to complete the soaking process. The highest cost (Rs 2996.25/kg) was observed for the SAS process compared to the other two methods owing to the installation cost. However, it required less energy compared to the hot water soaking process. For instance, the electricity cost of the hot water soaking process was Rs 50, and for the SAS process, it was Rs 9.00. Moreover, it reduced the energy usage with less environmental damage compared to the hot water soaking process.

In addition, most of the processors used firewood as a raw material during the hot water soaking process to soak the paddy. The ultimate outcome of that was the ash, and it directly affected the environmental quality and human health. Considering this comprehensive analysis, the SAS process reduced the soaking duration and energy consumption compared to the CS hot water soaking processes.

According to the present study, TS was higher in the hot water soaking process. Because of that, the expected BOD level should be higher in the hot water soaking process. However, in this investigation, a higher BOD level (1411.5 ± 17.68 mg/L) was observed in the CS process due to the extended soaking duration. Because of this finding, obviously, the BOD of the soaked water was in line with the microbial count. Higher microorganisms (23.75 × 10⁶/mL) were observed at the ambient temperature, and these microbes were reduced with the increasing temperature due to the thermal sensitivity. Furthermore, the leaching rate of the solid was gradually increased with increasing temperature and a higher leaching rate (0.081 g/L min) was detected in the hot water soaking process (60 °C). However, the hot water soaking process released low-quality effluent in terms of TS, TDS, EC, and turbidity content in the soaked water.

There were some mechanisms that contributed to the observed differences in these soaking methods. According to that, CS is the common process of the paddy parboiling, and it combines with the diffusion process. Slower diffusion was identified in the CS process compared to the SAS and hot water soaking processes. The aeration process facilitated the movement of water molecules and soluble substances through grains and resulted in faster moisture absorption. Moreover, the anaerobic microbial activities declined during the SAS process, and this improved the DO profile of the soaked water more than the CS. Anyhow, compared to the CS and SAS processes, the hot water soaking showed the greater solids leaching (TS, TDS, EC, and turbidity) due to the elevated temperature, because the higher the temperature, the greater the kinetic energy of the water molecules and the more enhanced the solubility of the substances. Based on all aforementioned information, the SAS process is more preferable than the CS and hot water soaking processes in terms of higher nutrient retention with minimum resource utilization.

Based on the findings of this study, the hot water soaking causes a considerable amount of nutrient leaching and an undesirable impact on the nutritional quality of paddy grains. Moreover, 2 h duration was required to attain 30% moisture content (wb) during the hot water soaking (60 °C) process, and it showed a higher leaching rate (0.081 g/L min) than the CS (28 °C). Additionally, the SAS approach showed lower levels of TS (1.717 ± 0.027 g/L) compared to the hot water soaking, showing its superior performance in striking a compromise between environmental sustainability and efficiency. When compared to the hot water soaking (379.5 ± 6.36 mg/L), the CS showed a higher BOD value (1411.5 ± 17.68 mg/L). In conclusion, effluent in CS and hot water soaking processes serve as a major indicator of environmental pollution.

To make clear the primary results of the soaking processes, experimental data are used to depict the overall effluent quality findings in Figure 19. Furthermore, not much research has been conducted to analyze the leaching kinetics of solids in paddy effluent, and this study organized to demonstrate the leaching rate in different soaking methods. Moreover, this investigation offers vital insights into the environmental effects of effluent produced during the parboiling process from various paddy soaking techniques with an emphasis on reducing the issues related to effluent release. Furthermore, this study demonstrates novel approaches that are needed in commercial-scale paddy parboiling to reduce soluble solids leaching and effluent strength and, subsequently, the ecological impact.

To ensure practical relevance, the study's findings suggest that the adoption of optimized soaking methods, such as SAS, could bring significant improvements in resource use and environmental impact within industrial applications. For instance, industries involved in paddy parboiling could benefit from increased water-use efficiency, reduced pollutant loads in effluents, and optimized soaking conditions. These practical recommendations can serve as valuable guidelines for promoting more sustainable and efficient practices in rice processing. Moving forward, further research should focus on accelerating these improvements, facilitating the broader adoption of environmentally responsible practices within the parboiling industry.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.