Sono-hydro priming process (ultrasound modulated hydration): Modelling hydration kinetic during paddy germination.

Application of ultrasound technology in modulating the hydration process during paddy germination was analyzed in this study. The effect of hydropriming (24 h) and sono-hydro priming (ultrasound priming, 12 h) on the hydration behaviour of paddies was determined at different temperatures (25-40 °C). Ultrasound pulse was applied for 10 min after every one hour for sono-hydro priming. Germination potential and microstructure analysis of treated paddies were also performed. Downward concave curve observed in hydration process of paddies indicates initial high-water absorptionthrough diffusion process. Sono-hydro priming process showed higher hydration rate compared to hydropriming. The changes in moisture content during hydration processes fitted to theoretical (Fick's model) and empirical model (Peleg model) exhibited high regression coefficient (R2 > 0.95) indicating suitability for predicting hydration behaviour in both paddies for germination. The Peleg model adequately predicted saturation moisture content and sono-hydro priming efficiently increased the water absorption rate. Effective moisture diffusivity determined from Fick's diffusion model increased for sono-hydro priming. Activation energy estimated from effective moisture diffusivity required in sono-hydro priming (Ea = 20.32 and 19.19 KJ/mol respectively) for pigmented rice and non-pigmented rice was lower than hydropriming (Ea = 27.11 and 32.15 KJ/mol respectively). Both hydration processes were endothermic and non-spontaneous inferred from thermodynamic properties. Sono-hydro priming exhibited < 95% germination potential with shorter soaking time (12 h) owing to the high mass transfer rate. SEM micrograph revealed water absorption through various micro-cavities during sono-hydro priming. Thus, sono-hydro priming potentially reduced the soaking process (approximately 50%) with higher germination rate in paddies beneficial for commercial malting of grains.

Introduction

Hydration of cereals and legumes is basically a process of soaking in water which increases the moisture content [1]. This hydration process is essential for the various industrial processes since it imparts a propitious effect on the physicochemical, textural and nutritional property [2] and is comprehensively used for hydration purposes in some necessary process specifically in germination and malting [1].

Germination is a traditional and natural process which eventuates after the hydration of grain. Hydration (soaking) during germination process aids in biochemical activation such as stimulation of phytohormones (gibberellic acid) which in turn induces the secretion of hydrolytic enzymes causing degradation of starch to simple compounds (glucose, maltose, maltodextrin etc.) which can be utilized by embryo for development [3].

Hydration is mainly a mass transfer operation and follows the principle of diffusion or capillary flow depending on the structure and composition of grains. The hydration kinetics is indeed a complex phenomenon that can be represented by two possible behaviours: downward concave shape (DCS) and sigmoidal shape [1]. The DCS is observed in most cereal grains and is characterized by a high hydration rate initially but decreases after reaching the saturation moisture content. On the other hand, sigmoidal behaviour has been observed absolutely for Fabaceae family grains and represented with a lag phase which ends with the seed coat hydration.

The intricacy of germination in grain involves long soaking requirements from 24 to 48 h for further metabolism to commence making it a very time-intensive process. The paddy composition is chiefly responsible for the long soaking process with husk as the outermost layer of the paddy, which provides the primary barrier to water absorption [4]. Further, starch is the main component in the endosperm resulting in a compact structure, which makes the hydration process excessively long. Thus, enhancement of the mass transfer process by modulating the germination process may be a possible solution for a shorter soaking period.

Recently different methods such as priming techniques have been studied for a reduction in the soaking time with improved germination rate. Priming can be defined as the exposure of grains to restricted water availability under controlled conditions which allows some of the physiological processes of germination to occur [5]. Some of the different chemical priming methods commonly applied are osmopriming, i.e. soaking in a solution of osmoticum, halopriming, i.e. soaking in salt solution and solid matrix priming, i.e. priming using solid carriers [5]. These priming techniques are reported to cause various changes in the physiological, biochemical and molecular structure such as increased protein synthesis, increased α and β amylase activity which in turn correlates to improved metabolic activity and higher seed vigour [6]. However, there is a significant disadvantage for the use of chemical priming methods for germination since the obtained malt may contain residues of these components after treatment, causing adverse health effects. Hydropriming method, i.e. soaking in water is one of the simplest priming methods which is economical and safe and has been used to increase the germination yield of various seeds such as okra, wheat etc. [5], [7].

Sono-hydro priming method (ultrasound technology in combination with hydropriming) can be applied to achieve a shorter soaking time with higher germination potential. Ultrasound technology enhances the mass transfer process in different food operations by two mechanisms: an indirect effect which involves the formation of micro-channel by the acoustic cavity and other one is the direct effect related to the inertial flow and generation of contraction and relaxation of the tissues for the flow of fluid [1]. Moreover, this technology is reported to improve both the hydration kinetics behaviour, i.e. DCS and sigmoidal one [8]. Application of ultrasound technology has been studied in the germination of pulses such as in mung beans and results revealed that ultrasound reduced around 25% of the hydration process along with an acceleration of the seed germination [9]. Further, ultrasound technology was also reported to increase the hydration behaviour in other cereals and pulses such as sorghum [10], common beans [8] and navy beans [11]. Thus, application of sono-hydro priming during the germination process shall potentially enhance the soaking period by modulating the compact structure of grain. Although ultrasound has been solely applied on various types of seed, sono-hydro priming has been rarely reported to stimulate the germination process of paddy by modulating the soaking process. Two different types of paddy from North-East India, i.e. pigmented and non-pigmented rice, will be taken into consideration in this study.

Moreover, understanding the kinetics of water absorption through the mathematical model is essential for the soaking process of paddy germination since it shall affect the impending processing operations. The water absorption kinetics in various cereals and legumes has been described by an empirical model, i.e. Peleg model or by Fick's diffusion model, which is an analytical model [12]. The Peleg model is an established non-exponential model which is commonly employed to describe the water absorption characteristics in food products. Similarly, Fick's model has been used in various agricultural products to model water diffusion characteristics [13]. However, a kinetic study employing Peleg and Fick's diffusion model for hydration behaviour during soaking of paddies pertaining to the germination process has not been studied in detail and requires further investigations.

The present study aims to employ sono-hydro priming technology in combination with different time and temperature conditions to enhance the soaking process during paddy germination. The objective of the study was: (a) to determine the effect of hydropriming and sono-hydro priming on hydration rate in paddies (b) to evaluate the applicability of Fick's diffusion model and Peleg model to predict the moisture absorption in paddies with both priming method (c) to evaluate the effect of hydropriming and sono-hydro priming on the germination potential of paddies and change in morphological structure.

Materials and methods

Raw material

Two paddies viz. Aijung rice, non-pigmented rice (NR) and Chakhao, black-pigmented rice (BR) was obtained from Tezpur, Assam and Manipur, respectively. The moisture content of the BR and NR paddy was observed to be 12.3 and 12.2%, respectively after harvesting (in 2018). On the other hand, the amylose content of BR and NR was found as 21. 92 and 8.16%, respectively. The paddies were washed to remove the husk and infested grain and only healthy grains were taken for the experiment. The chemicals used for analysis in this study were of analytical grade. The reagents were purchased mainly from Sigma, Merck India Pvt. Ltd. and Himedia.

Hydration process

Hydropriming process

The hydropriming process for BR and NR was carried out as mentioned by [7], [14] with some modification. The paddies were soaked for a different time interval (0 to 24 h) and temperature conditions (25 to 40 ˚C) to study the hydration process. For the process, 10 g of both paddies were selected (without any damage) and placed into different net bags. Thereafter, each bag was soaked in separate beakers containing distilled water (grain to water ratio 1:3 w/v) at four different temperatures, i.e. 25, 30, 35 and 40 ˚C. During the hydration process, the grains were periodically drained at an interval of 3 h, surface dried with a blotting paper and weighed. The grains were soaked again to continue the process up to 24 h. The moisture content (M at each interval was determined by the mass balance method, i.e. considering the initial grain mass (m), initial moisture (M) and the obtained mass of grain (m) determined during the hydration process after a specific interval. The application of this method enabled to determine the grain kinetic behaviour during the hydration process.

Sono-hydro priming

The sono-hydro priming of paddies was carried out by the method suggested by Patero et al. [10]. The hydration process was performed as per the procedure described for the hydropriming process with some changes. The different beakers prepared with the samples as mentioned in the hydropriming process were placed in an ultrasonic water bath (Aczet, Model: CUB-2.5 India), whose piezoelectric elements are arranged below the tub and generated mechanical waves are transmitted through water to the product. During the experiment, the operating frequency (40 kHz) and ultrasonic power (50 W) was fixed and the soaking temperature ranged from 25 to 40 °C. For each treatment, a pulse of ultrasound for 10 min was provided at an interval of 1 h and this process was continued up to 12 h of soaking. During this process, the grains were periodically (3 h) removed, drained, superficially dried with blotting paper, weighed and again placed in the beaker. The moisture content of the grain at each interval was evaluated by the mass balance as mentioned in the hydropriming process.

Further, in both the hydropriming and sono-hydro priming process, the soluble solids leakage from grain to the hydration water was neglected for estimating the moisture content by the mass transfer method. The experiments were conducted in triplicate to avoid any error and the average value of the moisture content were used for data analysis for modelling and representation.

Modelling of the hydration process

Fick's diffusion model

Fick's laws of diffusion and its derived equations have been utilized widely to model transport processes in the food. The solution was obtained assuming that paddies have invariable dimension, homogeneous characterstics, constant water diffusivity and the external mass transfer resistance was negligible. The infinite series diffusion equation was employed to model the water absorption process by paddy components [4].where MR = moisture ratio, dimensionless, M0 = initial moisture content, % d.b., Mt = moisture content at any time of soaking, Ms = saturation moisture content, % d.b., Ψi = constant for a given solid shape.

The shape of the paddy considered during mathematical modelling is mostly spherical [11] and cylindrical [4]. However, it was reported that compared to other simpler-shaped models, ellipsoid closely resembles the geometry of rice [15]. Therefore, in this study, an ellipsoid shape of the paddy was considered to solve the model.where Re = equivalent radius of an ellipsoid and λi = geometrical shape factor

The equivalent diameter of the ellipsoid is calculated as follows [16]where a and b are the semi-axes of the ellipse as shown in Fig. 1 and the dimensions (length, breadth and thickness) of around 100 paddies were measured using a Vernier calliper and the observations were recorded.

Fig. 1

Schematic of different dimensions of paddy as an ellipsoid.

The infinite series of the right-hand side of Eq. (1) converges rapidly to the first term when the Fourier number ( (Eq. (4)) becomes large [4] and the Eq. (5) is obtainedwhere is defined as the coefficient of the water absorption rate. Therefore, the suitable model could be determined from the experimental values of Ψ that were evaluated by applying a non-linear regression procedure. Further, the diffusion coefficients for paddies were estimated after fitting the absorption data to Eq. (5) and determination of Ψ and W (water absorption rate constant, h−1).

Peleg model

The Peleg model is a famous non-exponential, empirical model which has been employed for modelling the water absorption kinetics of agricultural products [17]. The significant advantage of this model is that it can predict the saturation moisture content of grains with short time experimental data [29].

The data obtained from the hydration process (hydropriming and ultrasound priming) were adjusted to the Peleg equation [12]

Mt is moisture content at time t (d.b. %), M0 is the initial moisture content (d.b. %), t is time (h), k1 and k2 are the Peleg rate and Peleg capacity constant, respectively. In Eq. (6), “±” becomes “+” if the process is absorption or adsorption and “−” if the process is drying or desorption. Curve fitting was performed by using Non-linear least squares method and Trust-region algorithm.

The rate of sorption (R) was determined from the first derivative of the Peleg equation

The Peleg rate constant k describes the sorption rate at the beginning (R), R at t = t

The Peleg coefficient constant k2 relates to maximum (or minimum) possible moisture content. As t = ∞. Eq. (6) gives the relation between saturation moisture content (M) and k.

The average moisture content was expressed as the dimensionless moisture content (Eq. (10)) and the values obtained were used for plotting the hydration curveswhere M is the moisture of hydrated paddies at time t (g/g d.b) and M is the initial moisture of the paddies (g/g d.b) and Eq. (6) was written as

Temperature dependency of Fick's diffusion coefficient and Peleg rate constant

It has been reported that Peleg rate constant (k1) and Fick's diffusion coefficient (Deff) was inversely related to temperature and its reciprocal defines the initial hydration rate [12]. Thus, the Arrhenius equation could be employed to explain the temperature dependence of the reciprocal of Peleg's constant k1 (water absorption rate) and Fick's diffusion coefficient (Deff) in the following manner:where: kref = reference hydration constant for Peleg model; Dref = reference moisture diffusion coefficient; Ea = activation energy (kJ mol−1); R = universal gas constant (=8.314 J mol−1 K−1); T = experience temperature (K). Logarithm was taken on both sides of the Eq. (12), the linear Eq. (13) is obtained:

The ln (1/k) and ln (Deff) was plotted versus (1/T) and a line with the gradient of (Ea/R) was obtained. The activation energy was calculated and the sensitivity of the constant to temperature can be analyzed. Curve fitting was performed by using MATLAB.

Thermodynamic properties of the process from the model

The thermodynamic properties of BR and NR paddies during hydropriming and sono-hydro priming process was determined employing Fick's model [8]. Fick's diffusion model was considered over the Peleg model to determine the thermodynamic properties since the former is a mathematical model which can provide higher predictability. The thermodynamic properties studied were activation enthalpy ΔH# (kJ mol−1), activation entropy ΔS# (kJ mol−1 K−1), and activation Gibbs free energy ΔG# (kJ mol−1). Here, R = universal gas constant (8.314 J mol−1 K−1), kb is Boltzmann’s constant (1.38 X 10-23 J K−1), hp = Planck’s constant (6.626 X 10-34 J s), T = temperature (K) and ln Z = is the intercept of the curve obtained from Eq. (13) [24].

Enthalpy of Activation (ΔH*)

Entropy of activation (ΔS*)

Gibbs free energy of activation (ΔG*)

The standard enthalpy of activation (ΔH#) was obtained from the conventional transition state theory and is related to the energy required to form the activated complex before the product is created. On the other hand, activation free energy (ΔG#) is associated with the spontaneity of the transition state formation and activation entropy (ΔS#) is related to the molecular order of activated complex [8].

Malting potential of paddy

The paddies (BR and NR) after hydropriming and sono-hydro priming process were tested for viability by evaluating the germination potential which is described by Germination Rate (GR), Dormancy Rate (DR), and Germination Capacity (GC) [3], [18]. It was performed by taking 100 paddies from both hydropriming and ultrasonic treatment and placed in Petri plates between layers of wet muslin clothes. These Petri plates were wrapped with aluminium foil to avoid moisture loss and after that germinated in a temperature-controlled cabinet at 35 °C. The germination status of the paddies was evaluated after every 24 h for 168 days and the percentage GR, DR and GC were estimated using the Eqs (17), (18), (19). The analysis was performed in duplicate to avoid an error.

Morphological analysis of paddy

Scanning electron microscope

The morphological properties of the native paddies and paddies treated at 30 and 35 °C during hydropriming and ultrasound priming process were investigated. These two temperatures were only considered for morphology study since inferior germination potential was observed at 25 and 40 °C. A longitudinal section was prepared by cutting the paddy with a dissection blade, as suggested by [19]. The samples were mounted on circular aluminium specimen holders with double carbon tape and examined using a scanning electron microscope (Make: Zeiss, Model: Gemini). An accelerating voltage of 3 kV and magnification of 5000X was applied for capturing the images.

Statistical analysis

The evaluation of the fitting capacity of the models, the statistical criterion of the root mean square error RMSE has been used:where denotes the experimental observations, are the predicted values, the total number of data points, the number of estimated model parameters.

Two -way analysis of variance (ANOVA) was employed using the SPSS statistical package (Ver.20) to determine the significant differences between treatments. The significance level was tested at p < 0.05.

Result and discussion

Hydration behaviour of paddy

Hydropriming and sono-hydro priming of paddies

The hydration process during grain germination is quite complex involving mass transfer process [9]. The grain follows a three-stage water uptake where the first step requires absorption of water to activate the germination metabolism. The second stage involves secretion of enzymes for digestion of reserve compounds and synthesis of simpler compounds and during this stage the water absorption is negligible. The third stage commences with the development of the radicle along with the formation of other structural components, which encourages higher water absorption during this period. However, as reported in the literature the only stage I is very imperative for processing of grains [9] and therefore, in this study the hydration modelling of this stage was evaluated in detail under different treatment conditions.

The water absorption behaviour of both the paddies during the hydration process (hydropriming and sono-hydro priming) is presented in Fig. 3. From the results, it was observed that during both the hydration process, the paddies followed a downward concave curve (DCS) behaviour which is characterized by a maximum water absorption initially and then eventually reduced to zero as the grain reaches the saturation moisture content. In the hydropriming process, the moisture content curve showed a sharp rise in the first 12 h at all soaking temperatures and later occurred at a slow rate up to 24 h. Similarly, in ultrasound priming, the moisture content increased significantly up to 4 h and thereafter declined gradually up to 12 h. According to reports, the hydration behaviour in grains is associated with the seed coat structure, whereas the hydration rate is related to the internal structure and composition [20]. Therefore, the DCS behaviour of paddies in this study may be explicated by the paddy structure, as suggested by [1]. Paddy structure consists of the hull and the pericarp which is fused with the seed coat forming the part of the bran. Although the hull provides some resistance to water diffusion, the bran consisting of different types of tissues is permeable to water. Thus, the high-water absorption initially in both paddies can be attributed primarily to the diffusion process causing water to move into the grain structure and declines thereafter as mass transfer reduces with the attainment of equilibrium.

Fig. 3

Hydration behaviour of paddies during hydropriming and ultrasound priming process of paddies. The curve represents the experimental value and the dots represents the predicted value from Peleg model. (a) hydropriming process of black rice (b) hydropriming process of non-pigmented rice (c) ultrasound priming process of black rice (d) ultrasound priming process of non-pigmented rice. Note: The average value of the moisture content was used for data analysis for modelling and representation.

According to reports, during the process of grain hydration, the internal resistance to mass transfer is a more significant hindrance as compared to the boundary resistance [10]. The results of this study showed higher water absorption in both BR and NR paddies during sono-hydro priming with a percentage change of 60 and 52%, respectively at 40 °C in comparison to the hydropriming process. This indicates that the sono-hydro priming process potentially overcomes the internal resistance of the paddies, which can be explained by the mechanism of the direct and indirect effects of ultrasound [8]. The direct impact of the ultrasound is the first action for augmentation of the hydration rate wherein the ultrasonic wave travels through the paddy, causing compression and expansion of the medium leading to the entry of water into grain pores by pumping. These effects are called the “sponge effect” (when the cells or the food matrix is compared to a sponge squeezed and released repeatedly) and the inertial flow (mass flow due to the wave propagation) [9]. Thereafter, the acoustic wave-induced cavitational activity (indirect effect) resulted in damaging of cellular material, creating microchannels for improved mass transfer [21]. This increase in mass transfer due to ultrasound treatment has also been observed in rough rice during soaking [22], mung beans [9] and common beans [8].

The soaking temperature, in combination with sono-hydro priming, had a notable influence on the hydration behaviour of both paddies. The high soaking temperature has also been attributed to the reduced grain diffusion resistance since higher temperature causes expansion and softening of the grains [23]. Moreover, increase in the temperature during soaking also causes breaking of molecular bonds in the endosperm, especially the glycosidic bonds in the starch granules which contain several hydroxyl groups capable of forming water-hydrogen bonds, thus leading to high water uptake [24]. A similar effect of temperature on hydration behaviour was reported in case of wheat [25], common beans [8], sesame seeds [26].

Modelling of the hydration process of paddies

Fick's diffusion model (Theoretical model)

The paddy hydration during hydropriming and sono-hydro priming were studied, and the experimental data were fitted to the Fick's model (diffusion model). To fit the model, the paddy was assumed to be ellipsoid and the physical dimensions were measured with the help of a Vernier calliper. The average length (l), width (w) and thickness (t) of BR paddy was observed as 9.25 ± 0.01, 2.8 ± 0.21 and 2.06 ± 0.04 mm, respectively and for NR it was observed as 7.61 ± 0.02, 2.22 ± 0.10 and 1.71 ± 0.03 mm. Further, the equivalent diameter for BR and NR was 2.75 ± 0.13 mm and 2.45 ± 0.03 mm, respectively.

The model was observed (Table 1) to adequately represent both the hydration process (hydropriming and sono-hydro priming) as the coefficient of determination (R2) was high and ranged from 0.933 to 0.998 and the root mean square was also low (0.020 to 0.101). The experimental data and the data generated from the Fick's law model established the goodness of fit of the model to the water absorption data obtained during hydropriming and sono-hydro priming at temperature range 25–40 °C as shown in Fig. 2. The residual was determined from the difference between the experimental and predicted data from the model and observed that the residuals were randomly dispersed around the zero lines. Thus, from the results, it can be concluded that Fick's diffusion model can be potentially used to describe the kinetics of hydration of paddy during hydropriming and sono-hydro priming for the germination process.

Table 1

Estimated values of parameters from Fick’s model and diffusivity coefficient of paddies during the hydration process.

Paddy	T(°C)	Ψi	Wa (h−1)	R2	RMSE	Deff (m2s−1)	Δ H#(kJ/mol)	ΔS#(kJ/mol-K)	ΔG# (kJ/mol)
HBR	25	1.068	0.096	0.931	0.100	3.522 × 10-7	24.638	−0.277	107.200
	30	1.083	0.130	0.952	0.088	4.748 × 10-7	24.597	−0.277	108.586
	35	1.067	0.143	0.969	0.069	5.271 X10-7	24.555	−0.277	109.972
	40	1.035	0.163	0.990	0.037	6.083 × 10-7	24.514	−0.277	111.359
HNR	25	1.1480	0.1098	0.935	0.098	3.592 × 10-7	29.675	−0.261	107.488
	30	1.1900	0.1335	0.906	0.120	4.010 × 10-7	29.634	−0.261	108.794
	35	1.1430	0.1579	0.940	0.092	4.839 × 10-7	29.592	−0.261	110.101
	40	1.0370	0.1959	0.976	0.057	6.303 × 10-7	29.551	−0.262	111.408
UBR	25	0.9495	0.2625	0.973	0.068	1.015 × 10-6	17.850	−0.292	104.773
	30	1.0100	0.2837	0.970	0.065	1.070 × 10-6	17.809	−0.292	106.232
	35	1.0230	0.3435	0.982	0.053	1.288 × 10-6	17.767	−0.292	107.692
	40	1.0070	0.3913	0.991	0.044	1.478 × 10-6	17.726	−0.292	109.152
UNR	25	0.9898	0.3035	0.974	0.062	9.996 × 10-7	16.631	−0.296	104.708
	30	1.0120	0.3547	0.990	0.039	1.155 × 10-6	16.590	−0.296	106.187
	35	1.0390	0.4092	0.973	0.067	1.315 × 10-6	16.548	−0.296	107.665
	40	1.0430	0.4497	0.949	0.091	1.442 × 10-6	16.507	−0.296	109.145

*Abbreviations: T = temperature, HBR = Hydroprimed pigmented paddy, HNR = Hydroprimed non-pigmented paddy, UBR = Sono-hydro primed pigmented paddy, UNR = Sono-hydro primed non-pigmented paddy, Ψi = constant of an approximate solution of diffusion equation, W = coefficient of water absorption rate and Deff = effective moisture diffusivity, Δ H# = enthalpy of activation, ΔS# = entropy of activation and ΔG# = activation free energy

Fig. 2

Hydration behaviour of paddies during hydropriming and ultrasound priming process of paddies. The curve represents the experimental value and the dots represents the predicted value from Fick’s diffusion model. (a) hydropriming process of black rice (b) hydropriming process of non-pigmented rice (c) ultrasound priming process of black rice (d) ultrasound priming process of non-pigmented rice. Note: The average value of the moisture content was used for data analysis for modelling and representation.

The estimated values of parameters of Eq. (5) for both BR and NR paddies with hydropriming and sono-hydro priming treatment which were obtained by non-linear regression are given in Table.1. The value for the constant of diffusion equation (Ψi) for both paddies during hydropriming and sono-hydro priming was obtained by regression and observed in the range of 0.9618 – 1.0430. The increase in soaking temperature showed a significant increase on the value of Ψi and it was due to the change in dimensional shape factor (λ1) as grain swelling increased due to water absorption at high temperature. The results are in accordance with [4] who reported similar Ψi value in case of paddy and brown rice. The ANOVA analysis showed that Ψi was significantly affected by variety, hydration process (hydropriming and son-hydro priming) and their interaction.

The coefficient of water absorption (W) in both paddies also showed significant (P < 0.05) increase with an increase in the soaking temperature in hydropriming and sono-hydro priming process. The sono-hydro priming method, in combination with a higher temperature (40 °C), accelerated the water absorption in both paddies by 2-fold. This might be due to the reduction in water viscosity at a higher temperature which improves the water flow as reported by [8] in case of beans. Moreover, sono-hydro priming showed a higher impact on the W value as compared to hydropriming within a short period of soaking time. This is due to the mechanism of ultrasound (direct and indirect effect) as described earlier along with temperature, which results in high mass transfer in grains. Miano et al. [9] reported a similar trend in hydration rate in mung beans due to the effect of ultrasound.

Evaluation of water diffusion during priming process

Moisture diffusivity (Deff) is a vital transport property required for the design and optimization of all the processes that involve internal moisture movement. The diffusion coefficient is the factor for proportionality representing the quantity of substance diffusing across a unit area through a unit concentration gradient in unit time [13]. The diffusion process, which is known to obey the Fick's law model, was observed to be a thermally activated process and significantly (P < 0.05) affected by temperature, hydration process and variety of paddy. The diffusion coefficient of BR and NR paddy increased as the soaking temperature was raised from 25 to 40 °C for both the hydration process (Table 1). According to reports, the change in temperature causes variation in the relative rate of diffusion, which eventually causes a modulation in the overall absorption behaviour [26]. The average effective diffusion coefficient for hydropriming and sono-hydro priming in BR paddy was observed as 1.00 × 10-6 and 1.21 × 10-6 m2s−1 and in case of NR it was observed as 4.62 × 10-7 and 4.90 × 10-7 m2s−1, respectively. The ANOVA analysis revealed that the hydration process (hydropriming and sono-hydro priming) showed a significant (P < 0.05) effect on the diffusion coefficient. The sono-hydro priming, along with the use of different temperature conditions, was observed to be more effective. The diffusion coefficient of paddies was higher within a shorter period of soaking time compared to hydropriming process. The high impact of ultrasound on the diffusion coefficient may be due to the mechanism known as 'rectified diffusion' [11]. The mechanism is that when high-intensity acoustic energy travels through a solid medium, a series of rapid and successive compression and rarefactions occurs caused by the sound waves and the rate depends on its frequency. This subjects the material to a quick series of alternating contractions and explanations known as the sponge effect. The sponge effect might be necessary for the creation of microscopic channels in paddy which in turn have reduced the internal resistance to the mass transfer.

Further, the alternating pressures and microstreaming phenomenon at the interfaces disturbs the diffusion boundary layer, which thereby decreases the resistance to convective mass transfer. Thus, sono-hydro priming of paddies increased the diffusivity due to the increasing mass transfer. Thakur et al. [4] reported the average value of diffusion coefficient of paddy, brown rice and husk as 4.91 × 10-11, 9.57 × 10-11 and 1.16 × 10-08 m2s−1, respectively. Similarly, Engels et al. [27] observed diffusion coefficient of rough rice as 6.67 × 10-9 and 9.03 × 10-9 m2s−1 at 40 and 50 °C, respectively. The effect of ultrasound power and soaking temperature on the diffusivity of chickpea was analyzed by Yildirim et al. [13] and observed significant (P < 0.05) increase in the water diffusion coefficient during soaking. However, in this study, we have found higher diffusion coefficient in paddies compared to these works of literature which might be due to the different processing conditions and variation in the chemical and molecular composition of the grains. However, the effect of ultrasound and temperature on the diffusion coefficient was observed to be the same, i.e. increase in diffusion rate.

Evaluation of the Peleg model (Empirical model)

Peleg's model applies to 'clearly curved' part of the sorption curve [12] and in this study the water absorption data of BR and NR at four different temperatures (25, 30, 35 and 40 °C) during hydropriming and sono-hydro priming was fitted to Peleg's equation (Table 2). The coefficient of determination (R2) for all the samples was observed to be high in both hydropriming (0.96 – 0.98) and sono-hydro priming (0.83 – 0.95). Further, the RMSE value, which indicates the absolute fit of the model, was observed to be low (0.23 – 0.59) in both the hydration process. The relationship between the experimental data and the data obtained from the model presented the goodness of fit (Fig. 3). This signifies that the model adequately characterized the hydration behaviour of the paddies.

Table 2

The Peleg coefficients, goodness of fit, sorption ratio and equilibrium moisture content of NR and BR during hydration process (hydropriming and ultrasound priming).

Paddy	T (°C)	k1	k2	R2	RMSE	Sorption Rate (h−1)	SMC (C)(g/g)	SMC (T)(g/g)	Deviation (%)
HBR	25	2.095	1.380	0.976	0.403	0.063	0.622	0.566	−9.939
	30	1.936	1.165	0.963	0.591	0.089	0.644	0.588	−9.481
	35	1.565	1.149	0.966	0.594	0.103	0.662	0.601	−10.18
	40	1.010	0.735	0.974	0.806	0.127	0.685	0.622	−10.05
HNR	25	3.158	1.728	0.986	0.232	0.045	0.509	0.461	−10.48
	30	2.530	1.612	0.986	0.219	0.064	0.600	0.552	−8.648
	35	1.999	1.198	0.968	0.539	0.082	0.621	0.560	−10.89
	40	1.718	0.985	0.976	0.558	0.143	0.628	0.578	−8.700
UBR	25	15.760	2.007	0.854	0.523	0.477	0.849	0.806	−5.290
	30	11.210	1.924	0.881	0.493	0.517	0.982	0.998	1.566
	35	9.728	1.858	0.911	0.426	0.563	0.994	1.012	1.747
	40	7.874	1.784	0.955	0.298	0.990	1.485	1.384	−7.265
UNR	25	22.100	2.595	0.853	0.403	0.317	0.703	0.670	−4.881
	30	15.570	2.102	0.836	0.549	0.395	0.744	0.719	−3.525
	35	12.240	2.012	0.895	0.431	0.500	0.959	0.915	−4.779
	40	6.979	1.983	0.942	0.309	0.582	1.139	1.085	−4.998

*Abbreviations: T = temperature, HBR = Hydroprimed pigmented paddy, HNR = Hydroprimed non-pigmented paddy, UBR = Sono-hydro primed pigmented paddy, UNR = Sono-hydro primed non-pigmented paddy, k and k = Peleg coefficients, , SMC (C) = calculated moisture content from Peleg model, SMC (T) = tabulated moisture content.

Peleg's constant k1 and the initial hydration rate

Peleg's constant k1 is related to the sorption rate or mass transfer rate [12]. Results (Table. 2) of the study revealed that k1 decreased significantly (P < 0.05) in both hydration processes for paddies (NR and BR) with an increase in the hydration temperature from 25 to 40 °C. This indicates an increased rate of water transfer with a rise in temperature and the highest observed at 40 °C. Other authors reported similar results in the case of bean and chickpea [17] and sesame seeds [26]. Thus, the reciprocal, i.e. 1/k1 corresponding to the initial hydration rate exhibited the positive effect of temperature on the water absorption rate of BR and NR in both hydropriming and sono-hydro hydration processes. This increase might be attributed to the morphological changes of paddy at high temperature, i.e. cell wall degradation felicitating high water uptake through capillary actions [9]. The ANOVA analysis showed that hydration process (hydropriming and sono-hydro priming), variety of paddy (pigmented and non-pigmented), temperature and their interaction showed a significant effect (P < 0.05) on the water absorption rate (k1).

Further, the sono-hydro priming process was observed to have a significant effect on the Peleg constant k1 as initial water absorption rate was found to be higher as compared to hydropriming process in both paddies. This was due to the fact that sono-hydro priming of paddies is a combination of temperature and ultrasound as compared to hydropriming, which involved the only application of temperature. Ultrasound treatment consists in applying cyclical pressure differences which aid in entrance of water in the grain by facilitating the removal of occluded air and this mechanism of ultrasound along with high temperature enhances the mass transfer rate. Miano et al., (2016) also reported that the use of hot soaking water and ultrasound technology improved the hydration process of white kidney beans.

Peleg's constant k2

The Peleg constant k2 is correlated to the maximum water absorption capacity [17], [12]. This constant is inversely related to the water absorption ability in food and food products. The results (Table 2) showed that similar to k1, the Peleg constant k2 also decreased significantly (P < 0.05) with an increase in the temperature from 25 °C to 40 °C in BR and NR for both hydration process (hydropriming and sono-hydro priming). This is the indication that similar to the increase in water absorption rate (k1), an enhancement of water absorption capacity (k2) was also observed in both the hydration process. According to reports, the changes in k2 is also dependent on the type of seed and the loss of soluble solids during the soaking period [12]. For bean and chickpea, the k2 values were reported to decrease with increase in temperature. Shafaei et al. [17] reported that in the case of bean varieties, k2 value was significantly affected by the rise in temperature, but it was not observed in chickpea. Similar results were also observed in sesame seeds during soaking at different temperatures (27–60 °C) and highest water absorption capacity was observed at 60 °C. The Peleg constant k2 was also significantly (P < 0.05) affected by the hydration process, variety and their interaction as obtained from ANOVA analysis. Among both the hydration process, sono-hydro priming showed higher percentage decrease in BR and NR (approx. 2-fold) at 40 °C when compared to one-fold decrease in hydropriming process of paddies. This signifies higher water absorption capacity of paddies due to mechanical effect caused by cavitation during ultrasound priming [21].

The reciprocal of Peleg constant k2 was employed to predict the saturation moisture content (SMC) at different temperatures. The SMC during soaking may increase, decrease or remain constant depending on the type of grain and studied temperature range [9]. In this study, the obtained SMC in both hydration processes was observed to significantly (P < 0.05) increase with an increase in soaking temperature for BR and NR paddies (Table 2). This increase of SMC with temperature in case of hydropriming process may be attributed to the degradation of structural components such as pectin and cellulose from cell walls which affects the cell integrity and plasticity [25]. Further, the reciprocal of k2 adequately predicted the SMC in case of sono-hydro priming process as only approximately 5% deviation was observed between the tabulated and calculated SMC in both paddies. However, a higher variation (10%) was observed in hydropriming process, which might be due to the requirement of longer hydration time for paddies to reach the SMC. The sono-hydro priming process also showed higher impact in case of saturation moisture content in both BR and NR as compared to hydropriming. In the case of NR paddy, more than 90% of, i.e. obtained by hydropriming process at 25 °C is achieved at initial 3 h of the sono-hydro priming process. The similar trend was also observed for BR paddy at all temperatures. Thus, the sono-hydro priming process reduced the time for reaching the SMC up to 75% when compared to the hydropriming process. Patero et al. [10] reported a reduction of 40% to achieve the SMC in ultrasound-assisted hydration as compared to the conventional method.

Similarly, 5% increase in SMC was observed in ultrasound hydration of corn kernels and explained that since ultrasound keeps the porous and internal cavities unobstructed, the water holding capacity increases [20]. However, depending on the type of grain some authors have reported a decrease in SMC with an increase in hydration temperature such as in common beans when treated with ultrasound [8] and adzuki beans [28]. Therefore, the SMC was also significantly affected by temperature, variety (pigmented and non-pigmented paddy), hydration process and their interaction as observed from the ANOVA analysis.

The effect of temperature on effective diffusivity is described using the Arrhenius-type relationship that aids to acquire a preferable agreement of the predicted curve with experimental data [23]. The activation energy (Ea) from Fick's diffusion coefficient and Peleg constant were calculated for both paddies with hydropriming and sono-hydro priming process (Fig. 4). The results showed positive values of activation energy, indicating that the seeds have gained energy during the hydration process. The results obtained from Fick's diffusion coefficient showed that sono-hydro priming reduced the activation energy in BR and NR from 27.11 and 32.15 KJ/mol in hydropriming process to 20.32 and 19.19 KJ/mol for sono-hydro priming, respectively. Similarly, the temperature dependence of Peleg constant was analyzed and presented in Fig. 5. The activation energy (obtained from Peleg model) for hydropriming process for BR and NR was observed as 37.25 and 34.55 kJ/Mol respectively. A higher decrease in activation energy, in this case, was also observed in sono-hydro priming process of BR (32.01 kJ/mol) and NR (36.06 kJ/mol). This result is per the ultrasound treated chickpeas and reported that ultrasound decreased the soaking time of chickpea [13]. Similarly, the reduction of the activation energy was published in the case of ultrasound treated common beans from 54.1 kJ/mol to 44.2 kJ/mol [8]. The lower activation energy in case of sono-hydro priming in both paddies suggests that there is a reduction on the energetic barrier to form the final products, i.e. hydrated paddy. Miano et al. [8] reported that ultrasound probably apparently abridged the maximum potential energy and then atoms reach during their rearrangement (atomic distances and bond angles). Moreover, the pre-exponential factor or frequency factor, related to the collisions rate irrespective of their energy was increased by ultrasound which suggests that this technology increases the collision frequency among the molecules for the activated complex formation.

Fig. 4

Arrhenius plot of Fick’s law model of diffusivity of paddies over different hydration temperature range (25–40 °C) during hydropriming and ultrasound priming process.

Fig. 5

Arrhenius plot for the Peleg rate constant k1 over different hydration temperature range (25–40 °C) during hydropriming and ultrasound priming process.

Further, it was observed that the activation energy value obtained from Peleg's model was higher in both hydration processes for BR and NR than the values obtained from Fick's model by employing diffusion coefficient. A similar observation was also reported by Sopade, (1992) where the value of activation energy in some cereals obtained from Peleg model was around 1000 times greater than values obtained from diffusion coefficient and attributed this difference to the empirical nature of the Peleg model.

Thermodynamic properties of the hydration process

The thermodynamic properties of the present study are mainly attributed to a reaction with intermediate formation (activated complex) rather than the final product. This state is also referred to as the transition state and to elucidate the results; the activated complex formation theory can be contemplated as suggested by Miano et al. [8]

The reaction takes place as the reactants interact before the formation of the products and this interaction results in specific changes such as molecular structure distortion, exchange of some atoms and changes in the interatomic distances and bond angle. Thus, these mechanisms cause the potential energy to reach a maximum value. The difference between the energies of the transition, and the initial states are closely related to the experimental activation energy for the reaction eventually leading to the formation of the activation complex. This high-energy complex represents an unstable intermediate and as the energy barrier is reduced, the reaction proceeds and product formation occurs [8].

This transition state hypothesis was applied to the study which can be explicated by assuming that the hydration of the paddies followed the following equation: during the process of hydration of paddy (PD), the activation complex [PD – H2O] is formed before achieving the final product, i.e. the hydrated paddy (HPD).

Miano et al. [8] reported that several mechanisms are involved in the formation of the activated complex such as molecular rearrangements, bond angles, interatomic distance changes etc. Further, according to transition-state theory, the activated complex is formed in a state of equilibrium with the atoms or molecules in the initial state. Therefore, its statistical and thermodynamic properties (enthalpy, entropy and Gibbs free energy of activation) can be specified.

The thermodynamic properties of paddies for both hydropriming and sono-hydro priming process was calculated from the mathematical model, i.e. the diffusion coefficient, which was obtained from Fick's model and data presented in Table 1. The activation enthalpy (ΔH#) for both the paddies with hydropriming and sono-hydro priming was observed to have a positive value indicating an endothermic reaction when an activated complex (PD – H2O) is formed [8]. Al-zubaidy et al. [30] also reported that ΔH# exemplifies the difference between the activated state and reactants and thus must possess a positive sign. The endothermic reaction in this study revealed that the formation of the activated complex might have been augmented due to the addition of energy in the form of temperature or ultrasound. The results showed that the activation enthalpy in case of sono-hydro priming was significantly lower as compared to hydropriming process in both paddies since the former reduced the activation enthalpy by 40 and 35% in case of BR and NR paddies at all the temperatures (25–40 °C). Thus, sono-hydro priming, along with different temperature conditions, have provided the energy required for the formation of the activated complex, which in turn have accelerated the hydration rate in both paddies. A similar positive value of enthalpy of activation was reported in case of hydration of common beans by ultrasound and high temperature. Al-zubaidy et al. [30] also reported the same value during the kinetic study of ascorbic acid degradation.

The activation entropy (ΔS#) on the other hand had a negative value in both paddies for both hydropriming and sono-hydro priming. The theory of the activated complex can be conferred to explain the mechanism, i.e. a substance in a state of activation can only have negative entropy of activation if degrees of freedom of translation or rotation is lost during the formation of the activated complex. The transitional state is then in a higher state of order than the reactant particles of which it is composed. It is also a known fact that aggregation reactions can be correlated to a marked increase in order resulting in a negative value of entropy of activation. Taking the above hypothesis in consideration, the effect of hydropriming and sono-hydro priming on ΔS# during hydration of both paddies can be elaborated. The negative value of ΔS# in paddies (NR and BR) with both hydration processes suggests that there is an increase in order, i.e. molecular structure is more organized. According to reports, during the hydration process, the H-bonds between water and other molecules are more organized than the ones among water molecules.

Further, results of activation entropy of BR and NR paddies with sono-hydro priming revealed lower values than the hydropriming priming. This might be due to the working mechanism of ultrasound technology, i.e. it creates chaotic and turbulent movement in the molecules, which makes it difficult to establish an organized molecular structure during the formation of the activated complex. Thus, to attain this organization, it was essential for the process to undergo a higher reduction of entropy to form the activated complex. Moreover, ultrasound processing leads to the formation of free radicals in water due to sonolysis generated by cavitation and thereby hinders the organization. Similar results were reported during hydration of rice in the parboiling process and indicated that entropy was negative and remained constant with temperature suggesting that the processes occurred with no significant increase in the disorder of the system [24]. Jideani et al. [12] reported a negative entropy during hydration of Botswana Bambara varieties indicating an increase in system order (a less random system).

The Gibbs free energy of activation (ΔG#) of paddies for both hydropriming and sono-hydro priming was observed to be positive, indicating a non-spontaneous reaction. The influence of temperature and hydration process conditions (hydropriming and sono-hydro priming) was minimal. The result was in accordance with Miano et al. [8] in case of common beans. Borges et al. [31] also reported that the values for ΔG# could be considered as a measure of the work performed by the system to promote hydration of the grain. The ΔG# was also observed to be positive in the hydration process of soybean and showed that changes in the values are related to the changes in the value of enthalpy and entropy of activation.

Thus, the thermodynamic properties of this study showed that the hydration process (hydropriming and sono-hydro priming) of paddies is non-spontaneous and requires additional energy from the surrounding area. Further, the molecular organization was observed to be high during this transition. According to Miano et al. [8] the thermodynamic properties of the activated complex is very less dependent on the change in temperature. This was also observed in our study as high temperature did not influence the molecular organization, spontaneity and energies to form the activated complex. Moreover, as mentioned by Miano et al. [8] sono-hydro priming along with temperature also aided in bringing close the water molecules and seed components.

Effect of sono-hydro priming on the malting process

Malting potential

The germination viability (GR, GC and DR) of two paddies (BR and NR) with sono-hydro priming is shown in Table 3 and Fig. 6. Germination is characterized by the emergence of the radicle or the growth of the embryo. In this study, the results showed that changes in the malting potential were significantly (P < 0.05) affected by the hydration conditions (hydropriming and sono-hydro priming) and temperature. The germination rate of paddies (BR and NR) for both hydration processes at 40 °C was observed to be lowest when germinated for 168 h (7 days). Therefore, the dormancy rate at this temperature was significantly high. This might be due to the application of high temperature, which causes reactions such as protein denaturation or molecular degradation leading to damage of seed vigour during the germination process [32]. Further, ultrasonication along with high temperature (40 °C) combination can be another reason for low malting potential since physical and chemical damages might take place owing to the swift changes in pressure and temperature generated by ultrasound waves. This might result in shear disruption, thinning of the cell membrane, localized heating and production of free radicals [32].

Table 3

Malting potential of paddies with ultrasound priming process.

Paddy	Malting time (days)	Germination rate (%)				Dormancy rate (%)				Germination capacity (%)
		25 °C	30 °C	35 °C	25 °C	30 °C	35 °C	25 °C	30 °C	35 °C	25 °C	30 °C	35 °C
HNR	Native	0 ± 0.00	0 ± 0.00	0 ± 0.02	0 ± 0.00	0 ± 0.0	100 ± 0.0	100 ± 0.0	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00
	01	1 ± 0.00	60 ± 0.19	53 ± 0.04	1 ± 0.00	99 ± 0.0	40 ± 0.03	47 ± 0.09	99 ± 0.18	−98 ± 0.03	20 ± 0.19	6 ± 0.01	−98 ± 0.01
	02	10 ± 0.02	82 ± 0.04	55 ± 0.13	1 ± 0.00	90 ± 0.0	18 ± 0.05	45 ± 0.06	99 ± 0.20	−80 ± 0.02	64 ± 0.20	10 ± 0.04	−98 ± 0.03
	03	17 ± 0.12	88 ± 0.10	72 ± 0.05	3 ± 0.01	83 ± 0.0	12 ± 0.01	28 ± 0.14	97 ± 0.28	−66 ± 0.19	76 ± 0.18	44 ± 0.07	−94 ± 0.02
	04	26 ± 0.14	90 ± 0.20	85 ± 0.09	5 ± 0.00	74 ± 0.0	10 ± 0.10	15 ± 0.8	95 ± 0.10	−48 ± 0.02	80 ± 0.03	70 ± 0.18	−90 ± 0.01
	05	32 ± 0.04	93 ± 0.13	90 ± 0.19	6 ± 0.00	68 ± 0.0	7 ± 0.00	10 ± 0.01	94 ± 0.19	−36 ± 0.19	86 ± 0.05	80 ± 0.10	−88 ± 0.10
	06	35 ± 0.21	95 ± 0.03	95 ± 0.01	7 ± 0.02	65 ± 0.0	5 ± 0.00	5 ± 0.00	93 ± 0.27	−30 ± 0.20	90 ± 0.10	90 ± 0.02	−86 ± 0.11
	07	42 ± 0.27	97 ± 0.00	98 ± 0.00	8 ± 0.01	58 ± 0.0	3 ± 0.00	2 ± 0.00	92 ± 0.22	−16 ± 0.02	94 ± 0.00	96 ± 0.01	−84 ± 0.03
UNR	01	4 ± 0.01	53 ± 0.06	55 ± 0.02	2 ± 0.02	96 ± 0.03	47 ± 0.01	45 ± 0.01	98 ± 0.01	−92 ± 0.01	6 ± 0.00	10 ± 0.01	−96 ± 0.01
	02	12 ± 0.02	60 ± 0.07	61 ± 0.03	5 ± 0.06	88 ± 0.07	40 ± 0.17	39 ± 0.04	95 ± 0.02	−76 ± 0.10	20 ± 0.01	22 ± 0.01	−90 ± 0.00
	03	22 ± 0.02	72 ± 0.19	76 ± 0.10	8 ± 0.19	78 ± 0.19	28 ± 0.21	24 ± 0.08	92 ± 0.01	−56 ± 0.17	44 ± 0.03	52 ± 0.02	−84 ± 0.15
	04	28 ± 0.01	80 ± 0.10	82 ± 0.09	9 ± 0.05	72 ± 0.04	20 ± 0.10	18 ± 0.18	91 ± 0.09	−44 ± 0.12	60 ± 0.09	64 ± 0.13	−82 ± 0.21
	05	35 ± 0.10	88 ± 0.21	90 ± 0.02	11 ± 0.10	65 ± 0.05	12 ± 0.09	10 ± 0.20	89 ± 0.19	−30 ± 0.04	76 ± 0.21	80 ± 0.18	−78 ± 0.19
	06	47 ± 0.18	90 ± 0.28	95 ± 0.06	14 ± 0.23	53 ± 0.14	10 ± 0.04	5 ± 0.01	86 ± 0.20	−6 ± 0.01	80 ± 0.13	90 ± 0.20	−72 ± 0.18
	07	52 ± 0.15	98 ± 0.00	99 ± 0.00	17 ± 0.10	48 ± 0.24	2 ± 0.00	1 ± 0.00	83 ± 0.22	4 ± 0.00	96 ± 0.05	98 ± 0.00	−66 ± 0.20
HBR	Native	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00	100 ± 0.00	100 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00	0 ± 0.00
	01	1 ± 0.03	6 ± 0.01	4 ± 0.02	1 ± 0.00	99 ± 0.07	94 ± 0.03	96 ± 0.01	99 ± 0.01	−98 ± 0.02	−88 ± 0.05	−92 ± 0.02	−98 ± 0.01
	02	5 ± 0.01	25 ± 0.03	30 ± 0.01	4 ± 0.00	95 ± 0.08	75 ± 0.07	70 ± 0.09	96 ± 0.02	−90 ± 0.01	−50 ± 0.01	−40 ± 0.19	−92 ± 0.03
	03	12 ± 0.05	33 ± 0.19	39 ± 0.18	6 ± 0.00	88 ± 0.03	67 ± 0.09	61 ± 0.04	94 ± 0.01	−76 ± 0.19	−34 ± 0.10	−22 ± 0.23	−88 ± 0.05
	04	21 ± 0.02	48 ± 0.12	45 ± 0.02	6 ± 0.01	79 ± 0.12	52 ± 0.17	55 ± 0.02	94 ± 0.04	−58 ± 0.16	−4 ± 0.00	−10 ± 0.03	−88 ± 0.19
	05	26 ± 0.10	52 ± 0.20	60 ± 0.19	7 ± 0.02	74 ± 0.19	48 ± 0.14	40 ± 0.10	93 ± 0.01	−48 ± 0.21	4 ± 0.00	20 ± 0.05	−86 ± 0.18
	06	32 ± 0.18	61 ± 0.22	64 ± 0.13	7 ± 0.00	68 ± 0.22	39 ± 0.28	36 ± 0.23	93 ± 0.03	−36 ± 0.10	22 ± 0.01	28 ± 0.19	−86 ± 0.04
	07	36 ± 0.11	66 ± 0.10	68 ± 0.12	7 ± 0.00	64 ± 0.04	34 ± 0.05	32 ± 0.18	93 ± 0.09	−28 ± 0.19	32 ± 0.02	36 ± 0.13	−86 ± 0.01
UBR	01	4 ± 0.00	8 ± 0.01	7 ± 0.01	2 ± 0.00	96 ± 0.01	92 ± 0.06	93 ± 0.03	98 ± 0.02	−92 ± 0.01	−84 ± 0.04	−86 ± 0.04	−96 ± 0.01
	02	8 ± 0.01	29 ± 0.12	21 ± 0.02	7 ± 0.00	92 ± 0.06	71 ± 0.08	79 ± 0.05	93 ± 0.01	−84 ± 0.03	−42 ± 0.02	−58 ± 0.02	−86 ± 0.05
	03	16 ± 0.02	37 ± 0.01	38 ± 0.12	8 ± 0.01	84 ± 0.16	63 ± 0.14	62 ± 0.08	92 ± 0.05	−68 ± 0.15	−26 ± 0.01	−24 ± 0.14	−84 ± 0.15
	04	23 ± 0.09	51 ± 0.19	57 ± 0.25	11 ± 0.02	77 ± 0.21	49 ± 0.08	43 ± 0.16	89 ± 0.03	−54 ± 0.08	2 ± 0.00	14 ± 0.24	−78 ± 0.11
	05	28 ± 0.18	57 ± 0.21	60 ± 0.17	16 ± 0.03	72 ± 0.13	43 ± 0.12	40 ± 0.20	84 ± 0.19	−44 ± 0.16	14 ± 0.12	20 ± 0.01	−68 ± 0.21
	06	34 ± 0.12	64 ± 0.11	68 ± 0.09	16 ± 0.00	66 ± 0.09	36 ± 0.24	32 ± 0.25	84 ± 0.02	−32 ± 0.02	28 ± 0.02	36 ± 0.11	−68 ± 0.20
	07	39 ± 0.10	68 ± 0.13	70 ± 0.16	18 ± 0.02	61 ± 0.05	32 ± 0.12	30 ± 0.20	82 ± 0.20	−22 ± 0.01	36 ± 0.13	40 ± 0.19	−64 ± 0.15

Abbreviations: UBR = Sono-hydro primed pigmented paddy, UNR = Sono-hydro primed non-pigmented paddy, HBR = Hydroprimed pigmented paddy, HNR = Hydroprimed non-pigmented paddy.

Fig. 6

Images of paddy germination before and after priming (hydropriming and sono-hydro priming) A = native non-pigmented paddy, B = germinated non-pigmented paddy after hydropriming, C = germinated non-pigmented paddy after sono-hydropriming, D = native black paddy, E = germinated black paddy after hydropriming, F = germinated non-pigmented paddy after sono-hydropriming.

The maximum GR and GC in BR and NR paddy for both hydropriming and sono-hydro priming process was obtained at hydration temperature of 30 and 35 °C. Thus, the dormancy rate was also very low. Earlier studies also reported maximum malting potential at both the temperatures in case of paddy [3]. This might be due to the higher activation of metabolites such as synthesis of Gibberellic acid (GA) at these temperatures, which further enhances the seed vigour for germination. The NR paddy was observed to have significantly (P < 0.05) higher malting potential compared to BR in both hydration processes which might be due to the variation in the composition that affects the seed vigour during germination.

Further, it was also observed that both hydration process also had a significant (P < 0.05) effect on the malting potential of the paddies since sono-hydro priming exhibited higher germination rate and capacity of paddies during the 7-day germination period at all temperatures as compared to hydropriming. Moreover, it was remarkable that paddies with sono-hydro priming showed a higher germination rate with a short hydration period (12 h) as compared to hydropriming hydration period (24 h). This might be attributed to the higher water absorption rate of paddies with sono-hydro priming, as observed in this study. Thus, the understanding of the correlation between the two parameters (hydration rate and malting potential) shall provide a better insight into the germination process. As mentioned earlier, the acoustic cavitation produced during sono-hydro priming causes cell wall damage and also shell fragmentation [21]. The shell fragmentation in paddy also might have increased the surface area, which in turn increased the mass transfer rate of water into the grain. The high hydration rate initiates faster germination process with stimulation of plant hormones such as gibberellic acid (GA) which further enhances the metabolic process in the aleurone layer by the synthesis of hydrolytic enzymes (α and β amylases). This amylase causes degradation of the reserved compound, i.e. starch to simpler molecules for the development of the embryo for germination. Thus, sono-hydro priming causes higher imbibition of water and as a natural response by seeds, there is an increase in the synthesis of GA occurs, which potentially enhances the malting potential. Our results are in accordance with the observations reported for pea germination, which documented increased germination percentage (83 to 97%) in pea due to the application of ultrasound. Similarly, Yaldagard et al. [21] reported an increase in germination rate together with decreased germination period in ultrasound treated barley seeds.

Microstructural changes

The Scanning electron micrographs of both paddies during hydropriming and sono-hydro priming were studied at two hydration temperatures, i.e. 30 and 35 °C in Fig. 7, Fig. 8. The longitudinal section of the native and treated paddies was analyzed for morphological changes during the process of hydration. The native microstructure of both BR and NR paddy was observed to be a compact structure with protein bodies and starch granules that appeared to be with a smooth surface. This compactness without intercellular space in the endosperm supports our mechanism of hydration mechanism in paddy, i.e. water transferred into the grain by the process of diffusion. Further, the paddies with hydropriming and ultrasound priming process at both temperatures showed a rough surface, swelling of granules due to water absorption and formation of pin-holes/cavity which might be attributed to the action of amylolytic enzymes by the mechanism of exo-corrosion and endo-corrosion [3]. In addition to enzyme action, some microcavities might also be formed during ultrasound treatment due to the generation of acoustic cavities occurring randomly inside the grain matrix.

Fig. 7

SEM microgrpahs of paddy: (a) Native NR paddy, (b) NR paddy at 30 °C hydropriming process, (c) NR paddy at 35 °C hydropriming process, (d) NR paddy at 30 °C ultrasound priming process, (e) NR paddy at 35 °C ultrasound priming proces.

Fig. 8

SEM microgrpahs of paddy: (a) Native BR paddy, (b) BR paddy at 30 °C hydropriming process, (c) BR paddy at 35 °C hydropriming process, (d) BR paddy at 30 °C ultrasound priming process, (e) BR paddy at 35 °C ultrasound priming process.

However, in sono-hydro priming enhanced mass transfer was observed which might be attributed to the turbulent and chaotic movement created by ultrasound waves resulting in higher water absorption through the cavities formed on the surface. According to Miano et al. [9] these microcavities grow in size with time and the mass transfer phenomenon enhances as a reasonable number of cavities are formed and channels are formed with connections between the cavities and the external medium. However, in the case of ultrasound of mung beans, the micro-channels formation was not detected from the SEM micrographs [9]. On the other hand, micro- channel formation was detected during hydration of sorghum employing ultrasound technology [14].

Conclusions

The present research study showed the application of sono-hydro priming at different temperature ranges to get an insight into the soaking process during germination in grains. Both the hydration process showed a downward concave curve (DCS) behaviour with a sharp increase in moisture content initially due to the diffusion process and later declined. The sono- hydro priming with different temperature combination showed higher mass transfer rate as compared to hydropriming process attributing to the direct and indirect effect. The Fick's diffusion model (analytical model) and Peleg model (empirical model) established the goodness of fit for the hydration process (hydropriming and ultrasound priming) of paddies with a high coefficient of determination and low RMSE value. A high effective moisture diffusion coefficient was observed in sono-hydro priming within a short soaking time as compared to hydropriming. Further, sono-hydro priming at high temperature increased water absorption rate (related to the parameter, i.e. Peleg constant k1) and high-water absorption capacity (related to the Peleg constant k2) in both paddies when compared to the hydropriming process. Sono-hydro priming process potentially reduced the time to reach the saturation moisture content in both paddies probably due to reduction in energetic barrier for the formation of the final products. The evaluation of the thermodynamic properties employing effective moisture diffusion coefficient showed that sono-hydro priming in paddies decreased the energy (activation enthalpy) required for the formation of the activated complex and the process was non-spontaneous. Therefore, the high mass transfer rate in the case of sono-hydro priming enabled the acceleration of germination potential in both paddies. The SEM micrographs during the sono-hydro process supported the theory of the formation of micro cavitation in paddies leading to high mass transfer. Finally, considering all the observations, it can be concluded that sono-hydro priming is a time and energy-efficient process that can be utilized for commercial production of malted grains which shall be used in the preparation of various food products such as weaning food, energy drink, confectionaries etc.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.