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Due to the importance of hydration stage for parboiling, related studies are insufficiently addressed in the literature. The objective of this study was to model the moisture absorption kinetics of paddy rice, as well as estimate the thermodynamic properties involved in the hydration. The observed mass transfer in the process was directly influenced by the temperature of the system, showing changes in absorption capacity over time. The models addressed allowed satisfactorily describing the moisture gain under various thermal conditions. Particularly, the model based on Fick's second law best represented the experimental data. The effective diffusion coefficient was adjusted to an Arrhenius equation and a new model was generated, capable of describing hydration. The coefficient presented positive dependence with the temperature increase and allowed the determination of different forms of process energy. It was concluded that the hydration of the rice in the parboiling process is nonspontaneous and exothermic. Practical applications This study allowed characterizing the moisture transfer in the hydration stage of the parboiling process, exploring the time and temperature relationship. From these data, mathematical modeling was used as a quality tool, guaranteeing for the parboiling industries control and optimization of the process in a cheap and easy way. In addition, this study presented relevant information on the thermodynamic properties, which made it possible to understand the different forms of energies and phenomena involved. This information can help, in a practical way, in the elaboration of industrial projects aimed at the parboiling of rice.

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The conventional parboiling process decreases the total phenolic compounds (TPC) and the antioxidant activity (AA) of rice. In order to change this nutritional disadvantage, in this research we have proposed an unusual solution. Here, we added grape pomace (agro-industrial waste) in the process during the parboiling as a source of bioactive compounds (BC). An increase in the concentrations of TPC, flavonoids anthocyanins and antioxidant activity was found. Moreover, there was slightly change in the final colour of the rice (which tended towards red) in addition to added proline (NMR spectra), which is an amino acid that is characteristic of grapes. Therefore, we concluded that it is possible to improve the functionality of parboiled rice with the addition of grape pomace, besides promoting the valorisation and use of an agro-industrial waste rich in secondary metabolites.

This study presents a generalization of a lumped parameters model through the use of fractional derivatives of arbitrary order. The conventional model has an analytical solution in terms of exponential function. However, when applied to kinetic data on moisture absorption in rice grains, its performance was worse when compared to the proposed generalized model. The results were analyzed for four hydration temperatures. Despite the similarity of the mass transfer coefficients for both models, the proposed generalization provided a statistically significant improvement in prediction of moisture values in relation to time and process temperature. Practical applications The model proposed in this paper is applicable in describing the hydration kinetics of rice which is an important step in parboiling process, for instance. The simplicity of the analytical solution of the model and the improvement of its prediction capability makes the presented approach useful to project of equipment intended to process rice.

This article presents the application of Nicolin–Jorge model in describing the hydration kinetics of rice and corn grains. The model was validated by hydration data of both grains for different hydration temperatures. The results show a satisfactory adequacy of the model to the data, even applying the data to the cylindrical and planar geometries, since the original model was proposed to describe the soybean hydration kinetics, considered spherical. Even as the parameters fitted for both cases have behaved differently depending on the temperature, the model predicted properly the reach of equilibrium moisture for very long hydration times. Practical applications The approach presented in this article is useful in describing the moisture absorption kinetics by rice and corn grains. The model applied fits satisfactorily the experimental data and is applicable when the hypothesis of constant mass transfer coefficient in moisture absorption is not enough to correctly describe the behavior of the kinetic curve of moisture as a function of time for such grains. The proposed approach is applicable to any project that requires prior knowledge of the moisture profile of the grains. The advantage of the model used is the simplicity of the analytical solution obtained and that the model possesses physical grounds, although the grains are considered to be a lumped parameter system, which is an adequate consideration for practical applications.

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