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Modeling and experimentation of continuous and intermittent drying of rough rice grains

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Abstract

Rice (Oryza sativa L.) is one of the most produced and consumed cereals in the world, being characterized as the main food of more than half of the world population, since it is source of energy, protein, vitamins and minerals. Freshly harvested rice from the field generally has a high moisture content to be stored safely and, therefore, a suitable drying process needs to be conducted to decrease the physical-chemical activity of the product and inhibit the associated microbial activities. In this work, experiments were carried out on the intermittent drying of rice grains, in order to evaluate the effects of different drying air temperatures (40, 50, 60 and 70 °C) and tempering times (30, 60, 120, 180 and 240 min). Intermittent drying was simulated by means of a new liquid diffusion model based on a prolate spheroid geometry. To validate the model, the solution was fitted to the experimental data for the drying air temperatures of 40 and 70 °C and tempering periods ranging from 0 to 180 min. The results show that, for all experiments of intermittent drying of rough rice, the effective operating time decreased compared to the continuous drying. In addition, intermittent drying produces lower temperature on the grain surface, which minimizes the thermal damages caused to the product during the process. Numerical simulations provide information on moisture distribution inside the rice grain during the periods of interruption in hot air application. After 15 min of drying at temperatures of 40 and 70 °C, followed by a 60-min pause at room temperature, it is possible to respectively reduce about 72 and 68% of the moisture content gradients inside the rice grain. With tempering of 180 min, it is possible to eliminate almost all moisture content gradients for the drying at 70 °C.

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Abbreviations

r 2 :

determination coefficient

d :

constant

h m :

convective mass transfer coefficient (cm s−1)

r, z :

cylindrical coordinates

s :

mean square deviation

D :

diffusion coefficient (cm2 s−1)

a ' , b ' , c':

dimensions of rice grains (mm)

M :

moisture content (kg/kg dry matter, d.b.)

f :

element of the set base function

M :

moisture content with respect to dry basis

\( {\overline{M}}^{\ast } \) :

dimensionless average moisture content

RH :

relative humidity

a, b, c :

semi-axes of the ellipsoid (mm)

A :

surface area (mm2)

T :

temperature (°C)

t i :

tempering start time (min)

v :

velocity of the drying air (m s−1)

V :

volume (mm3)

χ 2 :

(chi-square)

γ n :

n-th eigenvalue

ψ n :

n-th eigenfunction

τ :

tempering period (min)

*:

dimensionless quantities

eq:

equilibrium condition

exp:

experimental

0:

initial condition

pre:

predicted

S:

surface condition

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Authors and Affiliations

  1. Center of Sciences and Technology, Federal University of Campina Grande, Campina Grande, PB, Brazil

    Célia M. R. Franco, Antonio G. B. de Lima, Vera S.O. Farias & Wilton P. da Silva

Authors
  1. Célia M. R. Franco
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  2. Antonio G. B. de Lima
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  3. Vera S.O. Farias
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  4. Wilton P. da Silva
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Correspondence to Wilton P. da Silva.

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Franco, C.M.R., de Lima, A.G.B., Farias, V.S. et al. Modeling and experimentation of continuous and intermittent drying of rough rice grains. Heat Mass Transfer 56, 1003–1014 (2020). https://doi.org/10.1007/s00231-019-02773-0

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