DIC-Assisted Hot Air Drying of Post-harvest Paddy Rice

Chapter
Part of the Food Engineering Series book series (FSES)

Abstract

Hot air drying is a very important post-harvest stage in the processing of paddy rice that serves to decrease the initial moisture content, generally from 26–38 % db (dry basis) to 12–14 % db final moisture content. The drying and storage conditions (temperature, air moisture) affect all functional and structural properties (Pearce et al., Cereal Chemistry 78:354–357, 2001; Rice: chemistry and technology, St. Paul, 265–287). The drying time is relatively long (from 8 to 12 or even 24 h), with few tempering periods. After the drying stage, polishing is carried out. Despite the precautions taken and low drying kinetics, the broken ratio, which is mainly the result of an elevated internal moisture gradient (more than 15 %), results in a relatively low yield of head rice. Generally the shelf life of paddy rice is rather short, not exceeding a few months. The cooking time is long, close to 17 min, with a short time before overcooking occurs (4 min longer).

Due to the high temperature–short time treatment, the residence time of grains in the DIC reactor has to be short to prevent heat damage. The drying process is more homogeneous, with a uniform moisture content in the final product. The short duration of DIC-assisted drying favors a much higher drying capacity with the same initial standard drier, i.e., with small DIC equipment, large amounts can be handled rapidly just after harvesting and energy consumption significantly reduced.

Keywords

Starch Convection Steam Brittle Milling 

Nomenclature

Ai and qi

Coefficients of Crank solution according to the geometry of the sample

Deff

Effective diffusivity of water within husks (m2s−1)

deq

Equivalent thickness of rice grain husks

k

Slope of ln((W W)/(W W o)) versus time (s−1)

P

DIC steam pressure (MPa)

t

Time (s)

td

DIC thermal treatment time (s)

tD

Time taken to dry paddy rice from 32 to 12.5 % dry basis (min)

vm

Absolute velocity of solid porous medium (m s−1)

vw

Absolute velocity of water flow within husks (m s−1)

W

Final water content, dry basis, in solid matrix t→∞

W

Water content, dry basis, in solid matrix at time t

Wi

Initial water content, dry basis, in solid matrix

Wo

Theoretical value of water content, dry basis, in solid matrix at starting time (% dry basis)

ρm

Apparent density of dry material (kg m−3)

ρw

Apparent concentration of water in material (kg m−3)

τ

Corresponds to Fick’s number: \( \tau ={D}_{\mathrm{eff}}\left(\frac{t}{d_p^2}\right) \)

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Faculty of Agriculture, Department of Food ScienceZagazig UniversityZagazigEgypt
  2. 2.Laboratory of Engineering Science for Environment LaSIE FRE 3474 CNRSUniversity of La RochelleLa Rochelle Cedex 01France

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