Abstract
The objective of this work was to study the convective drying process used for cylindrical samples of two different pear varieties, one originating from Europe (Pyrus communis) and the other from Asia (Pyrus pyrifolia), submitted to two blanching treatments, as well as determine some coefficients referring to heat and mass transfer and thermodynamic parameters of the process, being: enthalpy (ΔH*), entropy (ΔS*) and Gibbs energy of activation (ΔG*). All samples were dried in a forced air oven at temperatures of (62, 76, 84 and 92)°C until reaching a constant mass. The heat and mass transfer coefficients referring to the constant and falling-rate drying periods were obtained by adjusting the experimental data to wet bulb temperature and liquid diffusion models, respectively. The heat and mass transfer coefficients varied linearly with temperature. Blanching altered the drying behavior of the two varieties in relation to the non-blanched samples, mainly at the elevated drying temperatures of 84°C and 92°C. ΔH* and ΔS* decreased while ΔG* increased with the elevation of drying temperature; therefore, it was possible to verify that variation in the diffusion process in pears during drying is dependent on energetic contributions of the environment.
Acknowledgments
The authors are thankful to the Brazilian agencies CNPq and FAPEMIG for the financial support.
Nomenclature
- A
Area, m2;
- D0
Pre-exponential factor, m2 s−1;
- Def
Effective diffusion coefficient, m2 s−1;
Drying rate, kg s−1;
- Ea
Activation energy, J mol−1;
- hc
Global heat transfer coefficient, W m−2 °C;
- hfg
Latent heat of vaporization, J kg−1;
- hm
Global mass transfer coefficient, m s−1;
- hp
Planck’s constant, 6.626 × 10−34 J s;
- kb
Boltzmann Constant, 1.38 × 10−23 J K−1;
- Pvbu
Vapor pressure for the wet bulb temperature;
- Pv∞
Vapor pressure;
- R
Cylinder radius, m.
- R
Universal gas constant, J mol−1 K−1;
- Rv
Universal Constant for water vapor, 0.462 J kg−1 K−1;
- RU
Moisture content ratio, dimensionless;
- T
Time, s;
- Ta
Drying temperature, K;
- Tbu
Wet bulb temperature, °C;
- T∞
Drying temperature, °C;
- U0
Initial moisture content of the product, kgw kgdm−1;
- Ue
Equilibrium moisture content, kgw kgdm−1;
- Ut
Moisture content of the product at a given time, kgw kgdm−1;
Greek letters
- ΔH*
Enthalpy of activation, J mol−1;
- ΔS*
Entropy of activation, J mol−1;
- ΔG*
Activation Gibbs energy, J mol−1;
- λn
Roots of the zero-order Bessel function, dimensionless.
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