CAPÍTULO IV RESULTADOS Y DISCUSIÓN
4.1. Resultados
4.1.1. Resultados de la entrevista y encuestas aplicadas
4.1.1.1. Resultado de las entrevistas al Sr. Alcalde del Cantón El Empalme . 38
From the discussion above, we identify the product temperature and moisture profile to be of significance during the CIR-HAD of sweet potato. Based on this, we quantify the effect of varying the model air temperature and the depth of IR penetration on the product temperature and drying kinetics of sweet potato (Figure 13).
Figure 13a shows that changes in the drying air temperature significantly affects the moisture content of sweet potato during CIR-HAD. An increase in the drying air temperature from 40 oC to 60 oC after 1000s resulted in about 2.2% reduction in the moisture content. A stronger sensitivity of sample temperature to drying air temperature is shown in Figure 13b.
Increasing the air temperature from 40 oC to 60 oC caused a significant increase in the sample temperature by about 46%.
The moisture changes of sweet potato during CIR-HAD was not too sensitive to the IR penetration depth at the start of the drying process (Figure 13c). However, the effect increased as drying continued, just as discussed above. On the other hand, the sample temperature was sensitive to IR penetration depth throughout the drying process (Figure 13d). Overall, the effect of IR penetration depth on the CIR-HAD drying of food became prominent as drying progressed, whereas the drying air temperature was effective from start till the end of the drying process. These results further explain the reason why moisture concentration in the product core is very high at the start of the drying process (Figure 8). Since IR internal generation of heat takes prominent as drying progresses, it makes sense to have less effect on moisture distribution at the start of the drying process.
Figure 13. Moisture content and temperature of the food as a function of time for different drying air temperature and infrared
penetration depth:[a] moisture profile (Tair=40-60oC);
[b]temperature profile (Tair=40-60oC); [c] moisture profile (IR-
depth=0.003-0.005m); [d] temperature profile (IR-depth=0.003- 0.005m)
5. Conclusions
A non-equilibrium multiphase model for combined infrared and hot-air drying (CIR-HAD) of sweet potato was developed. The model was validated with experimental moisture content and temperature drying data. We quantify the evaporation rate, gas pressure, liquid water and vapour fluxes during the CIR-HAD of sweet potato, which is not possible using the much simpler single-phase model. This will allow more rationale fine-tuning of drying conditions to give desirable quality parameters. To pave the way for further development of CIR-HAD technology, this study concludes with the following:
• A multiphase porous media model considering Lambert's law for electromagnetic heat generation can adequately be used to describe the drying behaviour and drying mechanism of food during CIR-HAD. • The temperature distribution is uneven within the food product during
the CIR-HAD duration.
• The phase change from liquid water to vapour occurs almost immediately after the start of the drying process for CIR-HAD.
• At the start of the drying process, the vapour fluxes are the only fluxes for moisture transport at the sample surfaces in all directions. The dominant mechanism for moisture transport in the sample core is water fluxes.
• Both water and vapour fluxes cause moisture transport around the surfaces and sides of the sample during the drying process.
• The moisture and temperature profile were sensitive to the model drying air temperature and IR penetration depth, with air temperature being the more dominant model parameter.
Declaration of Competing Interest
The authors confirm that there is no conflict of interest associated with this research
Acknowledgement
This work was supported by Universiti Putra Malaysia under the Geran Putra Berimpak grant (GBP/2017/9553800).
𝑐𝑐 Moisture concentration (𝑘𝑘𝑘𝑘/𝑚𝑚3)
𝑇𝑇 The temperature of each phase (K) 𝜌𝜌 Density �𝑘𝑘𝑔𝑔
𝑚𝑚3�
𝐶𝐶𝑝𝑝𝑠𝑠 Specific heat capacity of sweet potato � 𝐽𝐽 𝑘𝑘𝑔𝑔𝑘𝑘�
𝑘𝑘 Thermal conductivity �𝑊𝑊 𝑚𝑚𝑘𝑘�
𝑄𝑄𝑖𝑖𝑖𝑖𝑖𝑖 Volumetric infrared heat source (W/m3)
𝑞𝑞𝑖𝑖𝑖𝑖𝑖𝑖 Heat absorbed by sweet potato (W)
𝛼𝛼 Absorption coefficient
𝛿𝛿 Infrared penetration depth (mm) 𝜀𝜀𝐼𝐼𝐼𝐼 Infrared emitter emissivity
𝜀𝜀𝑠𝑠 The emissivity of the sweet potato
𝐹𝐹𝑠𝑠,𝐼𝐼𝐼𝐼 View factor between the surface of the IR
bulb and sample
𝐹𝐹𝐼𝐼𝐼𝐼,𝑤𝑤 View factor between IR emitter and the wall
𝐹𝐹𝑤𝑤,𝑠𝑠 View factor between the sample and the wall
𝜎𝜎 Stefan-Boltzmann radiation constant 𝑇𝑇𝐼𝐼𝐼𝐼 The temperature of the emitter (˚C or K)
𝑇𝑇𝑠𝑠 The temperature of the sample surface (˚C
or K)
𝑅𝑅𝜕𝜕 Total thermal resistivity
𝑚𝑚0 The initial mass of sample (kg)
𝑚𝑚𝑑𝑑 Mass of dried sample (kg)
ℎ𝑅𝑅 Heat transfer coefficient (W/m2K)
ℎ𝑚𝑚 Mass transfer coefficient (m/s)
𝑇𝑇𝑎𝑎𝑖𝑖𝑟𝑟 Drying air temperature (˚C or K)
ℎ𝑖𝑖𝑔𝑔 latent heat of evaporation �𝐽𝐽 𝑘𝑘𝑔𝑔�
𝐶𝐶𝑝𝑝 Specific heat capacity � 𝐽𝐽 𝑘𝑘𝑔𝑔𝑘𝑘�
𝑘𝑘𝑝𝑝 Thermal conductivity of product �𝑊𝑊 𝑚𝑚𝑘𝑘�
𝐴𝐴0 Sample initial surface area (m2)
𝐴𝐴 Sample surface area at a time, t (m2) 𝜌𝜌𝑤𝑤 Density of water �𝑘𝑘𝑔𝑔
𝑚𝑚3�
𝜌𝜌𝑠𝑠 The density of sweet potato �𝑘𝑘𝑔𝑔 𝑚𝑚3� 𝑅𝑅𝑎𝑎 The activation energy (𝑘𝑘𝐽𝐽/𝑚𝑚𝑚𝑚𝑚𝑚)
𝑅𝑅𝑔𝑔 Universal gas constant (𝐽𝐽/𝑚𝑚𝑚𝑚𝑚𝑚/𝐾𝐾)
𝐷𝐷0 Diffusion integration constant (m2/s)
𝜌𝜌𝑎𝑎 Density of air �𝑘𝑘𝑔𝑔 𝑚𝑚3�
𝜇𝜇𝑤𝑤 Dynamic viscosity of water (kg/m. s)
𝜇𝜇𝑣𝑣 Dynamic viscosity of vapour (kg/m. s)
𝑣𝑣 Drying air velocity (m/s)
𝑘𝑘𝑎𝑎 Thermal conductivity of air (W/ (K.m))
𝐶𝐶𝑝𝑝𝑎𝑎 Specific heat capacity of the air � 𝐽𝐽 𝑘𝑘𝑔𝑔𝑘𝑘�
𝑎𝑎𝑤𝑤 Water activity
∆𝑉𝑉 The sum of the gas, water and solid volume (𝑚𝑚3)
∆𝑉𝑉𝑔𝑔 The volume of gas (𝑚𝑚3)
∆𝑉𝑉𝑤𝑤 The volume of water (𝑚𝑚3)
∆𝑉𝑉𝑠𝑠 The volume of solids (𝑚𝑚3)
𝜑𝜑 Apparent porosity 𝑆𝑆𝑔𝑔 Gas saturation
𝑆𝑆𝑤𝑤 Water saturation
𝐾𝐾𝑒𝑒𝑣𝑣𝑎𝑎𝑝𝑝 Evaporation constant
𝑀𝑀𝑣𝑣 The molecular weight of vapour (kg mol-1)
𝑀𝑀𝑎𝑎 The molecular weight of air (kg mol-1)
𝑀𝑀𝑔𝑔 The molecular weight of gas (kg mol-1)
𝑃𝑃𝑎𝑎𝑚𝑚𝑎𝑎 Ambient pressure (Pa)
𝑃𝑃𝑤𝑤 The total flux of liquid water
𝑅𝑅𝑒𝑒𝑣𝑣𝑎𝑎𝑝𝑝 Liquid water evaporation to water vapour
(𝑘𝑘𝑘𝑘/𝑚𝑚3𝑠𝑠)
𝐾𝐾𝑔𝑔 Intrinsic permeability of gas (m2),
𝑃𝑃𝑣𝑣𝑎𝑎𝑖𝑖𝑟𝑟 Air vapour pressure (Pa)
𝐷𝐷𝑣𝑣𝑎𝑎 Binary diffusivity (m2/s)
𝐶𝐶𝑝𝑝𝑣𝑣 Specific heat capacity of vapour � 𝐽𝐽 𝑘𝑘𝑔𝑔𝑘𝑘�
𝐶𝐶𝑝𝑝𝑤𝑤 Specific heat capacity of water � 𝐽𝐽 𝑘𝑘𝑔𝑔𝑘𝑘�
𝐾𝐾𝜕𝜕ℎ,𝑠𝑠 Thermal conductivity of sweet potato �𝑊𝑊 𝑚𝑚𝑘𝑘�
𝐾𝐾𝜕𝜕ℎ,𝑔𝑔 Thermal conductivity of the gas �𝑊𝑊 𝑚𝑚𝑘𝑘�
𝐾𝐾𝜕𝜕ℎ,𝑤𝑤 Thermal conductivity of water �𝑊𝑊 𝑚𝑚𝑘𝑘�
𝑊𝑊𝑣𝑣 Vapour mass fraction
LWCD Liquid water capillary diffusivity
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