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. 2014 Jan 5;51(4):813–819. doi: 10.1007/s13197-013-1250-1

Drying characteristics of paddy in an integrated dryer

M R Manikantan 1, P Barnwal 1,3,✉, R K Goyal 2
PMCID: PMC3982010  PMID: 24741181

Abstract

Drying characteristics of paddy (long grain variety PR-118 procured from PAU, Ludhiana) in an integrated dryer using single as well as combined heating source was studied at different air temperatures. The integrated dryer comprises three different air heating sources such as solar, biomass and electrical. Drying of paddy occurred in falling rate period. It was observed that duration of drying of paddy from 22 to 13 % moisture content (w.b.) was 5–9 h depending upon the source of energy used. In order to select a suitable drying curve, six thin layer-drying models (Newton, Page, Modified Page, Henderson and Pabis, Logarithmic and Wang and Singh) were fitted to the experimental moisture ratio data. Among the mathematical models investigated, Wang and Singh model best described the drying behaviour of paddy using solar, biomass and combined heating sources with highest coefficient of determination (r2) values and least chi-square, χ2, mean bias error (MBE) and root mean square error (RMSE) values. However, Page model adequately described the drying behavior of paddy using electrical heating source.

Keywords: Paddy, Integrated paddy dryer, Drying models, Moisture ratio

Paddy or rough rice (Oryza Sativa L.), the most widely grown food grain crop, is the staple food of over 3 billion people. World and Indian rough rice production in 2010 was 700 million tonnes and 143.9 million tonnes respectively (FAOSTAT 2010). Rice provides more calories per hectare than any other cereal crops. Its nutritional value is high among cereals and grains. Though the protein content of rice is less than that of wheat, the protein digestibility and biological value of rice protein are the highest among wheat and other cereals. After harvesting, paddy normally goes through two moisture treatments; one is drying that may be required for safe storage, and the other is water absorption in preparation for further processing. Quality deterioration takes place if fresh paddy is not immediately dried to safe moisture level. Drying reduces bulk quantity, thus, facilitates in transportation, handling and storage. Although sun-drying is economical, mechanical drying speeds up the process, prevents losses, ensures use of safer drying temperatures and produces superior product compared to sun drying (Mudahar and Bains 1982). In view of high cost and non-renewable nature of the present conventional electrical energy source, different renewable energy sources such as solar and bio mass may be employed for paddy drying. The combining of conventional and non-conventional energy sources for paddy drying will be helpful for exploiting solar and bio mass sources as much as possible.

The drying characteristics of food is a complex phenomenon and requires simple representations to predict the drying behaviour and for optimizing the drying parameters. Thin layer drying equations were used for drying time prediction for generalization of drying curves (Karathanos and Belessiotis 1999). Though research in drying characteristics of paddy was carried out under individual heat source and data was reported on moisture loss and drying rates, systematic studies on the drying characteristics of paddy under integrated paddy drying system are lacking. The objectives of the present study were: i) to study the drying characteristics of paddy in an integrated paddy dryer using single as well as combined heating sources and ii) to evaluate a suitable thin layer drying model.

Materials and methods

A holding bin of one tonne paddy per batch capacity was fabricated and installed at Central Institute of Post Harvest Engineering and Technology, Ludhiana, Punjab, India. The paddy was re-circulated inside the holding bin using bucket elevator (capacity-165 kg/h) till it attained the safe moisture level of 12–14 % w.b. The hot air and the paddy were in cross circulation inside the holding bin. The width of the column where hot air and grain intersect was 20 cm. For heating the air, three heating mechanisms namely biomass fired furnace, solar air heaters and electric air heaters were considered to see the effectiveness of drying under individual as well as combined drying system. The heat energy required for drying the paddy from 22 % w.b. to a safe moisture level of 14 % w.b. was calculated using standard relationship (Chakraverty 1997; Barnwal et al. 2012) and the solar air heaters, bio mass furnace and electrical heating system was developed and integrated based on the calculated heat energy. The solar heater assembly made with 30 solar panels of 1.50 × 0.92 m size connected in parallel with three blowers of 2 hp each using 50 mm galvanized iron pipe (Goyal et al. 2004b). The bio mass furnace mainly consisted of a cross and counter flow shell, rectangular tube type heat exchanger, grate, hopper, air combustion chamber, main body frame and suction blower. The electrical heating system was made of 15 kW heating capacity coil (Goyal et al. 2004a). The integrated paddy drying system comprising holding bin, air heating system using solar, bio mass and electrical energy is shown in Fig. 1.

Fig. 1.

Fig. 1

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A schematic diagram of an integrated paddy dryer

Freshly harvested long grain paddy variety PR-118 was procured from experimental farm at Punjab Agricultural University, Ludhiana. Paddy was cleaned and graded using pedal cum power operated cleaner before the drying studies. The initial moisture content of the samples was determined as per standard hot air oven method (AOAC 1984). The drying experiment was conducted during February 2005 under three individual heating sources and one combined heating source (Solar and electrical system). The temperature of grain as well as air at different points was recorded at regular interval of 60 min.

Moisture ratio of samples during drying was calculated by the following equation:

graphic file with name M1.gif 1

where, MR is the dimensionless moisture ratio, M is the moisture content at time t, and Mo and Me are the initial and equilibrium moisture contents, respectively, on dry basis. During thin-layer drying of paddy in integrated paddy dryer, the samples were not exposed to uniform relative humidity and temperature continuously. So the moisture ratio was simplified according to Pala et al. (1996) and Doymaz (2004), to

graphic file with name M2.gif 2

To select a suitable model for describing the drying process of paddy, drying curves were fitted with six thin-layer drying equations. The evaluated moisture ratio models are presented in Table 1. Most of the models in Table 1 were derived by simplifying the general solutions of the Fick’s second law or the modification of the simplified models. Therefore, most of them were not arbitrarily chosen models; on the contrary, they were based on the physiological bases (Hacıhafızŏglu et al. 2008).

Table 1.

Thin layer drying models given by various authors

Model Name Equation Reference
Newton MR = exp (-kt) Liu and Bakker-Arkema(1997)
Page MR = exp (-kt n) Zhang and Litchfield (1991)
Modified Page MR = exp (-(kt)n) Overhults et al. (1973)
Henderson & Pabis MR = a exp (-kt) Henderson and Pabis (1961)
Logarithmic MR = a exp (-kt) + c Yaldiz et al. (2001)
Wang and Singh MR = 1+ at + bt 2 Wang and Singh (1978a, b)
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MR Moisture ratio; a, b, c, k, k 0, k 1 drying constants; t drying time

To determine the drying characteristics, paddy was dried in an integrated paddy dryer under individual as well as combined heating sources. About 100 kg cleaned and graded paddy sample was used for the drying experiment. Moisture content of paddy was determined at 60 min interval using standard hot air oven method (AOAC 1984). The drying was continued till the attainment of safe moisture level of 12–14 % w.b. Experiments were conducted in triplicate.

The non-linear regression analysis was done using MATLAB (version 6.5) software package. Coefficient of determination, r2 was one of the main criteria for selecting the best model. In addition to coefficient of determination, the goodness of fit was determined by various statistical parameters such as reduced chi-square, χ2, mean bias error (MBE) and root mean square error (RMSE) values. For quality fit, r2 value should be higher and χ2, MBE and RMSE values should be lower (Pangavhane et al. 1999; Sarsavadia et al. 1999; Togrul and Pehlivan 2002; Demir et al. 2004; Erenturk et al. 2004). The parameters were calculated by using the following expressions:

graphic file with name M3.gif 3
graphic file with name M4.gif 4
graphic file with name M5.gif 5

where,

MRexp

experimental moisture ratio

MRpre

predicted moisture ratio

N

number of observations

z

number of drying constants

Results and discussion

The time taken for drying of paddy under different sources is given in Table 2. The final moisture content of paddy ranged from 10.54 to 14.15 % (w.b.). It was also observed that the drying time for paddy ranged from 5 to 9 h depending on the source of energy used. It is evident that the drying air temperature has an important effect on drying. When the temperature was increased, the drying time reduced. The results are similar with the earlier observations on drying of Thai Hom Mali paddy (Doungporn et al. 2012), parboiled wheat (Mohapata and Srinivasa 2005; Kahyaoglu et al. 2012), garlic slices (Madamba et al. 1996), onion slices (Sarsavadia et al. 1999), egg plants (Akpinar and Bicer 2005), peach slices (Kingsly et al. 2007) and plum slices (Goyal et al. 2007).

Table 2.

Experimental data, drying rate and moisture ratio of paddy under different heat sources

Source Time (h) M.C. (%w.b.) Grain temperature (°C) Ambient temperature (°C) Hot Air inlet temperature (°C) Hot Air outlet temperature (°C) M.C. (d.b.) Drying rate, R (g water/min/kg bone dry material) Moisture Ratio
Solar and Electrical 0 21.52 37 30 50 35 27.42 - 1
1 20.28 42 29 51 38 25.44 0.330 0.928
2 19.01 45 30 52 39 23.47 0.329 0.856
3 17.96 45 33 52 38 21.89 0.307 0.798
4 16.63 45 33 52 38 19.95 0.311 0.727
5 15.17 44 32 52 38 17.88 0.318 0.652
6 14.29 44 31 52 38 16.67 0.299 0.608
7 13.92 42 30 51 38 16.17 0.268 0.589
Electrical 0 14.75 19 17 50 17 17.30 - 1
1 14.25 19 18 50 19 16.62 0.114 0.961
2 14.09 20 19 50 18 16.40 0.075 0.948
3 13.33 20 21 51 21 15.38 0.107 0.889
4 12.78 21 19 50 19 14.65 0.110 0.847
5 10.54 19 19 50 19 11.78 0.184 0.681
Solar 0 22.24 37 30 50 32 28.60 - 1
1 21.47 38 31 50 31 27.34 0.210 0.956
2 19.86 38 30 50 33 24.78 0.318 0.867
3 18.33 39 31 50 35 22.44 0.342 0.785
4 17.08 41 31 50 32 20.60 0.333 0.720
5 16.69 37 31 51 33 20.03 0.286 0.701
6 15.96 38 31 51 33 18.99 0.267 0.664
7 15.32 38 31 51 34 18.09 0.250 0.633
8 14.67 38 33 50 34 17.19 0.238 0.601
9 14.15 38 34 50 34 16.48 0.224 0.576
Biomass 0 21.22 20 30 50 29 26.94 - 1
1 20.80 21 28 50 31 26.26 0.112 0.975
2 19.03 22 28 51 30 23.50 0.286 0.873
3 18.71 24 27 51 31 23.02 0.218 0.855
4 17.77 24 25 52 29 21.61 0.222 0.802
5 17.50 25 24 52 29 21.21 0.191 0.788
6 16.40 29 23 51 29 19.62 0.203 0.728
7 14.60 23 22 51 31 17.10 0.234 0.635
8 14.32 20 21 50 31 16.71 0.213 0.621
9 13.92 20 21 50 31 16.17 0.199 0.600
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The drying rates and moisture ratio were computed and presented in Table 2. The moisture ratio for drying of paddy under different energy sources is given in Fig. 2. From Table 2 and Fig. 2, it is clear that moisture ratio decreased continuously with drying time. It is obvious that the constant rate period was absent, and drying of paddy took place in the falling rate period for the entire duration. Similar observations have been reported for the drying of soybean (Rafiee et al. 2009), mushroom (Arumuganathan et al. 2009), apricots (Doymaz 2004), figs (Piga et al. 2004), peach (Kingsly et al. 2007) and plums (Goyal et al. 2007). The drying in falling rate period shows that, internal mass transfer has occurred by diffusion.

Fig. 2.

Fig. 2

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Variation of moisture ratio of paddy with drying time under different energy sources in integrated paddy dryer

The average moisture ratio of paddy dried under different energy sources was test verified with six different drying models to find out their suitability to describe the drying process. The drying model constants, coefficient of determination (r2) and results of statistical analyses such as chi-square (χ2), mean bias error (MBE) and root mean square error (RMSE) values obtained from non linear regression analysis using MATLAB are summarized in Table 3. The best model to describe the drying behaviour of paddy was selected on the basis of high r2 and low χ2, MBE and RMSE values (Togrul and Pehlivan 2002; Demir et al. 2004; Erenturk et al. 2004). It is observed from the Table 3 that the value of r2 for all six models were greater than 0.90, indicating good fit. From the results, Wang and Singh model gave comparatively higher r2 value in all the energy sources except for electrical, where as the χ2, MBE and RMSE values were also found lowest. Wang and Singh (1978a, b) proposed a new empirical quadratic equation to fit the single-layer drying data for medium-grain rough rice. Thus, Wang and Singh model may be assumed to represent the thin layer drying of paddy under solar, bio-mass and combination of solar and electrical in an integrated dryer. This may be due to the uncontrolled nature of heating source such as solar and biomass which may not fit under the theoretical and semi-theoretical drying models. In the case of paddy drying using electrical energy, page model adequately described the drying characteristics. Das et al. (2004) showed that the Page model describes the experimental data adequately for drying of high moisture rough rice. Basunia and Abe (2005) and Cihan et al. (2007) found that the Page model yields an acceptable fit for the moisture content for their drying data of medium-grain rough rice dried under electrical based convection drying system.

Table 3.

Results of statistical analyses on the drying of paddy

Drying source Model Constants r2 χ 2 (X 10 −4 ) RMSE MBE(X 10 −4 )
Solar & Electrical Newton k = 0.079 0.944 1.64 0.012 −7.68
Page k = 0.078;
n = 1.014		 0.944 1.90 0.012 −4.68
Modified page k = 0.079;
n = 0.998		 0.944 1.92 0.012 −9.64
Henderson and Pabis a = 1.003;
k = 0.080		 0.994 1.88 0.012 −0.31
Logarithmic a = 0.905;
k = 0.093;
c = 0.100		 0.994 2.29 0.011 −0.53
Wang and Singh a  = -0.083;
b  = 0.003 		0.995 1.63 0.011 −0.15
Electrical Newton k = 0.139 0.969 17.80 0.034 −3.74
Page k  = 0.120;
n  = 1.119 		0.973 17.27 0.032 −0.74
Modified page k = 0.118;
n = 1.183		 0.969 19.74 0.034 −33.50
Henderson and Pabis a = 1.012;
k = 0.143		 0.971 18.61 0.033 −20.61
Logarithmic a = 1.354;
k = 0.097;
c = −0.349		 0.972 27.25 0.033 −9.15
Wang and Singh a = −0.132;
b = 0.006		 0.973 18.56 0.033 −74.26
Solar Newton k = 0.067 0.975 2.63 0.024 26.86
Page k = 0.165;
c = 0.449;
k = 0.087;
n = 0.861		 0.984 4.49 0.019 −20.37
Modified page k = 0.073;
n = 0.918		 0.975 7.12 0.024 25.15
Henderson and Pabis a = 0.988;
k = 0.065		 0.976 6.47 0.022 10.99
Logarithmic a = 0.568; 0.992 4.33 0.018 −17.32
Wang and Singh a  = −0.083;
b  = 0.004 		0.992 3.94 0.017 −0.12
Biomass Newton k = 0.051 0.973 5.51 0.022 −3.74
Page k = 0.049; n = 1.0178 0.973 6.17 0.022 0. 27
Modified page k = 0.050;
n = 1.008		 0.973 6.19 0.022 −4.06
Henderson and Pabis a = 1.002;
k = 0.051		 0.973 6.18 0.022 −0.12
Logarithmic a = 0.996;
k = 0.046;
c = 0.001		 0.973 6.91 0.022 2.92
Wang and Singh a  = −0.036;
b  = 0.335 		0.973 5.30 0.004 −0.05
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Conclusion

The effect of different energy sources on drying of paddy in an integrated dryer was investigated. From the above study on drying of paddy under different energy sources in an integrated dryer, the following conclusions were drawn:

  1. Increase in drying air temperature decreased the drying time.

  2. The drying process occurred in falling rate period.

  3. Wang and Singh drying model showed better fit with high coefficient of determination and low χ2, MBE and RMSE values during the drying of paddy under solar, biomass and combination of solar and electrical energy sources.

  4. Page model adequately described the drying characteristics of paddy using electrical energy source.

  5. The finding of this study will be helpful in drying of paddy under different heating systems.

References

  1. Akpinar EK, Bicer Y. Modelling of the drying of eggplants in thin-layers. Int J Food Sci Techol. 2005;40:273–281. doi: 10.1111/j.1365-2621.2004.00886.x. [DOI] [Google Scholar]
  2. Official methods of analysis. 4. Washington DC: Association of Official Analytical Chemists; 1984. [Google Scholar]
  3. Arumuganathan T, Manikantan MR, Rai RD, Anandakumar S, Khare V. Mathematical modeling of drying kinetics of milky mushroom in a fluidized bed dryer. Int Agrophysics. 2009;23:1–7. [Google Scholar]
  4. Barnwal P, Kadam DM, Singh KK. Influence of moisture content on physical properties of maize. Int Agrophys. 2012;26:331–334. doi: 10.2478/v10247-012-0046-2. [DOI] [Google Scholar]
  5. Basunia MA, Abe T. Thin-layer re-wetting of rough rice at low and high temperatures. J Stored Prod Res. 2005;41:163–173. doi: 10.1016/j.jspr.2004.01.005. [DOI] [Google Scholar]
  6. Chakraverty A. Post harvest technology of cereals, pulses and oilseeds. New Delhi: Oxford and IBH; 1997. [Google Scholar]
  7. Cihan A, Kahveci K, Hacıhafızŏglu O. Modelling of intermittent drying of thin layer rough rice. J Food Eng. 2007;79(1):293–298. doi: 10.1016/j.jfoodeng.2006.01.057. [DOI] [Google Scholar]
  8. Das I, Das SK, Bal S. Drying performance of a batch type vibration aided infrared dryer. J Food Eng. 2004;64:129–133. doi: 10.1016/j.jfoodeng.2003.09.020. [DOI] [Google Scholar]
  9. Demir V, Gunhan T, Yagcioglu AK, Degirmencioglu A. Mathematical modelling and the determination of some quality parameters of air-dried bay leaves. Biosystems Eng. 2004;88:325–335. doi: 10.1016/j.biosystemseng.2004.04.005. [DOI] [Google Scholar]
  10. Doungporn S, Poomsa-ad N, Wiset L. Drying equations of Thai Hom Mali paddy by using hot air, carbon dioxide and nitrogen gases as drying media. Food Bioprod Process. 2012;90:187–198. doi: 10.1016/j.fbp.2011.02.009. [DOI] [Google Scholar]
  11. Doymaz I. Effect of pre-treatments using potassium metabisulphite and alkaline ethyl oleate on the drying kinetics of apricots. Biosystems Eng. 2004;89:281–287. doi: 10.1016/j.biosystemseng.2004.07.009. [DOI] [Google Scholar]
  12. Erenturk S, Gulaboglu MS, Gultekin S. The thin layer drying characteristics of rosehip. Biosystems Eng. 2004;89:159–166. doi: 10.1016/j.biosystemseng.2004.06.002. [DOI] [Google Scholar]
  13. FAOSTAT (2010) Production of rice. http://www.faostat.fao.org
  14. Goyal RK, Manikantan MR, Barnwal P, Wanjari OD (2004a) Design and development of indirect type portable agricultural waste fired furnace. In: Proceedings of International Workshop and Symposium on Industrial Drying – 2004, 20–23rd December 2004, Mumbai, India
  15. Goyal RK, Wanjari OD, Ilyas SM, Manikantan MR (2004b) Design and development of solar air heater as renewable source of energy for drying. In: Proceedings of International Workshop and Symposium on Industrial Drying – 2004, 20–23rd December 2004, Mumbai, India
  16. Goyal RK, Kingsly ARP, Manikantan MR, Ilyas SM. Mathematical modelling of thin layer drying kinetics of plum in a tunnel dryer. J Food Eng. 2007;79:176–180. doi: 10.1016/j.jfoodeng.2006.01.041. [DOI] [Google Scholar]
  17. Hacıhafızŏglu O, Cihan A, Kahveci K. Mathematical modelling of drying of thin layer rough rice. Food Bioprod Process. 2008;86:268–275. doi: 10.1016/j.fbp.2008.01.002. [DOI] [Google Scholar]
  18. Henderson SM, Pabis S. Grain drying theory I. Temperature effect on drying coefficient. J Agric Eng Res. 1961;6:169–174. [Google Scholar]
  19. Kahyaoglu LN, Sahin S, Sumnu G. Spouted bed and microwave-assisted spouted bed drying of parboiled wheat. Food Bioprod Process. 2012;90:301–308. doi: 10.1016/j.fbp.2011.06.003. [DOI] [Google Scholar]
  20. Karathanos VT, Belessiotis VG. Application of thin-layer equation to drying data of fresh and semi-dried fruits. J Agri Eng Res. 1999;74:355–361. doi: 10.1006/jaer.1999.0473. [DOI] [Google Scholar]
  21. Kingsly RP, Goyal RK, Manikantan MR, Ilyas SM. Effects of pretreatments and drying air temperature on drying behaviour of peach slice. Int J Food Sci Technol. 2007;42:65–69. doi: 10.1111/j.1365-2621.2006.01210.x. [DOI] [Google Scholar]
  22. Liu Q, Bakker-Arkema FW. Stochastic modelling of grain drying, part 2: model development. J Agri Eng Res. 1997;66:275–280. doi: 10.1006/jaer.1996.0145. [DOI] [Google Scholar]
  23. Madamba PS, Driscoll RH, Buckle KA. The thin layer drying characteristic of garlic slices. J Food Eng. 1996;29:81–88. doi: 10.1016/0260-8774(95)00062-3. [DOI] [Google Scholar]
  24. Mohapata D, Srinivasa Rao P. A thin layer drying model of parboiled wheat. J Food Eng. 2005;66:513–518. doi: 10.1016/j.jfoodeng.2004.04.023. [DOI] [Google Scholar]
  25. Mudahar GS, Bains GS. Pretreatment effect on quality of dehydrated Agaricus bisporus mushroom. Indian Food Packer. 1982;28:19–27. [Google Scholar]
  26. Overhults DD, White GM, Hamilton ME, Ross IJ. Drying soybeans with heated air. Trans ASAE. 1973;16:195–200. doi: 10.13031/2013.37459. [DOI] [Google Scholar]
  27. Pala M, Mahmutoglu T, Saygi B. Effects of pretreatments on the quality of open-air and solar dried products. Food. 1996;40:137–141. [Google Scholar]
  28. Pangavhane DR, Sawhney RL, Sarsavadia PN. Effect of various dipping pre-treatment on drying kinetics of Thompson seedless grapes. J Food Eng. 1999;39:211–216. doi: 10.1016/S0260-8774(98)00168-X. [DOI] [Google Scholar]
  29. Piga A, Pinna I, Ozer KB, Agabbio M, Aksoy U. Hot air dehydration of figs (Ficus carica L.): drying kinetics and quality loss. Int J Food Sci Technol. 2004;39:793–799. doi: 10.1111/j.1365-2621.2004.00845.x. [DOI] [Google Scholar]
  30. Rafiee S, Keyhani A, Sharifi M, Jafari A, Mobli H, Tabatabaeefar A. Thin layer drying properties of soybean (Viliamz Cultivar) J Agric Sci Technol. 2009;11:289–300. [Google Scholar]
  31. Sarsavadia PN, Sawhney RL, Pangavhane DR, Singh SP. Drying behaviour of brined onion slices. J Food Eng. 1999;40:219–226. doi: 10.1016/S0260-8774(99)00058-8. [DOI] [Google Scholar]
  32. Togrul IT, Pehlivan D. Mathematical modelling of solar drying of apricots in thin layers. J Food Eng. 2002;55:209–216. doi: 10.1016/S0260-8774(02)00065-1. [DOI] [Google Scholar]
  33. Wang CY, Singh RP (1978a) Use of variable equilibrium moisture content in modeling rice drying. ASAE Paper No. 78–6505, ASAE, St Joseph, MI
  34. Wang CY, Singh RP (1978b) A single layer drying equation for rough rice. ASAE Paper No. 78–3001, ASAE, St Joseph, MI
  35. Yaldiz O, Ertekin C, Uzun HI. Mathematical modeling of thin layer solar drying of sultana grapes. Energy. 2001;26:457–465. doi: 10.1016/S0360-5442(01)00018-4. [DOI] [Google Scholar]
  36. Zhang Q, Litchfield JB. An optimization of intermittent corn drying in a laboratory scale thin layer dryer. Drying Technol. 1991;9:383–395. doi: 10.1080/07373939108916672. [DOI] [Google Scholar]

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