Thin layer drying of cob maize using a natural convection solar tunnel dryer.

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Mukuwa, Mukwangole
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The University of Zambia
A semi- cylindrical natural convection Solar Tunnel Dryer (STD) was used for experiments to dry partially field- dried maize cobs with a moisture content of about 30 % wet basis to a safe storage moisture content of 12.5 % wet basis. Air temperatures ranging from 15.8 oC to 20.8 oC above ambient temperature were recorded in the collector unit. The presence of Aflatoxins was investigated for maize dried in the Solar Tunnel Dryer (STD), Open- Sun Drying (OSD) and partially field dried maize gotten Straight From the Field (SFF). The chimney performance was also evaluated. Drying time was examined with moisture content ratio as exponential and polynomial correlations. Fourteen different mathematical models available in literature were compared using their coefficient of determination to estimate solar drying curves. Based on statistical analysis results, Midilli drying model has shown a better fit to the experimental drying data of maize with a coefficient of determination R2 = 0.99912 as compared to other models. The Aflatoxins concentration levels was 8.37 parts per billion (ppb) for maize dried in STD and 11.57 ppb for OSD maize, respectively. The maize which was partially field- dried and tested for Aflatoxins had a concentration of 8.63 ppb. In Zambia, Aflatoxins concentrations above 10 ppb are not allowable. Acceptable Aflatoxins concentrations in the USA is 20 ppb whereas in the European Union countries concentrations of 4 ppb are allowed for ready to eat corn and 10 ppb for corn destined for further processing. In one of the experiments, it took 30 hours to obtain a moisture content of 12.57 % from an initial moisture content of 20.98 % wet basis as compared to 46 hours for OSD. In this experiment, the results show that the designed STD reduced the drying time of maize by 16 hours as compared to OSD. For air movement to take place, the buoyant pressure head in the chimney should be positive. Average values of buoyant pressure head ranged from 0.24645 Nm2 , 0.44859 Nm2 and 0.10819 Nm2 for all the experiments carried out.
Thesis of Master of Engineering in Agricultural Engineering