EVALUATION OF DRYING CHARACTERISTICS AND QUALITY OF MAIZE DRIED BY TWO STAGE DRYING TECHNIQUES

Abstract

Drying of freshly harvested maize is a great concern in all maize producing countries. This research was conducted to investigate suitable and efficient two-stage drying options for high moisture maize. First option was employed by fluidized bed drying and tempering followed by sun drying method and second option was carried out in fluidized bed dryer using variable temperature i.e fluidized bed drying at high temperature and tempering followed by further fluidized bed drying at low temperature. First stage drying in fluidized bed dryer was carried out using three temperatures (120, 140 and 1600C) at constant bed thickness of 7.0 cm. After first stage drying, all dried samples were immediately tempered for 45 minutes at grain temperature. Further, all the tempered samples were dried in second stage by sun drying method and in the same fluidized bed dryer using 65 ± 20C, respectively in first and second option of two stage drying. Drying performances in terms of drying time, effective moisture diffusivity and specific energy consumption of the drying options and quality of dried maize were evaluated. Drying results indicated that total drying time required 4.25 hours in first option and 1.41 hours in second option of two-stage drying technique. On the other hand, total drying time for single stage drying (complete sun drying method) required 10 hours. Effective moisture diffusivity of maize kernel was found to be ranged from1.05×10-08 m 2 /s to 1.15×10-08 m 2 /s and 4.4×10-08 m 2 /s to 6.3×10-08 m 2 /s in first and second option, respectively. Specific electrical energy consumption in terms of primary energy and specific thermal energy consumption varied from 0.13 MJ/kg to 0.35 MJ/kg and 0.28 MJ/kg to 0.68 MJ/kg water evaporated for the first stage of fluidized bed drying. Physical properties such bulk density and porosity of the dried product varied between two stage drying and sun drying option. Total color difference of the dried maize obtained from both the two stage drying options was higher (6.32±1.11) in contrast to the sun dried sample (3.64±0.49). The values of % crude protein (6.24±0.27) and % starch content (69.20±0.96) increased during two stages drying whereas amylose content (19.05±0.30 mg/100g) and β-carotene content (0.14±0.02) were found to be decreased in compare to control drying. Even though both the options yielded almost same quality dried product yet second option is preferable for quick drying using lower energy. Therefore, the proposed two-stage drying can be introduced as alternating drying techniques for high moisture maize in commercial drying complexes of Bangladesh. However, further study is required for design and development of commercial dryers to be used at farm level.