TWO-STAGE DRYING FOR RAPID MANAGEMENT OF HIGH MOISTURE MAIZE

Abstract

Management of high moisture maize grains is a problem in Bangladesh. This study was conducted to investigate the feasibility of two options of two-stage drying of maize: i) fluidized bed drying (FBD) and tempering followed by sun drying method and ii) fluidized bed drying (FBD) and tempering followed by fixed bed drying method. An experimental fixed bed dryer was designed and fabricated for using in second stage drying under this study while a previously developed laboratory scale fluidized bed dryer was used for first stage drying of the samples. The fixed bed dryer consists of a rectangular shaped drying chamber with the dimension of 25 × 25 × 100 cm. It has a blower that produces maximum air flow rate 0.10 m3 /s and a heating unit composed of three electric heaters (1.5 kW each) separately connected. The blower was driven by a 2 hp motor of 1440 rpm. The fluidized bed drying of maize samples was carried out using three temperatures (100, 130 and 1500C) at three bed thickness (7, 9 and 11cm). 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 drying in the fixed bed dryer using 45 ± 20C temperature at 30 cm bed thickness and by sun drying method separately. Total drying time required 4.57 hours for first option and 2.57 hours for second option of two-stage drying technique. On the other hand, total drying time for single stage drying (complete sun drying method) required 11.25 hours. The proposed two-stage drying technique exhibited as a quicker method than existing drying method of maize. The results showed that most of the drying parameters of fluidized bed drying were suitable for maize drying in employing both the options. The highest nutritional quality in terms of protein content (6.65% to 6.67%) and ash content (1.17% to 1.18%) was obtained in the dried samples dried using 1300C at 9 cm bed thickness. Better grain colour was also found in that drying condition while the colour values were: L* = 67.5 to 70.9; b* = 43.1 to 43.3; and a* = 7.9 to 8.0. The calculation revealed that the microbial load of dried maize for the first option and second option methods were 2.68 × 105 to 1.59 × 106 cfu/g and 3.0 × 103 to 3.2 × 103 cfu/g, respectively. Thus, it is clear that both the methods were almost similar based on physical and nutritional quality but in term of microbial load, the second option was more hygienic than the first option. Hence, for quality and hygienic dried product, high moisture maize should be dried in fluidized bed dryer using 1300C temperature and 9cm bed thickness in first stage drying then by fixed bed dryer. Therefore, two-stage drying may be recommended for drying of high moisture maize and other cereals in commercial scale.