Nghiên cứu khoa học nông nghiệp " Fluidised-bed drying experiments " potx

Appendix 1: Fluidised-bed drying experiments INTRODUCTION In the previous study, Response Surface Method (RSM) experiment for optimum drying condition has been determined in terms of HRY using a high temperature fluidised bed dryer. The drying air temperature and time in the first pass of RSM experiments (75-87 o C and 2.5 mins) has been used effectively in terms of drying rate and HRY. At optimum condition, the head rice yield (HRY) has found almost similar to the controlled sample. However, sensory experiment has shown that the whiteness of optimum sample was lower than of the controlled sample. This has reduced the sensory score of the optimum sample. Statistical analysis has also indicated that first drying pass affected remarkably on the colour of the samples. In this period, the tempering time in the first pass was thought to be a factor affecting the colour of the cooked rice samples. The objectives of this experiments were to study the effect of tempering time and drying air temperature of the first pass on: (i) the whiteness of the cooked rice; (ii) moisture removal rate of paddy in the ventilation stage; and (iii) head rice yield. MATERIALS AND METHODS Fluidized bed dryer A batch lab-scale dryer (HPFD150) developed at the Nong Lam University, Vietnam, was used in this study. This dryer consists of three main components: (i) a cylinder shaped drying chamber 40 cm in height and 15 cm in diameter; (ii) two 5 kW electric heating units; and (iii) a centrifugal fan driven by a 0.75 kW motor. The inlet drying temperature range of 20-100 o C was monitored by a temperature controller (Hanyoung Electronics Inc., Model DX7, Seoul, Korea). The outlet drying temperature was monitored by a Daewon thermocouple. Rice samples Freshly harvested paddy of OM4900 was collected at Kien Giang Province during March 2009. These paddies were cleaned, transferred to laboratory and kept at 4 – 5 o C. Before drying, remove paddy from cooler and soak in water for 1.5 h, then drain for 30 min to obtain moisture content of 31-33 % wb. Experiments A two-factorial design was used to study the effect of fluidized-bed drying air temperature of 75 and 80 o C and tempering time (20-40 mins) in the first pass. Tempering was carried out at grain temperature immediately after drying (Table 1). Control sample was ventilated at 35oC until its moisture content reached 14% wb. 210 g rewetted paddy is used (thickness of 2 cm) for each drying treatment. In the first pass, paddy is fluidized bed drying at 80 o C for 2.5 mins. After drying, paddy is immediately transferred to glass containers and tempered in incubator, warmed up at grain temperature (73 o C), for designed tempering time. Noted that grain temperature is measured right after removed from outlet of fluidized bed dryer in the insulating box. The rice samples was continued to dry using tray-type dryer for ventilation at 35 o C. After drying, rice samples were packed in sealed plastic bags and stored at room temperature for 24 h before milling. Rice samples are determined final moisture content after each drying pass, and subjected to head rice yield, hardness and percentage of gelatinization measurements. RESULTS AND DISCUSSIONS At 80 o C, the overall moisture removal rate (OMR) at tempering time of 40 mins and 30 mins was 8-8.4%/h and 7.2-7.9%/h, respectively (Table 1). These values were 4.2-4.4%/h and 3.9- 4.6%/h, respectively for drying temperature of 75 o C. Therefore, the drying time was reduced significantly by the drying temperature 80 o C in comparison with 75 o C. The moisture removal rate in the ventilation stage (VMR) was also different significantly between 75 o C and 80 o C. Table 1: Drying conditions and moisture content of paddy after drying and ventilation Treatment MCI, %wb Td, oC MCF, %wb Tt, oC ttime, min tV (h) MCV, %wb Replication1 Control 33.4 7.0 14.0 A 32.4 75 15.9 20 4.8 13.3 B 30.9 75 17.0 30 4.5 13.4 C 32.2 75 15.5 73 40 4.5 13.5 D 31.6 80 15.2 20 2.5 13.2 E 32.0 80 15.7 30 2.6 13.4 F 32.6 80 16.1 75 40 2.4 13.2 Replication2 Control 32.0 6.8 13.3 A 30.2 75 15.3 20 4.3 13.9 B 32.0 75 17.8 30 3.9 14.0 C 31.8 75 16.2 73 40 4.2 13.1 D 33.1 80 16.1 20 2.5 14.4 E 33.6 80 18.1 30 2.5 14.0 F 32.7 80 17.2 75 40 2.3 13.7 Notes: MCI (Initial moisture content), Td (Fluidised-bed drying temperature), MCF (moisture content after first pass), ttime (tempering time), tV (ventilation time), MCV (moisture content after ventilation). Initial results shown that there was no significant different of the HRY between drying temperature 75 o C and 80 o C in the tempering time range of 20-40 mins. This HRY (56-57 %) was slightly lower than that of the controlled samples (60 % for low drying temperature at 35 o C) (Table 2). Table 2: Moisture removal rate and head rice yield Treatment OMR (%/h) VMR (%/h) HRY, %wb OMR (%/h) VMR (%/h) HRY, %wb Replication1 Replication2 Control 2.76 2.76 60.06 2.76 2.76 57.62 A 4.02 0.55 56.09 3.84 0.33 55.34 B 3.86 0.79 55.51 4.58 0.97 55.59 C 4.16 0.44 56.25 4.43 0.73 56.61 D 7.36 0.80 55.28 7.48 0.68 57.73 E 7.21 0.89 57.14 7.94 1.66 57.69 F 8.02 1.20 52.67 8.44 1.56 48.85 Notes: OMR (overall moisture removal rate), VMR (moisture removal rate in the ventilation stage), HRY (head rice yield). Sensory experiments are under analysed to be available in the next report. . Appendix 1: Fluidised-bed drying experiments INTRODUCTION In the previous study, Response Surface Method (RSM) experiment for optimum drying condition has been determined. temperature fluidised bed dryer. The drying air temperature and time in the first pass of RSM experiments (75-87 o C and 2.5 mins) has been used effectively in terms of drying rate and HRY. At optimum. rewetted paddy is used (thickness of 2 cm) for each drying treatment. In the first pass, paddy is fluidized bed drying at 80 o C for 2.5 mins. After drying, paddy is immediately transferred to glass