DEVELOPMENT OF A CLEANING AND GRADING MACHINE TO IMPROVE THE QUALITY OF MILLED RICE (Oryza sativa L.)

Abstract

Locally processed rice (Oryza sativa L.) is characterized with high percentage of broken grains and impurities which impair the quality and reduces the market value. Reducing the proportion of broken rice and impurities after milling remains a challenge. There is therefore a need to develop a machine which cleans and grades the rice kernels based on quality. The objective of this work was to develop a cleaner - grader to improve the quality of milled rice. The physical and aerodynamic properties of six rice cultivars (ITA 150, FARO 44, FARO 52, NERICA 1, IGBEMO 1 and IGBEMO II) were determined using ASAE Standards. Based on these properties, a three sieve oscillating cleaner - grader machine was developed. Samples from the six rice varieties were milled using No 1 rice huller and milling characteristics were evaluated according to NIS 230 Standards. The machine was tested to determine cleaning performance index and grading efficiency of the rice cultivars. The tilt angle of the top and bottom sieves were fixed at 2 and 15o, the intermediate sieve was set at 2, 4 and 6o and the blower was inclined at 0 and 5o. Data were analysed using ANOVA and descriptive statistics. The moisture content of the rice cultivars during milling varied from 10.7 to 12.5% and 11.0 to 13.6% in the raw and parboiled paddy respectively. The mean angle of repose was 29.3o ± 0.4 and the mean co efficient of friction was 0.3 ± 0.01. The spherical mean ranged from 2.9 to 3.5 mm and was significantly different (p˂0.05) in Igbemo II cultivar when compared with other cultivars. The sphericity was within 0.41 to 0.5; the bulk density ranged from 1.34 to 1.38 gcm-3, while the terminal velocity varied from 4.8 to 6.4 ms-1.The machine was developed to operate at a fan speed of 240 rpm, air velocity of 4.8 ms-1 with aperture diameter of 3, 5.5 and 7 mm for bottom, intermediate and top sieves respectively. The head rice yield ranged from 22.3 to 34.1% and 44.8 to 55% for the raw and parboiled milled respectively. The percentages of broken grains obtained during milling of raw and parboiled rice were further reduced by 41.5 and 64.8% respectively with the use of the machine. The cleaning performance index of the machine ranged from 0.6 to 0.9; the highest occurred at a tilt angle of 4o and blower inclination of 5o for raw ITA 150. The grading efficiency ranged from 61.4 - 84.8% and 69.0 to 94.5% for raw and parboiled milled rice respectively. The highest values of 84.8% and 94.5% were obtained in raw FARO 44 at a tilt angle of 2o and in parboiled ITA 150 at a tilt angle of 6o respectively with blower inclination of 5o. The use of the cleaning and grading machine after milling has substantially reduced broken kernels of locally processed rice in the raw and parboiled milled rice thereby enhancing the quality.