Development of Post–harvest Bulk Handling System of Onions (Ⅰ) : Characteristics of Friction and Airflow Resistance

Onion is one of the major vegetables and is also the number one crop from the perspective of storage amount in South Korea. However, the postharvest processing for onions has been accomplished mainly with net package, causing high cost and low efficiency. Thus, development of a continuous and integrated processing system is required to ensure buyer satisfaction and marketing success. The objective was to determine frictional properties and airflow resistance of onion, which are very important to understand the behavior of onions in developing its bulk handling system. Frictional properties of onions, friction coefficient and rolling resistance, were determined by the method of inclined surface. Those values were investigated on three different surfaces (rubber, plywood, and galvanized steel) for four size grades (extra-large, large, medium, small). The airflow resistance, or static pressure drop, was measured with different mixing rate of onion sizes, and equations were developed for prediction of pressure drop in bulk state. The static-friction coefficient ranged from 0.35 to 0.46 for all experiment condition. The highest value was obtained on the plywood surface (0.46 plus or minus 0.06) followed by the rubber (0.41 plus or minus 0.03) and galvanized steel surface (0.35 plus or minus 0.03). This friction coefficient decreased with the onion bulb size. The static-rolling resistance ranged from 8.4 degree to 17.6 degree for all sizes, and, unlike the friction coefficient, increased with the onion bulb size. The highest values of rolling angles were obtained on the rubber surface (15.43 plus or minus 4.78) followed by the plywood (13.77 plus or minus 3.77) and galvanized steel surface (10.84 plus or minus 4.26). The airflow resistance increased as the onion size decreased, and a low porosity produced a higher pressure drop in all mixing rate of onion size. The pressure drop in stacked onion was also proportional to the superficial air velocity. Equation developed for prediction of pressure drop with superficial air velocity (SV) and stacking depth (SD) (r super(2)=0.99) was better than one with stacking depth (SD) (r super(2)=0.95) only. These results are invaluable to design post-harvest bulk handling system of onions.