Effect of convective, infrared and microwave heating on drying rates, mass transfer characteristics, milling quality and microstructure of steam gelatinized Paddy

摘要

Paddy (var. Sworn Masuri) was dried by hot air (50 degrees C), infrared hot air (4,185W m(-2) radiation intensity and 40 degrees C) and microwave rotary drum (0.37, 0.78, and 1.23kW kg(-1) power density) to evaluate the most suitable method of drying. Before drying, the paddy grains were soaked in water (60 degrees C, 6h) followed by steaming (200kPa, 10min) to gelatinize the starch. The drying rate constants, apparent moisture diffusivity (D-a), convective mass transfer coefficient (h(m)), head rice yield (HRY) and the broken percentage were estimated. The X-ray diffraction analysis and scanning electron microscopy (SEM) of starch granules and outer husk of the dried paddy were carried out. Microwave rotary drum drying (MWD) was found to be faster as compared to hot air drying (HAD) and infrared hot air drying (IRHD). The h(m) in MWD was found to be 90.76-97.01% higher than HAD and 33.33-78.44% higher than IRHD. The HRY and specific energy consumption in MWD were, respectively, 5.87% and 89.16% lower than HAD. The microwave dried rice showed the lowest crystallinity percentage. The SEM of rice starch obtained by MWD showed cracks and fissures whereas IRHD induced microstructural changes only in paddy husk.Practical applicationsRice is a staple food of 90% Asian population. The parboiled paddy is dried in the open sun (2-3days) and by hot air (6-9h) on farms and in the rice industry. These are highly laborious and time-consuming methods. Therefore, the study was undertaken from industry suggestion to develop a cost-effective microwave-assisted drying method for parboiled paddy. The microwave rotary drum drying was found to give similar quality rice compared to hot air with a 90% reduction in specific energy consumption. Therefore, traditional open sun and hot air drying can be replaced by microwave rotary drum to save expenses towards the capital costs, energy, labor, and grain handling.