Development of a new pre-drying method of accelerated water absorption and partial gelatinization of starch in paddy using pulsed microwave-water applications in a microwave rotary drum dryer

Abstract

The present study was undertaken to develop a pre-drying process of an accelerated water absorption and partial gelatinization of starch in paddy using pulsed microwave-water applications. The experiments on microwave-assisted water absorption by paddy (var. Sworn Masuri) at five power density levels in the range 0.2–1 kW kg⁻¹ were conducted in a semi-pilot-scale microwave dryer. The paddy was exposed to pre-calculated intermittent water spray and pulsed microwaves up to 100 min. The water absorption kinetics were modeled using modified Azuara’s equation of mass transfer and the process was compared with traditional hot water soaking of the paddy at 60 °C for 3.33 h. The moisture content gained by paddy during microwave-assisted water absorption was faster (30% in 1.67 h) compared to hot water soaking (30% in 3.33 h). The consumption of water, specific energy, and water absorption time during microwave processing were reduced by 70%, 39.51%, and 50%, respectively. The method of slopes was used to estimate moisture diffusivity during microwave-assisted water absorption by paddy. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) of treated paddy were done to understand the changes in the crystallinity and the microstructure of starch molecules. The microwave-assisted water absorption process was found to accelerate the moisture gain with the effect of partial gelatinization of starch in the paddy. Further, the experiment can be continued in the same microwave dryer to fully gelatinize the paddy at 30% moisture content and dry the gelatinized paddy.

Keywords:

• Effective moisture diffusivity
• microwave absorption
• moisture content gain
• SEM
• XRD