Starch Gelatinization and Drying of Paddy using Microwave Heating

Abstract

Rice (Orzya Sativa L.) is one of the staple foods for half of the world’s population. It is an excellent source of carbohydrate and also contains some amount of protein, phosphorus, iron, calcium, and sodium, a trace of fat, vitamins, and minor minerals. Paddy processing consists of three different operations, namely: soaking in water, steaming or hot water boiling and drying. The hydrothermal treatment also known as parboiling gelatinizes the starch in the paddy before milling to achieve maximum recovery of head rice and minimizes breakage of grains. Traditional and modern methods of soaking, parboiling and drying are laborious, take longer time, energy consuming, cause incomplete starch gelatinization and generate huge amount of wastewater. Therefore, a study on the development of an alternative method of paddy parboiling and drying using microwaves was undertaken. The experiments on microwave assisted water absorption by paddy at five power density levels in the range 0.2 to 1 kW kg-1 were conducted in a semi pilot-scale microwave dryer. The water absorption kinetics were modelled using Azuara’s equation and the process was compared with traditional hot water soaking of the paddy. It was found that the moisture content of paddy was increased to 44.54 % db in 1.67 h at 1.0 kW kg-1 power density compared to 32.19 % db in 3.33 h in traditional hot water soaking. 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 average variable moisture diffusivity was found to be in the range 61×10-10 -1.92×10-10 m2 s-1 at all power densities, whereas in hot water soaking it was 1.66 × 10-10 m2 s-1. Further, the microwave gelatinization of microwave-assisted water absorbed paddy was carried out in the same dryer at different levels of microwave power density, time of treatment and water application rate. The experiments were planned using central composite design and the process was optimized. The effects of microwave power density (MPD), time of treatment (TT) and water application rate (WAR) on final moisture content (FMC), crystallinity percentage (CP) and specific energy consumption (SEC) were studied. The optimum condition in microwave assisted starch gelatinization process was found at 1 kW kg-1 MPD, 60 min TT and 90 mL/10 min WAR. The gelatinized paddy was then dried in the same dryer at five microwave power densities in the range 0.25-1.25 kW kg-1. The drying characteristics, internal and surface mass transfer properties, milling yield, cooking characteristics and kinetics of the milled rice were analyzed. The Page model was found to be the best model for describing the microwave drying characteristics with higher R2 and lower RMSE values. The crystallinity percentage of microwave dried rice was decreased with an increase in power density. At higher power density, the microstructure of starch granules showed formations of internal cracks and fissures. The head rice yield and colour change of the microwave gelatinized and microwave dried rice were in the acceptable range. The hardness of microwave dried rice was increased with increase in MPD. The crude fat percentage in microwave-gelatinized and microwave (MG-MW) dried rice was decreased from 3.27 % to 2.69 % with an increase in MPD. The cooking time of raw rice was 31.36 min whereas in MG-MW dried rice, it was in the range 24-30 min. Gruel solid loss in MG-MW dried rice was in the range 5.62 to 6.69 %. All the quality characteristics and cooking properties of MG-MW dried rice were compared with the rice obtained by traditional steam-gelatinization followed by hot air (HA), infrared hot air (IRHA) and microwave (MW) drying. The total specific energy consumption in microwave-assisted water absorption, gelatinization and drying was estimated to be 2.11 MJ per kg of dry matter processed compared to 26.08 MJ per kg dry matter processed in hot water soaking, steaming and hot air drying.