Aging characteristics of rice dried using microwave at 915 MHz frequency

Abstract

AbstractBackground and ObjectivesRice aging is an intrinsic phenomenon that leads to physicochemical changes in rice kernels. The goal of this study is to determine the physicochemical properties of rice dried using a 915 MHz microwave and then aged. Two rice cultivars, a medium grain cultivar Titan and long grain hybrid cultivar XL753, with a harvest moisture content of 20% ± 1% (wet basis) were used for the study. The rice samples were treated with two different drying methods, natural air drying as control and microwave heating at 915 MHz frequency followed by tempering and natural air cooling at 25°C and 56% relative humidity (RH). Microwave‐dried samples were treated at specific energies ranging from 360 to 630 kJ/kg of initial wet grain mass (kJ/kg‐grain). Head rice samples were stored for 6 months at 4°C and 25°C representing nonaged and aged samples, respectively. Physicochemical properties of the samples were determined.FindingsAged microwave‐dried rice had significantly higher setback viscosity, cooked rice hardness and gumminess, and significantly lower peak viscosity and solid loss than nonaged rice. The changes in the viscosity profiles of aged microwave‐dried rice were generally similar to the trends recorded for aged control rice. Furthermore, aging significantly increased the water uptake and kernel elongation ratio of microwave‐dried rice cultivar XL753 but had no significant impact on rice cultivar Titan. Statistical models indicate that the solid loss of microwave‐dried rice decreases by 0.083% while the final viscosity, kernel elongation ratio, and hardness increase by 30 cP, 0.014, and 41 g respectively, with every degree Celsius increase in aging temperature (between 4°C and 25°C), when all other variables are kept constant. Also, aged microwave‐dried rice that were treated at 360 and 525 kJ/kg‐grain were practically equivalent in terms of hardness and solid loss.ConclusionsNatural aging of microwave‐dried rice (dried at 915 MHz frequency) led to changes in some physicochemical properties which were generally similar to the aging patterns of the control rice. Microwave specific energy of 525 kJ/kg‐grain is recommended for rice drying because it produces rice with desirable drying and aging properties.Significance and NoveltyThis study provides processing recommendations that can be used to select microwave parameters for rice drying, which will achieve desirable rice properties.