Propriedades físicas, químicas e tecnológicas de arroz submetido ao envelhecimento natural e ao acelerado por microondas

Abstract

Natural aging of rice is the process by which grains remain stored for a certain period, which generally varies from six months to one year, under controlled temperature and humidity conditions. Stored rice (old crop) has advantages over freshly harvested rice (new crop), due to the higher head rice yield in the industry and the greater expansion of volume and water absorption in cooking. However, natural aging is a time-consuming and costly process, and requires prolonged storage of the grains. For these reasons, emerging technologies such as the application of microwaves have been studied as viable alternatives to accelerate rice aging. Paddy rice grains of the IRGA 424 RI cultivar, widely cultivated in the southern region of Brazil, were subjected to natural aging for one year in storage at 27 °C and accelerated aging by microwave irradiation at a power of 900 W for six exposure times (30, 60, 90, 120, 150 and 180 seconds). The results demonstrated that naturally aged rice presented better water uptake and kernel expansion ratio, with 378.15% and 425.68%, respectively. In addition, it presented lower peak, breakdown, final and retrogradation viscosities, higher paste temperature and lower percentage of solid loss (5.34%). The cellular structure of the grains became thicker (SEM) and the in vitro digestibility of starch was slower, with a higher percentage of resistant starch (32%) and lower glycemic index (79%). However, it presented higher percentage of free fatty acids (0.4%), lower relative crystallinity of starch (18.7%) and darker grains. Accelerated aging of the grains by microwave irradiation provided positive results. The 120-second microwave treatment was the closest to naturally aged rice. The grains presented water uptake ratio of 365.5% and kernel expansion ratio of 441.7%. Furthermore, they presented lower peak, breakdown, final and retrogradation viscosity, lower percentage of solid loss (5.5%) and lower free fatty acids (0.3%). The cellular structure presented a denser characteristic (MEV) and the in vitro digestibility of starch was reduced, with a percentage of resistant starch of 20.4% and glycemic index of 86.0%. However, the microwave treatment resulted in darker and more opaque grains and the relative crystallinity of starch was also reduced (19.5%). Nevertheless, the microwave treatment was effective in accelerated aging of rice.