Effect of parboiling on texture and starch digestibility of rice

Parboiling is a hydrothermal treatment pre-dominantly practiced in South Asian countries to improve the milling recovery of rice. Parboiling involves soaking the paddy rice, steaming, and drying. This process improves the nutritive value of cooked rice by seeping of water-soluble nutrients like B-vitamins from the bran into the endosperm and reducing the glycemic index (GI). GI is related to the level of resistant starch (RS) or the starch fraction not digested in the small intestine of humans. However, parboiling renders the cooked rice darker, flakier, and less tender, which may negatively impact its acceptability to Filipinos. This study was conducted to evaluate the effect of parboiling on the texture and digestibility of local cultivars of rice. Nine varieties with varying amylose content, pasting properties and gelatinization temperature were subjected to parboiling and their cooked texture and starch digestibility, as measured through the levels of RS, non-resistant starch, and total starch, were determined. Parboiled rice cooked using the standard 1:1.5 rice water ratio generally raised the instron hardness, up to 113% in NSIC Rc 222. Pronounced increase in RS levels was observed in intermediate to high amylose varieties. Generally, RS was higher in parboiled rice. RS levels of cooked parboiled rice ranged from 0.05-2.72 g/100g. Although Meniangan had the highest retrodegradation potential (2821.5 cP) as measured by Rapid Visco Analyzer, the high amylose NSIC Rc 222 displayed the highest RS content, either parboiled (2.72 g/100g) or non-parboiled (1.82 g/100g), increasing the rice: water ratio of parboiled NSIC Rc 222 to 1:2.5 decreased the hardness of cooked rice from 4.41 kg/sq cm to 1.79 kgS/sq cm. while the RS level remained relatively high. In conclusion, parboiling of intermediate to high amylose varieties could deliver healthier cooked rice with acceptable textural quality when appropriate ratio of rice and waters is used.