Steaming retains more phenolics, dietary fiber, and antioxidant activities than cooking for rice with different milling processes

Background and objectives The traditional heat processing methods, steaming and cooking treatment, can affect the nutrients in rice, such as phenolics, flavonoids, and dietary fiber. However, there is no systematic assessment that by which heat treatment method and conditions more nutrients in rice can be retained, especially in brown rice, two-times milling rice, four-times milling rice, polished rice, and color selection rice (from different milling processes with practical processing at rice milling factory). The aim of this study was to investigate the effect of steaming and cooking treatment on the changes in nutrients in rice with different milling processes, including dietary fiber and phenolic acids, flavonoids, anthocyanins, individual phenolic compounds, antioxidant activities (DPPH, ABTS, and T-AOC) in the free, and bound extracts. Findings Compared with unsteamed and uncooked rice, the total phenolic content (TPC), the total flavonoid content (TFC), and individual phenolic compounds in the free and bound fractions, total anthocyanin content (TAC), and antioxidant activity in the free fractions in brown rice, two-times milling rice, four-times milling rice, polished rice, and color selection rice were significantly reduced by steaming and cooking treatment (p < .05). The TAC and antioxidant activity in the bound fractions, soluble dietary fiber (SDF), insoluble dietary fiber (IDF), and total dietary fiber (TDF) in the five rice samples were increased after steaming and cooking. However, it was very interesting that steaming treatment resulted in a less reduction in phenolics and antioxidant activities than cooking. Conclusions From the view of retaining more nutrients, steaming treatment is more suitable for the white rice of the milling process with high milling degree, while cooking treatment is more suitable for the brown rice or rice of milling process with low milling degree. Generally, steaming treatment retains more phenolics, dietary fiber, and antioxidant activities than cooking rice with different milling processes. Significance and novelty This study may provide useful reference and offer a simple and practical method to retain more nutrients for preparation and consumption of rice. These results suggest that steaming treatment is an efficient method of daily processing to reduce the loss of nutrients in rice.