Effects of hydrothermal processes on antioxidants in brown, purple and red bran whole grain rice (Oryza sativa L.)

Authors: MIN, BYUNGROK - University Of Maryland Eastern Shore (UMES); McClung, Anna; Chen, Ming Hsuan

Submitted to: Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2014
Publication Date: 3/12/2014
Citation: Min, B., McClung, A.M., Chen, M. 2014. Effects of hydrothermal processes on antioxidants in brown, purple and red bran whole grain rice (Oryza sativa L.). Food Chemistry. 159:106-115.

Interpretive Summary: The consumption of whole grain cereals is strongly recommended in the Dietary Guidelines for Americans because of their protective effects against many chronic diseases such as cancer and cardiovascular diseases. Antioxidants in whole grain cereals are considered major contributors to this health-beneficial potential. Whole grain rice contains significant amounts of antioxidant compounds including vitamin E (tocopherols and tocotrienols), '-oryzanols and phenolics (especially proanthocyanidins and anthocyanins in red and purple bran rice, respectively). However, the concentrations of these natural antioxidants and, thus, their health-promoting potential, are dependent upon processing and cooking methods used with prior to consumption. Therefore, evaluation of changes in concentrations of natural antioxidants due to thermal processing is critical for establishing dietary guidelines for rice. In this study, five hydrothermal processes were used: wet-cooking brown rice, parboiling rough or brown rice, and wet-cooking the brown rice following the two parboiling methods. Wet-cooked brown rice retained the concentrations of tocotrienols, tocopherols and '-oryzanol, while wet-cooked parboiled rice had higher concentrations of these lipophilic antioxidants compared to un-cooked brown rice. In general, hydrothermal processes decreased the concentrations of water-soluble phenolic and flavonoid compounds, especially the anthocyanins in purple bran rice and the larger molecules of proanthocyanidins in red bran rice. However, proanthocynidins that are smaller in size and which are absorbable by the intestine, increased by several fold after hydrothermal processing. The hull, the outer covering of the grain, apparently provided protection from thermal degradation and/or leaching of water-soluble antioxidants during the parboiling process. These results suggest that parboiled and wet-cooked rice can be an excellent dietary source of lipophilic antioxidants but the parboiling process should be optimized to minimize the loss of soluble phenolics.

Technical Abstract: The impacts of parboiling and wet-cooking, alone and in combination, on concentrations of lipophilic antioxidants (vitamin E homologs and '-oryzanol), soluble (including proanthocyanidins and anthocyanins) and cell wall-bound phenolics and antioxidant capacities in whole grain rice from 6 rice cultivars having different bran colors were investigated. Parboiling rough and brown rice increased concentrations of lipophilic antioxidants but, in most rice cultivars, decreased concentrations of total phenolics and antioxidant capacities found in the soluble fraction . The retention of extractable anthocyanins was low, but was high for simple phenolics, after hydrothermal processes of purple bran rice. For proanthocyanidins of red bran rice, the extractable oligomers of degree of polymerization (DP) less than 4 increased up to 6-fold; while for oligomers (DP ' 4) and polymers there was a significant decrease that was positively correlated with the DP and temperature of the processing methods. The presence of hulls helped to retain water-soluble antioxidants during parboiling.