Composition and phosphorous profile of high protein rice flour and broken rice and effects of further processing

Authors: Liu, Keshun

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2018
Publication Date: 5/7/2018
Citation: Liu, K. 2018. Composition and phosphorous profile of high protein rice flour and broken rice and effects of further processing. Journal of the American Oil Chemists' Society. https://doi.org/10.1002/aocs.12040.
DOI: https://doi.org/10.1002/aocs.12040

Interpretive Summary: Rice is a major staple food grain for more than half the world's population. The world rice production in 2017/2018 was estimated at 481.0 million metric tons. Although protein constitutes a small fraction of milled rice dry mass compared to starch (about 8% vs. 80%), it is an important and high quality nutrient. Although rice protein isolates can be made in relatively large quantities by a common method of alkaline extraction followed by acid precipitation, for economic reasons rice is processed industrially by enzymatically converting starch in situ and recovering its hydrolysates as syrup and the residue is a co-product known as high protein rice flour (HPRF) with a protein content as high as 70%. In major rice producing countries, million tons of HPRF are produced annually and its use is limited to animal feed. The purpose of our study was to evaluate ways to increase the value of HPRF and expand its end uses. We found a 7-fold increase in contents of protein, oil, ash and phytate P in its co-product HPRF compared to the broken rice feedstock. Among the several options of further processing (dry fractionation, leaching in an aqueous medium, and leaching in an aqueous ethanol), leaching in an aqueous medium was most effective, achieving up to 10% increase of protein content and 10-20% reduction of phytate in a refined HPRF. Extraction of protein was least effective, since the protein in HPRF had been totally denatured during the industrial process.

Technical Abstract: High protein rice flour (HPRF), a co-product of an enzymatic process for commercial production of rice syrup, and broken rice (feedstock) were characterized for chemical composition and phosphorus profile, and investigated for effects of dry fractionation, wet extraction/leaching, and particle size reduction on protein enrichment and phytate removal. Results show that the HPRF contained 68.72, 7.54, 3.66, 8.27 and 11.56 (% dry matter) of protein, oil, ash, residual starch and other carbohydrate, respectively, equivalent to 90% starch reduction and more than 7 fold of increase in protein, oil and ash over broken rice. Sieving or particle size reduction and sieving caused about 5% increase in protein content. Protein extractability from HPRF was very low over a wide pH range, regardless of sample particle sizes, but leaching HPRF in an aqueous medium or aqueous ethanol enriched its protein by 10%. HPRF contained 4.41, 0.27, 3.00, and 7.68 (mg/g dry matter) of phytate P, inorganic P, rest of P and total P, respectively, representing 7.02, 2.70, 5.17, and 5.87 fold of increase over broken rice, respectively. Aqueous leaching led to 10-20% reduction in phytate P, but had little effect on total P content due to concomitant increase in rest of P. Therefore, leaching in an aqueous medium was relatively effective in enriching protein and reducing phyate content of HPRF.