Composition and phosphorous profile of high protein rice flour and broken rice and effects of further processing dry and wet 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: 5/8/2018
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. 95:957-967. 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 coproduct of enzymatic processing for rice syrup production, and broken rice (feedstock) were collected from a commercial source and characterized for chemical composition and phosphorus (P) profile. The effects of dry fractionation, wet extraction/ leaching, and particle size reduction on protein enrichment and phytate removal were also investigated. The HPRF contained 68.72, 7.54, 3.66, 8.27, and 11.56 (% dry matter) of protein, oil, ash, residual starch, and other carbohydrates, respectively, equivalent to 90% starch reduction and more than sevenfold increase in protein, oil, and ash over broken rice. Sieving or particle size reduction followed by sieving caused about 5% increase in the protein content. Protein extractability from HPRF was very low over a wide pH range regardless of sample particle sizes, indicating total protein denaturation by the commercial process, 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-1 dry matter) of phytate P, inorganic P, rest of P and total P, respectively, representing 7.0-, 2.7-, 5.2-, and 5.9-fold increase, respectively, over the four types of P found in broken rice. High phytate P in HPRF raises environmental concerns, since excess P excretion into manures is expected when fed to animals. About 15% phytate could be removed by leaching HPRF in an aqueous medium having a mild acidic to mildalkaline pH, but not with aqueous alcohol.