No-washing rice processing and technology
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The production of rice-free rice requires the use of high-quality rice and involves a more advanced processing method compared to traditional rice milling. In addition to standard steps like cleaning, husking, grain separation, and whitening, the process includes additional stages such as brown rice selection, deep grinding, white rice polishing, product grading, and sealed packaging. The overall process can be summarized as: rice cleaning → husking → coarse grain separation → brown rice sorting → whitening and grinding → deep milling → white rice polishing → final classification → sealed packaging. The key differences from conventional methods lie in the three critical steps: brown rice selection, deep milling, and white rice polishing.
Studies have shown that simply cleaning rice is not enough to remove all impurities, as new contaminants like rice husks, ash, and other particles are generated during processing. To address this, an extra step has been introduced to select and remove unpolished grains from the raw material, ensuring that only pure brown rice enters the milling machine. This step is essential for maintaining the quality of the final product. Additionally, it helps classify brown rice by size, improving uniformity and enhancing the overall quality of the end product.
The brown rice selection process starts with a planar rotary sieve to remove fine dust, lumps, ash, and many defective grains. Using 20 mm x 20 mm rectangular holes or Φ3.2 mm round holes, the removal rate of fine and coarse grains can reach 100%, while imperfect particles are removed at about 80%. Large grain brown rice extraction exceeds 55%. Further, light impurities and stone fragments are eliminated using a density separator and winnowing. In Chinese rice mills, these impurities can be concentrated in valleys after coarse separation and then further processed with winnowing and density sorting, significantly improving the quality of the unwashed rice.
Traditional whitening methods often use one or two machines, which can damage the surface of the rice and increase breakage rates. To avoid this, a multi-stage whitening approach is used, typically involving three passes. The first pass removes about 4.5%-5% of the brown rice, the second around 4.5%, and together they account for about 9%, equivalent to the entire bran layer. The third pass removes 2%-3%, mainly from the sub-aleurone layer, which is rich in protein, fat, and vitamins. This layer is highly nutritious and fully edible, making multi-machine whitening not only beneficial for quality but also for resource utilization.
To optimize multi-machine whitening, each mill must be set to an appropriate speed, and the ratio of sand rollers to iron rollers should be carefully adjusted. For example, a sand roller mill might operate at 1,400–14,050 rpm, while an iron roller mill runs at 900–950 rpm. In existing Chinese mills, adding a third blower mill can help achieve higher efficiency. By separating some of the second-stage rice for a third pass, large-scale production of rice-free rice becomes possible.
White rice polishing is a crucial step in producing rice-free rice. A polishing agent solution is sprayed onto the rice surface, forming a thin gel film that gives the grains a pearl-like luster. Some polishing machines use two chambers with iron and non-toxic nylon rollers. As the rice moves through the chamber, heat from the solution and the rollers causes the surface to gelatinize, creating a clear, glossy finish. Some Italian manufacturers even apply Vaseline oil to achieve a similar effect.
In Japan, a method involves treating polished rice with water at 55–60°C for 10–15 minutes to dissolve surface dirt or grease. Temperature and time must be strictly controlled; too low or short will reduce effectiveness, while excessive heat or time may cause ethanol to penetrate the rice, posing safety risks. After treatment, the rice is dried quickly to remove any remaining ethanol, using hot air, cold air, natural drying, or vacuum drying. For example, drying at 80°C for 5 minutes followed by 20°C for 10 minutes ensures optimal results.