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ice is a vital staple food, feeding half
of the world’s people and, according
to the Food and Agricultural
which leather strips were nailed. The
second was that the cone rotated at a
speed that was 25 percent slower than
the vert...
polishing. The screen basket covering the cam rolls
allows a pre-determined distance between them, with
a greater distance...
Cimbria develops and...
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Rice Polishing - 150 years of innovation


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“Nowadays, more and more rice mills are installing rice polishers that subject rice to multiple polishing passes. The degree of polishing has reached an all-time high”

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Rice Polishing - 150 years of innovation

  1. 1. R ice is a vital staple food, feeding half of the world’s people and, according to the Food and Agricultural Organisation, providing as much as 20 percent of the global population’s dietary energy supply. In 2015, global rice farming produced 743 million tonnes of paddy rice – which yielded 493 million tonnes of white rice. Of this, an estimated 300 million tonnes of rice is polished. Originally, rice was consumed as unrefined, whole grain brown rice. The evolution of polished rice has changed our relationship with this staple food and with it, consumer tastes and demands. Today there are more than 40 000 different varieties of rice, each with their own characteristics, and each forming an integral part of the culinary traditions of many different regions and cultures. For instance, sushi and biryani from Asia, paella and risotto from Europe, as well as rice pudding - a British classic. In demand Until the late 20th century, rice mills around the world, including top-quality millers, did not integrate polishing into the production process. However, owing to the steadily increasing demand for whiter, silkier rice, the polishing process is now considered to be a crucial stage in the milling process. Although it is widely accepted that brown rice has a much higher nutritional value than white rice, many consumers prefer the taste of the polished white alternative. Furthermore, it easier to digest, needs no pre-soaking, cooks quicker and uses less water. It should be noted that the cooking process might cause the rice to burst, making it look coarser, which consumers can find off- putting. However, this can be reduced, if the degree of polishing is adapted to suit the particular variety of rice. Polished rice has benefits for food producers and retailers too. It improves the appearance of the grain, making it more visually appealing at the point of sale, meaning it can command a higher price. It also removes traces of bran left after the whitening process. This is particularly important, as glycerides in the bran turn rancid when exposed to oxygen. If they are not removed, they reduce the shelf life and eventually result in a product that is unfit for consumption. However, the demand for high-gloss, transparent-looking rice in some parts of the world has been so high that unsafe, unapproved methods have been used to give the desired result. For instance, glazing the rice with non-food-safe additives, such as oil or talcum powder. Fortunately, an increasing number of rice mills are turning to innovative rice polishing technology to deliver new standards, improve quality, enhance food safety and deliver the degree of whiteness and silkiness that consumers demand. Humble beginnings Prior to the advent of modern polishers, several simpler methods were used including pounding the rice using a pestle and mortar, rubbing it on the floor, beating gently with clubs in jute bags and treading by humans and animals. These makeshift means, often carried out in poor hygiene conditions, not only required significant time and energy, but usually resulted in a poorly-finished and significantly damaged rice, with high levels of wastage. The first commercial rice polisher is widely believed to have been patented by the British engineer Sampson Moore. The inventor, a prominent engineer during the British Industrial Revolution, was credited in the London Gazette for his invention on June 21st 1861, for “improvements in the machinery or apparatus for dressing and polishing rice”. Since those early days, a range of machines have improved the efficiency and quality of milled, polished rice. Rice polisher development Unpolished rice naturally has a coarse surface, with ridges that protect individual grooves, where the bran sits. Prior to polishing, the rice must go through a whitening process, designed to level out undulations naturally found in the caryopses of all rice varieties and this helps to remove the majority of the bran. However, the abrasive elements used cannot be made fine enough to remove all of the bran without damaging the grain, which is why polishing is required. This gentler process, which removes dust, flour and bran residues, uses a pressing and rubbing technique to create friction. As the grains rub against each other, their surfaces are smoothed, removing the remainder of the bran, allowing more light to be reflected, which in turn makes the rice appear whiter and glossier. The first generation of polishers were adapted from whitening machines. They featured a similar vertical cone design but had two basic differences. The first was that the cone was made of a simplified steel wire construction and covered with wood, on to “Nowadays, more and more rice mills are installing rice polishers that subject rice to multiple polishing passes. The degree of polishing has reached an all-time high” - Sujit Pande, Rice expert, Buhler Rice Polishing 150 years of innovation 58 | March 2016 - Milling and Grain F
  2. 2. which leather strips were nailed. The second was that the cone rotated at a speed that was 25 percent slower than the vertical cone design. The process began with the rice entering the space between the cone and the wire screen, it was then gripped by the leather strips that rolled the grains over each other and against the leather and wire screen. Then, with the application of a small amount of pressure, the remaining bran particles were removed and the rice became shinier or more transparent. Unfortunately, this method caused breakages, particularly in long grain rice, thus reducing its value. Furthermore, the leather strap needed to be replaced periodically, increasing maintenance costs. These issues drove further innovations, including the creation of the horizontal polisher, which consisted of three principal parts: a feed screw, a cam roll and screen. Rice is fed into the machine by gravity, while both the feed screw and cam rolls rotate. The feed screw pushes the rice into the working chamber where a retainer plate, with an adjustable counterweight, creates a controlled pressure on the rice kernels. The fractioning effect is achieved by rolling or rubbing the rice and through displacement. A screen basket covers the cam rolls, allowing a pre-determined distance between them. Air suction enhances the compactness and cools the rice while sucking away the residue. Performance matters Modern day polishing machines now offer a significant improvement in efficiency and wastage reduction. However, there are a number of factors that impact performance. Broken grains of rice can very much impair the polishing process because surface fractioning cannot be achieved if the grains are sandwiched between broken kernels. The polishing effects are improved if ‘brokens’ are sifted out beforehand. Polishing time is a deciding factor for both silkiness and breakage. A shorter polishing time results in less silkiness. A longer time improves silkiness but increases the risk of breakages. Screen basket and cam distance influences the degree of T: +1 270-631-1303 E: BUCKET ELEVATOR SUPPORT TOWERS / CATWALK SUPPORT TOWERS / GOAL POST TOWERS / SUPER STRUCTURES / CATWALKS / PLATFORMS A PITTSBURG TANK & TOWER GROUP CO. www.allstatetower.comFAMILY OWNED SINCE 1919 Ad_allState.indd 3 17/12/2015 11:06 Diagram of a horizontal polisher Milling and Grain - March 2016 | 59 F
  3. 3. polishing. The screen basket covering the cam rolls allows a pre-determined distance between them, with a greater distance (i.e. a wider chamber) reducing the degree of polishing. Rotational speed determines the degree of polishing. A higher rotational speed increases the number of individual impacts, which results in a higher degree of polishing. However, it also increases the amount of rice breakages. Air suction is used to compact and cool the rice. If the volume of air used is excessive, it can reduce the compactness and the effectiveness of the polishing process, thus reducing the shine on the rice. The importance of water Older machines that relied solely on friction to achieve a lustrous look and feel, generated a lot of heat, causing a large proportion of the rice to break. Manufacturers tried to counteract this by incorporating a hollow shaft, through which air was blown to cool the rice. Unfortunately this proved ineffective. One solution is the use of atomised water, to humidify the rice grains and thereby increase friction. In modern water mist polishers, temperatures remain lower, to prevent the rice surface from drying out. The addition of water also helps to create a slip layer between the bran fragments and the rice kernel, improving the removal of bran, resulting in a smoother appearance, longer shelf life and a higher yield of unbroken rice. However, if the rice still contains bran particles, the polishing effect can be reduced when a water jet polisher is used because the fat in the bran solidifies at low temperatures. Any bran particles that come into contact with water disintegrate, triggering enzymatic activities, which turn the rice yellow and release a rancid smell. It is therefore essential to the remove the bran before water polishing. Bühler: Planning today for the demands of tomorrow Integrating food safety into every aspect of rice production is vital for each player in the supply chain. Bühler, the global leader in rice processing solutions, always adheres to good machine and engineering practices to ensure food safety can be maintained easily, throughout the lifespan of its products, which include the range of rice UltraPoly™ and SuperPoly™ polishers. A polisher must have little or no residue left in the machine during and after operation, and be easy to clean at regular intervals. Ideally, there should be no crevices or sharp edges on the housing to avoid accumulation of dust. Bühler designs its polishers to enhance food safety - a key feature in Bühler’s UltraPoly™ is a unique replaceable tooth design on the cam, which provides a slot for cleaning inside the cam, thus improving hygiene standards. It is also important that equipment is designed to have the best balance between polish performance, energy consumption and yield. Bühler is committed to advancing its knowledge and developing the relevant technology to help resolve these issues. One such development is Bühler’s design to optimize the entire polishing chamber - including the cam roll, sieve geometry, water addition system and aspiration. Traditionally, aspiration was only understood as a means of removing bran during polishing. However, research reveals that the air flow within the polishing chamber has a significant influence on the finished result of the polishing. Using computational fluid dynamic analysis (CFD), Bühler has optimized the air flow, ensuring that even with a relatively short polishing chamber, the desired polish can be achieved. Bühler has successfully incorporated this innovation in its SuperPoly™ horizontal polisher, which enables processors to deliver a highly polished and shiny rice kernel without increasing wastage or energy consumption. Furthermore, Bühler has developed a high-capacity polisher that is able to match the polishing performance achieved by small capacity polishers; something the industry has required for a while. The challenge has always been the distance between the screen basket and cam –a wider chamber increases capacity, but diminishes polishing. To overcome this, many polishers contain two chambers within the same frame. However, these have the inherent disadvantage of higher power consumption per ton of rice processed. Bühler’s UltraPoly™ range of polishers takes a more innovative approach. Patented screen design based on years of research, gives an excellent and efficient polishing performance, even at high capacity. While these innovations are a step change in rice polishing, measuring the reflective quality of the rice, often referred to as its silkiness, is still a challenge for rice millers. There is no current measuring unit, or equipment, for determining the level of polish achieved, meaning the degree to which the grain reflects light is based purely on an expert’s sight evaluation. This subjective method can be a source of dispute, which in turn can impact the selling price. While rice polishing technology has clearly made great headway, there is still room for improvement. With its 150 years of expertise, Bühler will continue to lead the way in developing new technologies to counter this issue and be at the forefront of other future innovations in rice processing – from handling, storage and milling solutions, supporting and working in partnership with rice processors to overcome future challenges. Diagram of a vertical polisher Rice inlet Cone covered with wood Pulley Shaft Leather Screen Rice outlet 60 | March 2016 - Milling and Grain F
  4. 4. GROWING INTO THE FUTURE TAKING CARE ADDING VALUE SOLUTIONS FOR HANDLING AND STORAGE OF GRAIN AND SEED Cimbria develops and manufactures an entire range of equipment and solutions for seed processing. Thorough technical engineering experience and in-depth product knowledge enable us to supply solutions for cleaning, grading and treatment of various seed and grain products. Special focus is kept on effective sorting and cleaning, gentle handling, crop-purity, safe and dust-free operation and low running costs. CIMBRIA.COM CIMBRIA UNIGRAIN A/S Praestejorden 6 | DK-7700 Thisted Phone: +45 96 17 90 00 E-mail: CONVEYING | DRYING | SEED PROCESSING | ELECTRONIC SORTING | STORAGE | TURNKEY