Rice milling developments for a changing world market
April 01, 1994
by Teresa Acklin
Systems are designed to enhance quality, competitiveness.
World rice trade generally has been characterized by steady annual growth and modest volumes. In the past three decades, trade has grown by 2% annually, but on average, only 3% of each year's total world production has been traded internationally.
Amid this pattern of relative stability, the market is changing. Competition has increased steadily, and this trend is likely to intensify as countries gradually remove their trade barriers and new markets emerge.
Rice consumers and buyers increasingly are demanding a high quality product at an attractive price. As a result, recent developments in rice milling focus on enabling millers to improve milled rice quality and reduce their milling costs.
Rice mill automation
Automation helps millers improve rice quality, attain consistently high yields and reduce operating costs.
Automation in rice milling is relatively new, as development of an automated system first required overcoming a few technical problems unique to the rice milling process. Through research, these technical problems have been resolved, making automation viable for rice millers.
The extent of automation can range from controlling individual machines to automating an entire mill facility. At the simplest level, automation regulates individual machines or machine groups.
An example is an electronic control unit for the rice whitening machine. Tlte control unit continuously measures whiteness and compares it to the pre-set value; as necessary, the control unit readjusts the pressure level to obtain the desired degree of whitening. At the same time, the unit computes the machine load to maintain the correct product flow.
Automation for an entire facility generally is accomplished today through the use of the programmable logic controller. Operation and control of the plant is monitored through either a luminous switchboard or a monitor screen; the switchboard covers a large wall and offers a view of the total process, whereas the screen displays sections of the mill diagram.
The monitor screen offers much more flexibility than the switchboard. Plant modifications can be programmed easily into the display, and information on measured values, process parameters, flow paths and operations modes (operating, stand-by, failure, offs can be displayed right on the screen.
A monitor and keyboard system also requires less space than a switchboard. And at a moderate additional cost, monitors may be installed throughout the plant as terminals for the most important work sections. Operators can manage the plant or a specific section from any one of the terminals.
Automation also protects plant and equipment by triggering either an alarm or a shut-down, depending on pre-set conditions, when a problem arises. Automated weighing systems can detect and signal worn-out screens, choking and product flows that do not meet prescribed tolerances.
In addition to process control, automated systems also can measure and record data and generate on-screen or printed reports on information such as quantities and actual yields. The automated mill system also can be integrated with management information systems used for inventories, sales and deliveries and accounting.
The vertical whitening process uses a configuration that can improve milling performance.
The milled rice preferred by many of the world's consumers consists of a smooth, white, whole grain, which requires removing the outer layer of brown rice bran. To achieve the desired product, modern whitening equipment uses a sequential combination of two forces: a high-speed abrasive action, which removes most of the bran, followed by friction achieved through pressure to produce the smooth, white finish.
This type of rice whitening equipment often is configured horizontally, which refers to the position of the main shaft and to the flow of rice. This configuration may create uneven density and pressure within the milling chamber because gravity will pull the rice to the chamber bottom.
The variability in density and pressure can reduce operating efficiency because the chamber's upper portion is underutilized. Uneven pressure also may contribute to broken grain.
In the vertical configuration, the shaft and milling chamber are oriented vertically, as is the rice flow. In systems with a bottom to top flow, a spiral shaft forces the rice grains to flow upward against the weight of the rice above.
The interaction of pressure and weight through all rice layers requires lower pressure and can maximize use of milling surfaces. The lower, more-even pressure also can reduce the generation of broken rice and wear on pits.
The vertical whitener can increase flow rates, which can save energy, and the upright orientation can save space. The vertical whitener also may reduce the amount of conveyors or elevating equipment required because the units may use a bottom-up flow.
The vertical milling system also can support use of a water polishing machine. This system removes loose bran and dust from the kernel grooves created by the abrasive milling stage. The result is a clean, shiny kernel with visual appeal to consumers.
Water polishing also softens the kernel, which makes the bran easier to remove. This means lower pressures can be used, again reducing the possibility of broken grain.
Polished rice also has a longer storage life because bran traces are removed; residual bran can become rancid, imparting an objectionable odor and taste to milled rice. Clean, bran- ordust-free kernels also discourage the growth of molds or fungi in storage.
Information on automation for this article was contributed by Buhler GmbH, Braunschweig, Gerrnany, a subsidiary of Buhler Ltd., Uzwil, Switzerland; information on vertical whitening was contributed by Satake (U.S.A.), Inc., Houston, Texas, U.S., a subsidiary of Satake Corp., Tokyo.