|Fig. 1: A rice paddy. (Source: A. J. Oswald under terms of the Creative Commons 2.0 License.)|
There are many industries that are energy consuming and rice milling is one that is on the higher end of energy consumption of these industries. Rice is one of the most important crops in the world, and millions of people rely on rice to survive. To mill rice there are a lot of utilities used such as water, steam, electricity, and labor. Of these utilities electricity is the highest and is used in many of the different processes of milling rice such as motors, pumps, blowers, conveyors, etc. There are also different ways of milling such as modern mills, conventional mills, and hand pounding. The breakdown of overall rice production of these methods is 50%, 40%, and 10% respectively.  How much more efficient though is modern milling compared to conventional, is the gap so large that all milling should be brought into the modern style, and what are ways that we can reduce energy consumption?
The modern mill is comprised of a paddy that uses rubber roll shellers that efficiently carries out the harvesting process. Machines in a modern mill carry out most of the harvesting process. The process encompasses cleaning, drying, dehusking, aspirating the husk, separating paddy and brown rice, and polishing the rice.  The modern mill also uses the method of parboiling. This method, which drastically reduces soaking time and improves rice quality, is done by soaking, steaming, and drying. The modern milling method is estimated to have an energy requirement of 9.54 × 105 J/kg of paddy.  The broken percentage of husks is between 10-20%.
The conventional mill is comprised of hullers, which remove the husk, bran, and part of endosperm in a single operation. In huller mills a metallic roller is used as a milling unit cleaned by a fixed or vibratory screen and/or fan.  The conventional mill also does not use the same method of parboiling as does the modern mill. The energy requirement is estimated to be 16.4 × 105 J/kg of paddy for conventional mill and 3.75 × 105 J/kg of paddy for pressure parboiling.  The broken percentage of husks is between 20- 30%.
Is this difference between methods so large that there should be a change? In the all India area (where massive amounts of rice are produced), production, and yield of rice in the year 2009-2010 was 45.32 million hectares, 91.12 million tonnes, and 2,089 kg/ha, respectively.  The difference between the two methods is 10.61 × 105 J/kg, and with the amount of rice being produced this is way too large of a number. This is not to mention the percentage of broken husks as well as the point that the difference is greater than the energy requirement alone of the modern method. For these reasons I believe that all milling methods should be turned into modern milling.
There are many areas where rice milling could be made more efficient or less energy demanding, and these areas tend to be in the conversion of fuel and methods aspects of milling. If mills saved the husk instead of using it as a fuel it would lead to co-generation of thermal energy in the machinery such as furnaces and boilers. This would lead to a reduction in the use of fuels and electricity. Paddy cleaners, blowers, de-huskers, and polishers operate throughout the year and may be made more efficient in different seasons based on different conditions. The hot water after soaking may be wasted as a drain that represents enthalpy loss. All of these aspects along with analyzing the efficiency of every process in the milling cycle could be improved upon to make rice milling less energy consuming.
Rice milling is an extremely important industry for the global economy of food stock, and there are many ways that it can be improved. Along with changing the different ways of milling to the most efficient way we have so far, we also can make the milling process more efficient by using the process to improve upon itself along with figuring out which parts can be directly improved upon.
© Colin McCall. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 S. K. Goyal, S. V. Jogdand, and A. K. Agrawal, "Energy Use Pattern in Rice Milling Industries - A Critical Appraisal," J. Food Sci. Technol. 51, 2907 (2012).