Milling rice paddy removes the husk and bran layer to produce white rice. Rice is best milled at 13-15% moisture content. Best results are attained when the process is completed in a number of stages. Grain temperatures should not exceed 45°C during the process. An efficient mill removes the husk (20%), the bran or meal (8-10%), and leaves 70% as white rice. Rice grown in irrigated systems should attain 60% white rice as head rice (unbroken, white kernels), and rainfed systems 40-50% as head rice. Rice is milled in several ways-
- Hand pounding using a mortar with a pestle results in very high numbers of broken rice and leaves brown rice (meal layer still attached). Cleaning of the husk is done by winnowing.
- A one-step milling process (steel rollers are available in Assam) where the husk and bran are removed in one pass and white rice is produced directly from the paddy. The single-pass rice mill is an adaptation of the Engleberg coffee huller. This process results in many broken kernels, low white rice recovery (50-55%), and head rice yields less than 30%. The fine broken grains are often mixed in with the bran and the ground rice husk. This happens due to steel roller. Now-a-days single pass rubber roller machines are available which can be a better option.
- A two-step milling process wherein the husk and bran are removed separately. These mills are often called compact rice mills, and in many countries, have superseded the Engleberg mill. The two-stage mill has separate hulling and polishing processes. Rubber rollers remove the husk and the brown rice is polished with a steel friction whitener. These mills have a capacity of 0.5-1 t/hour paddy input and are often used for custom milling in rural areas. The milling performance of the compact rice mill is superior to the single-pass huller with milling recoveries normally above 60%.
- A multi-stage milling process wherein rice passes through a number of different operations. The milling process in larger commercial mills combines a number of operations and produces higher quality and higher yields of white rice from paddy.The process involves:
- Pre-cleaning the paddy prior to milling
- Removing the husk or outer layer from the paddy
- Polishing the brown rice to remove the bran layer
- Separating the broken grains from the whole kernels
- Bagging the milled rice, and
- Managing the by-products
Market intelligence
The value of milled rice in the market is determined by a number of physical and chemical characteristics, and the consumers, which varies within a country, and between the countries.
Physical characteristics
- Milling degree or colour: The degree of milling or amount of the brown rice removed affects the colour of white rice and often the price. Under-milled rice absorbs water poorly, does not cook well, and is normally cheaper.
- Head rice or broken percentage: Milled rice, having 75-80 % head rice (whole kernels), also includes broken kernels. High head rice yield is one of the most important criteria for measuring milled rice quality. High-quality rice normally has less than 5% broken.
- Whiteness or translucency: This characteristic is a combination of varietal physical characteristics and the degree of milling. During milling, the whitening and polishing process greatly affects the whiteness of the grain and its transparency.
- Chalkiness: Grain appearance is affected by the amount of chalkiness or white belly. Chalkiness is caused by interruption of the final grain filling. Though chalkiness disappears upon cooking (and has no direct effect on cooking and eating qualities), excessive chalkiness often downgrades the quality and reduces milling recovery.
Chemical characteristics
- Gelatinization temperature or cooking time: Environmental conditions such as temperature during ripening influence gelatinization temperature. There is normally a preference for rice with intermediate gelatinization temperature.
- Amylose content or stickiness: The amylose content of rice usually ranges from 15-35%. High-amylose rice has high volume expansion, the grains cook dry, are less tender, and become hard upon cooling. Low-amylose rice cooks moist and sticky. Intermediate-amylose rice (21-24%) is preferred in most rice-growing areas of the world.
- Gel consistency: Gel consistency measures the tendency of the cooked rice to harden on cooling. Varieties with a softer gel consistency are preferred if rice is to be consumed after cooling, or if cooked rice with higher degree of tenderness is desired.
