Rice milling quality can be influenced by any factor that affects kernel strength, which is ultimately responsible for the kernel withstanding the processes of hulling and bran removal without breaking apart.
Rice milling yield is quantified by two numbers (for example: 58/70). The number 70 in this example is themilled rice yield, which is determined by the amount of white rice remaining after the hulling and milling processes. It is calculated by dividing the mass of the milled rice by the original mass of the rough rice prior to processing, and typically ranges from 68 to 72 percent.
The number 58 in the example is thehead rice yield, and it is determined by the amount of milled rice kernels that are “whole.” Whole milled kernels remain three-fourths or more of their original length.
Head rice yield is calculated by dividing the mass of head rice by the original mass of rough rice. Head rice yield can vary from zero, meaning all kernels broken, to as high as the milled rice yield (as much as 68-72 percent with no kernels broken).
Milling quality is often expressed as a ratio of head rice yield to milled rice yield. For example, a 58/70 value would indicate a head rice yield of 58 percent, milled rice yield of 70 percent, and 12 percent broken kernels -- the difference between the two values.
Harvest moisture content may have the greatest effect on rice milling quality that is within a producer’s control. Head rice yield varies with the moisture content at which rice is harvested. Under typical Arkansas weather conditions the harvest moisture content at which head rice yield is maximum is approximately 19 to 21 percent for long-grain cultivars and 22 to 24 percent for medium-grains.
As rice matures, kernels on a panicle exist at very different moisture contents, representing various maturity and kernel strength levels. Some kernels are immature or “green,” with greater than 22 percent moisture. Others are “dry” kernels, with lower than 14 percent moisture.
Immature kernels are typically weak in structure and often break during milling. Rapid rewetting of dry kernels, through exposure to rain or high relative humidity air, causes a rapid expansion at the kernel surface resulting in the formation of fissures.
Harvesting at moisture contents greater than, or less than, optimal can cause a greater number of broken kernels, resulting in decreased head rice yield. To achieve the best head rice yield, it is recommended to harvest rice at the moisture contents mentioned above. However, increased harvest moisture content means increased drying costs, which could mean harvesting at a lower harvest moisture content to optimize overall economic return rather than simply maximizing head rice yield.
Nighttime air temperature can also have a major impact on rice milling quality. High nighttime air temperature during certain kernel reproductive stages will dramatically increase chalkiness and reduce head rice yields. The most dramatic impact of high nighttime temperatures on milling quality is a reduction in peak head rice yields, which is to say that the maximum possible head rice yield is reduced.
Disease and insects can also have detrimental effects on rice quality. Rice blast, sheath blight, and kernel smut can reduce milling quality. Kernel smut can also cause discoloration of kernels, resulting in problems during parboiling. The rice stink bug is a primary insect pest affecting rice quality, whereby the insect bores into the kernel during development, leaving a black spot on the kernel surface known as “peck.”
Any factor that causes a reduction in kernel strength and limits the ability of kernels to withstand the hulling and milling processes will impact milling yield. While nighttime air temperatures are generally out of our control, additional factors responsible for milling quality reductions can be effectively managed. Proper scouting and timely management of diseases and insects during the growing season can help to substantially reduce the impact of these pests on yield and quality.
Perhaps as important as nighttime air temperatures, but under grower control, is harvest moisture content. If weather permits, rice producers have the ability to harvest rice in a timely fashion. The moisture content at which rice is harvested can have a dramatic effect on milling quality, with head rice yield reductions occurring by harvesting at moisture contents greater or lower than optimal.
Jarrod T. Hardke, Rice Extension Agronomist, and Terry Siebenmorgen, director of the Rice Processing Program, are both with the University of Arkansas Division of Agriculture.