Production factors affecting rice milling quality

Fissures form

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.