Rice quality is influenced by numerous factors and key to determining the price a grower will receive for the harvested crop. The quality of milled rice depends upon grain uniformity, shape, color, clarity, the amount of chalk as well as contamination with other seed or foreign material. One of the most important quality factors is the percentage of whole (unbroken) grains remaining after milling.

Rice milling yields are calculated on a percentage basis. The first step in the milling process is removal of the hulls, which typically comprise approximately 20% of the weight of paddy rice. If, for example, you started with 100 pounds of dried rough (paddy) rice, you now have 80 pounds of brown rice.

The next step in milling is removal of the bran layer, which typically is about 6%-10% of the weight of paddy rice (we will assume 8% for this example). The remaining milled grains weigh 72 pounds. Next is removing the broken grains (we will assume 9% of the total weight of paddy rice). Therefore, we have 63 pounds of whole milled grains remaining from the original 100 pounds of rough rice.

This sample of rice is then reported as a 63-72 miller. Again, this means from the original 100 pounds of rough rice, after milling, we have 72 pounds of milled rice of which 63 pounds are whole grains. In Southwest Louisiana these numbers are often converted to a barrel basis which is the typical unit of per acre yield. A rice barrel unit is 162 pounds. So to convert milling numbers to a barrel basis you would simply multiply by 1.62. Therefore, the 63-72 miller would convert to a 102-116 miller on a barrel basis. Also, whole grain is commonly referred to as head rice in this region.

Many factors influence the eventual whole grain yield of rice. Genetics is one, with some varieties having a higher average whole grain yield than others. Environmental factors also influence whole grain yields. One critical factor is drying of the harvested rice. Rough rice must be dried down to 12%-13% grain moisture shortly after harvest for long-term storage. However, rice is often harvested with grain moisture as high as 20%-21%. If rice is dried at too high a temperature, this can often cause damage that will lead to more grain breakage during the milling process. Also, if rice is harvested at high grain moisture and grain drying is not initiated for an extended period of time, heat begins to build within the rice and can lead to damage that will decrease milling yields as well as cause other quality problems. This was a problem for a number of farmers last year who had harvested rice just before Hurricane Rita’s journey through the area. The resulting loss of power had many growers scrambling to find generators to power drying units and avoid the total loss of this harvested grain.

The question often arises as to why rice is harvested at high moisture if it must then be artificially dried down to 12%. The reason is that typically the highest whole grain milling yields are realized with this system. As rice grains continue to lose moisture below 20% during the natural drying process in the field, there is a potential for grain fissures (stress fractures) to develop in the endosperm which comprises the largest part of the grain. The exact mechanism that causes fissures is not completely understood. However, it is known that fissures typically occur more often when rice kernels are exposed to moisture after they have dried below a certain moisture level. When rice grains dry, they will shrink. And when exposed to moisture, they will expand. Apparently, at higher moisture levels, the grains can shrink and then expand without damage (fissures). However, as the moisture level of the grain is reduced, the endosperm becomes less pliable, and the stresses associated with the cycles of shrinking and then expansion can lead to fissures occurring in the kernels. The importance of this is that kernels are very susceptible to breaks along these fissures during harvesting, handling, milling and transporting. These breaks will lead to a lower percentage of whole grains after the milling process, which can greatly reduce the value of the milled rice.

Rice varieties have differences in their genetic resistance to fissuring. Incorporating high levels of resistance to fissuring into future varieties is an important goal of our breeding program. A variety that consistently maintains high whole grain milling yields, even at low grain harvest moisture, is said to have good milling stability.

Cypress is an older (released in 1992) variety that set the standard for milling stability. Cypress can often be harvested at low grain moisture and still produce high whole grain milling yields. Another variety that has similar milling stability is the Clearfield variety CL161. The newer LSU AgCenter long-grain rice variety releases have fairly good milling stability (Cocodrie, Cheniere, CL131 and Trenasse) but not quite up to the standards of Cypress and CL161.

One major goal of our breeding program is to develop the technology to screen for genetic fissure resistance so that future varieties can possess this trait, which is of utmost importance to our industry. Consistently high whole grain milling yields will be of benefit to rice growers, millers, processors and ultimately to consumers.

Permission granted 03/22/06 by B. Leonards (LA Farm & Ranch) to republish article on the LSU AgCenter Website.