Examine the impact of glass transition temperature on product quality.

Commercial dryers use multi-pass techniques to remove moisture from freshly harvested rice. Between drying passes, the rice remains in bins for a certain period of time to allow moisture content (MC) gradients within kernels, created during the drying process, to subside.

This tempering decreases MC gradients by allowing moisture to migrate from the core to the outer layers of the kernel. The MC gradients cause differential stress inside the kernel, which, if sufficiently large, causes the kernel to fissure. Moisture migration during tempering also improves the energy utilization in subsequent drying passes.

Fissured kernels may break during the milling process and could reduce head rice yield. This yield is the current standard in the rice industry used to measure rice-milling quality. It is defined as the weight percentage of rough rice that remains as head rice--kernels that are at least three-quarters of the original kernel length--after complete milling.

A true understanding of rice kernel fissuring and breakage caused by drying and tempering must involve both engineering and cereal science. Particular emphasis must be placed on the change of states of starch that occurs at the glass transition temperature. This transition from a glassy to a rubbery state, or vice versa, plays an important role in fissuring and breakage. Glass transition behavior is a critical parameter in the transformation process and can be used to predict textural properties and physicochemical stability of products during storage.

Researchers at the University of Arkansas (Department of Food Science, 2650 N. Young Ave., Fayetteville, AR 72704) have attempted to determine the effect of the glass transition temperature on milling quality during rice drying and tempering. They have determined the minimum tempering time required for various drying conditions to optimize milling quality. After drying rice under different conditions, investigators found that a certain percentage of harvest moisture content can be removed per drying pass without damaging the rice kernel, as long as they allow for sufficient tempering. The required tempering durations were shorter for long-grain rice than for medium-grain rice.

Investigators found that drying air temperatures as great as 60 C can be used without reducing the head rice yield, as long as sufficient tempering at a temperature above the glass transition temperature of the rice is allowed.

Further information. Terry Siebenmorgen, Department of Food Science, Cornell University, 116 Stocking Hall Ithaca, NY 14853-7201; phone: [phone]; fax: [phone]; email: [e-mail].