79 Incorporating the glass transition temperature in rice drying and tempering to optimize milling quality.

A. G. CNOSSEN, T. J. Siebenmorgen, and J. D. Reid. Dept. of Food Science, University of Arkansas, 272 Young Ave., Fayetteville, AR 72704.

A complete and fundamental understanding of rice kernel quality reduction, indicated by head rice yield (HRY) reduction, during drying and tempering must include both engineering and cereal science principles. Understanding of the response of kernels to various drying and tempering environments must include considerations of material properties at the temperature and moisture content (MC) of various sections of the kernels. Especially the change of state of starch, as it goes through a glass transition temperature (T(g)), is hypothesized to be of importance. This state transition can create HRY reduction if there is a sufficient MC gradient inside the kernel, created during drying. The objective of this research is to investigate the effects of glass transition on HRY reduction during drying and tempering. To test our hypothesis, drying and tempering tests were conducted during the 1998 harvest season on rice from two locations in Arkansas, harvested over a range of MCs (22 to 18%). The rice was dried with three different air conditions, two with drying air above T(g) of the rice, and one below T(g). Samples were dried under each condition for different durations to create different magnitudes of moisture gradient. After drying, the samples were tempered for varying time periods ranging from 0 to 4 hours at the same temperature as the dryer. Subsequently, samples were gently dried to 12.5% MC and milled for HRY determination. Results indicate that tempering duration after a drying pass has a significant influence on HRY and high tempering temperatures will help maintain high HRYs.