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THE GLASS TRANSITION TEMPERATURE CONCEPT IN RICE DRYING AND TEMPERING: EFFECT ON MILLING QUALITY

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Transactions of the ASAE.  VOL. 43(6): 1661-1667 . (doi: 10.13031/2013.3066) @2000
Authors:   A. G. Cnossen, T. J. Siebenmorgen
Keywords:   Rice, Glass transition temperature, Drying, Tempering, Head rice yield

A true understanding of rice kernel fissuring and breakage, as a result of drying and tempering, must include both engineering and cereal science principles. Particular emphasis must be placed on the change of states of starch occurring at the glass transition temperature (T g ). This transition from a glassy to rubbery state, or vice versa, has been identified to play an important role in rice fissuring and breakage. A hypothesis has been developed explaining rice kernel fissuring during drying and tempering. The objectives of this research were to determine the effect of the T g during rice drying and tempering on milling quality. Additionally, the minimum tempering time required for various drying conditions, to optimize milling quality, was determined. Rice was dried under three conditions, two with a drying air temperature above T g and one below T g , for four durations and then tempered for 0 to 240 min. The experimental procedure was designed to directly test the T g hypothesis by cooling rice to a temperature below the T g after each tempering duration. Results for both medium-grain rice and long-grain rice at 19.6 to 23.7% harvest moisture content (MC) * show that 5 to 6 percentage points MC can be removed per drying pass without damaging the rice kernel, as long as sufficient tempering is allowed. Required tempering durations were shorter for long-grain rice as compared to medium-grain.

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