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

Abstract

A true understanding of rice kernel fissuring and breakage, as a result of drying and tempering, must includeboth engineering and cereal science principles. Particular emphasis must be placed on the change of states of starchoccurring at the glass transition temperature (T g ). This transition from a glassy to rubbery state, or vice versa, has beenidentified to play an important role in rice fissuring and breakage. A hypothesis has been developed explaining rice kernelfissuring during drying and tempering. The objectives of this research were to determine the effect of the T g during ricedrying and tempering on milling quality. Additionally, the minimum tempering time required for various dryingconditions, to optimize milling quality, was determined. Rice was dried under three conditions, two with a drying airtemperature above T g and one below T g , for four durations and then tempered for 0 to 240 min. The experimentalprocedure was designed to directly test the T g hypothesis by cooling rice to a temperature below the T g after eachtempering 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 longas sufficient tempering is allowed. Required tempering durations were shorter for long-grain rice as compared tomedium-grain.