The glass transition temperature concept in rice drying and tempering: Effect on milling quality

A true understanding of rice kernel fissuring and breakage, as a result of drying and tempering, must include both engineering and cereal science prin ciples. Particular emphasis must be placed on the change of states of starc h 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 i mportant role in rice fissuring and breakage. A hypothesis has been develop ed explaining rice kernel fissuring during drying and tempering. The object ives of this research were to determine the effect of the T-g during rice d rying and tempering on milling quality. Additionally the minimum tempering time required far various drying conditions, to optimize milling quality wa s determined Rice was dried under three conditions, two with a drying air t emperature 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 ea ch tempering duration. Results for both medium-grain rice and long-grain ri ce at 19.6 to 23.7% harvest moisture content (MC)* show that 5 to 6 percent age points MC can be removed per drying pass without damaging the rice kern el, as long as sufficient tempering is allowed. Required tempering duration s were shorter for long-grain rice as compared to medium-grain.