تاثير دما و مدت فرايندهاي خشك كردن و استراحت دهي بر ترك خوردگي شلتوك و مدل سازي پديده شناختي آن

چكيده انگليسي :

135Effect of temperature and duration of drying and tempering processes on rough rice fissuring and its phenomenological modeling Ali Ghasemi a ghasemi@ag iut ac ir January 2013 Department of Agricultural Machinery Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisors Morteza Sadeghi sadeghimor@cc iut ac ir Abstract Rice is the second most important agricultural crops Because about one third of the world population de pend on it as one staple food Due to the necessity of self sufficiency in rice production reducing losses duringmilling process of rough rice into white rice is of great importance Rice is commonly harvested at high mois ture contents MCs 25 to 35 w b To perform the milling process and prevent product deterioration duringstorage it is required to reduce the initial MC One of the factors affecting the quality of rice during milling isinternal fissured kernels created during and after the drying operation The fissures reduced the resistance of thekernel to failure significantly During drying the outer surfaces of the kernels obtain MCs less than those forinner parts This difference in MC between various parts of the kernel is caused internal MC gradients Mois ture gradients within the grain caused tensile stresses at the surface and compressive stresses at the center of akernel which in turn cause fissures to develop when the amount of stresses exceeds the strength of the ricekernel Under intermittent drying method in order to subside the moisture gradients within the kernel aftereach drying pass the rough rice is kept in a place at a certain temperature for certain durations so that the mois ture is not allowed to transfer into the medium closed jar This process is called tempering The mechanicaland thermal properties of a rough rice kernel undergo major changes provided the kernel goes through a glasstransition crossing the glass transition temperature Tg line due to its temperature and MC changes At a tem perature and MC below the Tg line starch exists as a glassy material As the kernel passes the Tg line the starchgoes from a glassy into a rubbery state and vice versa Since this state transition occurs in the temperatureranges typically happened during rice drying determination of Tg line for rough rice varieties and simulationof intermittent drying kinetics is vitally important in order to predict the kernel behavior and obtain the optimalconditions for drying and tempering operations In this study the state diagram Tg variations with MC wasobtained for Hashemi long grain rough rice variety The Tg values at five MCs were measured by differentialscanning calorimeter DSC method Moreover the effect of intermittent drying process on fissuring of Hash emi and Koohsar medium grain varieties was investigated The number of fissured kernels having one twoand more than two fissures was determined in 100 kernels immediately after the end as well as 48 h after dry ing process Then an index namely stress cracks index SCI was accordingly computed The optimal dura tions and temperatures for drying and tempering processes were determined using the experimental results andanalysis of MC distributions within a rice kernel by a simpli ed sphere drying model A hot air convectivedryer equipped with relative humidity and temperature control systems was used to conduct the experiments The experiments were performed under thin layer drying mode at relative humidity of 40 and air velocity of0 5 m s 1 with two replications In one case the experiments were carried out at drying and tempering tempera tures of 30 45 and 60 C drying durations of 20 40 and 60 min and constant tempering duration of 80 min Inorder to investigate the influence of drying and tempering durations on kernel fissuring the second case of thetests were also performed at constant drying and tempering temperature of 60 C drying durations of 20 40and 60 min and tempering durations of 0 continuous drying 40 80 120 160 200 240 min

قاسمي، علي

تاثير دما و مدت فرايندهاي خشك كردن و استراحت دهي بر ترك خوردگي شلتوك و مدل سازي پديده شناختي آن

خشك كردن متناوب , دماي گذار شيشه اي , شاخص ترك تنشي , گراديان هاي رطوبتي , مدل هندسه كروي