Researches On Moisture State And Moisture Distribution In Carrot During Drying Based On Low-field NMR

In recent years,all kinds of combinated drying technology has been widely appliedwith its short drying time,low energy consumption and good drying quality.There aremainly three fractions of water (combined water,bound water,free water)in fruits andvegetables, free water and bound water are removed during drying process,whilecombined water is not taken away under normal circumstances.Combinated dryingtechnology has made full use of the advantages of various drying technology,bydifferent drying technology in different stages of the drying progress,to dewater most offree water and combined water by evaporation or sublimation,respectively.The study ofmoisture states and moisture distribution during drying is for the determination ofdrying technology,the and conversion point,the establishment of mathematical model toprovide theoretical bases.At present,the domestic and overseas scholars have gained themap of moisture distrbution of materials by nondestructive testing technology,such ashydrogen nucleus magnetic resonance (HNMR) technology,computer tomography (CT)and technology of gamma rays. However, HNMR technology which have features offast measurement,high accuracy and more information used in fruits and vegetablesdrying to the study has obvious advantages.The thesis was to reasearch the changes of internal water in carrot by using NMRand MRI technology during drying at different temperatures,Drying of carrot slices wasperformed at constant temperatures of40℃,50℃,60℃,70℃and80℃using air oven.The first part of the article was to study the correlation coefficient of nuclearmagnetic resonance (NMR) signal amplitude and moisture content of drying base(db),and fit the calibration curve.With the regression analysis by the software SPSS13.0, thecorrelation coefficient was0.999(R2=0.9991),and α<0.01,moisture content of dryingbase was between5.10%~90.71%,so there was a significantly linear relationshipbetween the NMR signal amplitude and db.The equation of calibration curve wasy=3.1784x+107.97,which x was db,and y was NMR signal amplitude.The moisturecontent of unknown carrot samples and each states during drying could be caculated bythe calibration curve.The second part of the article was to reasearch the effects of drying at differenttemperatures on the moisture state and moisture distribution in carrot slices.Five carrot slices were prepared before the experments,the diameter and thickness of them were25mm and10mm,respectively,and were dried at different temperatures till the moisturecontent of carrot slices reduced to below10%.Three peaks were showed in the T2inversion spectrum,that was to say, there was three-component water proton system incarrot slices,which were correlated with three fractions of water(combined water,boundwater and free water),Moisture states and moisture distribution were changed duringdrying and the moisture which had high degree of freedom into the moisture that hadlow degree of freedom; The drying rate was accelerated with the rise of tempera-ture,however,drying rate was restrained because the material surface crusting hinderedthe outmigration of water at80℃.The third part of the article was to study the sample magnetic resonance imaging ofdifferent stages during drying at temperatures of40℃and70℃.Moisture distributionwas inhomogeneous in sample which can be observed by the images.Through the colourmap,we could clearly see the brightness of image become weak with the extension ofdrying time.The water of the outer layer was evaporated firstly due to temperaturegradient and moisture gradient. The migration of water was obstucted by the seriouslyshrinkage and the crusting of outer layer of the sample at the later stage of drying, inaddtion,at the later stage,the combination of water and dry matter was more closely,thedrying rate was in reduction rate stage.However,the organization of the sample wasdestoryed because of the high temperature at the condition of70℃.From the images wecould observe that the brightness of image reduce faster at the condition of70℃thanthat at the temperature of40℃.The fourth part of the article was to study the adaptability of inversion softwarealgorithm under different signal-to-noise ratio,when SNR>40,the inversion results weresteady by simultaneous iterative reconstruction algorithm (SIRT),the SNR of the signalin the experments were greater than40,so the inversion results were credible;Whilewhen SNR<40,false peaks or sharp peaks appeared in the inversion spectrum.Thesignal-to-noise ratio of short relaxation components was lower than that of the longrelaxation components,so the inversion result of short relaxation components wasserious interference.Low-field nuclear megnetic resonance techniques revealed the change of moisturein carrot during drying,and it can provide an effective method to study the parameter offruits and vegetables drying.