现代食品化学研究进展-1-水

Chapter1waterWATER22March-WorldDayforWater2006WaterandCulture.CopingwithWaterScarcity—2007Wateralwaysfindsawayout.WaterisgoldWateristhesourceofalllife.Waterproductsanditsprocessing.purifiedwater,mineralwater…HealthwaterMYNAMEISWATERwhoami?mynameiswaterwithoutme,cropsdieanimalsdie,peopledieindustriesdiethat’smewaterbutwhatsurprisemeisdoyoupeopleknow?whoami?idoubt,youletyourindustrialwastepollutemewhenicomeinabundancetowateryourcropsyoucomplainbutdoyouknowwhoami?mynameiswaterihavecometosavelivesnottodestroythatismewaterWaterProverbsTokeepwaterinone'smouthTokeepasecret.ItislikedrinkingaglassofwaterItisveryeasytodo.TomakeaholeintothewaterMakingvainattempts.You'reinhotwaterYou'reintrouble.AfishoutofwaterNotfeelingathomewhereyouare.StillwatersrundeepThequietpeoplecanbethesmartestandwisest.一、MoisturetransferMoisturetransferinposition水分的位转移Moisturetransferinphase水分的相转移（一）水分的位转移ReasonTemperatureandwateractivityaredifferentinfood,sowaterchemicalpotential(化学势)isdifferent.水分要沿着化学势降低的方向移动，从而发生位转移，直到各个部分的化学势力完全相等为止。Influencefactorstemperatureandwateractivity.（二）水分的相转移EvaporationCondensation空气的湿度有关几个概念绝对湿度-Absolutehumidity相对湿度-Relativehumidity饱和湿度-Saturatedhumidity饱和湿度差-Saturationhumiditydifference空气的饱和湿度与同一温度下空气的绝对湿度之差。决定食品水分蒸发量的一个重要因素。主要受空气温度、绝对湿度和流速的影响。二、分子移动与食品稳定性Molecularmobilityandfoodstability（一）Understanding1玻璃态（glassstate）：是聚合物的一种状态，它既象固体一样有一定的形状，又象液体一样分子间排列只是近似有序，是非晶态或无定形态。处于此状态的聚合物只允许小尺寸的运动，其形变很小，类似于玻璃，因此称～。2橡胶态（rubberystate）：大分子转变成柔软而具有弹性的固体时的状态（未融化），分子具有相当的变形，是一种无定形态。（一）Understanding3玻璃化转变温度（glasstransitiontemperature,Tg）：非晶态食品从玻璃态到橡胶态的转变称玻璃化转变，此时的温度称～。Alltheamorphous(无定形)materialschangefromtheglassytorubberystateataglasstransitiontemperature(Tg)whichisspecificforeachmaterials.Theglasstransitionofaliquidtoasolid-likestatemayoccurwitheithercoolingorcompression.Amorphous：是物质的一种非平衡，非结晶的状态（一）UnderstandingThetransitioncomprisesasmoothincreaseintheviscosityofamaterialwithoutanypronouncedchangeinmaterialstructure.Theconsequenceofthisdramaticincreaseisaglassexhibitingsolid-likemechanicalpropertiesonthetimescaleofpracticalobservation.Thistransitionisincontrasttothefreezingorcrystallizationtransition,whichisafirst-orderphasetransitionintheEhrenfestclassificationandinvolvesdiscontinuitiesinthermodynamicanddynamicpropertiessuchasvolume,energy,andviscosity.Inmanymaterialsthatnormallyundergoafreezingtransition,rapidcoolingwillavoidthisphasetransitionandinsteadresultinaglasstransitionatsomelowertemperature.Othermaterials,suchasmanypolymers,lackawelldefinedcrystallinestateandeasilyformglasses,evenuponveryslowcoolingorcompression.DeterminationTheglasstransitiontemperature(Tg)istakenasthemidpointintheendothermaltroughasmeasuredfromtheextensionsofthepre-,andpost-transitionbaselinesonreverseheatflowversustemperaturegraph.Atthispoint,thespecificheatcapacitychangeassumeshalfthevalueofthischangeupongoingthroughthetransition.DifferentialScanningCalorimetry(DSC)Refertothefigureontherightplottingtheheatcapacityasafunctionoftemperature.Inthiscontext,TgisthetemperaturecorrespondingtopointAonthecurve.ThelinearsectionsbelowandaboveTgarecoloredgreen.TgisthetemperatureattheintersectionoftheredregressionlinesAresearcherswhousedDSCtomeasuretheglasstransitiontemperatureofcarbohydratematerialfoundthattheglasstransitiontemperatureappearedtobeloweratslowerscanningratesandhigherathigherscanningrates(Noeletal.,1991;ChuyandLabuza,1994).ChuyandLabuza(1994)explainedthatthelowerobservedtemperaturevaluesattheslowestheatingratemaybeduetothelongerresidencetimeofthesampleatanytemperatureduringheating,whichincreasestheprobabilityforviscousflow,crystallisationoftheamorphouscarbohydrateandcollapse.DSCmeasuresadiscontinuityinthespecificheat,itsapplicationisnotsuitableforsomematerialswheretheglasstransitionisnotaccompaniedbyasubstantialandsharpspecificheatchange,suchassugar-freeproteinandhigh-molecular-weightcarbohydratesystems.TMCTisbasedontheprinciplethatanamorphoussampleundercompressionandtemperaturerampingsuddenlytransformsfromaglassystatetorubberystatewhichcanbemanifestedbyasuddendisplacementofcompressionprobe.温度由低到高，无定形聚合物经历为：玻璃态---橡胶态---粘流态glassstate---rubberystate---viscousstateTTg，分子运动能量低，大段分子不能运动，呈玻璃态；T=Tg，分子热运动增加，玻璃态逐渐转变为橡胶态，此时大分子聚合物处于玻璃态转变区域；TgTTm（熔融温度），分子的热运动能力足以使链段自由运动，但链间存在较强局部性相互作用，整合分子受到抑制，处于橡胶态平台区；T=Tm，分子热运动能量是大分子聚合物整链开始滑动，橡胶态向粘流态转变，处于橡胶态流动区；TTm，大分子聚合物链自由运动，处于粘流区。Statediagrams状态图Thefoodstartsasatruesolution(A),whichiscooleduntilitreachesthemeltingpointline(B).Atthistemperature(alittlelessthan0oC)crystalsofpureiceformleavingthefoodmaterialmoreconcentrated.FurthercoolingconcentratesthefoodmoreandmorewithlittlechangeintemperatureuntilitreachesTEtheeutectic(共晶)point.Thermodynamicallythefoodsolutewouldliketostartcrystallizingatthispointbutcannot.Insteadthefoodcontinuesalongthelineintothesupersaturatedregion(arubberforms)andmobilityisrapidlyreduced.Attheglasstransitiontemperature,thesolutefinallylocks-upandformsaverylowmobilityglassandmostdiffusionlimitedfoodchemicalreactionseffectivelystop.Coolingbeyondthis