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.Below the glass transition temperature, the T2 of the fast-decaying Gaussian component was found to be independent of temperature; butafter passing through the rigid lattice limit temperature, TRLL, it was found toincrease with temperature, which clearly indicates the onset of mobility in themore rigid protein chain component.Interestingly, the decrease of TRLL with in-creasing water content was found to be the same for gluten, soluble glutenin,and gliadin.Since comparison with DSC indicates that TRLL corresponds to theonset of the glass transition, this shows that glutenin, gliadin, and gluten havesimilar glass transition temperatures.Further insight is obtained by following the proton transverse relaxation asdry gluten is hydrated with D2O.The proton transverse relaxation then becomesdouble exponential, with fast- and slow-decaying components.The ratio of thesecomponents changes with temperature until, at 90°C, most of the signal arisesfrom the slowly relaxing mobile component, and this change is retained oncooling.This is surprising, because the heat treatment also causes the gluten toharden, presumably by formation of interchain disulphide bonds.The data sug-gest that the molecular domains between the cross-links within glutenin retainthe more mobile, random-coil structure of the denatured material.The proton longitudinal relaxation of dry gluten also shows double expo-nential behavior, but, unlike the transverse relaxation, the components are notsensitive to gluten type.However, when the interchain gluten disulphide bondsare removed, single-exponential T1 relaxation is observed (28, 30).Data analysissuggests that the double exponential behavior results from restricted motion aboutCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. disulphide linkages in a region that has dimensions of the order of 10 nm andsupports the dynamic model by which more mobile domains punctuate more rigidregions created by interchain disulphide bonds.Water NMR relaxometry based on deuterium (D2O) and oxygen-17 (H217O)have also been applied to the gluten (31).The increase in deuterium signal ingluten with increasing water content correlated remarkably well with the decreasein relative stiffness, R(M), at a water content, M, in dynamic mechanical analysis(DMA).In fact,% detected signal 100 [1 R(M)] (2)This was interpreted as showing that the onset of motion (plasticization) in thegluten corresponded to an increase in the deuteron relaxation time and hence ofobservable signal intensity.It is particularly surprising that two methods of suchwidely differing time scales give such close correspondence.The interaction of the gluten proteins with the starch component is also apotentially important determinant of product quality.Li et al.(32) used rotatingframe relaxation measurements to probe this interaction, though the starch actu-ally came from maize, not wheat.The T1Á(H) values for starch and gluten in a1:1 mixture were separated by first spin-locking the proton magnetization forvariable times with a fixed spin-lock field strength and then transferring the resid-13ual magnetization with a fixed contact time to C spins so that the starch and13gluten C spectral resonances could be distinguished.Surprisingly the T1Á valuesfor the gluten and starch were found to be similar in the mixture and in theseparated components, indicating limited interaction, at least at 20% moisturecontent.The moisture dependence of the gluten T1Á values appeared to be greaterthan that of the starch, suggesting that the gluten has the higher water affinity,and this was supported by the observation that heating a drier mixture containingonly 2% moisture resulted in a large increase in the T1Á(H) values of the glutenbut only slight increases in those of starch.Dough quality can also be altered by the addition of reducing or oxidizingagents.Typical oxidants include benzoyl peroxide, azodicarbonamide, acetoneperoxide, chlorine dioxide, and potassium bromate, while typical reducing agentsinclude cysteine and sodium bisulphite.These presumably function by changingthe development of the dough protein network, though their exact mode of actionis unknown.Loaf volume, uniformity, texture, water-holding capacity, doughstrength, and mixing tolerance of the dough can all be affected by such additives(33).To study this important phenomenon, deuterium NMR has been used toinvestigate the effect of two oxidants, potassium bromate and ascorbic acid, onwater mobility in wheat gluten (34).The signal was simplified by making doughwith D2O, and comparison was made with the dough s thermomechanical proper-ties.It was found that at any given hydration level over the range 10 58% D2O,the signal intensity from the oxidant-treated samples was significantly lower thanCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved. that of the untreated control, and the glass rubber transition region extended tohigher temperatures in the treated samples.This suggests that oxidation leads toa more rigid gluten fraction, extending the glass transition to a higher temperaturerange.Deuterium NMR has been used to monitor the water-sorption capacity ofgluten as the dry material is heated in 10°C intervals from 5 to 90°C in a sealedtube containing excess D2O (35)
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