Effect of solutes and matrix structure on water mobility in glycerol-agar-water gel systems: a nuclear magnetic resonance approach

Nuclear magnetic resonance spectroscopy (NMR) has been widely used to determine water molecular mobility in food systems. This study aimed to examine the effects of matrix structure and solutes on the dynamics of water molecules in model mixed systems, glycerol-agar-water gels, using low- and high-resolution NMR. Simple models to explain water relaxation rates and self-diffusion coefficients in mixed systems were developed using the experimental values obtained for the individual binary systems (glycerol-water solutions and agar-water gels). The spin-lattice relaxation of mixed systems was influenced by interactions of both glycerol and agar with water, while the spin-spin relaxation of mixed systems was dominated by the interaction of agar with water. Water diffusion was influenced by not only molecular interactions between all components but also the gel matrix structure. These models are able to differentiate the effect of solutes from that of matrix structure on water molecular dynamics.     

基于低场核磁和差示量热扫描的面条面团水分状态研究

为了解低水分面条面团中水分的存在状态,明确真空度及和面时间对水分状态的影响,该研究以3个小麦品种（济麦20、宁春4号、济麦22）磨制的面粉为材料,采用真空和面制作低水分面条面团（含水率35%）,采用低场核磁共振技术（LF-NMR,low-field nuclear magnetic resonance）和差示量热扫描（DSC,differential scanning calorimetry）2种技术,测定不同真空度（0、0.06、0.09 MPa）和搅拌时间（4、8、12 min）下面团中水分的形态和分布,并进一步分析2种技术测定水分形态结果的相关性。结果表明,在低水分面条面团中,水分主要以弱结合水形态存在。不同品种的小麦粉面团的水分形态及分布存在差异,强筋小麦粉（济麦20）制作面团的水分自由度较低。真空和面（0.06 MPa）可以促进水分与面筋蛋白的相互作用,降低面团中水分子流动性,促进水分结构化;而非真空或过高真空度均会导致面团中水分自由度增加。济麦20、济麦22小麦粉和面时间为8 min时,面团水分流动性较低;而宁春4号小麦粉面团在4 min时,水分自由度较低;继续搅拌,深层结合水减少、弱结合水增多。LF-NMR和DSC测得面团水分状态的结果具有一致性。LF-NMR测得的弱结合水峰面积百分比与DSC测得的可冻结水百分比具有相同的变化趋势（r=0.695）,且深层结合水峰面积百分比与非冻结水百分比具有相同的变化趋势（r=0.564）。研究结果为认识制面过程中水分的作用,优化和面工艺和调整产品特性提供参考。     

Salt diffusion and distribution in meat studied by 23Na nuclear magnetic resonance imaging and relaxometry

This study introduces the use of combined 23Na magnetic resonance imaging (MRI) and 23Na NMR relaxometry for the study of meat curing. The diffusion of sodium ions into the meat was measured using 23Na MRI on a 1 kg meat sample brined in 10% w/w NaCl for 3-100 h. Calculations revealed a diffusion coefficient of 1 x 10(-5) cm2/s after 3 h of curing and subsequently decreasing to 8 x 10(-6) cm2/s at longer curing times, suggesting that changes occur in the microscopic structure of the meat during curing. The microscopic mobility and distribution of sodium was measured using 23Na relaxometry. Two sodium populations were observed, and with increasing length of curing time the relaxation times of these changed, reflecting a salt-induced swelling and increase in myofibrillar pore sizes. Accordingly, the present study demonstrated that pore size and thereby salt-induced swelling in meat can be assessed using 23Na relaxometry.     

Cooking effects on water distribution in potatoes using nuclear magnetic resonance relaxation

Continuous low-field (LF) (1)H NMR relaxometry was used to monitor the structural changes during cooking of potatoes with two different dry matter (DM) contents. A principal component analysis of the relaxation decay curves revealed major events related to water mobility during cooking, which occur at 53 and 60 degrees C for potatoes with medium and low DM contents, respectively. Exponential analysis of the relaxation decays reveals two major water populations in the potato: a slow-relaxing (assigned to water in cytoplasm and extracellular cavities) water component, T(22) ( approximately 350-550 ms), and a fast-relaxing component (primarily assigned to water associated with starch and cell walls), T(21) ( approximately 45-65 ms). Significant DM dependent shifts in both the T(21) and T(22) relaxation time constants were observed during cooking, indicating that starch gelatinizes between 53 and 70 degrees C with water exchanging with the hydroxyls of starch (transition in T(21)) and cells start to disrupt with an increase in diffusion volumes at approximately 60 degrees C (transition in T(22)). The study reveals that continuous LF NMR measurement is an excellent and highly sensitive method to study changes in water mobility and water populations during the cooking of potatoes.     

Water absorption of freeze-dried meat at different water activities: a multianalytical approach using sorption isotherm, differential scanning calorimetry, and nuclear magnetic resonance

Hydration of freeze-dried chicken breast meat was followed in the water activity range of aw=0.12-0.99 by a multianalytical approach comprising of sorption isotherm, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). The amount of frozen water and the shape of the T2-relaxogram were evaluated at each water content by DSC and NMR, respectively. Data revealed an agreement between sorption isotherm and DSC experiments about the onset of bulk water (aw=0.83-0.86), and NMR detected mobile water starting at aw=0.75. The origin of the short-transverse relaxation time part of the meat NMR signal was also reinvestigated through deuteration experiments and proposed to arise from protons belonging to plasticized matrix structures. It is proved both by D2O experiments and by gravimetry that the extra protons not contributing to the water content in the NMR experiments are about 6.4% of the total proton NMR CPMG signal of meat.     

低场核磁共振分析聚乙二醇对萌发期水稻种子水分吸收的影响

为研究聚乙二醇(PEG,polyethylene glycol)处理下水稻种子萌发过程中内部水分分布和变化规律,进而揭示水稻耐旱性在水分吸收规律上的重要特征。应用低场核磁共振的T2弛豫谱和质子密度加权成像分析了PEG处理下水稻种子萌发过程中的水分变化,研究了利用蒸馏水(对照)和质量分数分别为10%、20%PEG6000处理对两个水稻品种旱9710、辽星1发芽指标的影响,以及对两个水稻品种萌发0、6、22、48、72 h吸水量的影响,确定单位质量核磁信号幅值与水稻种子湿基含水率的回归函数关系。发芽指标检测结果显示:旱9710耐旱性高于辽星1。质子密度加权成像结果显示:在水分吸收初期,水分子直接通过种子表面裂缝进入种子体内,胚乳中的淀粉粒等物质开始吸水膨胀,种子体积增大。24 h后,种子内营养物质向种胚流动。PEG处理下,水稻种子吸水量明显减少,发芽速度明显降低,且PEG质量分数越高,发芽速度越慢。基于核磁共振理论及T2弛豫谱的多组分特征,当反演频率为10 000时,水稻种子萌发过程中的水分分为束缚水与自由水两部分。T2弛豫谱结果表明:在蒸馏水和质量分数分别为10%、20%PEG处理下,种子湿基含水率和核磁信号幅值均逐渐增长。PEG处理下,核磁信号幅值增长相比对照处理显著降低(P0.05)。PEG处理抑制了两个水稻品种种子对水分的吸收,PEG质量分数越高,抑制作用越强。PEG处理24 h后,耐旱性强的水稻品种吸水率相比对照处理的降低幅度小于耐旱性弱的品种。回归分析表明,3种处理下,核磁信号幅值和湿基含水率具有一致的线性关系(R2=0.983),由回归方程可以求得水稻种子萌发过程中各状态水分的含量。试验为研究水稻种子萌发过程对干旱胁迫的反应机制,开发种子水分微观活体无损检测技术等方面的研究提供理论支持和数据参考。     

用核磁共振研究浸种方法对水稻种子吸水量的影响

为寻求较佳浸种方法,该文应用低场核磁共振检测技术,研究了不同的浸种方式及浸种溶剂对水稻种子吸水量的影响。试验利用横向弛豫时间 T2反演谱分析了水稻种子的水分状态变化及吸水特性,发现浸种过程改变了水稻种子内部的水分分布情况,水稻种子吸水量对初始含水率差异不显著(P>0.05),但对各种浸种方法差异显著(P<0.05)。研究表明,采用连续浸种4 h、浸种3 h-晾干1 h-浸种1 h、浸种2 h-晾干1 h-浸种2 h及浸种2 h-晾干2 h-浸种2 h这4种不同的浸种方式时,浸种2 h-晾干1 h-浸种2 h的间歇浸种方式吸水率较高;采用清水、强氯精300倍液、饱和澄清石灰水、质量分数为40%福尔马林的50倍液、100倍液及200倍液6种不同的浸种溶液时,应用质量分数为40%福尔马林50倍液药剂时吸水率较高。低场核磁共振检测技术揭示了水稻种子含水量的影响因素,为浸种过程中吸水量的测定提供了一种有效的方法。     

基于核磁共振技术检测小麦植株水分分布和变化规律

为研究活体冬小麦植株水分的分布状况和连续变化过程,该研究利用核磁共振无损、非侵入的技术优势,分析了小麦各器官T2弛豫谱特征及其反映的代谢特性,分别推求出小麦叶片、茎秆和穗的信号幅值与被检测器官纯水含量以及鲜质量的回归函数关系,在此基础上建立了测量活体冬小麦植株各器官湿基含水率的检测方法。对活体植株各器官湿基含水率核磁共振检测方法的可靠性验证表明,由核磁共振法和烘干法测定的小麦叶片、茎秆和穗的湿基含水率均方根误差分别为：5.3%、3.5%、3.3%。然后将该检测方法用于监测同一株冬小麦各器官湿基含水率的长期变化和日变化过程,结果显示,乳熟期至成熟期,小麦各个器官的湿基含水率均逐渐减小,而叶片湿基含水率的日变化则呈现先减小后增大的趋势。乳熟期叶片的湿基含水率由8:00逐渐减少,且在14:00－16:00达到最低值后开始恢复,于20:00恢复至当日初始水平。成熟期叶片湿基含水率由8:00逐渐减少,但在20:00不能恢复至日内的初始水平。由于这套基于核磁共振技术的小麦湿基含水率检测方法能够对同一植株进行活体无损连续监测,因此该研究的结果能够更直接更准确地揭示冬小麦植株体内水分的连续变化规律和植株衰老过程,从而为研究冬小麦健康生长耗水规律和制定合理的灌溉制度提供理论基础。     

利用低场核磁共振分析蓝莓贮藏过程中水分含量及迁移变化

为了探究蓝莓在不同贮藏温度下,其内部水分含量及迁移状况随贮藏时间的变化规律,利用低场核磁共振（Low Field Nuclear Magnetic Resonance,LF-NMR）及其成像技术（Magnetic Resonance Imaging,MRI）采集0、8、23 ℃贮藏0、3、6、9、12 d的蓝莓波谱信息以及质子密度图像信息,并分析其规律变化。试验结果表明：采后蓝莓内部水分极易受到贮藏温度与贮藏时间的影响;同时,弛豫谱峰面积和弛豫时间可以有效判定蓝莓贮藏过程中水分含量及迁移变化。随着贮藏时间的延长,其液泡水含量A23与总水分含量A2呈现出整体显著下降（P<0.05）的趋势;在蓝莓贮藏至12 d过程中,弛豫时间随贮藏时间的延长而不断右移,细胞壁水含量A21变化不明显,细胞质水含量A22呈现小幅增加趋势;但23 ℃贮藏至9 d后,蓝莓发生腐烂,细胞壁水和细胞质水迅速增加,贮藏至12 d时液泡水急剧减少,转化为细胞质水和细胞壁水,试验发现23 ℃贮藏的蓝莓货架期为1周左右;与前者相比,蓝莓低温贮藏至12 d时,液泡水含量仍占总水分含量的89%以上,说明蓝莓在低温贮藏下,其内部水分迁移缓慢、流失量较少,其贮藏时间更长、保鲜效果更佳。研究结果为蓝莓在不同温度下贮藏保鲜提供了理论支撑和数据参考。     

用低场核磁共振研究盐溶液漂洗对带鱼鱼糜凝胶品质的影响

为了比较单一成分盐溶液(质量分数为0.30%Na Cl、0.06%Ca Cl2或0.50%柠檬酸钠)和复合盐溶液(0.15%Na Cl、0.04%Ca Cl2、0.35%柠檬酸钠)漂洗对带鱼鱼糜凝胶水分分布及其凝胶特性的影响,该文运用低场核磁技术测定鱼糜凝胶的水分弛豫时间和水分质子密度,并结合鱼糜凝胶含水率、持水性、凝胶强度及其电镜扫描图,对盐溶液漂洗效果进行探究。鱼肉经单一成分0.30%Na Cl、0.06%Ca Cl2或0.50%柠檬酸钠溶液漂洗,柠檬酸钠组鱼糜凝胶中的氢质子束缚力最大,水分难以迁移,自由水的相对含量最低;与单一成分漂洗相比,经过复合盐溶液漂洗后,鱼糜凝胶水分迁移力相对较低,自由水含量较少,而水分质子密度较高,水分分布均匀;盐溶液漂洗后鱼糜凝胶的持水性、含水率、凝胶强度、凝胶显微结构与鱼糜凝胶中的水分分布相关,利用复合盐溶液漂洗带鱼鱼肉,能更好地改善带鱼鱼糜凝胶品质。研究结果为改善鱼肉漂洗工艺、提高鱼糜品质提供参考。     

1H nuclear magnetic resonance relaxometry study of water state in milk protein mixtures

1H NMR signal was used to characterize highly hydrated milk protein dispersions (3-20% dry matter) with various micellar casein concentrations (3-15%), whey protein concentrations (0-3%), lactose concentrations (0-7.5%), CaCl(2) concentrations (0-2 mM), and pH (6.2-6.6). The results showed the predominant effect of micellar casein concentration on water state and were consistent with the three-site relaxation model in the absence of lactose. The relaxation rates observed for these dispersions were explained by the free water relaxation rate, the hydration water relaxation rate, and the exchangeable proton relaxation rate. Hydration water was found to be mainly influenced by casein micelle concentration and structure. The variations in hydration with pH were consistent with those observed for classical measurement of voluminosity observed at this range of pH. The effects of lactose and whey protein content are discussed.     

利用LF-NMR探讨冻融处理影响甘薯膨化产品品质的机理

为探究冻融处理对甘薯变温压差膨化干燥产品品质影响的机理,应用低场核磁共振技术（low-field nuclear magnetic resonance,LF-NMR）研究了冻融后甘薯中水分存在形式和各组分含量,比较了冻融次数对甘薯膨化干燥产品硬度、色泽和多孔性等品质指标的影响。结果表明：冻融后甘薯LF-NMR自旋-自旋弛豫时间T2谱中出现4个水分峰,其横向弛豫时间分别为T21（0.25～0.55 ms）、T22（1～2.5 ms）、T23（5～12 ms）、T24（40～200 ms）。随着冻融次数增加,自由水含量（mT24）先增加后减少,结合最紧密的水含量（mT21）先减少后增加;干燥产品多孔性和复水性逐渐增大,ΔE逐渐变小,L*和硬度先减小后有稍许增大。相关性分析表明,多孔性与T24、mT23呈现显著正相关（P＜0.05）,相关系数分别为0.995、0.989;mT22与ΔE的相关系数为0.984。该研究为阐明冻融处理对果蔬变温压差膨化干燥品质变化机理分析提供了理论依据。     

Effects of casein and fat content on water self-diffusion coefficients in casein systems: a pulsed field gradient nuclear magnetic resonance study

The water self-diffusion coefficients in casein matrixes were measured using a pulsed field gradient spin-echo nuclear magnetic resonance technique (PFG-SE NMR). The dependence of the water self-diffusion coefficient on the casein concentration and the aqueous phase composition is reported in both a rehydrated native phosphocaseinate dispersion and a concentrated casein retentate. A model has been proposed to explain the different behavior of the water self-diffusion coefficient in the two casein systems. This model demonstrates that the water self-diffusion cannot be simply explained by the water content only. So, taking into account the specific effect of each constituent of the aqueous dispersing phase, the water self-diffusion reduction induced by the casein micelle can be modeled. The effect of fat on the water self-diffusion coefficients was investigated. Anhydrous milk fat-reconstituted retentate samples were used in order to estimate the obstruction effect of fat globules in the modeling process. The dependence of the self-diffusion coefficient of water on the fat and casein content is reported. A general model included the effect of the aqueous phase composition, and the obstruction effects of casein micelles and fat globules were proposed. This model was validated for water self-diffusion coefficients in industrial fatty retentates.     

基于低场核磁共振的热风干燥猕猴桃切片含水率预测模型

为研究猕猴桃切片热风干燥过程中水分迁移规律,该试验通过对猕猴桃切片进行热风干燥,考察不同干燥温度（70、80、90 ℃）、切片厚度（3、4、5 mm）下的干燥特性。试验采用直接干燥法测定含水率,运用低场核磁共振技术（Low-Field Nuclear Magnetic Resonance,LF-NMR）分析热风干燥过程中猕猴桃切片内部水分分布状态与变化规律,建立动力学模型,验证并预测。结果表明：猕猴桃切片热风干燥开始为外部控制,随后属于内部扩散控制,水分有效扩散系数范围为1.58×10-7～4.18×10-7 m2/s,扩散效率随温度升高而增大。升高温度能显著提高猕猴桃干燥速率,可加快结合水、不易流动水以及自由水的迁移。自由水和结合水先于不易流动水发生变化,自由水含量在干燥前期逐渐下降,此过程中不易流动水和结合水含量均表现为先升高后降低的趋势。当自由水被脱除后,不易流动水和结合水含量依次达到最大值;此后,随着干燥的进行,不易流动水逐渐被脱除,此时结合水含量开始下降直至干燥结束。整个干燥过程中,猕猴桃切片部分自由水先转化为不易流动水和结合水,结合水与不易流动水相互转化,循环往复伴随整个干燥过程。以干燥过程中的自由水、结合水、不易流动水的核磁峰值总和、切片厚度和干燥温度为自变量,猕猴桃切片含水率为因变量,进行多元线性回归分析,建立含水率预测模型,模型的拟合优度为0.982。结果表明,低场核磁共振技术结合数学模型可用于描述猕猴桃切片热风干燥过程,可实现对猕猴桃切片干燥过程中含水率的快速、无损检测,研究结果可为猕猴桃热风干燥工艺和过程设计提供理论依据。     

利用低场核磁共振研究冷却条件对猪肉保水性的影响

为了通过水分分布信息,深入了解冷却条件对猪肉保水性的影响,以三元杂交猪为对象,利用低场核磁共振(LF-NMR)技术研究了不同冷却条件下猪肉的水分分布及流动规律。结果表明:冷却猪肉低场核磁共振自旋-自旋弛豫时间T2谱中出现4个水分峰,其横向弛豫时间分别为T20(150～500μs),T21(1～3ms),T22(40～50ms),T23(150～400ms)。滴水损失、蒸煮损失与T22,T23呈现显著正相关(P<0.01),自由水含量(pT23)与滴水损失的相关系数为0.858。冷却过程中不易流动水(pT22)先增加后减少,自由水(pT23)先减少后逐渐增加。与常规常湿冷却相比,两段式快速冷却使胴体的冷却失重降低了0.3%,高湿冷却使胴体的冷却失重降低了约0.5%,显著提高了保水性(P<0.05)。核磁成像显示,相对湿度95%处理组冷却猪肉表面的自由水比相对湿度80%处理组多。该研究结果对猪肉冷却工艺确定具有一定的指导作用。     

Solution-state nuclear magnetic resonance study of the similarities between milled wood lignin and cellulolytic enzyme lignin

The structures of milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) have been analyzed using traditional chemical methods and solution-state NMR techniques. Comparisons of the results obtained reveal that subtle differences exist between the two lignin preparations. Thioacidolysis produced higher monomer yields from CEL than MWL, suggesting MWL has a more condensed structure. Quantitative (13)C NMR determined the degree of condensation in MWL to be 0.43 unit per aromatic moiety as compared to 0.36 in CEL. The MWL also contained a lower amount of beta-O-4' substructures per aromatic ring than CEL, 0.41 versus 0.47, respectively. Carbohydrate analysis revealed that the MWL may contain a higher proportion of middle lamella material as compared to the CEL. Because the middle lamella is considered to have a more condensed lignin structure, on the basis of the bulk polymerization theory, these results could explain the differences in beta-O-4' and degree of condensation.     

基于31P核磁共振分析的植物体内磷分级研究

为了深入了解植株残体中磷的分级情况以及在降解过程中的供磷情况,采用31P核磁共振(31 phosphorus nuclear magnetic resonance,31P-NMR)技术,分析了油菜、蚕豆不同部位的磷的分级情况,并采用室内模拟的方法评价不同植物材料还田后的有效性。结果表明,不同植物材料中都以正磷酸盐所占比例最高,在60%～67%之间,其次为磷酸单酯,占浓缩液全磷的比例为20%～28%;高磷植株的正磷酸盐和磷酸单酯含量显著高于中磷和低磷植株,这与植物吸收磷的特性以及磷在植物体内的储存方式有关。结果表明,31P-NMR分析技术能较好地反应植物材料还田后的供磷潜力。     

基于~(31)P核磁共振分析的植物体内磷分级研究

为了深入了解植株残体中磷的分级情况以及在降解过程中的供磷情况,采用~(31)P核磁共振(31phosphorus nuclear magnetic resonance,~(31)P-NMR)技术,分析了油菜、蚕豆不同部位的磷的分级情况,并采用室内模拟的方法评价不同植物材料还田后的有效性。结果表明,不同植物材料中都以正磷酸盐所占比例最高,在60%~67%之间,其次为磷酸单酯,占浓缩液全磷的比例为20%~28%;高磷植株的正磷酸盐和磷酸单酯含量显著高于中磷和低磷植株,这与植物吸收磷的特性以及磷在植物体内的储存方式有关。结果表明,~(31)P-NMR分析技术能较好地反应植物材料还田后的供磷潜力。     

Self-diffusion coefficient of water in tofu determined by pulsed field gradient nuclear magnetic resonance

The self-diffusion coefficient of water in soybean protein dispersion and tofu was measured by pulsed field gradient (PFG) NMR. A soy protein isolate (SPI) dispersion (6 and 12%, w/w) in water, calcium cross-linked precipitate, and tofu were used for comparison. The self-diffusion coefficient of water (D) in the SPI dispersion, 2.23 x 10(-9) m2/s, was estimated lower than that of free water, 2.6 x 10(-9) m2/s at 25 degrees C, and decreased as the SPI concentration increased. It further decreased by the addition of calcium chloride, reflecting the obstruction effect induced by the precipitates in addition to the hydration and hydrodynamic interaction in the protein dispersion. The two water regions in tofu were interpreted by the two-site K?rger model: D1 and D2 of soft tofu were 2.26 (+/-0.11) x 10(-9) and 6.84 (+/-0.34) x 10(-11) m2/s, respectively. The relative amount of proton (water) was p1 = 0.98 and p2 = 0.02 at 100 ms of diffusion time. The self-diffusion coefficients of water decreased in pressed tofu, and their relative amounts of water changed to p1 = 0.93 and p2 = 0.07. It was suggested that D1 corresponded to obstructed water in the network structure and D2 corresponded to hydrated water on the surface layer of pores formed in the protein network of tofu. The pore sizes estimated from the diffusion length of obstructed water were 21.3 microm in soft tofu and 20.8 microm in pressed tofu. The removal of fat from pressed tofu led to a decrease in D2 from 6.26 (+/-0.31) x 10(-11) to 3.53 (+/-0.18) x 10(-11) m2/s, and the relative amount of hydrated water increased from 0.07 to 0.14, which indicated hydrophobic hydration.