核磁共振研究蔗糖对面团中分子流动性的影响

利用脉冲梯度场核磁共振技术(PFG-NMR)研究蔗糖对面团自旋-自旋弛豫特性的影响。分子的自旋-自旋弛豫特性与分子的流动性呈正相关。采用CPMG脉冲序列检测面团中质子的自旋-自旋弛豫时间(T2)。在面团中流动性不同的水分,表现出不同的弛豫时间。实验结果表明,蔗糖的亲水性能够显著改变面团的自旋-自旋弛豫时间。通过磁共振图像可以发现蔗糖能够明显促进面团中质子的均匀分布,增强其流动性。当水分含量一定时,面团的自旋-自旋弛豫时间随蔗糖含量的增加而增加。     

南酸枣糕烘干过程中水分的迁移和分布

利用核磁共振及其成像技术,探究南酸枣糕在烘干过程中水分含量、迁移和分布情况。通过分析南酸枣糕烘干过程中核磁共振的自由衰减弛豫时间、自旋-自旋弛豫时间和核磁共振成像图发现:南酸枣糕的水分含量与体系的质子密度高度线性相关,其相关系数达到0.990 6。因此,可以通过建立水分含量与核磁共振质子密度的标准曲线实现南酸枣糕水分含量的核磁共振定量测定;在烘干过程中,随着时间的延长,南酸枣糕中的束缚水的流动性(T21)呈降低的趋势,自由水的流动性(T22)呈先增加后降低的趋势,核磁共振成像图逐渐变暗;从成像图也可直观地看出,南酸枣糕内部弛豫信号比外部强,证实南酸枣糕干燥是一个不均匀的干燥过程。     

核磁共振技术在脐橙保藏和质量评估中的应用

采用低场核磁共振技术研究脐橙储藏过程中水分的状态、变化以及水分的迁移.利用Carr-Purcell-Mei-boom-Gill(CPMG)序列采集样品的核磁共振受激回波信号,采用多重指数衰减模型对收集的信号数据进行分析并计算组织中不同水分自旋-自旋弛豫时间.探讨脐橙在储藏过程中品质变化与核磁共振弛豫参数变化的相关性.发现在脐橙的后成熟期、稳定期和腐烂期,对应的自旋-自旋弛豫时间变化呈现一个上升-稳定-急速下降的过程,且在脐橙腐烂前出现了一个自旋-自旋驰豫时间急剧下降的过程.该方法可以作为脐橙质量评估的有效手段,对于脐橙的质量评估和保藏方法的评估等具有指导意义.     

利用核磁共振及成像原理研究鸡蛋的煮熟过程

利用核磁共振技术研究在不同的煮熟时间下带壳鸡蛋的弛豫特性.采用IR、CPMG脉冲序列来测定不同煮熟时间下质子的自旋-晶格弛豫时间(T1)和自旋-自旋弛豫时间(T2).在蛋清和蛋黄不同的凝固和成形过程中,其表现出不同的弛豫时间.实验结果表明,蛋清先于蛋黄逐渐变性,弛豫时间T23对应的质子密度逐渐降低,最后达到稳定的状态,说明整个带壳鸡蛋的流动性下降.通过核磁共振成像能够直观地说明带壳鸡蛋在煮熟过程中内部结构的变化情况.     

NMR和MRI技术对面包和奶酪二元体系在贮藏过程中水分迁移的研究(英文)

采用低场核磁共振成像技术中的spin3D、CPMG等序列对面包和奶酪二元体系进行成像并测定自旋-自旋弛豫时间等参数,并对其在贮藏过程中水分迁移行为及规律进行了研究,为核磁共振技术在食品中的应用打下良好的基础。     

水分和温度对猪肉质构及感官品质的影响

本文以猪里脊肉为原料,在感官评定、仪器分析的基础上,结合低场核磁共振测定结果,考察了水分条件及加热温度对猪肉制品品质的影响。实验数据表明:猪肉的感官硬度、紧密性及咀嚼感随含水量降低而升高,感官弹性略有改善,但100℃热处理的不同干燥样之间没有显著差异(p0.05);质构指标硬度、咀嚼性和剪切力值等与肉品的水分含量呈极显著负相关关系(p0.01);通过降低肉的含水量(36%)可改善121℃热处理导致的肉质软烂和硬度降低。通过LF-NMR分析发现:随干燥程度增大,各状态水分的T2弛豫时间均向快弛豫方向移动,加热温度越高,T21状态的水分弛豫时间移动越明显;干燥脱水造成的水分分布变化不及高温加热造成的影响显著;不易流动水与肉的质构特性关系相当密切,控制其含量可影响脱水肉制品的食用品质。     

不同烟草材料中水分赋存状态的低场核磁共振分析

为探讨不同烟草材料内部水分赋存状态,采用低场核磁LF-NMR手段分析了叶丝、梗丝、再造烟叶丝3种烟草材料的自旋-自旋弛豫时间T_2反演谱,并分析了不同烟草材料含湿量、微观结构对T_2反演谱中水分子信号峰弛豫时间、弛豫峰面积的影响规律。结果表明:(1)在10%~35%含水率范围内烟草材料的T_2反演谱图通常呈现2~4个质子信号峰,其中弛豫时间在0.01~10 ms范围T_(21)峰可反映烟草中水分的氢质子弛豫信号强度。(2)随着烟草材料含水率的增大,叶丝、梗丝、再造烟叶丝的T_(21)峰弛豫时间在0.183~1.430、0.166~1.560、0.166~1.650 ms范围内逐渐增大,烟草材料中迁移性较强的水分比例增大。(3)T_(21)弛豫峰面积与不同烟草材料含水率均存在显著的线性相关,该T_2反演信号可用于烟草材料含水率的定量分析。(4)天然烟草材料中30μm以上细胞尺度上的孔隙被破坏后,水分弛豫时间明显减小,表明天然烟草材料中水分赋存状态受其微观多孔结构影响明显。     

反复冻融过程中调理狮子头脂肪氧化与低场核磁共振弛豫特性的相关性

该文研究冻融过程中调理狮子头脂肪氧化与低场核磁共振弛豫特性的相关性。利用低场核磁共振技术监测调理狮子头冻融过程中水分的存在形式及相对含量,比较冻融次数对酸价、过氧化物值及硫代巴比妥酸值(thiobarbituric acid reactive substances,TBARS)的影响,分析脂肪氧化指标与其相应低场核磁共振弛豫信号间的相关性。结果表明,随着冻融次数的增加,结合水(T21)与不易流动水(T22)弛豫时间后移,不易流动水(P22)与自由水(P23)含量不断减少;酸价和过氧化值先增后减,TBARS值不断增加。冻融次数与T23(自由水弛豫时间)、P21(结合水含量)、T22、P22和TBARS值极显著相关(P <0. 01),与过氧化值显著相关(P <0. 05); TBARS值与T22、P22极显著相关(P <0. 01); TBARS值与弛豫特性的回归模型决定系数为0. 814,回归方程拟合度较高。该实验结果为肉制品脂肪氧化的监测及科学贮藏提供理论依据。     

KCl部分替代NaCl对低钠盐肉脯质量的影响

KCl与NaCl由同族元素组成,且与NaCl有相同的离子价态及相似的化学性质,因此KCl常作为NaCl替代物用于低钠盐食品的加工。本研究在肉脯生产过程中以KCl部分替代NaCl,通过测定产品的出品率、水分含量、水分活度、剪切力和色差值,并采用低场核磁共振(low-field nuclear magnetic resonance,LF-NMR)技术检测肉脯中水分的弛豫时间(T_2)和分布情况,研究KCl部分替代NaCl对于产品质量的影响。结果表明:随着KCl替代比例的增加,产品的出品率和水分含量显著降低(P0.05),剪切力显著增加(P0.05),水分活度、红度值(a~*)和亮度值(L~*)差异不显著(P0.05);产品水分的T_(21)、T_(22)弛豫时间均显著增加(P0.05),表明水分的移动性增强,肉脯的保水能力下降。低钠盐肉脯中,KCl的替代比例在20%~30%之间较为合适,此时产品具有较高的品质和感官可接受性。     

利用核磁共振技术监测海带湿面贮藏品质

利用低场核磁共振技术监测2种贮藏条件下海带湿面样品弛豫特性的变化,探讨水分及其迁移变化规律,测定产品质构性质的变化,分析二者的相关性。真空包装的海带湿面样品分别于常温、4℃和-18℃条件下贮藏。以弛豫时间T2、质子密度M2、质构特性为主要指标。监测贮藏期间其品质变化规律。试验结果表明:常温、4℃和-18℃3种贮藏条件下水分迁移及质构特性变化的关键时期依次为28~35,35~42,49~56 d。弛豫特性与质构特性呈显著相关。     

NMR State Diagram Concept

ABSTRACT:  This article introduces a new concept, the NMR (nuclear magnetic resonance) state diagram, in the context of food shelf-life stability as affected by the molecular mobility of the food matrix. Our literature review shows that some shelf-life–related changes cannot be explained or predicted by the current a _w and glass transition temperature concepts. This article presents the theoretical principles and some experimental evidence of the NMR state diagram concept that could be complementary to the a _w and glass transition concepts. An NMR state diagram is a curve of NMR relaxation time versus temperature. Some of the curve features were found to correlate highly to the physiochemical states and changes of food polymers, for example, caking, stickiness, and firming. The potential applications of this concept in quality and safety of food products, especially dry and intermediate moist foods, may include ingredients screening, prediction of physiochemical changes, chemical degradation, and microbiological activity. The goal concept of this article is to provoke more in-depth studies to analyze the relationships among NMR relaxation, molecular mobility, and stability of foods.     

Food Stability Beyond Water Activity and Glass Transtion: Macro-Micro Region Concept in the State Diagram

The water activity concept proposed that a food product is the most stable at its monolayer moisture content. Recently, the limitations of the water activity concept were identified and the glass transition concept was proposed in order to overcome the limitations of water activity. Based on the glass-transition concept, a food is the most stable at and below its glass transition point. Recently it has also become evident that the glass transition concept is not universally valid for stability determination when foods are stored under different conditions. The glass transition concept was used to develop the state diagram by drawing another stability map using freezing curve and glass line. Currently, other components indicating different characteristics are being included in the state diagram. It is being emphasized in the literature to combine the water activity and glass transition concepts. In this paper, an attempt is made to combine these two concepts in the state diagram and to propose a macro-micro region concept for determining the stability of foods.     

Water/moisture and fat analysis by time-domain NMR

Time-domain nuclear magnetic resonance (TD-NMR) is widely used within food industries for QC/QA applications. Precise determination of moisture and water is of fundamental interest. To determine moisture and water in the presence of lipids, the minispec makes use of differences in their respective NMR relaxation properties. The minispec applications analysing bound water are well-known and have become International standard methods. Different methods to analyse food samples with high water content or free water, like some dairy products, are addressed in this paper. Explicitly, the well-known pre-drying methods are compared to NMR diffusion experiments and a combined relaxation time analysis. Finally the single-sided NMR approach is discussed to analyse packages food.     

Food stability determination by macro–micro region concept in the state diagram and by defining a critical temperature

In the 1950s the concept of water activity was proposed for determining food stability. This concept is now being used although it has some limitations. Indeed, these limitations mean that the concept is not universally applicable and in fact is invalid under certain conditions. In order to address the limitations of the water activity concept, the glass transition concept was proposed in the 1960s, although significant application of the concept only started in the 1980s. Recently, it has become evident that the glass transition concept is also not universally valid for stability determination in all types of foods when stored under different conditions. Currently in the literature the need is emphasized to combine the water activity and glass transition concepts since both concepts could complement each other. The glass transition concept was used to develop the state diagram by drawing another stability map using freezing curve and glass transition line. In this paper an attempt is made to review the published methods used to combine both concepts. These approaches are graphical plot of glass transition conditions and water content as a function of water activity, and macro–micro region concept in the state diagram. In addition, a new approach is proposed in this paper by defining a critical temperature for stability and then relating it with water content, and other hurdles affecting food stability. The water mobility concept is also reviewed to provide another dimension of food stability in order to determine a more complete picture.     

Implication of water activity and glass transition on the mechanical and optical properties of freeze-dried apple and banana slices

The interactions between water content, matrix structures and chemical and physical events in complex foods are usually regarded in terms of water activity and glass transition. In this work, the water content and water activity values that define important changes in the textural and optical properties of freeze-dried banana and apple slices have been related to the critical water content and water activity that cause the glass transition in both products. From the obtained results, it may be concluded that both approaches, glass transition and water activity, may be considered to prevent changes in the mechanical properties, while only the concept of water activity is needed to control the browning reactions that take place in the fruits.     

Pulse NMR Study of Glass Transition in Maltodextrin

A pulse nuclear magnetic resonance (NMR) technique was used to study the glass transition in maltodextrins. The spin-lattice relaxation time (T₁ and spin-spin relaxation time (T₂) of different molecular weight maltodextrins at different moisture contents and temperatures were measured using the pulse NMR tech nique. T₁, and T₂ were plotted against temperatures. From the resultant curves, the state transitions were observed and the corresponding state transition temperatures were determined. The state transition temperatures were very close to the glass transition temperatures (T_gs) determined with differential scanning calorimetry, suggesting a strong relationship between glass transition and proton relaxation behavior. Results indicate pulse NMR-based instrumentation could be very effective for the study of glass transition in food polymers.     

Applications of macro–micro region concept in the state diagram and critical temperature concepts in determining the food stability

Food stability determination from a scientific basis rather than empiricism is a challenge to food scientists and engineers. Over the years, water activity and glass transition concepts are the two most successful theoretical foundations developed in determining the food stability. The macro–micro region concept in the state diagram and critical temperature concept are further extension of the water activity and glass transition concepts. This paper presents the applications of critical temperature concept in determining food stability during storage. In addition, it presents the applications of macro–micro region concept in analyzing air- and freeze-drying processes, maintaining rice quality using different drying strategies, analyzing structural changes during baking process, and determining chemical stability during storage.     

Mobility and Stability Characterization of Model Food Systems Using NMR, DSC, and Conidia Germination Techniques

Mold conidia germination was used as a microbial probe of food stability in sucrose, starch, and sucrose/starch systems. A group of nuclear magnetic resonance (NMR) and differential scanning calorimeter (DSC) techniques were used to fully characterize the water and solid mobility and glass transition temperature (Tg) of the systems, respectively. Water content, a_w, and ²H NMR R₁ and R₂ relaxation rates did not predict mold germination time. We concluded that the self-diffusion coefficient, (translational mobility of water), the DSC Tg (overall system mobility), and to a more limited extent, the ²H NMR R*₂ relaxation rate and the ¹³C T_1p (solids mobility), could provide alternative measures to supplement a_w for predicting food stability and safety.     

Glass Transition and Food Technology: A Critical Appraisal

ABSTRACT: Most low water content or frozen food products are partly or fully amorphous. This review will discuss the extent to which it is possible to understand and predict their behavior during processing and storage, on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture properties (crispness) also appear to be greatly affected by sub-Tg relaxations and aging below Tg. Secondly, glass transition is only one among the various factors controlling the kinetics of evolution of products during storage and processing. For processes such as collapse, caking, crystallization, and operations like drying, extrusion, flaking, Tg data and WLF kinetics have good predictive value as regards the effects of temperature and water content. On the contrary, chemical/biochemical reactions are frequently observed at temperature below Tg, albeit at a reduced rate, and WLF kinetics may be obscured by other factors.     

Glass transition and caking of spray-dried lactose

Simple test methods to determine the onset temperatures of viscous flow and caking of spray-dried amorphous lactose are described. The extent of viscous flow was measured as an increase in the density of lactose plugs within a cylindrical aluminium compaction apparatus after incubation for 3h at a specified temperature. Caking was characterized by an increase in the hardness of the lactose plugs formed. The onset temperature of viscous flow decreased with increasing water activity, A_w, and corresponded to the onset temperature of glass transition, T_g1. Glass transition temperatures were determined using both differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spin relaxation. The results suggest that elevation of the powder temperature above T_g1 promotes viscous flow and increases the potential for caking of amorphous food powders.