Slade L, Levine H
Nabisco Brands, Inc., Fundamental Science Group, East Hanover, New Jersey 07936-1944.
Adv Exp Med Biol. 1991;302:29-101. doi: 10.1007/978-1-4899-0664-9_3.
Descriptions of the functional significance of carbohydrates based on the familiar equilibrium thermodynamics of very dilute solutions fail for pragmatical time scales and conditions, which are far from equilibrium. This is not too surprising, since limiting partial-molar properties reflect the independent behavior of solute in the limit of infinite dilution where free volume is maximum at a given temperature, while Tg'-Wg' properties reflect the cooperative behavior of solute-plasticizer blends at the limiting minimum value of free volume to observe relaxation within experimental time scales. Carbohydrate-water systems, with well-characterized structure and MW above and below the entanglement limit, provide a unique framework for the investigation of non-equilibrium behavior. Thermal analysis by DSC reveals the central role of water as a plasticizer for carbohydrates and of the glass transition as a physicochemical parameter that governs their properties, processing, and stability. A classical polymer science approach is used to study structure-property relationships of carbohydrates as water-compatible food polymers, which are treated as homologous systems of polymers, oligomers, and monomers with their plasticizers and solvents. Mechanical relaxation behavior is described by a "transformation map" of the critical variables of moisture content, temperature, and time. The glass curve is a reference contour, which represents the limiting isogram for free volume, local viscosity, relaxation rates, and rotational and translational mobility. Map domains are discussed as aspects of "water dynamics," to dispel the myth of "bound water," and "glass dynamics," to relate to macroscopic structure and collapse phenomena. A particular glass with invariant composition and Tg (prepared by freeze-concentration) is identified as a pivotal and practical reference state. The Tg observed during DSC analysis is often an effective Tg, resulting from instantaneous relative relaxation rates and non-uniform distribution of total sample moisture. Non-equilibrium melting, annealing, and gelation/recrystallization of kinetically metastable, partially crystalline carbohydrate systems exhibit non-Arrhenius kinetics which depend on the magnitude of delta T above the appropriate Tg, as defined by WLF relaxation transformations. Thermally reversible aqueous gels (crystallized from an under-cooled, rubbery melt) are described by a "fringed micelle" structural model for a three-dimensional polymer network, composed of microcrystalline junction zones crosslinking plasticized amorphous regions of flexible-coiled, entangled chain segments.
基于极稀溶液中常见的平衡热力学对碳水化合物功能意义的描述，在远离平衡的实际时间尺度和条件下并不适用。这并不太令人惊讶，因为极限偏摩尔性质反映了溶质在无限稀释极限下的独立行为，此时在给定温度下自由体积最大，而Tg'-Wg'性质反映了溶质-增塑剂共混物在自由体积极限最小值下的协同行为，以便在实验时间尺度内观察到松弛。具有特征明确的结构且分子量高于和低于缠结极限的碳水化合物-水体系，为研究非平衡行为提供了独特的框架。差示扫描量热法（DSC）进行的热分析揭示了水作为碳水化合物增塑剂的核心作用，以及玻璃化转变作为控制其性质、加工和稳定性的物理化学参数的作用。采用经典的聚合物科学方法来研究碳水化合物作为与水相容的食品聚合物的结构-性质关系，这些碳水化合物被视为聚合物、低聚物和单体与其增塑剂和溶剂的同源体系。机械松弛行为通过水分含量、温度和时间等关键变量的“转变图”来描述。玻璃化曲线是一条参考轮廓线，它代表了自由体积、局部粘度、松弛速率以及旋转和平移流动性的极限等值线。图中的区域被作为“水动力学”的方面进行讨论，以消除“结合水”的神话，以及“玻璃动力学”的方面，以关联宏观结构和塌陷现象。一种具有不变组成和玻璃化转变温度（通过冷冻浓缩制备）的特定玻璃被确定为一个关键且实用的参考状态。DSC分析过程中观察到的玻璃化转变温度通常是一个有效玻璃化转变温度，它是由总样品水分的瞬时相对松弛速率和不均匀分布导致的。动力学亚稳的部分结晶碳水化合物体系的非平衡熔化、退火以及凝胶化/重结晶表现出非阿累尼乌斯动力学，这取决于高于由WLF松弛转变定义的适当玻璃化转变温度的ΔT的大小。热可逆水性凝胶（从过冷的橡胶状熔体中结晶）由三维聚合物网络的“缨状微胞”结构模型描述，该网络由微晶连接区交联柔性盘绕、缠结链段的增塑无定形区域组成。
