果胶微波降解过程中的分子链变化及其化学反应基础

Pectin has been widely used in food, pharmaceutical and daily chemical industries. The degradation of pectin during various kinds of processing methods has raised considerable concerns. Our previous study has found that pectin was degraded and its functional properties were reduced by microwave treatment (MT). However, the effect of MT on pectin’s structure and conformation as well as its chemical reaction basis remain unclear. In present work, pectins are degraded under different MT conditions and fractionated into different fractions. The effect of MT on structure of pectin is characterized by chemical analysis (including periodate oxidation, Smith degradation, partial acid hydrolysis and methylation analysis) and instrumental analysis (including UV, FT-IR, NMR and MS). The effect of MT on conformation (such as radius of gyration, hydrodynamic radius and flexibility) of pectin is evaluated by techniques such as intrinsic viscosity determination and dynamic/static light scattering method. After that, molecular construction and conformational simulation are performed based on the selected characteristic structure parameters. The relationship between the change of structure and the change of conformation is discussed. Finally, the chemical reaction basis of these changes is explored by determining the type and extent of various chemical reactions (hydrolysis, β-elimination, demethoxylation, and free-radical-induced degradation). This project can provide theoretical evidence to explain the change of pectin’s functional properties after MT, and offer an academic reference for investigating the chemical nature of other degradation methods.

果胶在食品、医药和日化行业应用广泛，其在各种加工过程中的降解备受关注。前期研究发现微波处理造成果胶降解和功能性质下降，但微波对果胶分子链结构和构象的影响及其化学反应基础尚不清楚。本项目拟采用不同微波条件处理果胶，经分级得到降解组分。采用化学（高碘酸氧化、Smith降解、部分酸水解、甲基化分析等）及仪器分析法（紫外、红外光谱、核磁共振、质谱等）考察微波对果胶分子链结构的影响。采用特征粘度法，动/静态光散射等技术考察微波对果胶分子链构象（旋转半径、水动力学半径、柔韧性等）的影响。筛选特征性结构参数，进行分子构建和构象模拟，探讨微波诱导的结构和构象变化的相互关系。通过测定微波过程中各化学反应（水解反应、β-消除反应、去甲酯化反应和自由基诱导降解反应）的发生程度，探寻分子链结构和构象变化的化学反应基础。为解释微波处理后果胶功能性质的改变提供理论依据，为深入研究其他降解手段的化学本质提供理论借鉴。

果胶广泛应用于食品、医药等行业，然而热、机械、酶等作用常导致果胶发生降解和功能性质改变。关于微波降解对果胶分子链的结构和构象变化及引起分子链变化的化学反应基础缺乏具体研究。本项目以微波为处理手段，测定不同处理条件下果胶分子链的变化，分析微波降解果胶可能涉及的化学反应，表明随着微波处理时间的延长，果胶的分子量逐渐下降，在酸碱条件下分子量下降程度越大；在酸性环境下，微波降解果胶还原糖含量增加，双键含量和酯化度没有明显变化，表明主要发生酸水解反应；在碱性条件下，微波降解果胶还原糖含量增加，双键含量增加，酯化度降低，表明果胶经微波处理后发生β-消除反应的同时伴随着去甲酯化反应。以冰水浴微波、沸水浴及加入自由基抑制剂处理为对照，测定处理后果胶还原糖含量结果表明微波降解果胶除热效应外，还存在自由基降解等其它效应。构象分析表明随着微波处理时间的延长，果胶分子的构象从无规则线圈变为刚性链状，特性粘度降低。以果胶分子量和酯化度为特征参数，通过分子动力学模拟构建了微波处理后果胶结构和构象的关系。流变结果表明在未调节pH条件下微波处理后的果胶呈现典型的凝胶状（tanδ<1），质构结果表明果胶的凝胶强度、弹性、硬度、粘性、咀嚼性均逐渐减低；而在酸碱条件下，处理后的果胶成流体状态，tanδ增加显著，且其值大于1。微波处理后果胶分子链键的断裂和化学反应的发生，引起果胶分子链结构和构象的变化，从而造成果胶澄清度、流变和凝胶等功能性质的改变，本项目为微波降解作为果胶大分子结构解析的新降解手段提供理论参考，为解释微波处理后果胶功能性质的改变提供理论依据，为深入研究其他降解手段的化学本质提供理论借鉴。