水分对淀粉微波加热过程的介电增强作用及结晶结构的影响

The issue of moisture migration and loss within starchy foods often appears during thermal processing. Due to the dominant role of water in the dielectric response of food system, the microwave processing quality of starchy foods, therefore, would be significantly affected by moisture changes. For instance, microwave-baking bread trends to present a deflated collapse, and microwave-steamed buns seem to have a hard centre. In order to achieve a high-level control upon the microwave processing of starchy foods, scholars respectively proposed hypotheses of targeted activation, thermal disturbance and multi-polarization relaxation, to explain the incentives of dielectric enhancement influenced by moisture. We are convinced that changes of water distribution and fluidity during gelatinization would result in the distinct dielectric response of different states of water. Thus, dielectric polarization of starch is strengthened, which could lead to a thermal runaway in the late microwave heating with a deterioration of texture. In this study, we investigate the starch with different water activity to reveal the reason of the dielectric enhancement in starch on the level of polarization and microwave absorbing, e.g., whether it is due to the distinction of water states or changes of polarization and microwave absorbing properties within hydrated starch. The consequent changes of crystal structure are also studied. This research will provide a theoretical basis for improving the quality of starchy foods during microwave processing and for their industrial control.

淀粉类食品在热加工过程中会出现水分的迁移和流失，而水在食品体系中具有突出的介电响应特性，因此水分变化显著影响淀粉类食品的微波加工品质，易于出现微波烘焙面包质构瘪塌、微波蒸制馒头硬心等现象。为实现含水淀粉类食品的高品质微波化控制，在传质传热模型研究的基础上，针对水分在淀粉体系中介电增强作用的诱因，国内外学者提出了靶向活化假说、热搅动假说、以及多重极化弛豫假说。我们认为糊化过程中水与淀粉的水合过程使得水在淀粉中的分布和流动性改变，造成不同状态水的介电响应改变，改变了淀粉的介电极化机制，引发微波加热中后期产生热失控，导致质构劣化。本研究以传统淀粉类食品的水分活度为标准，旨在从极化、吸波角度揭示微波场下水分对淀粉的介电增强作用是否取决于水状态的差异，是否取决于水合淀粉极化机制及吸波性能的改变；同时，还将揭示由此引发的结晶结构变化。本研究将为微波加工淀粉类食品的品质提高以及工业化控制提供理论依据。

淀粉是主食品的重要结构物质，而水作为加工过程密不可分的组成部分，一定程度上决定了食品的加工品质；并且凭借水分和淀粉的微波响应差异，考察水分在淀粉食品中的干预作用，成为了开发和完善微波加工技术的关键基础。此前，微波场下围绕水对淀粉作用的研究仍以物性特征和结构学特征的考察为主，缺乏基于介电本质的基础研究。本项目在前期已完成的水分对淀粉基本介电性质影响研究的基础上，阐明了淀粉内部水状态随水分活度的变化规律，发现了谷类和薯类淀粉伴随水分变化质子类群的演化差异；明确了水分与淀粉溶胀过程电磁响应机制的关系，评价了不同形式水引发的变化与吸波性能的相关性，发现了诸如水分含量与物料厚度的特异性关系，明确了其非线性变化与“真实单层”理论的相关性；评价了微波加热过程水分对淀粉分子自由基及相关结构的干预机制，并获得了蛋白质、金属离子等关键组份对含水淀粉质物料体系的影响规律。