【作者】 李晶；

【导师】 李斌； 王凌； 李艳； 陈义杰；

【作者基本信息】 华中农业大学 ， 食品科学， 2014， 博士

【摘要】 魔芋葡甘聚糖(Konjac Glucomannan, KGM)是一种亲水性的天然高分子中性多糖。在KGM溶胶中加入碱并加热脱去KGM分子上的乙酰基团,可以得到类似于橡胶的具有高弹特性的脱乙酰魔芋葡甘聚糖(Da-KGM)凝胶,该凝胶在低温区域存在一系列可在食品加工中被应用的独特现象,如冷冻及解冻中发生凝胶强度剧增或纤维化等。对脱乙酰魔芋葡甘聚糖凝胶而言,经冷冻处理后其结构和性能的变化迄今无系统的研究。鉴于其潜在、特异的应用价值,本文考察了冷冻处理对Da-KGM凝胶全质构(TPA)参数和脱水收缩率的影响,并结合流变学、FT-IR、 DSC、SEM和XRD等手段表征了脱乙酰KGM溶胶和凝胶在冷冻前后结构和性能的变化,以期探明冷冻处理影响凝胶结构和性能的机制,为实际生产中对凝胶质构的调控提供借鉴。在此基础上,利用冷冻处理改变凝胶结构、性能的特性,创制了一系列功能材料,为开发生物可降解的多孔材料提供新素材,从而扩大其应用范围,促进魔芋的高附加值加工和利用。本文主要研究结果如下：1.不同浓度的Da-KGM凝胶在冷冻后全质构(TPA)指标均发生了变化,样品浓度越高,冷冻后硬度增幅越大。当KGM初始浓度为5wt%时,冷冻后硬度增幅达到2950%。对比冷冻后凝胶的脱水收缩率发现,脱水后凝胶浓度的增幅远小于硬度的增幅,表明冷冻加工中凝胶的质地的改变主要来源于凝胶结构的改变而非脱水作用。凝胶硬度增加的现象与冻融工艺相关。在实验设定的温度范围内,于-30℃C冷冻的凝胶硬度最大。凝胶样品的硬度在冻融1次后达到最大值,此后随着冻融次数的增多而降低,同时观察到析水率的增加。上述结果表明恰当的冷冻工艺是调控凝胶质构的有效手段。2.随着碱浓度的增加,KGM的脱乙酰度随之上升,凝胶的硬度和析水率也呈现出上升的趋势。说明脱乙酰进程是影响冷冻Da-KGM凝胶质构特性和脱水收缩作用的重要因素。比较不同脱乙酰度对魔芋凝胶共晶点和解冻温度的影响发现：随着脱乙酰度的增加,凝胶的共晶点有缓慢下降的趋势,由不脱乙酰时的-11.9℃下降到-13.4℃C；而解冻温度呈现先略微升高后降低的过程。3.SEM观察发现,不同初始浓度、不同冷冻处理温度和不同加碱量对冷冻脱乙酰KGM凝胶的微观结构均有影响。这与上述因素对冰晶生长的影响有关。冷冻Da-KGM溶胶的流变学实验表明,在冷冻过程中,冰晶生长导致的浓缩和挤压效应促进了Da-KGM分子的自聚集而导致了缠结程度的降低。4.通过在凝胶制备中加入尿素、SDS、NaCl等对可能影响多糖分子间作用力的物质并结合解胶实验,提示维系冷冻Da-KGM凝胶的作用力不仅有氢键作用力,可能还有疏水相互作用。5.为了增加冷冻加工中材料的孔隙率和均匀性,拓展冷冻KGM凝胶的应用范围,本研究尝试采用表面活性剂模板法结合冷冻加工制备了魔芋多孔材料。表面活性剂SDS的加入对多孔材料的孔隙率、吸蒸馏水率和吸生理盐水率都有影响。冷冻条件(时间和温度)对魔芋多孔材料的结构和性能都有影响。随着冷冻时间的延长,多孔材料的拉伸强度(湿强)逐渐增大,断裂伸长率和孔隙率逐渐降低,在-10℃下冷冻的多孔材料的拉伸强度高于其它组,当冷冻时间为9d时达到了所有样品中最大的拉伸强度(49.12KPa)。冷冻温度越高,孔隙率越高。从XRD和DSC分析的结果可以看出,冷冻处理对KGM的晶体结构和热特性造成了改变。6.以上述多孔材料为基础制备的复合PVA-ε-PL魔芋多孔材料具有缓释性能,对金黄色葡萄球菌(Staphylococcus aureus)和大肠杆菌(Escherichia coli.)均有较好的抑制作用。对单宁酸的吸附实验表明,在pH2.0的溶液中多孔材料对单宁酸的吸附量达到了最大的679.3mg/g,准二级动力学方程对多孔材料吸附单宁酸的拟合度更高,对单宁酸的吸附更宜在低温下进行。等温吸附实验表明多孔材料对单宁酸吸附属于多层吸附。在此基础上,利用吸附在多孔材料上的单宁酸原位还原硝酸银制备了载纳米银的多孔材料,该材料展示出了良好的催化能力和抑菌能力。这些研究为开发一类可降解生物抗菌发泡材料、催化材料和多层吸附材料奠定了基础。7.利用简便、温和的冷冻干燥法制备了魔芋葡甘聚糖-蒙脱土复合气凝胶。这种气凝胶具有较小的密度(<0.09g/cm3)。纳米蒙脱土(Sodium Montmorillonite, Na+-MMT)的加入可大大提高气凝胶的机械性能和热稳定性并改变气凝胶的微观结构。随着体系中碱量的增加,复合气凝胶的压缩模量和比模量随之上升,且碱的加入提高了复合气凝胶的热稳定性。当KGM和Na+-MMT初始含量一定时,随着冷冻温度的降低,复合气凝胶的压缩模量和比模量都逐渐增大。当KGM和Na+-MMT的初始添加量分别为2wt%和4wt%、预冻温度为-80℃时,气凝胶的压缩模量和比模量都达到了实验中的最大值,分别为3.480MPa和53.80MPa cm3/g。更多还原

【Abstract】 Konjac glucomannan (KGM) is a kind of natural neutral hydrophilic polysaccharide from the tuber of amorphophallus konjac plant. High elastic rubber-like deacetylated konjac glucomannan (Da-KGM) gel could be obtained by adding a certain amount of alkali in konjac glucomannan sol to remove the acetyl groups attached to the saccharide units and then heating the mixture for a certain time. A series of unique phenomenon for this gel exist in the low-temperature range such as increase of gel strength after the freezing and thawing treatment that can be applied in food processing. There is no systematic study of change of structure and properties for Da-KGM gel so far. In view of its potential and special application, this paper examined the effects of freezing treatment on texture (TPA) parameters and syneresis of Da-KGM gel, characterized the structure and properties of the KGM sol and gel after the freezing treatment using rheology, FT-IR, DSC, SEM and XRD, in order to explore the mechanism that freezing process affect the structure and properties of the Da-KGM gel. This study could provide reference to the control of gel texture in actual production process. Furthermore, the theoretical research was extended to the preparation of a series of functional materials in order to expand its scope of application, and promote high value-added processing and utilization of konjac glucomannan.The main results are as follows:1. The parameters of textural property analysis (TPA) of Da-KGM gel with different concentrations were changed after freezing process, and the increase in hardness were greater when the concentration of the sample were larger. When the initial concentration of KGM was5wt%, hardness of frozen gel increased by2950%. It was found that the increase of the gel concentration is far less than the increase of the hardness after freezing, indicating that the change of gel texture mainly resulted from the change of gel structure rather than syneresis. Frozen treatment exerted a great impact on the textural properties of the samples. In the set temperature range of the experiment, hardness was the largest as the gel was frozen in-30℃. The hardness of the sample reached a maximum value after1cycle of freezing-thawing, then decreased with increase of freezing-thawing cycles, along with the increase of syneresis. The results show that appropriate freezing process is an effective measure to regulate the gel texture.2. With increase of alkali concentration, the deacetylation degree (DD) of KGM raised, and gel hardness and syneresis also showed an upward trend. The other parameters of TPA also changed with the amount of added alkali varied. This indicated that the deacetylation process was one of the most important factors affecting the texture properties and syneresis of frozen Da-KGM gel. The eutectic point of deacetylated konjac gel dropped from-11.9℃to-13.4℃while the thawing temperature increased at first and then decreased in this process with increasing degree of deacetylation.3. The concentration of the initial KGM, the freezing temperature and the amount of alkali all affected the microstructure of the Da-KGM gel, as these factors are associated with the growth of ice crystal. Rheology experiments of frozen Da-KGM sol indicated the freeze concentration and squeezing effect due to growth of ice crystals promoted self-aggregation of Da-KGM molecules resulting in decrease of entanglement among glucomannan molecules.4. Substances that may affect intermolecular forces such as urea, SDS or NaCl were added during the gel preparation. The TPA and peptization reulsts suggested not only hydrogen bonding but also hydrophobic interaction were the maintain force of the frozen Da-KGM gel.5. In order to increase the porosity of KGM material fabricated by freezing process and expand the application scope of frozen KGM gel, surfactant template method combined with freezing process was utilized to prepare KGM porous materials. The addition of SDS affected the porosity of the porous material, absorption rate of distilled water and saline. The freezing conditions (time and temperature) have impact on the structure and properties of KGM porous materials. The tensile strength of the foam increased while the break elongation and porosity decreased with the extension of the freezing time. The tensile strength of the foam frozen at-10℃was obviously higher than that of other group and reached a maximum tensile strength (49.12KPa) when the freezing time is9d. The porosity is higher when the freezing temperature is higher. XRD and DSC analysis indicated freezing induced change of crystal structure and thermal properties of KGM.6. The composite PVA-ε-PL antibacterial material based on konjac porous material could slowly release ε-PL and exhibited good inhibition effect against Staphylococcus aureus and Escherichia coli. Adsorption experiments of tannic acid showed the maximum adsorption capability of tannin from aqueous solution was653.97mg/g. Adsorption kinetic parameters of tannic acid by konjac porous material were better described by pseudo-second-order equation. Tannic acid adsorption by this porous material is more favorable at low temperature. Isotherm experiments indicated tannic acid adsorption by this porous material is a multilayer adsorption process. On this basis, a nano Ag-loaded foam was prepared by in situ reduction of Ag+using the tannin acid adsorbed on konjac porous material and exhibited highly effective catalysis activity and strong antibacterial ability. These studies mentioned above can lay the foundation for the exploitation of a class of degradable biological antibacterial, catalytic and multilayer adsorption porous materials.7. KGM-montmorillonite composite aerogel can be prepared using the facile freeze-drying method. The obtained composite aerogel is white with a low density (<0.09g/cm3). Addition of sodium montmorillonite (Na+-MMT) into the aerogel greatly improved the mechanical properties and thermal stability and changed microstructure of the aerogels. The compression modulus and specific modulus of the composite aerogel increased with increase of alkali within the system, and the addition of alkali improved the thermal stability of the composite aerogel. The compression modulus and specific modulus of the composite aerogel gradually increased with decreasing of freezing temperature when the initial content of KGM and Na+-MMT were fixed. When the initial content of KGM and Na+-MMT were2wt%,4wt%respectively, and pre-freezing temperature was-80℃, the compression modulus and specific modulus of composite aerogel reached a maximum value in the experiment, namely3.480MPa and53.80MPa cm3/g, respectively.更多还原