【作者】 刘兴训；

【导师】 余龙；

【作者基本信息】 华南理工大学 ， 淀粉资源科学与工程， 2011， 博士

【摘要】 淀粉的热降解在食品加工与非食品加工中是一个普遍现象,例如在淀粉基食品的油炸和烘焙过程中,淀粉基食品和材料的挤出加工中也都会产生热降解。另一方面,通过淀粉的热降解也可得到各种有用的产品,如糊精等。因此,全面理解淀粉的热降解和热稳定性对于食品的热加工,淀粉及淀粉基材料的生产与性能评估和淀粉基废料的热处理与能源转化都有着重要的工业应用意义。另外,研究淀粉的热降解和热稳定性对于完善淀粉的相变理论,深入理解淀粉的化学改性有着重要的理论意义。人们对于淀粉的热降解和热稳定性已进行了相当广泛和深入的研究,但是这些成果并不能满足以淀粉为原料的新型产品的开发,同时一些基础性问题依然存在争议。淀粉热降解和热稳定的复杂性源于其结构的复杂性和亲水性（保持一定的水分含量）。不同于合成高分子,淀粉来源于天然植物,其结构取决与植物的种类和种植环境。淀粉从化学结构上可分为两大类:直链淀粉和支链淀粉。淀粉中的链支比对于淀粉的物理化学性能起关键作用。另外,淀粉基产品中的水分含量对其性能也起着重要作用。因此,通过系统的研究一系列不同链支比的同一种天然淀粉,找出其热降解和热稳定的规律,将对该领域的研究起指导作用。另一方面,以往对于淀粉的热降解和热稳定的研究都是在开放环境中进行的,最常用的方法就是热重分析仪（TGA）。在实际中,淀粉的热加工很多是在密闭环境中,因此,建立一个能够研究淀粉在密闭体系中的热降解和热稳定的方法将对这一领域的研究具有突破性的意义。本论文以不同链支比的天然玉米淀粉（0/100,蜡玉米; 23/77,普通玉米; 50/50,高直链玉米G50; 80/20,高直链玉米G80）为模型材料,采用热重分析仪（TGA）、傅里叶红外光谱（FTIR）、核磁共振（NMR）、热裂解-气质联用（Py-GC/MS）、热重气质联用（TGA-GC/MS）、热重质谱联用（TGA-MS）、热重红外光谱技术（TG-FTIR）系统地研究了不同链支比的淀粉在开放环境中的热稳定性行为及热降解机理;首次通过构建封闭环境,利用差示扫描量热仪（DSC）研究了淀粉在恒定水份中的热稳定性行为;利用凝胶色谱-多角度激光光散射技术（GPC-MALLS）和密炼机研究了剪切力环境下淀粉的分子量的变化。通过新的研究方法的建立,对不同链支比的玉米淀粉的热稳定性进行了系统的研究,主要成果如下:1、多尺度系统的研究了玉米淀粉的链支比对其结构的影响,在颗粒结构上,光学显微镜和SEM的研究发现高支链淀粉颗粒比高直链淀粉在形状上更规则,随着直链淀粉含量的增多,颗粒尺寸越来越小;在半结晶层结构上,SAXS,FTIR,FT-Raman结果表明直链淀粉含量影响淀粉的晶体结构,随着直链淀粉含量的增大,SAXS光散射的强度降低,半结晶层重复单元的厚度（d-space）减少,晶体的相对结晶程降低,有序程度减少,原因是直链淀粉影响了淀粉结晶区的生成;在链结构水平上,GPC-MALLS结果表明随着直链淀粉含量的增多,淀粉的分子量降低。DSC和激光共聚焦显微镜（CLSM）表明直链淀粉的含量影响淀粉的糊化行为,在一定的增塑剂含量下,淀粉的糊化温度随着直链淀粉含量的增多而上升。2、使用TGA系统的研究各种因素,如初始水分含量、热降解气氛等,对不同链支比的玉米淀粉的热降解行为的影响。研究结果表明,淀粉在开放环境中的热降解行为与升温速率,热降解气氛和直链淀粉的含量有关,初始水分含量并不影响淀粉的热降解温度。淀粉的热降解温度随着直链淀粉含量的增大而减少。相比惰性气氛,氧气的参与加速了淀粉的热降解,并在550°C左右处出现了灼热燃烧（glowing combustion）。基于自由模型的活化能的计算结果表明,在惰性环境下,其热降解活化能随着直链淀粉的含量增多而降低。3、使用FTIR,NMR和TGA-FTIR,TGA-MS,TGA-GC/MS和Py-GC/MS等先进仪器对淀粉降解过程中的固体残渣和气体产物进行系统的定性或者定量的分析。研究结果确认淀粉在降解过程中会产生呋喃、小分子的酮醛物质以及CO₂,CO,H₂O等气体,并定量描述了CO₂,CO,H₂O等随着温度的变化趋势。该结果表明淀粉的热降解一个复杂的多步的化学反应过程,主要的机理可能是分子内或者分子间的脱水造成的自由基反应。4、通过DSC不锈钢高压盘构建封闭环境,在世界上首次利用DSC研究了不同链支比的玉米淀粉在封闭环境下的热降解行为。这一封闭系统可以保证升温过程中恒定水份。结果表明淀粉在封闭环境下有两个热降解温度,第一个在相对低的温度,为淀粉分子链的断裂,第二个在相对高的温度,为葡萄糖环的热降解。淀粉在水分含量保持一致的情况下,在大约260°C时会发生氧化和降解反应,葡萄糖在该温度的热降解验证了该论断。另一方面,淀粉在较低的温度下表现为分子链的断裂,随着直链淀粉含量的增多,降解温度移向低温区。DSC结果还表明热降解行为受加热速率和水分含量的影响,水分含量加速了淀粉的热降解行为,其原因是水在高温下提供了大量的自由基,加速了淀粉的自由基降解反应。5、利用带双转子的密炼机研究了淀粉在剪切力下的相变行为,并利用GPC-MALLS对不同挤出时间收集的淀粉样品进行分析。结果表明密炼机可以用于淀粉的相变研究,初始温度影响淀粉的最终扭矩和最终温度。GPC-MALLS结果表明,高直链淀粉（G80）和高支链淀粉（Waxy）在剪切力下发生分子量的降解,高温加速了分子链的降解程度。高支链淀粉的降解速率大于高直链淀粉,该结果可能与支链淀粉的分子尺寸和支叉结构有关。6、按照AS ISO 14855生物降解性能测定方法和TGA热降解法研究了淀粉和木粉对聚乳酸的生物降解性能和热降解性能的影响,使用TGA-FTIR研究这些有机填料的热降解产物对于聚乳酸热稳定性的影响。结果表明,聚乳酸/淀粉共混物和聚乳酸/木粉共混物生物降解速率要低于纯纤维素但是高于纯聚乳酸。淀粉和木粉都可以加速聚乳酸的降解,主要是因为淀粉和木粉在降解过程中产生的极性基团,如CO, CO₂, H₂O, C₂H₄O₂和CH₂O等可以加速聚乳酸分子链的断裂。另外,聚乳酸/淀粉共混物的热降解温度要低于聚乳酸/木粉共混物。一个原因是淀粉的热降解温度低于木粉。淀粉的尺寸较小,和聚乳酸的接触面积更大,是加速混合物的热降解的另一个可能原因。更多还原

【Abstract】 The thermal decomposition of starch is an important and common issue for food and non-food industries. It has been widely observed during thermal processing of foods, such as frying or baking of starch-based foods, and extrusion of starch-based foods and materials. On the other hand, the thermal decomposition of starch can also be used to produce some useful products, such as dextrin. The study of the thermal decomposition and stability of starches has great commercial benefits as it will provide a useful practical guideline to manufacture starch-based products, and to predict the properties of these products. On the other hand, the study of thermal decomposition of starch also has significant scientific importance since it will help to further understand the phase transitions and decomposition mechanisms of starch during thermal processing, and mechanisms of starch modifications.Actually the thermal decomposition of starch has been widely and extensively studied. However, the results and conclusion cannot meet the requirements of developing new starch-based products. Due to its complexity starch exhibits characteristics of complex thermal stability. Unlike synthetic polymers, starch derived from natural plants and its structure depends on the species and growing environments of plants. Starch can be chemically fractionated into two types of glu-can polymer: amylose and amylopectin. The amylose/amylopectin ratio plays a critical role in the chemical and physical properties of starch. Therefore, the investigation of the mechanisms on the thermal decomposition of same kind of starches with different amylose/amylopectin ratios can provide a useful practical guideline to other types of starch. In the other hand, previous research works mainly focus on the thermal decomposition and thermal stability based on an open system, such as thermogravimetric analysis （TGA）. Development a new method to monitor the thermal decomposition and thermal stability of starch in a sealed system can make a significant breakthrough in the thermal process area as most of thermal processes are performed in a sealed system. In this dissertation, the native cornstarches with different amylose/amylopectin ratio （0/100, Waxy; 23/77, Maize; 50/50, G50; 80/20, G80） was used as a model materials. The TGA, Fourier Transform Infrared Spectrometry （FTIR）, Nuclear magnetic resonance （NMR）, Pyrolysis Gas Chromatography Mass Spectrometry （Py-GC/MS）, TGA-GC/MS, TGA-MS, TGA-FTIR was used to study the thermal decomposition behavior and the thermal decomposition mechanism in an open system; High-pressure stainless steel pans with a gold-plated copper seal were used to establish the sealed system, the thermal decomposition behavior with constant moisture in a sealed system was firstly studied by Differential scanning calorimetry （DSC）; The Gel Permeation Chromatography-Mutil-Angle Laser Light Scattering （GPC-MALLS） was used to investigate the molecular weight under shear system which was provided by a mixer with twin-roller rotors as a function of different extrusion time. The new methods has been developed and applied to study the starches. The main achievements can be concluded as following:1. The physical and chemical properties of a certain starch depend on its amylose/amylopectin ratio. The SEM and optical microscopy were used to further study the granule structure of starch. The results show that the granules of the amylopectin-rich starches were more regular in shape than those of amylose-rich starches, and the particle size of granules were waxy>maize>G50>G80. The SAXS, FTIR and GPC-MALLS show that the intensions of SAXS decrease with increased amylose content; the d-space （repeat distances of an amorphous and crystalline lamellae）, relative crystallinity, order of starch and molecular weight decrease with increasing amylose content. The results of DSC and confocal laser scanning microscopy （CLSM） revealed that the gelatinization of starch was a multi-phase process, and affected by plastics, such as water or glycerol. Generally, the gelatinization temperature decreases with increasing amylose content.2. TGA has been widely used to study the thermal decomposition of starch in an open system. TGA results show that heating rate, heating condition and amylose content can affect the thermal decomposition temperature. Under inert gas condition, dehydration and decomposition have generally been considered as two separate processes associated with the degradation mechanisms of starch, the initial moisture content did not affect the decomposition temperature. Oxygen accelerated the decomposition. When the temperature is more than 550℃glowing combustion will happen. Apparent activation energy of starch decomposition under nitrogen gas condition calculated by Model-free model is Waxy>Maize>G50>G80.3. The FTIR and NMR was used to character the finial products after thermal decomposition of starch, while the advanced technologies such as TGA-FTIR, TGA-MS, TGA-GC/MS, Py-GC/MS were used to study the gases from thermal decomposition. The result showed that some small molecular such as ketone, aldehyde, and CO. CO₂, H₂O were produced during the thermal decomposition process. The main decomposition mechanism of starch is probably the free radical reaction which is introduced by the dehydration reaction between starch hydroxyls.4. DSC with high-pressure stainless steel pan sealed by gold-plated copper to form a sealed system, was firstly used to study the thermal degradation of starch in a sealed system. The system keeps moisture constant during heating. The result showed that the two decomposition temperatures were observed in the sealed system: the first at lower temperature represents long chain scission; and the second at higher temperature involves decomposition of the glucose ring. The starches were oxidized and decomposed at temperatur≥e 260℃with constant moisture content while the decomposition of glucose appeared at the same temperature. On the hand, the endotherm at lower temperature was assumed to be resulted from the breakage of chains. Glucose did not show this endotherm, which supports this conclusion. The temperature of the chain decomposition increases with increasing amylopectin content, which may be due to the higher molecular weight and the stable microstructure of amylopectin. DSC results also showed that the water accelerated the thermal decomposition of starch, which could be explained by the free radical created due to higher temperature.5. Hakke mixer with twin-roller rotors was used to study the rheological properties and phase transition of starch under shear condition. The GPC-MALLS was used to determine the molecular weight of starch during the different extrusion time. The result shows that the initial temperature affects the finial torque and temperature. The GPC-MALLS shows that the molecular weight of both G80 and waxy can decrease with increased extrusion time, the initial temperature accelerate the degradation of starch. Moreover, the degradation rato of waxy is high than G80. It maybe due to the larger molecular size and branched structure of waxy.6. Effect of starch and wood flour on the bio- and thermal degradation of polylactic acid （PLA） was studied by composting under controlled conditions in accordance with AS ISO 14855 and TG respectively. TG-FTIR was used to investigate the effect of degraded products from these fillers on the thermal degradation of PLA. It was found that the biodegradation rate of PLA/starch blends and PLA/wood-flour composites were lower than that of pure cellulose but higher than that of pure PLA. Both starch and wood-flour can accelerate the thermal decomposition of PLA by releasing chemicals, in particular, those with polar groups, such as CO, CO₂, H₂O, C₂H₄O₂ and CH₂O, which act as chain scissors for PLA. The lower decomposition temperature of starch compared with that of wood-flour resulted in the lower decomposition temperature of PLA/starch blends compared with that of PLA/WF composites. In comparison with wood-flour, the smaller particle size of starch also accelerated the decomposition of PLA as it provided a larger contact interface with the PLA matrix, which enhanced its function.更多还原