Thermal Decomposition Of Starch And Starch Based Materials

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.