奈米/次微米澱粉粒子的製備、特性、穩定與交聯反應之研究

Abstract

澱粉是天然量大且可再生之植物資源，由於其分子量大與結構的緊密使其不容易進行反應，造成利用其為原料所生產的生物可分解包材機械性能不佳，限制其應用與推廣。奈米化是增加澱粉表面積與修飾表面性質的好方法，可以增進反應活性提高交聯度，進而改善澱粉薄膜的機械性質。 為了證明此假說，本研究第一部份進行奈米/次微米澱粉粒子的製備與特性分析，使用介質研磨製備樣品，應用示差熱掃描分析儀與動態流變儀分析粒徑與分子量對相關物化特性之影響。粒徑分析結果顯示經過30分鐘研磨，數目平均粒徑會從9.61降低至0.26 μm，顯微觀察也證實奈米/次微米澱粉粒子的存在。介質研磨提供的機械能會導致澱粉顆粒破碎，而產生超過43.7%的澱粉損傷與55%的糊化度，並且重量平均分子量會從9.98 ×106降低至7.63 ×106 g/mole。經過研磨處理之澱粉，由於糊化度的提高造成糊化溫度與所需熱焓的降低，分子量降低表現對熱與剪切作用穩定之流變性質，而高度澱粉損傷也改變膨潤力與水溶性，顯示可以利用介質研磨修飾澱粉分子量進而改變其功能性。 第二部份進行奈米/次微米澱粉粒子的表面性質與顯微形態研究，利用表面元素分析儀、表面電位儀、核磁共振儀與X光粉末繞射儀來量測奈米化澱粉顆粒的表面性質與結晶構型。應用靜態雷射光粒徑儀與掃描式電子顯微鏡分析其粒徑分佈與顯微形態，進而探討可能之研磨機制。研磨120分鐘後，體積平均粒徑由17.3降低至0.7 μm且帶有-12 mV表面電位，表面積與顆粒數分別增加25與15,000倍，並且表面氧元素增加60%使碳氧比由1.04 降至0.84，雙股螺旋含量與相對結晶度分別降低70%與60%，並伴隨內生性黏度由181降低至98 mL/g。由粒徑分佈與顯微形態顯示研磨機制可能為先剝離再粉碎，粒徑降低與所造成的高度澱粉損傷可能使結構改變而增加表面羥基數目，顯示奈米/次微米澱粉應能增加澱粉反應活性。 第三部份進行奈米/次微米澱粉粒子的穩定化研究，以平均粒徑、表面電位、離心與濁度穩定性篩選適當乳化劑後，發現5% w/w脂肪酸甘油脂的添加有助於奈米/次微米澱粉懸浮液之穩定。根據顯微觀察、繞射圖譜等分析，顯示穩定之機制可能是因為複合物的形成產生靜電立體作用，因此研磨180分鐘後可得到體積平均粒徑0.3 μm表面帶-16 mv之研磨樣品。由粒徑分佈結果也顯示，乳化劑的添加可以提高研磨效率與奈米粒子產率，並且不會影響介質研磨之粒徑降低機制。研磨所造成之澱粉損傷(30分鐘研磨達33.8%)雖較未添加組低，但仍造成顆粒完整性喪失與相對結晶度的變化，同樣影響其糊化溫度與熱焓，雖然水溶性指標與膨潤力沒有顯著差異，但一樣也顯著影響澱粉成糊與流動性質。 第四部份進行奈米/次微米澱粉對交聯反應與澱粉薄膜機械性質影響研究，藉由示差熱掃描分析儀、動態流變儀來量測交聯後澱粉的熱與流變性質變化，以及電子顯微鏡觀察薄膜形態，質地分析儀量測機械強度，了解奈米/次微米澱粉可以增進反應活性(提高交聯度)，進而改善澱粉薄膜的機械性質。結果顯示以三偏磷酸鈉作為交聯劑進行交聯反應，澱粉的交聯程度隨研磨與交聯時間的增加而增加(反應300分鐘後，研磨90分鐘的樣品較未研磨交聯程度提高約3.7倍)，表示奈米化確實可以增進反應活性而提高交聯度，而粒徑對交聯反應速率的影響主要在前30分鐘。在熱性質方面，結果顯示尖峰溫度與熱焓値隨交聯程度的增加而增加，但粒徑越小交聯程度的影響越不顯著。在流變性質(流體流動與成糊特性)顯示，隨交聯程度的增加其糊化與回凝的黏度也隨之降低。交聯反應可提升澱粉薄膜的機械強度，但是延展性仍嫌不足。甘油的添加則可更強化澱粉薄膜的延展性，但是其機械強度則會些微下降。 以上數據顯示，透過介質研磨的物理修飾作用，可以降低分子量與結晶性，增加表面積與官能基，進而提高反應性與增進薄膜機械性質。

Starch is an abundant and renewable plant resource but it is quite inert to reaction due to its high molecular weight and granular crystallinity. Thus, starch films exhibit poor mechanical properties that limit the utilization of starch as biodegradable packaging. Nanonization would be beneficial for increasing reactivity of starch with increasing surface area and surface properties modification and thus enhancing the cross-linkages of starch molecules to improve the mechanical properties of starch films. In order to prove the hypothesis, the first part of the research was to explore the feasibility for preparing nano/submicron starch particles and understanding the change of functionality after size reduction. Media milling was employed to prepare the sample with different operating parameters. The effects of particle size and molecular weight on the related physicochemical properties of milled starch were also studied. HPSEC-MALLS-RI, DSC and dynamic rheometer were utilized to determine the molecular weight, thermal and rheological properties of milled products. After 30-min milling, the number average diameter of starch particle was reduced from 9.61 to 0.26 μm. SEM/TEM observation confirmed the presence of nano/submicron starch particles. As starch granules being disintegrated, the mechanical energy imparted by media milling resulted in more than 43.7% starch damage and 55% degree of gelatinization. The weight average molecular weight was reduced from 9.98 ×106 to 7.63 ×106 g/mole. The milled starch exhibited lower gelatinization temperature and heat for gelatinization probably were due to an increase in degree of gelatinization. The reduction of molecular weight let the rheological properties of milled samples become more shearing and thermal stable, and it also affected the swelling power and water soluble index by increasing the degree of starch damage. The result showed that it was possible to manipulate the molecular by media milling and to modify the functional properties of starch. The second part of the research was to study the surface properties and morphology of nano/submicron starch particles made by media milling. ESCA, Zetasizer, NMR and XRD were utilized to determine the surface properties and microstructure of milled starch. Particle size distribution and morphology were examined by SLS and SEM in order to explore the size reduction mechanism of starch products prepared by media milling. The volume average diameter was reduced to 0.7 μm with a surface charge of -12 mV after being milled for 120 min. The milling resulted in an increasing of surface area around 25 folds and number of particles about 15,000 times. On the surface, the oxygen content was increased 60% and the ratio of carbon to oxygen was decreased from 1.04 to 0.84. The milling also resulted in loss 70% of double helix content and 60% of relative crystallinity along with the reduction of intrinsic viscosity from 181 to 98 mL/g due to mechanical degradation. The mechanism of milling was associated with surface erosion/shattering and appeared to follow the concept of amylopectin cluster. Size reduction and starch damage lead to molecular dislocation would increase the hydroxyl group on surface that could enhance the reactivity of starch. The third part of the research was to stabilize the nano/submicron starch particles. After screen the suitable emulsifier according to the average particle size, surface charge, centrifugation and turbidity stability, the addition of 5% w/w monoglyceride was found to give the best result. The volume average diameter was reduced to about 0.3 μm with a surface charge of -16 mV after being milled for 180 min. The change of transmission electron micrograph, XRD diffraction and DSC thermogram showed that the stabilized mechanism probabily was due to the formation of inclusion complex with electrosteric effect. The addition of emulsifier also can increase the efficiency of size reduction and the yield of nano particles without affecting the milling mechanism by examined the PSD. The starch damage was decreasing when compared with the milled starch without emulsifier addition (33.8% at 30 min) but the thermal and rheological properties changed with the same tendency, and the swelling powder and water soluble index didn’t show significantly difference between them. The final part of the research was to evaluate the cross-linking and mechanical properties of starch films made of nano/submicron starch particles. The thermal and rheological of cross-linked starch, morphology and mechanical properties of starch films were studied with DSC, dynamic rheometer, SEM and texture analyzer. The results showed that the degree of cross-linkage was increased with the milling and reaction time by using SMTP as reagent (the degree of cross-linking for 90-min milling sample increased about 3.7 times when compared with native starch after 300-min reaction) that confirmed nanoization can enhance the reactivity, particularly significant for the first 30 min of milling. The increasing of cross-linkage will increase the peak temperature and enthalpy for thermal property but decrease the gelatinized viscosity and setback for flow and paste properties. Cross-linkage could improve the tensile strength and the elong at break, with the addition of glycerol can further increase the elong at break but would decrease the tensile strength. The study confirmed that physical modification can be done by media milling with controlling of molecular weight and crystallinity. The reactivity of starch can be increasing by increasing the surface area and functional group to improve the mechanical properties of starch films.