TEMPO 觸媒氧化澱粉與 pH 應答型微凝膠之製備及物化性質

Abstract

本研究將以TEMPO/NaClO系統氧化一般玉米澱粉(normal corn starch, NC)，製備得TEMPO 觸媒氧化澱粉，並保留澱粉套膜，經預糊化與三偏磷酸鈉交聯之方式，將氧化澱粉澱粉粒由無法隨意改變體積之硬粒子，變成在不同條件溶液中具明顯體積變化之軟粒子，稱之為澱粉微凝膠(starch microgel)。此澱粉微凝膠能感應外在環境中pH及離子強度的改變，而產生體積的變化，使其具有包覆攜帶與釋放的功能。試驗中，亦將利用咖啡因及溶菌酶測試其裝載與釋放物質之能力。試驗結果得知，利用TEMPO/NaClO氧化系統能得到較一般NaClO氧化法carboxyl groups 含量較高之氧化澱粉，且與常見之NaClO氧化法相比，尖峰黏度、膨潤力及溶解度皆有顯著提升之現象。在TEMPO/NaClO氧化反應的過程中，會發生澱粉粒結構受損與澱粉分子量降解的情況。其糊液黏度及熱性質測定結果顯示，成糊溫度及起始溫度隨著氧化度提高而有下降趨勢，尖峰黏度、最終黏度及熱焓值亦隨氧化時間延長而有下降趨勢。完全糊化之氧化澱粉糊液，其光穿透度隨氧化度或氧化時間延長有上升趨勢。若在TEMPO/NaClO氧化反應後加入NaBH4，將有助於改善糊液黏度測定時氧化澱粉分子降解之情況。所有氧化澱粉之樣品中，OS1501-BH4及OS2501-BH4註在較高溫度(95°C)下仍可維持穩定之膨潤力，表示其熱穩定性較佳，適合用來製備微凝膠。將OS1501-BH4及OS2501-BH4分別以60°C及55°C預糊化10 min，並加入不同比率之三偏磷酸鈉 (trisodium trimetaphosphate, STMP) 進行交聯試驗。結果顯示，STMP交聯澱粉微凝膠在較高pH (pH 8)及較低離子強度(0 mM)下擁有較佳之膨潤能力。隨著STMP添加量上升，膨潤力則下降趨勢。將STMP交聯氧化澱粉微凝膠裝載不同大小之化合物，例如：咖啡因及溶菌酶時，小分子之咖啡因無法被微凝膠內部之網狀結構所限制，使其自由進出微凝膠，導致不具控制釋放之效果。試驗結果亦顯示，大分子之溶菌酶較適合被澱粉微凝膠攜帶，其若懸浮在低離子強度之溶液中，能保留裝載之70%物質至模擬小腸環境(不含酵素)中才被釋放，以達到控制釋放之效果。 註:OS1501-BH4及OS2501-BH4表示在氧化反應中，NaClO添加量為1.5及2.5 mmol/ g starch，進行氧化反應1h，並加入NaBH4選擇性還原carbonyl groups者。

TEMPO catalyst oxidized starch was prepared through TEMPO / NaClO oxidized system with normal corn starch (NC). After pregelatinization and cross-linking with sodium trimetaphosphate (STMP), the oxidized starch granules changed from hard particles into soft particles which could display significant volume change in different solutions. They could be called microgels. The envelope of oxidized starch starch granules were still retained, so they could be distinguished as independent particles. The starch microgels can sense the change of pH and ionic strength in the external environment, and display significant volume change. They have the ability to load and release substances. Caffeine and lysozyme will be used to test their loading and releasing ability. The experimental results show that TEMPO/NaClO oxidation system can obtain oxidized starch with higher carboxyl groups content than traditional NaClO oxidation method. Compared with the NaClO oxidation method, the peak viscosity, swelling power and solubility were significantly improved. In the process of TEMPO/NaClO oxidation, the structure of starch granumles were damaged and the molecular weight of starch molecules were degraded. As shown in the results of pasting properties and thermal properties, the peak viscosity and initial temperature decreased with the increase of oxidation degree. The peak viscosity, final viscosity and enthalpydecreased with extension of the oxidation time. The light transmittance of completely gelatinized oxidized starch paste increased with the increase of oxidation degree or oxidation time. Adding NaBH4 after TEMPO/NaClO oxidation could improve the degradation of oxidized starch molecules during pasting properties test. OS1501-BH4 and OS2501-BH4* maintained a stable swelling power at higher temperatures (95°C). It indicatedthe better thermal stability. They were suitable for the preparation of microgels. OS1501-BH4 and OS2501-BH4 were pregelatinized at 60°C and 55°C for 10 min, and cross-linked with different ratios of sodium trimetaphosphate. The results showed that microgel displayed higher swelling power at higher pH (pH 8) and lower ionic strength (0 mM). The swelling power decreased with increase of the STMP ratio. STMP cross-linked oxidized starch microgels loaded two different compounds with different sizes. They were caffeine and lysozyme. The experimental results showed that loading content decreased with increase of the STMP ratio. Because of the small size, caffeine couldn’t be stayed inside the microgels. Caffeine entered and exited microgels freely, so it didn’t be controlled release. On the other way, the size of lysozyme was more suitable to be carried by the microgels. If they were suspended in a low ionic strength solution, 70% of the loaded components can be released in simulated intestinal conditions (without enzyme). *OS1501-BH4 and OS2501-BH4 mean that starch was oxidized with NaClO (1.5 and 2.5 mmol / g starch) for 1h. After reaction, NaBH4 was added to selectively reduce carbonyl groups.