以氧化和交聯雙重修飾糯性玉米澱粉製備pH應答型微凝膠

Abstract

本研究擬分別以TEMPO (2,2,6,6-tetramethyl-pyperidine-1-oxyl)-NaClO-NaClO2氧化系統及以三偏磷酸鈉(sodium trimetaphosphate, STMP)製備氧化及交聯糯性玉米澱粉；再以經氧化及交聯雙修飾之糯性玉米澱粉製備pH應答型微凝膠。氧化澱粉之糊液黏度測定結果顯示，當澱粉與氧化劑濃度提高時，澱粉氧化度亦有相對提高的趨勢；當氧化度提高時，其糊化起始溫度降低、尖峰黏度提高，且以最高劑量組合(S500T20，澱粉及TEMPO濃度分別為5%及0.2 mmol/g starch)最為顯著。熱性質分析結果與糊液黏度測定結果相似，支鏈澱粉雙股螺旋結構之解構溫度(onset temperature, To; peak temperature, Tp)隨著澱粉濃度和氧化劑劑量提高而降低，熱焓值(enthalpy, ΔH)亦隨氧化度之提高而降低，尤以S500T20之熱焓值降低最為顯著。氧化澱粉之分子結構分析結果顯示，隨著氧化度的提高，澱粉分子有明顯降解情形；當於高氧化劑量(T20)時，澱粉分子除發生降解外，也不排除可能有分子鏈間的交聯反應，導致出現氧化度越高，測定之分子量反而提高。由FTIR圖譜中之-C-O-C-鍵結特徵峰(1319 cm-1)亦可證實，除了在澱粉及氧化劑量濃度均低的組合(S050T07，澱粉及TEMPO濃度分別為0.5%及0.07 mmol/g starch；S050T10，澱粉及TEMPO濃度分別為0.5%及0.1 mmol/g starch)外，高濃度組合確實同時產生氧化與交聯反應。交聯澱粉試驗之結果顯示，當反應時間長於60 min，STMP交聯劑與澱粉之間傾向交聯反應，並隨著交聯劑濃度的提高，相對交聯程度亦顯著提高，此可由其偏光性增強及熱性質To與Tp提高證實。雙修飾澱粉試驗結果顯示，無論雙修飾順序如何，相同氧化條件之雙修飾澱粉有相近的氧化度，但先氧化後交聯之樣品(OS-CS)，其顆粒完整性低，並喪失糊液黏度。雙修飾澱粉的膨潤力主要受第二次化學修飾影響顯著，而溶解度則與經第一次修飾後的澱粉特性相似。因此，於微凝膠試驗時，為確保澱粉粒之膨潤完整性，故以先交聯再氧化處理(CS-OS)之雙修飾澱粉進行。先經交聯(CS015-60)再氧化(S500T20)所製備微凝膠(CSH-OS)，以90oC預糊化後，其澱粉粒套膜仍可適當維持。初步試驗結果顯示，微凝膠之體積(以膨潤力與收縮率為指標)與流動性(以流變性質為指標)會受環境pH值和離子強度影響，此與應用相關之性質值得再深入探討。

This study intends to prepare two types of single chemical modified starch, which are the oxidized and the cross-linked waxy corn starches by using TEMPO (2,2,6,6-tetramethyl-pyperidine-1-oxyl)-NaClO-NaClO2 oxidation system and trimetaphosphate (STMP), respectively. Furthermore, the dual modified waxy starch were prepared by either oxidizing first followed by cross-linking or the reverse. Finally, the characters of pH-responsive microgels made of dual modified waxy corn starch were investigated. It found that the gelatinization temperature decreased and the peak viscosity increased aas the degree of oxidation (DO) increased. The highest DO of the oxidized starch could be obtained in the reaction condition containing 5% starch and 0.2 mmol / g starch of TEMPO (S500T20). The de-association temperatures, onset temperature (To), peak temperature (Tp) and the enthalpy of the amylopectin decreased as the concentrations of starch and oxidant increased as well. The averaged molecular weught (Mw) of oxidized starch deceased significantly, indicate that the degradation of molecules occurred, while the higher Mw of the oxidized waxy corn starch with higher DO was investigated. This is attributed to the cross-links between inter- or intra- starch molecular chains through the acetyl formation of carboxyl group in aldehyde of oxidized glucose unit and the hydroxyl group in glucose unit. This is evidenced by characteristic FTIR absorption peak of C-O-C bond (1319 cm-1). For the cross-linked starches made of STMP, the cross-linking was obvious when reaction time was above 60 min, and the relative degree of cross-linking was significantly increased with the increased with the increased amount of STMP. The cross-linked starch had more intensive polarized signal and higher To and Tp determined by differential scanning calorimetry (DSC). The similar DO of dual modified starches could be obtained under the same reaction condition (i.e. concentration of starch and oxidants) no matter what the order of modification (cross-linking followed by oxidation (CS-OS) or reverse (OS-CS)). The dual modified starch (OS-CS) tended to rupture during gelatinization with very low viscosity. The swelling power of dual modified starch was dominated by the first chemical modification, while its solubility was dependent on the second modification. The microgel (CSH-OS) made of pregelatinized (at 90oC) dual modified waxy corn starch, which was prepared by cross-linking (CS015-60) first followed by oxidizing (S500T20) show the intact envelope of swollen starch granule. This microgel shows the pH-responsive behavior of swelling and shrinkage and the characteristic rheological properties. The specific properties of the microgel prepared in this study is worthy of further investigation for practical application.