4. 以相分離法製備微膠囊改善囊壁耐熱性
4.2 以調 pH 值法製備微膠囊
以三仙膠、褐藻膠或HPMC 等多醣類，分別與魚皮明膠混合，以調 pH 值方式製備微膠囊。發現三仙膠或褐藻膠與魚皮明膠在 70℃混合，經調整 pH 至魚皮明膠等電點以下，皆可發生相分離現象，下層為澄清液，上層為 白色的絮凝物(圖二十二 C)。由於絮凝物的產生，是因為在低於明膠等電點 的環境下，帶淨正電荷比例隨pH 值降低而增加，因此帶正電荷的明膠和帶 負電荷的陰電性多醣類兩種相反電荷的大分子，可藉由靜電交互作用產生
凝聚情形(Zhang et al., 2001；Haug et al., 2003)。Haug 等(2003)亦指出，魚皮
明膠和 κ-鹿角菜膠(κ-carrageenan)混合膠體，在魚皮明膠的 pH 值小於等電 點和低離子強度下，有利於兩分子間凝集(agreegation)，並有相分離現象，
尤其在接近兩聚合物的規則/不規則排列(order/disorder)之相轉移溫度(60℃) 最易發生。
HPMC 與魚皮明膠混合液在 pH 值 4-5 時，則整個系統呈現乳白色的乳 化液，未包覆的浮油在液面大量出現(圖二十二 A)，顯示無法形成囊壁有效 包覆油脂。以立體顯微鏡觀察相分離後的絮凝物，發現魚皮明膠與三仙膠 絮凝物呈現較分散的雲霧狀，魚皮明膠與褐藻膠絮凝物呈現聚集的團狀，
魚皮明膠與HPMC 並無發現絮凝現象，呈現混合後的乳化球滴(圖二十三)。
這些現象導因於 HPMC 為非離子型的纖維衍生物(Sovilj and Petrovic, 2006)，無法與帶正電荷的魚皮明膠進行靜電交互作用，因此沒有明顯發生 相分離情形，楊(1990)亦發現與 HPMC 結構和特性類似的羧甲基纖維素，
與明膠混合後調整pH 值，並無明顯相分離的情形，也有大量浮油之現象。
以三仙膠與魚皮明膠為囊壁物質包覆蠟時，亦可發生相分離現象，下 層為澄清液，上層為稍黃色的絮凝物(圖二十二 C)，以位相差顯微鏡觀察微 膠囊時，呈現團狀物，並無明顯的顆粒存在(圖二十四)，與上述經鹽析及固 化後，包覆蠟所製備之微膠囊結果類似，亦顯示蠟並不適合在相分離方法 製備微膠囊，因此後續實驗以沙拉油作為微膠囊核心物質。以三仙膠與魚
皮明膠包覆沙拉油的微膠囊呈現圓滑完整的球型，大小約5-20µm(圖二十五 A)，褐藻膠與魚皮明膠為囊壁物質的顆粒較小(約 0.5-5µm)、分佈較平均，
但呈現較不規則的顆粒狀(圖二十五 B)。以 HPMC 與魚皮明膠為囊壁物質 時，發現有未包覆的油滴(圖二十五 C)，進一步證實選用魚皮明膠和 HPMC
做為囊壁物質時，因二者之間無法以調整pH 值方式有效的結合，導致包覆 效果差。
魚皮明膠與三仙膠以調 pH 值法製備之微膠囊，由 SEM 可觀察到由許 多微膠囊聚集構成較大的凝集體(圖二十六)，顯然這些微膠囊在相分離後，
微膠囊間的囊壁物質仍可繼續交互作用，在凍乾時容易發生聚集及堆疊的 現象。
以DSC 測定多醣類與魚皮明膠為包覆材質之微膠囊熱安定性時，發現 以魚皮明膠與三仙膠製備的微膠囊囊壁融解溫度約134℃(圖二十七 A)，高 於以噴霧乾燥法製備的微膠囊囊壁融解溫度。沈(2006)認為造成明膠與多醣 體為囊壁物質的微膠囊具有熱安定性的原因，是由於pH 值調到約 4 左右，
使明膠帶正電荷，與陰電性多醣產生靜電交互作用，並且二者都能存在於 界面，而穩定微乳化的耐熱性。Tolstoguzov(1991)則推測提供此混合凝膠囊 壁物質熱安定性的原因，是混合陰電性多醣與蛋白質於水溶液中時，由於 陰電性多醣的親水性比蛋白質高，其分子會趨向包圍或纏繞於蛋白質之外 部，而具有保護作用。
但是由圖二十七B中發現，魚皮明膠混合褐藻膠的囊壁物質，其融解溫 度仍只有130℃左右，推論是多醣類分子中單醣的組成和結構不同，因此在 特性上和蛋白質的交互作用表現不同，三仙膠較其它陰電性多醣類有較高 的分子量(3×105至8×106 Da)、耐酸水解性及耐熱性(Steve, 2005)。楊(1990) 以明膠和三仙膠、褐藻膠、鹿角菜膠、果膠等七種多醣類，以調整pH值法
進行相分離，製備熱安定性高之不溶性香料微膠囊，也發現明膠及三仙膠 為製備微膠囊最好的囊壁物質。
由於以調整pH 值法進行相分離製備的微膠囊，熱安定性仍低於多層乳 化液以鹽析方法製備之微膠囊，因此在製備魚皮明膠與多醣為囊壁之微膠 囊時，調整pH 值相分離後，進一步以 2%TG 催化魚皮明膠分子之間的交聯，
固化囊壁物質，發現囊壁的融解溫度提高至140℃以上(圖二十七 C, D)，顯 示 2%TG 促進魚皮明膠交聯可提高微膠囊熱安定性，此現象與鹽析後的絮 凝物，再將魚皮明膠囊壁經交聯後可提高其耐熱性的結果一致。
為瞭解魚皮明膠與多醣調整pH值產生靜電作用而相分離後，是否仍能 與其他耐熱性物質形成複合囊壁，實驗中另設計在魚皮明膠與三仙膠相分 離後，加入幾丁聚醣溶液，冷卻後再以三聚磷酸鈉於低溫下催化魚皮明膠 與幾丁聚醣之間的交聯，所製得微膠囊，吸熱峰位置可提高至約 154℃(圖 二十七E)，顯示添加幾丁聚醣交聯後形成的複合囊壁可提高微膠囊耐熱 性。盧等(2003)發現單獨明膠在 134.81℃有一個相當明顯而尖銳的波峰，而
當明膠中加入幾丁聚醣相互混合，在低溫與低pH值下，利用三聚磷酸鈉進 行交聯，形成複合膜材後，吸熱峰往高溫移動(Tm=163.5℃)，可以相互增加 其熱安定性。在本實驗中所測得微膠囊融解溫度，並不如預期結果明顯提 高耐熱性，推論原因為魚皮明膠與三仙膠進行相分離時部分正電荷已用於 與三仙膠結合，因此利用三聚磷酸鈉與幾丁聚醣產生的離子交聯反應量受 限。
伍、結論
魚皮明膠在添加HPMC 並靜置交互作用後，凝膠能力增強，熱安定性 提高，且混合凝膠可保持原有魚皮明膠的冷凝膠和HPMC 的熱凝膠性質，
適合作為耐熱性微膠囊的囊壁物質。
以魚皮明膠及HPMC 混合凝膠為親水相，蠟為疏水相製備的乳化液，
經噴霧法製得的微膠囊耐熱性仍低，此與魚皮明膠分子和HPMC 間缺乏強 力的鍵結或交互作用力有關。因此，以魚皮明膠為囊壁物質，利用鹽析進 行相分離所製備的微膠囊，經交聯劑固化後，使魚皮明膠分子藉由交聯作 用形成安定的囊壁結構，提高微膠囊耐熱性。若進一步以魚皮明膠和HPMC 的混合凝膠為囊壁物質，製備而成多層囊壁微膠囊，在本研究中呈現最高 的耐熱性。顯示添加熱凝膠的HPMC，加上促進魚皮明膠分子交聯的 TG，
對提升高溫下微膠囊的結構安定性，要比明膠及陰電性多醣體利用靜電作 用形成的囊壁有較好的效果。
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