檸檬酸修飾澱粉之製備與澱粉膜物化性質分析

Abstract

本研究以檸檬酸(citric acid, CA)對一般玉米澱粉(normal corn starch, NS)經二階段加熱處理進行澱粉修飾，並取雙酯化程度(degree of di-esterification, DDE)為4.81、7.93及10.45%之檸檬酸修飾澱粉(citric acid modified starch, CS)進行澱粉膜製備，而為了提升檸檬酸修飾澱粉之成膜性質，在製膜過程中分別以鹼及高溫進行處理。第一階段低溫(60℃)熱處理於pH 4.5下反應4 hr後，可達最高尖峰糊液黏度(peak viscosity, PV)，顯示此條件下檸檬酸已與澱粉進行取代反應，使水合能力上升。二階段加熱處理製得之檸檬酸修飾澱粉分析結果顯示，隨著第二階段乾熱處理溫度與時間提升，單取代程度(degree of mono-esterification, DME)逐漸下降，雙取代程度逐漸上升，且尖峰糊液黏度逐漸下降，顯示修飾效果逐漸由取代轉為交聯反應；十字偏光性質減弱，糊化熱焓值與相對結晶度下降，顯示檸檬酸修飾過程會破壞澱粉結晶結構；澱粉糊精化反應(dextrinization)更加劇烈，造成澱粉鏈斷裂，分子量降解。以鹼處理製備之檸檬酸酸修飾澱粉膜，雙取代程度下降至1.68-2.65%，顯示雙酯鍵在鹼性環境下會幾乎被完全水解；鹼處理使檸檬酸修飾澱粉膜抗拉強度(tensile strength, TS)、斷裂伸長率(elongation at break, EB)及水氣滲透性(water vapor permeability, WVP)下降，透光度(light transmittance)上升，且隨著檸檬酸修飾澱粉雙酯化程度提升，趨勢更為顯著，此乃因澱粉分子量降解程度提高，膜內澱粉分子結構差異所致。以高溫處理製備之檸檬酸酸修飾澱粉膜，雙取代程度下降至6.91-12.90%，顯示雙酯鍵在高溫處理下會斷裂，但仍有一定程度的保留。高溫處理後，檸檬酸修飾澱粉膜相較於一般玉米澱粉膜抗拉強度較高，此乃因交聯反應形成澱粉鏈之間架橋所致；而斷裂伸長率提升可歸因於降解後的小分子澱粉，作為塑化劑提升澱粉鏈移動性；疏水性雙酯鍵的存在則會增強水氣阻隔性；透光度隨著檸檬酸修飾澱粉樣品雙酯化程度提升，呈現先升後降之趨勢，顯示雙酯鍵數量多者，較不易受高溫處理而完全糊化，膜內較大之澱粉分子易造成光散射。

The study is aimed to prepare citric acid (CA) modified starch (CS) with normal corn starch (NS) through two-stage heating treatment, and the CS with degree of di-esterification (DDE) of 4.81, 7.93, and 10.45% were chosen for starch films preparation. To enhance the film-forming property, alkaline and high temperature treatments were applied during film-forming process. After the first stage low temperature (60℃) heating treatment under ph 4.5 for 4 hr, the highest peak viscosity (PV) could be obtained, suggested that CA had start substituting to NS, and the hydration abilities were increased. As the treatment temperature and time period of the second stage dry henting treatment increased, DME, PV decreased, and DDE increased, indicating that the efficiency of cross-linking was gradually larger than substitution; the brightness of birefringent pattern diminished, the enthalpies and crystallinities decreased, which suggested that the modification of CA altered the crystalline structure of NS; the dextrinization of starch was more significant, which resulted in the break of starch chains. Analyzing the alkaline treated CS films, DDE decreased to 1.68 to 2.65%, which suggested that the di-ester bonds were almost completely hydrolyzed. In addition, the tensile strength (TS), elongation at break (EB), and water vapor permeability (WVP) decreased as DDE of CS increased because the degrees of starch molecules degradation enhanced, which leads to the structure difference of molecules in films. Analyzing the high temperature treated CS films, DDE decreased to 6.91 to 12.90%, which suggested that the di-ester bonds were partially hydrolyzed. The high temperature treatment leaded to the increase of TS of CS films due to the cross-links formed between starch chains. The enhancement of EB could be attributed to the degraded starch molecules served as plasticizers to improve the chain mobility. Besides, the di-ester bonds increased the water barrier property. In addition, the light transmittance increased initially and then decreased as the DDE of CS increased, which suggested that the CS with more di-ester bonds was difficult to completely gelatinized and the larger starch molecules in films contributed to the light scattering.