利用交聯化改良普魯藍薄膜之物理特性

Abstract

普魯藍是由黑酵母菌 (Aureobasidium pullulans) 所生產的胞外多醣，以麥芽三糖為單元透過α-1,6鍵結組成線性結構。純化過後的普魯藍能夠以鑄模法或熱擠壓法製成薄膜，具有透明、無毒和生物可分解的性質，並提供良好的機械性質與氣體阻隔性，可做為食品包材的選擇，惟普魯藍薄膜易溶於水的特性導致其在包裝上的應用受限。本研究利用戊二醛作為交聯劑，能夠以較溫和的反應條件與普魯藍分子上的羥基反應形成縮醛的化學鍵結，藉此加強薄膜的耐水性與物理性質，並配合醱酵高分子量普魯藍與塑化劑添加分析這些因子對於薄膜的影響。以不同交聯劑濃度處理以觀察薄膜性質之變化並利用傅立葉轉換紅外線光譜分析結構，結果顯示當交聯劑與普魯藍重量比例達0.01g/g時便可使薄膜耐水性顯著提升，並且隨著提高交聯劑濃度能夠進一步提升耐水性，此外提高交聯劑濃度的同時也抑制了薄膜吸水的程度，顯示交聯鍵結確實形成，這些現象可由FT-IR圖譜中O-H振動吸收波峰 (3300 cm-1) 強度減弱獲得印證。在低交聯劑濃度 (0.005-0.015, g/g) 時，薄膜機械強度隨之增加，然而，交聯劑濃度高於 0.025 (g/g) 以上時，機械強度反而降低，此時薄膜過於硬脆不利於使用。醱酵普魯量分子量為2,570 kDa，大於商業生產普魯藍分子量，實驗結果發現以高分子量普製膜可以在各項性質中獲得較好的表現，例如較好的機械強度以及較佳的水氣阻隔能力；添加甘油於薄膜中可有效提升薄膜的延展性，卻也增加了薄膜的親水性。而甘油的添加會干擾低濃度交聯劑的反應，因此未來可針對交聯劑與甘油的比例進行最適化的調整，以實際應用於食品包裝上。

Pullulan is an extracellular polysaccharide originated from Aureobasidium pullulans. The linear structure of pullulan consist of maltotrioses with α-1,6 glycosidic linkages. The purified pullulan films formed by casting or extruding methods possess several advantages including high transparency, low toxicity, good biodegradability, good mechanical properties, and low oxygen permeability, which are preferable features for packaging of food. However, the applications of pullulan films on packaging are limited due to its high solubility in water. In this study, glutaraldehyde was used as cross-linking reagent to improve the water resistance and physical properties of pullulan film. It reacts with the hydroxyl group on pullulan under mild reaction condition, forming acetal linkages between molecules of pullulan. The glycerol as plasticizer and high molecular weight fermented pullulan previously investigated in our lab, were also applied in this study to evaluate their functions and mechanisms in film forming. Effects of cross-linking reagent with different concentration on physical properties, including water absorptions, swelling behaviors, water vapor permeability and tensile strengths of films were determined, and the chemical structure of cross-linked pullulan film were also analyzed by Fourier transform infrared spectroscopy (FT-IR). When the ratio of crosslinking reagent to pullulan reaches 0.01 g/g, the water resistance was improved significantly. Better resistance to water and less swelling degree were observed when crosslinking reagent increased. The decreasing O-H stretching vibration (3300 cm-1) of films in FT-IR spectra indicate a higher degree of crosslinking was formed with increasing concentration of crosslinking reagent. The mechanical properties increased at 0.005-0.015 g/g of glutaraldehyde per gram of pullulan, and start to decrease when higher than 0.025 g/g of glutaraldehyde per gram of pullulan was used. However, the fragile property in films with high degree of cross-linking is unfavorable for industrial uses. In addition, the fermented pullulan possesses a high molecular weight of 2,570 kDa, which is much higher than commercial pullulan, and it exerts better performance in all properties we had tested. The addition of glycerol as plasticizer enhanced the extensibility of films as well as the hydrophilicty, resulting in higher water vapor permeability. It might be attributed from the disturbance of crosslinking. In summary, the crosslinking strategy effectively increase the weakness of pullulan films, yet needs futher studies such as optimization of the ratio of crosslinking reagent/glycerol for future application on food packaging.