酵素法製備不同鏈長脂肪酸修飾樹薯澱粉之性質探討

Abstract

本研究以天然樹薯澱粉為反應基質，與三種不同鏈長飽和脂肪酸(羊蠟酸(C10:0)、肉荳蔻酸(C14:0)與硬脂酸(C18:0))分別於水相及有機相行酯化反應，並且以工業中常用的脂肪水解酶lipase PS (from Burkholderia cepacia)催化酯鍵合成，可得脂肪酸修飾澱粉(fatty acid modified starch, FAMS)。後續對產物性質探討的試驗中，首先以鹼水解滴定法與核磁共振光譜測定FAMS之取代度，並以FTIR圖譜辨別澱粉與脂肪酸之間酯鍵的生成，接著以光學顯微鏡與掃描式電子顯微鏡(SEM)觀察產物外觀的變化與澱粉結晶區特有的十字偏光影像，再以X-ray繞射圖譜鑑定酯化修飾前後澱粉結晶區的變化，最後測定FAMS之糊液黏度、熱性質與接觸角，探討物化性質的改變與脂肪酸鏈長的關聯。結果顯示，於有機相系統製備的FAMS取代度皆高於水相系統製備者，取決於酵素催化的反應種類與澱粉型態的不同，相較於天然樹薯澱粉，所有的FAMS在核磁共振光譜中於0.85、1.22-1.67與2.2 ppm產生新的波鋒，在FTIR圖譜1730-1750 cm-1亦產生新的吸收峰，兩者皆直接證實天然澱粉與脂肪酸的酯鍵生成。由光學顯微鏡與SEM觀察FAMS的結果可知，水相製備流程不會破壞澱粉結晶區，所有澱粉粒仍保有完整的十字偏光現象，但是表面有些微損傷。X-ray繞射圖譜顯示，以未純化的酵素溶液製備之FAMS於22.0°出現新的特徵峰，推測是殘留於FAMS之游離脂肪酸所造成。從FAMS糊液黏度曲線來看，起始糊化溫度(PT)隨著脂肪酸鏈長增加而上升，尖峰糊液黏度(PV)隨著脂肪酸鏈長增加而降低。由接觸角測量的結果可知，FAMS錠片與蒸餾水探針的接觸角大於天然澱粉粒與天然澱粉溶出物，表示酯化修飾提升了澱粉的疏水性，而且提升的程度隨著脂肪酸鏈長增加而上升，但是與FAMS取代度無顯著關聯。

Starch esterification was proceeded with native cassava starch (NCS) and three saturated free fatty acids (FFA): capric acid (C10:0), myristic acid (C14:0) and stearic acid (C18:0). The fatty acid modified starch (FAMS) was prepared by using lipase PS (from Burkholderia cepacia) as the catalyst. To characterize FAMS, the degree of substitution (DS) of FAMS was determined by both alkaline titration and NMR methods and the ester bonds formed between hydroxyl groups of NCS and carboxyl groups of FFA were conformed by FTIR. The granular morphology and crystalinity of FAMS were examined by using the optical, polarized and scanning electronic microscopes and X-ray diffractometry. The pasting and thermal properties of FAMS were also investigated by using RVA and DSC. The surface polarity of FAMS was assessed by the contact angle measurement. FAMS prepared in the DSMO system had higher DS value than those prepared in the buffer system. Comparing with NCS, FAMS had new chemical shift peaks at 0.85, 1.22-1.67 and 2.2 ppm in 1H-NMR spectra and new absorbance at 1730-1750 cm-1 in FTIR spectra, which confirmed the ester bonds formed between hydroxyl group of starch and carboxyl group of FFA. Esterification process did not significantly destroy the starch granular morphology and its crystallinity, while slight damages could be investigated on the starch granule surface. The new diffractive absorbance (22.0°) in X-ray diffractograms revealed that FFA residues were found in FAMS prepared with commercial enzyme solution and colleced by using the centrifuging method. The increases of pasting temperature (PT) and decreases of peak viscosity (PV) were found in FAMS, which were positively related to the chain length of FFA but DS. The high PT, low PV, small or no BkD (breakdown), low FV (final viscosity) and small SB (setback) were the characters of C14:0 and C18:0 FAMSs. The results of contact angle measurements revealed that FAMS prepared in both buffer and DMSO systems were more hydrophobic than cassava starch, and the hydrophobicity of FAMS increased with the increase of chain length of FFA.