乙醯酯化馬鈴薯澱粉與樹薯澱粉儲藏穩定性之探討

Abstract

乙醯酯化澱粉目前已被廣泛的運用於食品工業當中，作為增稠劑、黏著劑或穩定劑之用。但近來有業者發現乙醯酯化澱粉之加工性質會隨著儲藏時間的增加而變差，此現象在高溫的環境下特別明顯。故本試驗旨在對馬鈴薯澱粉及樹薯澱粉進行乙醯酯化修飾，並對其進行不同溫度之儲藏試驗，以了解儲藏過程中乙醯酯化澱粉之性質改變情形，並藉此近一步了解乙醯酯化澱粉在儲藏過程中性質改變之原因。研究的第一部分利用不同醋酸乙烯酯的添加量對澱粉進行修飾，發現反應試劑的添加量與取代度有良好之相關性，且取代度與澱粉糊液之尖峰黏度與成糊溫度也有良好之相關性，並成功製備取代度約為0.08之乙醯酯化馬鈴薯澱粉(acetalyed potato starch, AP)與樹薯澱粉(acetylated tapioca starch, AT)。第二部分利用自行製備之澱粉於25、35、40與45°C下進行儲藏，發現AP與AT在儲藏過後皆有尖峰黏度下降、回凝黏度下降、pH值下降與電導度值上升之現象，且以上現象皆隨著儲藏溫度與時間的增加而加劇。利用水洗步驟降低AP與AT之電導度後進行儲藏，則上述現象變化更為劇烈，顯示水洗步驟會降低乙醯酯化澱粉之儲藏穩定性，且水洗步驟對AT之影響較對AP之影響為大。第三部分以變化最劇烈之儲存環境，45°C儲藏0天與55天之樣品，進行流變性質、膨潤力、溶解度、分子量、熱性質、結晶性質、表面型態、NMR與表面電荷之觀察與測定。藉由流變性質、膨潤力、溶解度與分子量之測定結果，推測在經過儲藏後，乙醯酯化澱粉之分子結構受到破壞，且以NMR確認乙醯酯化澱粉在儲藏後，有取代度下降伴隨醋酸生成之現象。熱性質與結晶性質之結果發現，乙醯酯化樹薯澱粉在經過儲藏後有澱粉分子重新排列之現象。但表面電荷量下降與澱粉粒表面變化之現象則僅發生在乙醯酯化馬鈴薯澱粉，此現象應是儲蔵期間馬鈴薯澱粉分子上之磷酸根掉落所導致。在天然組澱粉(天然澱粉)與控制組澱粉(修飾環境處理澱粉)的儲藏試驗部分，發現天然馬鈴薯澱粉之糊液黏度、pH值與電導度會隨著儲藏時間的延長而改變，且以控制組馬鈴薯澱粉性質改變最為劇烈，但流變性質、膨潤力、溶解度、分子量、熱性質與表面電荷則與修飾澱粉有相同之變化現象，代表這些性質改變主要發生在修飾後之澱粉因儲存而改變。天然樹薯澱粉與控制組樹薯澱粉在經儲藏後性質皆無顯著變化。最後由ICP-OES測定水洗步驟對澱粉內部離子含量之影響，發現馬鈴薯澱粉可以保留較多之鹽類，且不易被電導度計測到。綜合以上結果，推測乙醯酯化澱粉加工性質之改變可能是因為乙醯基斷裂成為醋酸後，除了增加環境之電導度值與降低其pH值外，更破壞澱粉分子之結構。而醋酸酯化澱粉中存在之鹽類，可以減緩酸釋放之速度，使澱粉之儲藏穩定性提高，因此，澱粉修飾後之過度水洗步驟反而對其儲藏穩定性有不良的效應，尤其是對磷酸根含量較低之樹薯澱粉影響更劇。

Acetylated starches with a relatively low degree of substation (DS) are widely used in the food industries as a thicker, viscosity modifier and stabilizer. Recently, it has been noticed that the viscosity of commercial acetylated potato starch and tapioca remarkably drop during transportation and storage, especially on hot summer days. The aim of this study is to prepare acetylated potato starch and tapioca starch with different amounts of vinyl acetate. Next, stored modify starch at different temperatures to reveal the instability of acetylated potato starch and tapioca starch by monitoring the viscosity, pH value and conductivity charges during storage. Finally, we detected the changes of physicochemical properties of starches after storage to figure out the mechanism or factors caused the instability of acetylated starches during storage. First, we modified potato and tapioca starch with different amounts of vinyl acetate. The amount of vinyl acetate had good correlation to DS, with good correlations to peak viscosity and pasting temperature. Secondly, acetylated potato starch (AP) and acetylated tapioca starch (AT) were stored at 25, 35, 40 and 45°C up to 2 months and their DS, conductivities, pH and pasting properties during storage were investigated. In order to realize if impurities or ions will affect the stability of AP and AT, other two low conductivity counterparts (AP-w & AT-w) were also prepared by continuous washing process. DS and pH of all acetylated starches decreased with time, while the conductivity increased with time during storage. The decreases of PV and SB were observed in all acetylated starches and the rate of decreasing in viscosity increased with the increases of storage temperatures. The changes of DS, pH, conductivity and viscosity of AP-w and AT-w were more obvious than AP and AT at all storage temperatures. Next, the rheology properties, swelling power, solubility, molecular weight, thermal properties, XRD, morphology, NMR and surface charge of AP, AP-w, AT and AT-w before and after 45°C 55 days’ storage were investigated. It was found that molecular structure of all acetylated starch was disrupted during storage. The acetylated starch showed a decrease of DS and followed by producing acetic acid which could be detected by NMR. Besides, AT and AT-w might rearrange during storage which could be detected by DSC and XRD. Surface of AP-w starch granule would become smooth after storage but not in AT-w. Surface charge decreased after storage which might result from the release of phosphate groups from potato starch. Storage test of native starch (NP and NT) and control starch (CP and CT) were also carried out. Both NP and CP were instable during storage, especial for the CP. It could be detected by the decreases of viscosity and pH value and the increase of conductivity, while the other properties were almost the same before and after storage. All physicochemical properties of NT and CT remained the same after storage. It revealed that potato starch was instable inherently and only modified tapioca starch would become instable after modification. Finally, we used ICP-OES to investigate the amount changes of ions before and after washing process. Potato starch could reserve more ions than tapioca starch. To sum up, acetyl group of acetylated starch would be hydrolyzed and produced acetic acid during storage. Acetic acid might disrupt starch molecules, resulting in the changes of physicochemical properties of acetylated starch. Acetylated potato starch was more instable than acetylated tapioca starch, although washing process significantly affected tapioca starch. We proposed that the phosphate groups in potato starch held more counter ions which slow down the degradation of acetylated potato starch molecules.