Physicochemical properties of corn starch from microwave-assisted modification in acetic acid solution
|Keywords:||玉米澱粉;微波加熱;醋酸;酯化;corn starch;microwave heating;acetic acid;esterification||Issue Date:||2010||Abstract:||
酸處理與乙醯酯化為兩種常用的澱粉修飾方法，酸修飾可降低糊化澱粉黏度，而乙醯酯化修飾，可以降低糊化溫度、增加澄清度、黏度及冷凍解凍穩定性。傳統乙醯酯化修飾方法於鹼溶液中進行反應，不但反應時間長，且會產生多量廢液。因此本論文嘗試於醋酸溶液中以微波加熱方式修飾澱粉，希望利用微波之電磁波促使極性分子震盪，利用極性分子間的摩擦，在短時間提高溫度，達到修飾澱粉的目的。本實驗探討利用不同濃度醋酸溶液(0 (水、對照組), 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 17.44 N)浸潤玉米澱粉，於粉體狀態下進行微波處理30秒，發現各試驗組產生的水解程度低，澱粉流失率皆為2 %以下，即使是於17.44 N (99.7 %)的冰醋酸中澱粉顆粒仍無顯著的水解液化。以光學及電子顯微鏡觀察修飾後的澱粉顆粒，發現醋酸濃度範圍為0 N (控制組) ~ 10 N的各試驗組，澱粉顆粒均產生明顯的膨潤與十字偏光性消失現象，且隨醋酸濃度增加程度愈加明顯，在5 N及10 N是驗組多數澱粉顆粒已看不到十字偏光性；此澱粉顆粒結晶度降低現象也可由X射線繞射及示差熱掃描分析結果得到證實。當醋酸濃度範圍為0 N (控制組) ~ 10 N的各試驗組，在示差熱掃描分析中熱焓值顯著下降，並有顯著的回凝吸收峰產生，顯示澱粉顆粒產生部分糊化。以連續糊化測定儀觀察於醋酸中進行微波處理之澱粉的成糊性質，發現各處理組別有不同程度成糊黏度下降現象，1.0與2.0 N 試驗組的澱粉有類似稀沸澱粉的成糊性質。進一步利用低剪力的動態流變分析，觀察加熱過程澱粉顆粒膨潤性質，結果顯示5 N及10 N兩處理組在較低溫度(< 55℃)時澱粉顆粒已產生膨潤現象，且由高效能分子篩層析分析觀察到澱粉分子產生較顯著的降解，但仍保持澱粉高分子量的特性。在高濃度的10.0 N及17.44 N兩試驗組，微波處理對澱粉顆粒影響不顯著，顯示經微波處理後仍保持較完整的結晶型態。因此使用濃度10.0 N及17.44 N兩試驗組添加醋酸酐進行酯化反應，以30秒時間的微波處理其乙醯酯化取代度分別可為0.004與0.008。
Acid treatment and acetylation are two common modifications used in food industry. Acid modification reduces the paste viscosity of starch and acetyl esterification decreased pasting temperature and increased clarity and stability of paste and of its freeze-thaw cycles. The traditional acetylating reaction conducts in alkali condition and it is a time consuming process and generates large volume of waste solution. This thesis attempted to establish a microwave assisted reaction model for starch modification in acetic acid solution. The hypothesis is using microwave energy that causes a rise in the temperature within starch granules as a result of rapid changes of the electromagnetic field at high frequency to achieve the goal of modification. The reactions were conducted on damped powders of starch granules that were impregnated with different concentration of acetic acid solution (0 (water as control), 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 17.44 N) and were heated in a domestic microwave oven for 30 seconds. All samples showed low hydrolysis, the loss ratio of all starch samples to be smaller than 2%, even in glacial acetic acid (17.44N, 99.7%). By microscopic examination, swelling and birefringence loss of starch granules significantly appeared as concentration of acetic acid solution in the range of 0 ~ 10.0 N. The change extent increased as the concentration of acetic acid increased. The majority of starch granules lost their birefringence in the experimental group of 5.0 and 10.0 N. The decreasing of crystallinity was verified by x-ray diffraction and differential scanning calorimetric (DSC) analysis. This microwave assisted modification in acetic acid solution caused partially gelatinization of starch granules indicated by enthalpy reducing and the appearance of retrogradation endothermic peak in DSC analyses. The enthalpy of starch gelatinization significantly decreased as the increasing of concentration of acetic acid in the range of 0 ~ 10.0 N. The microwave assisted modification also reduced the pasting viscosity of starch with different extent. The pasting profile of starch prepared from 1.0 and 2.0 N experimental groups were similar to the one of thin-boiling starch. Dynamic rheological analysis showed that starch granules of 5.0 and 10.0 N had been swollen under low temperature (<55℃), starch molecules degraded more significant when compare to other treatments, but still maintain high molecular weight. In the experimental groups of high acetic acid concentration (15.0 and 17.44 N), the effects on starch became less significant and the granules remained intact. The acetic acid was used as solvent to conduct an acetylation reaction of starch with acetic anhydride. The degree of substitution of starch acetate was 0.004 and 0.008 for using 10.0 and 17.44N acetic acid, respectively, with 30 seconds microwave heating time.
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