微波輔助酸處理對不同直鏈澱粉含量玉米澱粉的理化性質之影響

Abstract

澱粉是顆粒型態的多醣，雖然分散容易但在加熱糊化時黏度極高且糊化後分子不穩定易產生回凝，使應用性受限，因此工業上有許多修飾形式，其中酸水解為常見修飾澱粉的方法，此外酸水解也作為研究澱粉顆粒結構的方法。微波具有穿透性而能整體加熱，選擇性吸收及省時的優點，本研究結合微波輔助減少酸水解時間及廢液產生，以不同濃度鹽酸水溶液(0 N(blank)、0.05 N、0.1 N、0.5 N、1.0 N、2.0 N)調整澱粉顆粒水分含量至34~43 %，加熱30秒(溫度約為56 ℃)，探討玉米澱粉受微波輔助酸影響，結果顯示不同直鏈澱粉含量之玉米澱粉對微波加熱耐受性不同。在水(0 N)下，糯性玉米澱粉、一般玉米澱粉、Hylon V澱粉的溶出率為21.9 %、0.9 %、0.68 %，在酸的作用下，其溶解率以高至低仍為糯性玉米澱粉、一般玉米澱粉、Hylon V澱粉。顆粒上出現部分膨潤及中心臍偏光消失，顯示微波作用在非結晶區；隨著酸濃度增加，偏光更加明顯消失且出現顆粒碎片。在結晶性方面，糯性玉米澱粉在水、0.05 N、0.1 N，一般澱粉在0.05 N、0.1 N、0.5 N出現構型改變且結晶度降低，而後結晶度增加(reordering)由一般玉米澱粉的熱焓值(△H1)作印証，ㄧ般玉米澱粉在0.5 N條件鏈長適合與直鏈澱粉反應而有較高的熱焓值(△H2)，Hylon V澱粉結晶度與熱焓值皆降低。在黏度方面，低剪力下一般玉米澱粉修飾後顆粒產生膨潤而有較高彈力參數(G’)，釋出較多直鏈澱粉而具高setback，在0.5 N G’明顯低於天然澱粉且糊化起始溫度(To)提前，表示澱粉顆粒受到嚴重損傷，1.0 N與2.0 N無G’，冷卻後殘存顆粒或顆粒碎片凝聚產生G’，在高剪力下比起天然澱粉呈現較低的黏度與setback。Hylon V修飾澱粉的G’降低，且酸濃度增加G’與To明顯降低，直鏈澱粉凝膠而產生離水測得最後G’。分子量方面，ㄧ般玉米澱粉與Hylon V澱粉隨酸濃度增加分子量顯著降低，一般玉米澱粉在2.0 N呈現ㄧ窄分子量分佈，推測分子重排成整齊結構，與熱焓和X-ray繞射圖譜結晶強度較高成正相關。

Starch is in granular form in nature and easily to disperse in cold water. After cooked, starch produces high viscosity and tends to retrograde. Acid modification is conducted to overcome the high viscosity and retrograding tendency of native starch for industrial applications and is also one of means to study microstructure of starch granules. Microwave heating is a more efficient process than traditional heating since it ensures homogenous operation on the whole volume of substance, greater penetrating depth, and selective absorption by water in food systems. Microwave assisted acid modification of starch granules was the main theme of this study to reduce the reaction time and acid waste. Waxy, normal and Hylon V (high-amylose) corn starches were impregnated with different concentrations (0 N(control), 0.05 N, 0.1 N, 0.5 N, 1.0 N, 2.0 N) of HCl solution to adjust water content of starch to the range of 34 ~ 43 % before being heated in a domestic microwave oven for 30 seconds. After heating, all sample temperatures were around 56 ℃ which was below the gelatinization temperature of native starch samples. The susceptibility significantly varied among of selected starch samples on microwave assisted acid modification. The solubility of starch granule increased as concentration of HCl increased but varied among different starch samples. Microwave alone without acid was sufficient to cause 21.9 % by weight loss of waxy starch but only had slightly effects on normal and Hylon V starch samples with solubility of 0.9 and 0.68 %, respectively. Increased acid concentration significantly caused higher solubility and reduced the yield, viscosity and apparent molecular weighted of starch. The microwave-heating caused partially swelling and birefringence loss from region around the helium toward periphery of starch granules with significantly corresponding of HCl concentration increasing. Substantial amount of broken and disintegrated debris appeared as 1.0 N and 2.0 N of HCl solutions were used to treat the starch granules. According the size-exclusion chromatographic analysis, the products from high concentration groups (1.0 N and 2.0 N of HCl solution) had been converted to small molecular amylodextrin, and significantly lost their viscosity and gel forming capability on dynamic rheological and Rapid Visco-Amylographic analysis. The microwave energy could alter starch molecular structure and selectively heated on lose packed and amorphous region of starch granules and caused heterogeneous acid hydrolysis revealed by x-ray diffraction and differential scanning calorimetric analysis.