球磨修飾不同直鏈澱粉含量之玉米澱粉的外觀形態、內部結構及理化性質

Abstract

本試驗旨在探討不同直鏈澱粉含量之玉米澱粉經球磨處理後對澱粉顆粒形態、分子結構、熱糊性質及熱性質之變化的相關性。以行星式球磨機與瑪瑙球磨珠對糯性、一般及高直鏈玉米澱粉進行研磨，使用掃描式電子顯微鏡(scanning electron microscopy, SEM)、廣角度X-ray繞射分析儀(wide angle X-ray diffractometer, WXRD)及小角度X-ray散射分析儀(small angle X-ray scattering, SAXS)探討球磨玉米澱粉之顆粒形態及結晶結構之特性。並使用高效能分子篩層析儀(high-performance size-exclusion chromatography, HPSEC)估算其直鏈及支鏈澱粉之重量平均分子量及分布，並以去分支酵素(isoamylase)水解澱粉，測量其支鏈澱粉之分支鏈長分布。以調幅視差熱掃描熱分析儀(modulated differential scanning calorimetry, MDSC)分析澱粉樣品之熱性質，及以快速糊液黏度測定儀(rapid visco analyser, RVA)測定澱粉樣品之糊液黏度性質。由SEM觀察發現，球磨對於高直鏈玉米澱粉的破壞有限，對於糯性及一般玉米澱粉的破壞則較為顯著。隨著球磨時間的延長，糯性及一般玉米澱粉的分子量有顯著下降，而高直鏈玉米澱粉則無顯著變化。推測球磨對於低直鏈澱粉含量之樣品有較高的研磨效率。然而在去分支球磨糯性及一般玉米澱粉樣品中，前者之支鏈澱粉分支鏈長分布於球磨前後無顯著差異，球磨1及2 hr之一般玉米澱粉之支鏈澱粉分支鏈長分布則有顯著差異，其直鏈澱粉含量下降且DP~18之鏈段含量上升，推測球磨能造成直鏈澱粉以及支鏈澱粉於非結晶區中的鏈結斷裂。糯性及一般玉米澱粉在經過2 hr研磨後，其MDSC圖譜中的吸熱峰消失，RVA測試則發現，球磨澱粉糊液的尖峰黏度、最終黏度及回凝黏度皆下降。此外，球磨澱粉(於54% RH平衡之樣品)的WAXD及SAXS之繞射鋒皆隨著研磨時間的延長而下降。以上結果顯示，球磨能有效降低澱粉的結晶度及澱粉分子之雙股螺旋結構的整齊排列程度，其中球磨對於糯性玉米澱粉有最高的破壞程度。

The purpose of this study was to investigate the effect of different amylose contents on the granular morphologies, molecular structures, pasting and thermal properties of ball-milled corn starches. Waxy, normal and Hylon VII corn starches were ball milled with a planetary ball mill. The surface appearance, granular morphologies, and crystallinities of the ball-milled starches were characterized with scanning electron microscopy (SEM), wide angle X-ray diffractometer (WXRD) and small angle X-ray scattering (SAXS). The weight-averaged molecular weight of amylose and amylopectin as well as the chain length distribution of isoamylase debranched amylopectin of the ball-milled starches were determined with high-performance size-exclusion chromatography (HPSEC). The thermal and pasting properties of the ball-milled starches were obtained respectively with modulated differential scanning calorimetry (MDSC) and rapid visco analyser (RVA). The SEM investigation revealed that after ball milling, the damage of Hylon VII corn starch was limited, while that of waxy and normal corn starch was significant. As ball milling time prolong the molecular weight of normal and waxy corn starch decreased gradually, while Hylon VII corn starch was remained the same. It suggests that ball milling was more effective in reducing the starch with lower amylose content than higher amylose content. However, in the sample of debranch ball-milled waxy and normal corn starches, the chain length for waxy corn starch didn’t have significant difference whether ball-milled or not, but normal corn starch did. The content of amylose of ball-milled normal corn starch was decreased, while the content of chain length with DP~18 was increased for 1 or 2 hr ball milling. It might be because of the ball milling will cleavage the amylose and the chains in amorphous region of amylopectin. For normal and waxy corn starch, the gelatinization enthalpies disappeared after ball milling for 2 hr, and the peak viscosity, final viscosity, breakdown viscosity, and set back decreased after ball milling. In addition, the WAXD and SAXS peaks of all these starches (conditioned at 54% RH) decreased while the ball milling time increased. These revealed that the ball milling was effective in reducing the crystallinity and the double helical order arrangements of all the starches, among which the waxy corn starch had the highest degree of disintegration.