浆液体系中木薯淀粉乙酰化反应均匀性及基团分布特征

Acetylization uniformity and distribution of acetyl group of cassava starch granule in aqueous slurry systems

以木薯淀粉为原料、乙酸酐为酰化剂,在浆液体系中制备乙酰化淀粉,采用4.3 mol · L^-1 CaCl2溶液对乙酰化淀粉颗粒进行化学表面糊化处理,获得不同表面糊化程度的剩余淀粉颗粒,通过SEM、XRD表征剩余颗粒的形貌及结晶结构,采用皂化法测定剩余颗粒的乙酰基含量,并利用最小二乘法拟合取代基含量在淀粉颗粒径向上的分布曲线,考察乙酰化反应均匀性和乙酰基分布的影响因素。结果发现:乙酰基团在淀粉颗粒中呈现外高内低的非均匀分布,30%以上的乙酰基分布在对比半径0.9~1.0的外部区域,而在0~0.7比较大的范围内仅占28%左右;反应温度升高、反应时间延长和酰化剂用量增大,乙酰基含量在淀粉颗粒内的分布离散程度减小,有利于乙酰化反应和乙酰基团分布更趋均匀;随着表面糊化程度增大,剩余颗粒均能保持木薯淀粉的形貌和“A”型结晶结构特征,但粒径稍有减小,结晶度整体呈现下降趋势,部分稍有突跃。以上结果说明,结晶结构与非晶结构在木薯淀粉颗粒内交替存在,且各区域结晶区和非晶区比例存在一定差异,适当改变反应条件可改善木薯淀粉颗粒内乙酰化反应和乙酰化基团非均匀分布的状况。

Using cassava starch as raw material and acetic anhydride as acylating agent, acetylated starch was prepared in the slurry system, and acetylated starch granules were chemically surface-gelatinized with 4.3 mol·L^-1 CaCl2 solution to obtain different surface gelatinization degree. The morphology and crystalline structure of the remaining granules were characterized by SEM and XRD. The content of acetyl group in the remaining granules was determined by saponification method, the distribution curve of substituent content in the radial direction of starch granules was fitted by least square method, and the factors influencing the homogeneity of acetylation and the distribution of acetyl group were investigated. The results showed that the distribution of acetyl groups in starch granule was inhomogeneous which was external high and internal low, more than 30% of the acetyl groups were distributed in the external region of the contrast radius from 0.9 to 1.0, while only 28% of the acetyl groups were distributed in the larger range from 0 to 0.7;with the increase of reaction temperature, reaction time and the amount of acylating agent, the dispersion degree of group distribution in starch granule decreased, which was favorable for more uniform of acetylation and distribution of acetyl groups. With the degree of surface gelatinization increased, the remaining particles could still maintain the morphology and“A”type crystalline structure of cassava starch, but the particle size decreased slightly, the crystallinity generally showed a downward trend while some of them had a sudden jump. The above results indicate that alternating crystalline and amorphous structures exist in the granule of cassava starch, and the proportion of crystallization and amorphous zones in various regions is different. Appropriate changes in reaction conditions can improve the inhomogeneous status of acetylation and distribution of acetylation groups in the granule of cassava starch.