Lopez-Rubio Amparo, Flanagan Bernadine M, Shrestha Ashok K, Gidley Michael J, Gilbert Elliot P
Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia.
Biomacromolecules. 2008 Jul;9(7):1951-8. doi: 10.1021/bm800213h. Epub 2008 Jun 5.
Resistant starch (RS) is defined as the fraction of starch that escapes digestion in the small intestine, serving as a fermentation substrate for beneficial colonic bacteria. Several studies have been focused on the description of the RS fractions from different starch varieties, but little attention has been paid to the digestion process itself that, from the present work, seems to play a key role in the generation of enzyme-RS (ERS), as determined in vitro. High-amylose starch samples, extruded at two different processing conditions, have been characterized at different stages of in vitro digestion using scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), infrared spectroscopy (FT-IR), solid state (13)C NMR spectroscopy, and X-ray diffraction (XRD). Control samples kept for 18 h in the digestion solution without starch hydrolyzing enzymes (alpha-amylase and amyloglucosidase) were used for comparison purposes. An increase in molecular order was favored by the hydrolytic action of the enzymes, reflected in an increase in double helical order observed by NMR, higher crystallinity measured by XRD, and corresponding changes in FT-IR spectra. An increase in the intensity of the scattering objects was also observed by SAXS as a function of digestion. SAXS from the dry ERS fractions reveals the 001 reflection of crystallites formed during the digestion process, corresponding to a characteristic dimension of the resistant crystalline fraction of approximately 5 nm. The changes found suggest that enzyme resistant starch does not refer to a specific structure present in predigested starches, but may in fact be formed during the digestion process through the rearrangement of amylose chains into enzyme-resistant structures of higher crystallinity. Therefore, the resistance to enzyme digestion of a specific processed starch is the result of a competition between the kinetics of enzyme hydrolysis and the kinetics of amylose retrogradation.
抗性淀粉（RS）被定义为在小肠中未被消化的那部分淀粉，它作为有益结肠细菌的发酵底物。多项研究聚焦于描述不同淀粉品种中的抗性淀粉组分，但对消化过程本身却鲜有关注。而从本研究来看，消化过程似乎在体外测定的酶抗性淀粉（ERS）的生成中起关键作用。对在两种不同加工条件下挤压而成的高直链淀粉样品，使用扫描电子显微镜（SEM）、小角X射线散射（SAXS）、红外光谱（FT-IR）、固态（13）C核磁共振光谱和X射线衍射（XRD）等技术，对其体外消化的不同阶段进行了表征。为作比较，使用了在不含淀粉水解酶（α-淀粉酶和淀粉葡糖苷酶）的消化液中保存18小时的对照样品。酶的水解作用有利于分子有序度的增加，这体现在核磁共振观察到的双螺旋有序度增加、X射线衍射测量的结晶度提高以及FT-IR光谱的相应变化上。SAXS也观察到散射物体的强度随消化过程增加。来自干燥ERS组分的SAXS显示了消化过程中形成的微晶的001反射，对应于抗性结晶组分的特征尺寸约为5纳米。所发现的变化表明，酶抗性淀粉并非指预消化淀粉中存在的特定结构，而实际上可能是在消化过程中通过直链淀粉链重排成更高结晶度的抗酶结构而形成的。因此，特定加工淀粉对酶消化的抗性是酶水解动力学和直链淀粉回生动力学之间竞争的结果。
