Molecular structures and physicochemical properties of native and modified barley starches with various amylose content.

摘要

Traditionally, barley has been used primarily in malting and animal feeds, and to a lesser extent in human foods. In order to increase the utilization of barley in terms of human consumption, it is necessary to understand the physicochemical properties of its constituents. Starch is a major component of barley grain, and its functionality predominately affects the textural properties of barley containing foods. The following study examined the granular and molecular characteristics as well as functional properties of native and modified barley starches varying in amylose content from 0 to 42%.; Normal and waxy types of starch granules exhibited bimodal size distributions, with large (10--25mum) and small (10mum) granules having different relative proportions. High amylose starch granules were distributed unimodally with the highest proportion of granules measuring 3mum. The weight average molecular weight (Mw) of amylopectin and amylose, obtained from high performance size exclusion chromatography (HPSEC) coupled to a multiangle light scattering (MALS) and a refractive index (RI) detection system, ranged from 136 x 106 to 305 x 106 (g/mol) and from 2.73 x 106 to 5.67 x 10 6 (g/mol), respectively. Amylopectin from waxy types of starches had the highest Mw, whereas amylopectin from high amylose starch had the lowest. Amylose and amylopectin polymers from normal barley starch were completely separated using flow-field flow fractionation (flow-FFF) with the application of two cross-flows, 0.35mL/min followed by 0.1mL/min. Choosing an appropriate rate for the second cross-flow allowed for better insights into the molecular weight distribution of amylopectin polymers, and revealed that it had a more compact molecular conformation than that of amylose polymers.; Substantial starch solubilization was observed during alpha-amylolysis of starch granules, with waxy starch solubilized 36--56%, normal starch 24--39%, and high amylose starch 13--20%. On the other hand, relatively little solubilization (9%) was found during the acid/alcohol treatment. Despite the severe granular deformation, especially in the enzyme-treated zero amylose starch, starch polymers retained their macromolecular structures, indicating that both crystalline and amorphous regions were degraded by alpha-amylase. On the other hand, acid appeared to preferentially hydrolyze the amorphous regions, causing significant degradation of starch polymers. These results were further supported by X-ray diffractometry, DSC, and solid-state CP/MAS 13C NMR spectroscopy. The enzyme (50U/g, 24h) and acid (2%HCl/MeOH, 10h) treated high amylose barley starches showed increased potential for gelation.; Starch and beta-glucan blends (5% w/w total carbohydrate concentration; ratio of starch:beta-glucan---100:0, 85:15, 70:30, 55:45) exhibited increases in the apparent viscosity and viscoelastic properties compared to solutions of individual polymers at the corresponding concentrations, indicating some interactions and co-entanglements between starch and beta-glucan polymers. In the high concentration systems (15% w/w total carbohydrate concentration; ratio of starch:beta-glucan---100:0, 95:5, 90:10), the addition of beta-glucan had relatively little effect on the viscoelastic properties of the blends, indicating that the blend networks at higher concentrations were governed mostly by strong junction zones formed by the starch polymers. However, some interactions between starch and beta-glucan polymers were also detected in the modified high amylose starch and beta-glucan blend systems (95:5 blends). The starch digestibility of the blend was inversely proportional to the elastic properties of the networks.