Internal microstructures of amylopectin and their contribution to the physical properties of waxy-cereal starches

Abstract

Amylopectin, one of the two major components of starch, is known as an assemblage of molecular arrangement of a cluster. In this study, typical A-type starches, waxy corn, waxy rice, waxy barley, and waxy wheat starch, were partially hydrolyzed to isolate the clusters using α-amylase. These clusters were further hydrolyzed to the building blocks, small and tightly branched units of the amylopectin, and analyzed to figure out the characteristics of internal microstructure of amylopectin. This study aimed to investigate the effects of internal microstructure of amylopectin on the physical properties, such as relative crystallinity, thermal, pasting, and rheological properties. The degree of polymerization of clusters where external residues had been removed by phosphorylase and β-amylase was similar to each other, and the a- to b-chain ratio ranged from 1.11 for waxy corn starch to 1.40 for waxy barley starch. The internal-microstructural properties of four waxy-cereal starches were categorized into two types. Waxy corn and waxy rice starches presented a relatively small amount of building blocks, a smaller density, and long inter-block chain length in clusters. Meanwhile, waxy barley and waxy wheat starches were composed of a large amount of building blocks, and showed a higher density and shorter inter-block chain length in clusters. Therefore, a difference in the arrangement of double helices in crystalline lamellae was predicted. The disordered double helix content estimated by X-ray and 13C CP/MAS NMR was lower in waxy corn and waxy rice starches, but was higher in waxy barley and waxy wheat starches. This result indicated that external double helices of amylopectin of waxy barley and waxy wheat starches were disorderly arranged because large number of building blocks were connected by short distance in internal chains of amylopectin. In onset and peak temperatures determined by differential scanning calorimetry, earlier gelatinization of disordered double helices was observed. Differences were also shown in pasting temperature derived from Rapid Visco Analyzer and viscoelasticity measured by rheometery. In conclusion, the density of building blocks and chain length between building blocks in amorphous lamellae can affect the arrangement of double helices in crystalline lamellae and the internal microstructure of amylopectin can cause different physical properties of starch.