Understanding the structural features of high-amylose maize starch through hydrothermal treatment

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Abstract

In this study, high-amylose starches were hydrothermally-treated and the structural changes were monitored with time (up to 12 h) using scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). When high-amylose starches were treated in boiling water, half-shell-like granules were observed by SEM, which could be due to the first hydrolysis of the granule inner region (CLSM). This initial hydrolysis could also immediately (0.5 h) disrupt the semi-crystalline lamellar regularity (SAXS) and dramatically reduce the crystallinity (XRD); but with prolonged time of hydrothermal treatment (≥2 h), might allow the perfection or formation of amylose single helices, resulting in slightly increased crystallinity (XRD and DSC). These results show that the inner region of granules is composed of mainly loosely-packed amylopectin growth rings with semi-crystalline lamellae, which are vulnerable under gelatinization or hydrolysis. In contrast, the periphery is demonstrated to be more compact, possibly composed of amylose and amylopectin helices intertwined with amylose molecules, which require greater energy input (higher temperature) for disintegration.

Introduction

Starch is a biopolymer that naturally exists in the form of granules in plants; and the granule is structured in a highly complex way, which has not been fully understood so far [1], [2], [3]. Starch has great potentials in a diversity of applications, with some new applications in foods (resistant starch, microporous starch, etc.) [4], [5], pharmaceutical materials (drug delivery systems) [6], [7], [8], and biodegradable plastics (starch-based materials) [9], [10], [11]. A clear understanding of starch structures and how its structures change under different treatments is indispensable for the utilization of starch with desired properties.

It is worth noting that in many applications like those abovementioned, high-amylose starch (a type of starch by genetic modification) is preferably used. High-amylose starch has some special properties, such as its heat resistance [12], which is reflected by high gelatinization temperature and the retainment of granules in boiling water [13], [14], [15], and its digestion resistibility [16], [17], due to which this starch has been used as resistant starch and in drug delivery systems. In addition, high-amylose starch is especially suitable for producing thermoplastic materials [18], [19], because amylose as a linear molecule can provide better mechanical properties resulting from easier formation of crystallites and entanglements. However, when preparing materials, a significant problem would be just the resistance to processing or treatment, because the complete destruction of original starch supramolecular structures is required to form a continuous integrated phase [12], [13], [14], [15]. In other words, due to its special structural organization, native high-amylose starch is more resistant to hydrolysis or disintegration by small molecules such as water, enzymes, etc., which negatively impacts on its application.

These particular behaviors of high-amylose starch can hardly be explained by the theories obtained from waxy and regular starches, of which the structural features have been more intensively studied [1], [2], [3]. Despite the lower crystallinity, high-amylose starches have a rather compact structure without weak points or voids, which may explain those behaviors to some degree [16], [17], [20], [21]. Nevertheless, the reason for such a compact granule organization, and how this compact structure is altered during hydrolysis by e.g. water, are not well understood.

There have already been many studies on the compact granule organization of native high-amylose starches [22], [23], [24], [25], [26], [27]. Leach and Schoch [25] found that different types of starches showed different degrees of enzyme susceptibility, with waxy maize starch granules being the most susceptible and high-amylose maize starch the least susceptible. It has been proposed that amylose concentrated at the periphery of starch granules could interacts with amylopectin to form a hard shell [28], [29], [30], and that amylose could form crystalline structure especially in high-amylose starch granules [31], [32], both of which could make the starch granules more compact. In addition, some researchers [33], [34] showed that there was no correlation between crystallinity and enzyme susceptibility, and the hydrolysis residues were composed of both amorphous material and B-type crystallites; therefore, they proposed that the distribution of B-type crystallites within the granule and their influence on local granule organization is important. However, to the best of our knowledge, there has been no further exploration on the structural organization of native high-amylose starch granules which account for its compactness.

The current study aims to further explore the structural features of native high-amylose starch, which was done by monitoring the structural changes of high-amylose starch during hydrothermal treatment.

Section snippets

Materials and Chemicals

Two varieties of commercially-available maize starches used in this work, Gelose 50 (G50), and Gelose 80 (G80), were supplied by National Starch Pty Ltd. (Lane Cove NSW 2066, Australia). Both of the two starches were chemically unmodified; and their amylose contents were 56.3% and 82.9%, respectively, and their degrees of crystallinity were 31.3% and 28.3%, respectively [35], [36], both as measured previously. 8-Aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS) and sodium

Morphological changes of high-amylose starch after hydrothermal treatment

Fig. 1 shows the morphological images of G50 and G80 before and after hydrothermal treatment for 30 min by SEM. Both native G50 and G80 granules displayed a nearly spherical shape with smooth surface. For some of the granules, a small depression on the surface could be clearly identified. After hydrothermal treatment for 30 min, hollowed or half-shell-like granules could be formed for both G50 and G80, and almost all granules turned into this morphology. The diameter of the pit was about 4–5 μm

Conclusions

By hydrothermal treatment, the current study further explores the special structural organization of high-amylose starch granules responsible for their resistance to gelatinization or hydrolysis. SEM and CLSM images show that the hydrolysis mostly started from the inner region, generating hollowed granules. This initial hydrolysis could disrupt the semi-crystalline lamellar structure as shown by SAXS and dramatically reduce the crystallinity as demonstrated by DSC and XRD. According to the

Acknowledgments

The research leading to these results has received funding from the National Natural Science Foundation of China (NSFC) (Project Nos. 21106023 and 31271942), the “Science and Technology Planning Project of Guangdong Province” (2013B010403030), the “Science and Technology Planning Project of Guangzhou City” (2013J4300043), the “Science and Technology Projects supported by Guangzhou Education Bureau” (Project No. 1201410965), and the “Training Programs of Innovation and Entrepreneurship for

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