Insights into waxy maize starch degradation by sulfuric acid: Impact on starch structure, pasting, and rheological property

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Highlights

  • Sulfuric acid hydrolysis decreases starch molecular size and size distribution.

  • Amylopectin chains with DP > 36 are degraded while those with DP24–36 are formed.

  • Starch crystallinity is decreased while starch granular structure is retained.

  • Molecular causes for the variations of starch viscosity are put forward.

Abstract

This study investigated the effects of acid degradation of amylopectin on the structure, pasting, and rheological properties of waxy maize starch. It is found that: 1) the amount of amylopectin short-chains with degree of polymerization (DP) ~ 15–50 increased while that of amylopectin long-chains with DP ~ 50–200 decreased by acid hydrolysis; 2) acid hydrolysis produced smaller amylopectin molecules with a narrower size distribution; 3) acid hydrolysis had a minor effect on the crystalline and granular structures of native starch; 4) the pasting viscosity of acid hydrolyzed starch during heating and the consistency coefficient, K, of starch gels increased, whereas the flow behavior index, n, decreased. Correlation analysis was used to clarify the molecular causes for the variations of pasting and rheological properties of acid hydrolyzed starch.

Introduction

Starch is the main carbohydrate source for supplying energy for human. It is a natural, renewable, and biodegradable biopolymer which is widely used in food industry. Native starch consists of two glucose polymers: highly branched amylopectin and nearly linear amylose. Starch has a multi-level structure, e.g. the individual branch (level 1), fully branched molecules (level 2), alternating crystalline and amorphous lamellas (level 3), etc. [1]. The multi-levels of structure determine the physicochemical properties, such as swelling, gelatinization, pasting, rheology and retrogradation. However, the natural properties of native starch, e.g. the poor mechanical strength and thermal property, the high susceptibility to retrogradation, limit its industrial applications. Starch modification is necessary to improve its properties and overcome its natural limitations. It commonly involves physical treatment (e.g., annealing, dry heating treatment, and pre-gelatinization), chemical technology (e.g., acid hydrolysis, oxidation, and substitution), and enzymatic modification [2,3].

Acid hydrolysis of native starch is commonly used to produce acid-thinned starch for improving viscosity and making it suitable for specific food applications, e.g., in the manufacture of gelled sweets [4]. Acid hydrolyzed starch is made by soaking starch granules in dilute mineral acid solutions (hydrochloric or sulfuric acid) below the gelatinization temperature for certain periods. Nearly all starches show the typical two-stage pattern: a fast initial hydrolysis rate and then a much slower rate [5]. The acid hydrolysis of native starch is affected by a wide range of factors, such as the starch source, hydrolysis time and temperature, and acid type and concentration. Among these factors, starch source is the inherent one determining the structure and physicochemical property of hydrolyzed starch [4]. One of the most commonly used starch sources around the world is maize starch, including several commercially available varieties differing in amylose content. Normal maize starch contains about 20–30% of amylose, the high-amylose maize starch has about 50%–70% amylose, and waxy maize starch has no amylose and consists mainly of amylopectin [6].

Acid hydrolysis of maize starch with various amylose contents has been performed to clarify the structure and physicochemical properties of acid hydrolyzed starch. For example, high-amylose starch always shows high resistance to acid hydrolysis, whereas low-amylose starch is highly susceptibility to acid hydrolysis, possibly because of the tightness of the double helices within starch crystallites [5]. In two recent studies [4,6], the granular and molecular structures and its corresponding physicochemical properties of acid hydrolyzed maize starch with varied amylose content were explored in details. The results show that both acid concentration and hydrolysis time decrease the molecular mass of starch, the chain length of amylose and amylopectin fractions systematically, indicating a clear genotype-dependent effect [6]. Acid thinning slightly decreases the stability of high-amylose maize starch gels but does not change those of waxy maize starch gels [4]. It has also shown that the degradation of the amylose fraction is associated to a large extent with the degradation of amylopectin. The ratio of Mw,AM/Mw,total starch is nearly constant over a wide range of starch molecular mass range [6]. The multi-scale structural changes of maize starch with different amylose content is also investigated by Chen et al. [7]. The molecular size, chain-length distribution (CLD), and crystalline structure show typical starch variety-dependent effects.

Much effort has been made to further understand the structure and properties of different maize starches in terms of amylose content by subjecting them to acid hydrolysis for obtaining a better industrial performance. However, the presence of amylose prevents a clear understanding of starch degradation features in terms of structure and property. For example, two main populations of starch chains with the peak maximum at degree of polymerization (DP) ~ 13–15 and 25–27 are observed for hydrochloric acid-hydrolyzed starches [3]. Our previous results showed that acid hydrolysis of normal maize starch with sulfuric acid for only 1 h leads to a decreased amount of long amylose chains with DP ~ 500–30,000 and an increased amount of short starch chains with DP ~ 50–500 [2]. For these newly formed short chains during acid hydrolysis, it is difficult to distinguish it originating from amylopectin and/or from amylose chains. Sulfuric acid hydrolysis of waxy maize starch could supply valuable information regarding this question. In this study, waxy maize starch was subjected to various concentrations of sulfuric acid to explore the changes of the multi-levels of structure and physicochemical properties, and to clarify the effects of acid hydrolysis on “structure-property” relationships.

Section snippets

Materials

Waxy maize starch was purchased from the Baolingbao Biology Co., Ltd., Shandong, China. Dimethyl sulfoxide (DMSO) was obtained from Merck Co. Inc. (Darmstadt, Germany) and Isoamylase (200 U/mL, Megazyme China, Beijing) was purchased from Megazyme (Wicklow, Ireland). The pullulan standards (peak molecular ranging from 342 to 2,350,000) were purchased from PSS (Mainz, Germany). All other reagents were analytical grade.

Acid hydrolysis

Acid hydrolysis was carried out following the previous method [2] with

Effects on starch molecular structure

The typical molecular size distributions of native starch and hydrolyzed starch samples are shown in Fig. 1A. Obviously, there is only one peak visible for all starch samples, because they are all waxy maize starch containing only amylopectin. As described elsewhere [2,9], the amylopectin peak is mainly ranging between Rh ~ 1–1000 nm with the peak maximum at Rh ~ 100–200 nm. Table 1 lists the average Rh values, denoted by Rh¯. The values show that acid hydrolysis generally decreases the

Conclusions

This study investigated the impact of sulfuric acid hydrolysis on starch structural, pasting, and rheological property of waxy maize starch. In structural terms, acid hydrolysis decreased the average molecular size and size distribution, producing a narrower polydispersity. Amylopectin CLDs were significantly affected by acid hydrolysis, indicating that long amylopectin chains were degraded while more short-chains were formed. The crystallinity was generally decreased by acid degradation,

CRediT authorship contribution statement

Hongyan Li: Data curation, Validation, Supervision, Paper draft writing; Revision

Minghao Xu: Experiment conduction, methodology, resources

Shu Yan: Experiment conduction, methodology, resources

Ruoxin Liu: Experiment conduction, methodology, resources

Zichu Ma: Revision, methodology, resources

Yangyang Wen: Experiment conduction, methodology, Writing and reviewing, Editing, Revision

Jing Wang: Supervision, Project administration, Writing and reviewing, Editing, Revision

Baoguo Sun: Supervision,

Acknowledgements

This work was supported by National Natural Science Foundation of China (31901729), National Key Research and Development Program of China (2018YFD0400600), School Level Cultivation Fund of Beijing Technology and Business University for Distinguished and Excellent Young Scholars (BTBUYP2020), and High-level Teachers in Beijing Municipal Universities (IDHT20180506).

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