Effects of starchy seed crystals on the retrogradation of rice starch
Introduction
Rice, which primarily consists of starch, is one of the major crops in the world and the staple food in the East Asia region. Starch is synthesized in semi-crystalline granules by the ordered packing of two glucose polymers, i.e., the linear amylose and the highly branched amylopectin (Li and Gilbert, 2018, Yu et al., 2018). Starch is widely used in many foods and non-food industries despite its tendency to retrograde after gelatinization. The properties for starch application, especially for food processing, are mainly determined by the structure changes in starch during gelatinization and retrogradation. These properties contribute to the edibility, nutritional value, acceptability, and shelf life of starchy foods (Amagliani et al., 2016, Matignon and Tecante, 2017). On the one hand, starch retrogradation significantly challenges the food industry because of its undesirable contribution to the staling of rice cakes and other starchy foods, which can cause reduced shelf life, lower consumer acceptance, and great waste. On the other hand, starch retrogradation is beneficial in some applications for modifying the structural, functional, and sensory properties of products, such as parboiled rice, Chinese rice vermicelli, and dehydrated mashed potatoes (Wang et al., 2015, Xie et al., 2017). Starch retrogradation is also significant in producing nutritional and functional starches because of the enzyme resistance of retrograded starch and sustained release of glucose into the blood system (Jagannadham, Parimalavalli, & Babu, 2017).
Until now, many researchers have studied the promotion or inhibition effects of exogenous components on starch retrogradation, such as lipids, carbohydrates, enzymes, dextrin, and other additives (Chen et al., 2015, Purhagen et al., 2011). Most of their studies are focused on the intervention effects of one or several substances on starch retrogradation. Starch retrogradation is essentially the recrystallization process of hydrated starch molecular chains (Gudmundsson, 1994, Miles et al., 1985). During starch crystallization, the primary nucleation takes place first during cooling from the equilibrium gelatinization temperature to a lower temperature. Stable crystalline nuclei are formed in the dilute solutions by homogenous or heterogeneous nucleation. Crystalline lamellae develop from the nuclei and a three-dimensional superstructure forms. Crystallization on the growth surface is continued by inducing more polymer segments, which increases the crystallinity and thickness of the already formed lamellar crystals. These nuclei are the starting point of crystal growth, which closely affects the process of reorganization of chain segments and crystal grain growth (De Yoreo and Vekilov, 2003, Dhanvijay and Shertukde, 2011, Marsh and Blanshard, 1988). Although numerous exogenous components have different function ways, the nature of their functions is that they interfere with the recrystallization process of starch.
The objective of this work was to determine the possible common mechanism of partially exogenous components affecting starch retrogradation by investigating the recrystallization behavior changes. Since native rice starch was A-type crystal and retrograded starch was mainly B-type crystal, A-type and B-type seed, which were considered as the crystal nuclei induced by most exogenous components, were prepared and added to the retrograded starch system to observe recrystallization behaviors. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and fitting by Jade 6.0 software were applied. Evidence derived from these methods might allow us to obtain a comprehensive understanding of the effect of seed on the rearrangement of starch molecules and crystal growth from the perspectives of nucleation, crystal growth and structure.
This work provides new insights into the nature of the effects of exogenous components on starch retrogradation. Based on the findings we could get a better understanding of how starch retrogradation is affected during different kinds of processing, as well as using this as a theoretical foundation for the preparation of starch products with high quality.
Section snippets
Materials
Normal rice starch was donated by Jiangxi Jinnong Development Co., Ltd. (Nanchang, China). The amounts of moisture, protein, free lipids, and amylose in this rice starch were 9.40%, 0.21%, 0.45%, and 23.45%. All other chemicals and reagents used in the study were purchased from Sinopharm Chemical Reagent Co., Ltd. (Suzhou, China), and were of analytical grade.
Preparation of A-type and B-type seed
A-type seed was prepared by hydrolysis of rice starches based on the method of Cai, Wei, Jin, and Tian (2017) with minor modification.
XRD pattern analysis of the seed
The XRD patterns and RC of samples are illustrated in Fig. 1. Normal rice starch produced a typical A-type XRD pattern with strong peaks at 2θ close to 15.0°, 17.1°, 18.2°, and 23.1° (Fig. 1a). This result was accorded with the report described by Hu et al. (2014). Since the A-type seed was prepared by acid hydrolysis of native rice starch, the RC increased from 16.42% to 22.54% (Fig. 1b). The amorphous region of rice starch was preferentially hydrolyzed, leading to the significant increase in
Conclusions
There were many differences between the A-type and B-type seeds in terms of crystalline structure. A-type seed did not apparently affect the nucleation and recrystallization rate of retrograded starch crystal, but the crystalline structure of starch got looser and more defective and the imperfect lamellar structure got thinner. This might be caused by the hindered movement of chain segments in the rice starch gel system. The added B-type seed could not only act as crystalline nuclei, but also
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (No. 31972027) and the Ten Thousand Talent Program Youth Top-notch Talent Project of China.
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