Structural comparisons of pyrodextrins during thermal degradation process: The role of hydrochloric acid
Introduction
Pyrodextrin is an intermediate product during the preparation of resistant dextrin, which is commonly made by heating dry starch with or without acid. The classification of pyrodextrin is arbitrary, generally, pyrodextrin is classified into three categories: yellow dextrin, white dextrin, and British gum according to the process conditions, e.g. heating time, temperature and acid concentrations (Li et al., 2020, Tomasik et al., 1989). There are various applications of pyrodextrin, such as adhesives, coatings, binders, oral delivery vehicles in industry and soluble dietary fiber in food (Bai & Shi, 2016).
Hydrolysis, transglycosidation and repolymerization are three main chemical reactions during the dextrinization of starch. Starch is degraded into small molecules by heat and acid at the initial stages of dextrinization due to the partial hydrolysis of glycosidic linkages of α-1,4 and α-1,6, leading to an increase in solubility and a decrease in molecular weight and viscosity (Han et al., 2018, Tomasik et al., 1989). Transglycosylation was achieved by a recombination of the degraded small fragments with nearby free hydroxyl groups, with intermolecular or intramolecular bond formation (e.g. α-1,6, β-1,6, α-1,2, β-1,2, and 1,6-anhydro-β-d-glucopyranosyl end groups), producing new branched structures (Bai & Shi, 2016). In particular, the highly branched structure with newly formed non-starch glycosidic linkages is the reason for its resistance to amylase and is crucial to health properties of pyrodextrin (Chen et al., 2020, Lehmann and Robin, 2007). Repolymerization was also proved by the increased viscosity and decreased reducing sugar content during pyroconversion (Laurentin et al., 2003, Wurzburg, 1986).
By far, the physicochemical characteristics and molecular structures of pyrodextrin produced using different processes are reported in numerous literatures. Dry heating treatment with the characteristics of nontoxicity and no harmful byproducts is commonly used in preparing pyrodextrin. Dry heating treatment causes starch chains degradation and smaller fractions generation. Zou, Xu, Tian, and Li (2019) observed that the aggregation of starch granules after continuous and repeated dry heating treatments for several hours. Liu, Hao, Chen, and Gao (2019) found that the crystallinity type of waxy potato starch was transformed from B to B + A type, and a reduction in relative crystallinity after dry heating treatment. Our previous study showed the molecular weight of starch decreased dramatically after heating treatment, but pyrodextrin still retained A-type crystal (Lei et al., 2020). Acid heating treatment using organic acid or mineral acid as a catalyst is also applied to produce pyrodextrin. Bai, Cai, Doutch, Gilbert, and Shi (2014) described that with the heating time prolonging, the solubility increased up to 100%, followed by a decrease in molecular size, crystal size, and melting enthalpy. Lin, Lin, Zeng, Wu, and Chang (2018) used acetic acid to catalyze the production of pyrodextrin, and elucidated that with the heating temperature increasing, the solubility increases, molecular weight and reducing sugar content decrease, and the presence of catalyst accelerates the above changes.
It is evident that the structure and property of pyrodextrin is influenced by the characteristics of materials, heating temperature, pyrolytic time, concentration and the type of acidic catalyst. For example, our previous work investigated the variations of molecular structure and physicochemical property under different pyroconversion processes in terms of heating temperature, and revealed the possible reaction mechanism under different pyroconversion phases (Li, Ji et al., 2020). However, very few studies compared the structural and property differences of pyrodextrins with and without acid, especially how acid and heating conditions contribute to these changes. To further understand the role of acid during the pyroconversion, HCl was adopted as the catalyst to investigate the structural differences between pyrodextrins with and without acid, the possible role of HCI during pyroconversion is also put forward.
Section snippets
Materials
Corn starch was obtained from Gaoguang company (Shandong, China). Dimethyl sulfoxide (DMSO) was from Merck Co.Inc. (Germany). Isoamylase (200U/mL) was purchased from Megazyme (Wicklow, Ireland). The pullulan standards with different molecular weight (from 342 to 2.35 × 106) were obtained from PSS (Mainz, Germany).
Preparation of pyrodextrin
Pyrodextrins were prepared according to the method by Bai et al. (2014) with some modifications. 50 g corn starch was dissolved in 75 mL water, 0.5 M HCl was adopted to adjust the pH
Effects on starch molecular structure
SEC weight distributions of native maize starch and pyrodextrins are presented in Fig. 1A. The molecular size distribution of branched samples is plotted as the weight distribution, wlog(Rh), against the hydrodynamic radius Rh. Table 1 shows the values of the average Rh, expressed as Rh. Apparently, native maize starch shows bimodal distributions with amylopectin (Rh ~ 100–1000 nm) and amylose (Rh ~ 1–100 nm) while the irregular size distributions are observed in pyrodextrin profiles. For
The role of dry heating during pyroconversion of native starch
Dry heating treatment is considered to have the same functions as chemical cross-linking modification, but simpler, safer and more environmental-friendly. From the above results, it is found that thermal treatment affects starch structure, e.g. molecular size, CLDs, and crystallinity. Specifically, the change of molecular size is initially observed after DH for 2 h, accompanying with a small reduction of long-amylose chains as represented by R1. With continuous heating to 4 h, is
Conclusion
This research reveals the influence of HCl as the catalyst on the multi-level of structure and solubility of pyrodextrins prepared from normal corn starch. Extension of pyrolytic time leads to a reduction in molecular size and crystallinity, a narrowed range in DP values and an increase in solubility, whereas the granular structure is less effected. Especially, the average molecular size of pyrodextrins prepared with HCl decreases drastically than that without HCl; dry heating treatment mainly
CRediT authorship contribution statement
Huijia Mao: Methodology. Zhijun Chen: . Jie Li: . Xueyang Zhai: . Hongyan Li: Data curation, Validation, Supervision. Yangyang Wen: Methodology, Resources. Jing Wang: Supervision, Project administration. Baoguo Sun: Supervision, Project administration, Funding acquisition.
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 work was supported by National Natural Science Foundation of China (31901729), School Level Cultivation Fund of Beijing Technology and Business University for Distinguished and Excellent Young Scholars (BTBUYP2020).
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