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. 2025 Jul 21;29:102824. doi: 10.1016/j.fochx.2025.102824

The multi-scale structures and physicochemical properties changes of wheat starch during high-temperature Daqu preparation process

Yang Xu a, Bohan Zhang a, Derang Ni a, Yubo Yang a, Fan Yang a, Xiangli Kong b,⁎, Huabin Tu c,⁎
PMCID: PMC12309921  PMID: 40741349

Abstract

High-temperature Daqu, a microbial-rich fermentation starter used in Jiangxiangxing Baijiu production, undergoes complex wheat starch transformations during its preparation. In this study, the multi-scale structure and physicochemical properties changes of wheat starch in high-temperature Daqu making process were investigated by using a variety of analytical techniques. These results showed that the changes of wheat starch during the fermentation process are more significant compared to the storage process. The surface of wheat starch particles was eroded during the fermentation process, resulting in a distinct porous structure. This structural alteration created channels for enzymatic entry into starch granules, facilitating starch degradation and consequently leading to a significant decrease in the molecular weight of starch after fermentation. The annealing effect of starch during high-temperature Daqu fermentation process led to an increase in the relative crystallinity of wheat starch, accompanied by the formation of more V-type crystalline structures. These changes collectively result in significant alterations in the thermal and paste properties of the starch. Additionally, a characteristic selective degradation of wheat starch was observed during the fermentation process, the small-sized starch granules (d < 10 μm) and amylose were preferentially utilized by microorganisms. This study provided a scientific basis for further understanding the change of wheat starch in high-temperature Daqu preparation and its impact on Jiangxiangxing Baijiu production.

Keywords: Daqu, Wheat starch, Fermentation, Multi-scale structure, Physicochemical properties

Highlights

  • •

    Wheat starch structural changes are greater in high-temperature Daqu fermentation.

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    Small-sized starch & amylose preferentially degraded by microbes in fermentation.

  • •

    Annealing effect enhances starch crystallinity and promotes V-type starch formation.

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    Providing valuable insights into high-temperature Daqu fermentation mechanisms.

1. Introduction

Chinese Baijiu, as one of the six major distilled liquors in the world, is a traditional Chinese liquor with complex brewing technology and unique flavor characteristics, which is loved by drinkers (Xu et al., 2022; Zheng & Han, 2016). Unlike the malt fermentation method employed in spirits such as whiskey and gin (Li, Zhang, & Sun, 2023), the brewing of Chinese Baijiu predominantly utilizes Jiuqu, a complex starter culture containing various microorganisms and enzymes, to facilitate the saccharification of starches in the raw materials, followed by the fermentation of sugars to produce alcohol (Sakandar et al., 2020; Tu et al., 2022). According to different raw materials and production techniques, Jiuqu can be divided into Daqu, Xiaoqu, and Fuqu (Xia et al., 2023). Among them, Daqu contains more microbial species, which provides an important basis for the complexity of Baijiu flavor substances. Therefore, most Chinese famous Baijiu usually use Daqu as a saccharification and fermentation agent (Li, Liu, et al., 2023). Daqu can be divided into low temperature Daqu (< 50 °C), medium temperature Daqu (50–60 °C) and high temperature Daqu (> 60 °C) three types based on the different fermentation temperatures (Ali et al., 2024; Zhou et al., 2022). Different temperature Daqu not only affected the flavor type of Baijiu, but also affected its quality. Of which high-temperature Daqu is mainly used in the production of Jiangxiangxing Baijiu (one of the four basic flavor types of Chinese baijiu), which mainly plays the roles of saccharifying starter culture, providing microbial sources, production raw materials, producing flavor substances and so on (Luo et al., 2024). Therefore, the high-temperature Daqu significantly affects the quality of the final product and plays a vital role in the production of Jiangxiangxing Baijiu.

High temperature Daqu is an important source of microorganisms, enzymes and materials in the process of Jiangxiangxing Baijiu fermentation. The microorganisms in Daqu mainly include bacteria, fungi, yeast, and actinomycetes, and their metabolic activity and community succession are key factors determining the quality of Daqu (Niu et al., 2025). The enzymes in Daqu mainly includes amylase, cellulase, protease, and esterase, which mainly decompose starch, protein and cellulose in the raw materials into small molecule nutrients required for microbial growth and metabolism (Huang et al., 2024). The materials in Daqu mainly refer to the chemical components, including starch, protein, flavor substances and so on (Xia et al., 2022). Among them, starch plays a crucial role in the production of Baijiu, serving not only as the fundamental raw material but also as a core function in the processes of saccharification and fermentation. Starch is hydrolyzed into glucose by enzymes such as α-amylase, β-amylase, and glucoamylase during the fermentation process. Subsequently, under the action of microorganisms like yeast, glucose is further transformed into ethanol and flavor compounds. It has been reported that the structure of starch has a significant impact on the efficiency and quality of fermentation (Yang et al., 2024). The structure and properties of starch directly affect the yield and flavor quality of Baijiu. Therefore, it is of great significance to study the structure and properties of starch in high-temperature Daqu for further understanding the brewing process and improving the yield and product quality of Jiangxiangxing Baijiu. However, most researches mainly focus on microorganisms and enzymes in high-temperature Daqu (Zhu et al., 2022), and a small number of studies focus on flavor composition (Hou et al., 2024). There is no research on starch in high temperature Daqu, and the changes in the structure and physicochemical properties of starch during the high temperature Daqu making process are still unclear.

Previous studies have shown that fermentation can affect the composition and structure of starches from wheat (Zhao et al., 2019), sorghum (Yang et al., 2024), sweet potato (Ye et al., 2019), rice (Tu et al., 2021), etc., ultimately leading to significant changes in the properties of the starch. Zhao et al. (2019) employed a variety of technical methods to investigate how fermentation affected the structure and physicochemical properties of wheat starch. The results indicated that fermentation altered the multi-scale structure of wheat starch, reducing the molecular weight, changing the particle surface, increasing crystallinity and so on. Additionally, fermentation also changed the pasting characteristics of wheat starch, decreasing the peak viscosity and setback value. This study demonstrated that fermentation could improve the quality of wheat starch-based food products. Despite the aforementioned studies have explored starch fermentation in other systems, the unique conditions of high-temperature Daqu fermentation—such as sustained temperatures exceeding 60 °C, prolonged fermentation duration (40 days), and the complex microbial consortia present—distinguish this process from others. These extreme conditions likely induce distinct structural and physicochemical changes in wheat starch, which not only affect the quality of high-temperature Daqu but also influence the yield and quality of Jiangxiangxing Baijiu. However, the patterns of changes in the structure and physicochemical properties of wheat starch during the high-temperature Daqu making process are still not well understood, which hinders a deeper comprehension of the production technology of high-temperature Daqu.

Hence, this study collected samples at various stages of the high-temperature Daqu production process and employed multiple analytical techniques to investigate the particle morphology and size, crystal structure, molecular structure, thermal properties, and pasting characteristics of wheat starch. The aim is to uncover the patterns of structural and physicochemical property changes in wheat starch during Daqu production process, to provide insights into the fermentation mechanism of wheat starch in high-temperature Daqu making production, and to provide a scientific basis for optimizing the production process of Jiangxiangxing Baijiu and improving product quality.

2. Materials and methods

2.1. Samples

The samples in high-temperature Daqu making process were provided by Kweichow Moutai Co., Ltd. (Zunyi, Guizhou, China) were used in this study. The production of high-temperature Daqu mainly involves two stages: fermentation and storage, encompassing a total of eight processes in its preparation. Each process must pass strict quality control in order to produce high quality Daqu in line with the production of Jiangxiangxing Baijiu. The eight processes are grinding wheat, mixing, shaping, loading into the fermenter, stacking fermentation, removing the straw, storing and grinding, respectively (Wang et al., 2019). The detailed production process and parameters for high-temperature Daqu were as follows: Wheat was first pulverized using a grinder, then mixed with 37–40 % water and Muqu (the previous year's high-quality Daqu) followed by thorough stirring to obtain a homogeneous mixture; this mixture was loaded into molds and manually compacted into a turtle-shell shape. The molded Daqu bricks were subsequently transferred into fermentation chambers to initiate the fermentation process, which represented the most critical and core phase of production, typically lasting approximately 40 days. During fermentation, the temperature within the Daqu bricks reached 60–65 °C around 7–9 days, triggering the Fir-F (first flipping) operation that involved systematic positional exchange of bricks between the upper/lower layers and inner/outer sections of the fermentation stack. Following this flipping, the brick temperature experienced a temporary decrease but rapidly recovered to exceed 60 °C. A Sec-F (second flipping) operation was conducted after a further 14–18 days of fermentation, then the temperature within Daqu bricks gradually declined. When the temperature approached ambient room temperature and the moisture content decreased to approximately 15 %, the Daqu blocks were removed from the fermentation chambers. The fermented Daqu bricks were stored for six months to yield mature high-temperature Daqu, which was ground before use in Jiangxiangxing Baijiu production. Fig. 1 shows the production process of high-temperature Daqu. This study focused on key samples (wheat, before fermentation, Fir—F, Sec—F, after fermentation, and Daqu) and systematically investigated the changes in starch structure and physicochemical properties during high-temperature Daqu fermentation and storage. All the samples were stored at −80 °C before analysis.

Fig. 1.

Fig. 1

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The production process of high-temperature Daqu.

2.2. Chemicals

Papain was purchased from Yuanye Biotechnology Co., Ltd. (Shanghai, China). Isoamylase (from Pseudomonas, EC 3.2.1.68) and pullulanase (from Klebsiella planticola, EC 3.2.1.41) were purchased from Megazyme (Bray, Co. Wicklow, Ireland). Sodium hydroxide solution (for IC, 50 %) was purchased form Sigma-Aldrich Co., ltd (Shanghai, China). Ethanol, sodium hydroxide, lithium bromide (LiBr), dimethyl sulfoxide (DMSO), sodium acetate (NaOAc) were all of analytical grade and obtained from Sinopharm Reagent Company (Beijing, China).

2.3. Starch extraction

To enable direct comparison of structural and physicochemical properties changes, starch was extracted from two sources: native wheat starch and wheat starch fermented during the high-temperature Daqu preparation process. The untreated native wheat starch served as the control to establish the baseline for structural properties. In contrast, the fermented wheat starch constituted the test group, allowing investigation of the changes in starch structure and physicochemical properties occurring during the high-temperature Daqu preparation process. The detailed extraction steps are as follows:

The starch in wheat was extracted by following the methods of Gao et al. (2016) with slight modifications. The wheat seeds were rinsed, shelled and ground into powder with a high-speed universal disintegrator. The powder was then suspended in a solution containing 0.25 % NaOH, and the suspension was left undisturbed at room temperature for 24 h. Compared with the method proposed by Gao et al. (2016), the decrease in NaOH concentration from 0.3 % to 0.25 % is due to the higher proportion and denser protein structure in tartary buckwheat, which requires a higher concentration of sodium hydroxide solution to separate starch granules. However, the protein in wheat is relatively easy to remove, so a high concentration of sodium hydroxide solution is not needed. Then select a 270-mesh sieve based on the particle size of wheat starch to sieve the suspension. The upper brown layer was removed, the residue decanted at the base was repeatedly washed with deionized water for several times until a neutral pH was achieved. The residue decanted at the base was collected and dried at 35 °C to obtain the wheat starch.

The starch in high-temperature Daqu or the intermediate product of the fermentation process was extracted using a protease-based approach. Firstly, 50 g of high-temperature Daqu or the intermediate product of fermentation process was added to 100 mL of 95 % ethanol, and the mixture was immersed for 30 min. The sample was then processed using a crusher for disruption, and then filtered through a 270-mesh sieve. The filtrate was centrifuged at 2810 ×g for 5 min. The residue decanted at the base was collected, then washed with deionized water for three times until the supernatant exhibits a lighter color, removing the upper layer of impurities after each wash. Subsequently, 200 mL of deionized water was added to the precipitate, the mixture was transferred to a conical flask and agitated in a water bath at ambient temperature. A 0.25 % (w/v) papain solution (activity: 800 U/mg) was used for enzymatic hydrolysis. Based on the results of preliminary optimization experiments, 4 h was selected as the enzymatic hydrolysis time, under which condition starch purity reached its peak at 92.1 % (see Supplementary Fig. S1). Upon completion of the enzymatic hydrolysis, the mixture was centrifuged at 2810 ×g for 5 min to facilitate separation. The residue decanted at the base was collected and dried at 35 °C to obtain the starch from the high-temperature Daqu or the intermediate product of the fermentation process.

2.4. Scanning electron microscopy (SEM)

The morphological characteristics of wheat starch in high-temperature Daqu making process were observed using SEM (Hitachi S 3000 N, Tokyo, Japan) under an accelerating voltage of 15 kV. The starch samples were immobilized onto the surface of copper stubs with double-sided tape and then coated with gold under vacuum.

2.5. Particle size distribution analysis

The size distribution of wheat starch in high-temperature Daqu making process was determined using a laser diffraction particle size analyzer (Beckman Coulter LS13 320, USA).

2.6. X-ray diffraction (XRD)

The crystalline structures of the wheat starch in high-temperature Daqu making process were acquired by using XRD (Bruker D8 Advance, Germany) operated at 40 kV and 40 mA with Cu-Kα radiation (λ = 0.1542 nm). The diffraction angle (2θ) scanning was from 4° to 40° with a scanning step of 0.02 and a scanning rate of 1°/min. The relative crystallinities of the starch samples were calculated by the ratio of the sample crystalline region area to the total diffractogram area using JADE software 5.0 (Materials Date Inc., Livermore, CA, USA).

2.7. Fourier transform infrared spectroscopy (FT-IR)

In order to analyze the short-range ordered structure of the wheat starch in high-temperature Daqu making process, the infrared spectra of the starch samples were record by a FT-IR spectrometer (Thermo Scientific NICOLET iS50FT-IR, Hudson, USA). A blend consisting of 4.0 mg of starch sample and 200 mg of spectroscopic-grade KBr was finely ground and subsequently pressed into a KBr pellet for FT-IR analysis. The spectra ranged from 400 to 4000 cm−1, and the resolution was 4 cm−1 using 32 scans.

2.8. Differential scanning calorimetry (DSC)

The thermal properties of wheat starch in high-temperature Daqu making process were determined using a differential scanning calorimeter (TA Instruments DSC 2500, USA) with an internal coolant. For all the DSC measurements, 2 mg of starch sample was accurately weighed into an aluminum pan, then 6 μL of deionized water was introduced. After sealing with a capper, the sample was balanced at room temperature for 4 h prior to analysis. The starch samples were heated from 40 °C to 90 °C at a heating rate of 10 °C/min with a nitrogen flow rate of 20 mL/min. An empty aluminum pan was used as a reference. The onset temperature (To), peak temperature (Tp) and conclusion temperature (Tc) were determined based on the analysis of the DSC curve, and the gelatinization enthalpy (ΔH) was calculated based on the dry starch weight.

2.9. Rapid viscosity analyzer (RVA)

The pasting properties of wheat starch in high-temperature Daqu making process were investigated using the RVA TecMaster (Pertrn Instruments, Inc., Hägersten, Sweden) according to its manufacturer's instructions. Initially, a 3.0 g starch sample (containing 14 % moisture) was blended with distilled water to reach a total mass of 28.0 g within an RVA container. Firstly, the mixture was heated to 50 °C and maintained for 1 min, and the temperature was raised to 95 °C over a period of 7.5 min and held constant for an additional 5 min. Then cooled back to 50 °C within 7.5 min and preserved at this temperature for 2 min. The mixing speed was set at 960 rpm for the initial 10 s, after which it was reduced to 160 rpm for the remainder of the process. The RVA profiles were documented, and the values for peak viscosity, through viscosity, breakdown, final viscosity and setback were determined.

2.10. Molecular weight and distribution analysis

The weight average molecular molar mass (Mw) and its distribution of wheat starch in high-temperature Daqu making process were analyzed using a size exclusion chromatography (SEC) system (Agilent 1260 series, Agilent Technologies, Waldbronn, Germany) coupled with a multi-angle light scattering (MALS) detector (DAMN HELEOS-II, Wyatt Corp., CA, USA) and a refractive index (RI) detector (Optilab, Wyatt Corp., CA, USA). The analysis method was referred to that previously reported by Zhao et al. (2019) with several modifications. Approximately 4 mg of starch sample was dissolved in 10 mL of DMSO solution containing 7 mg/mL of LiBr. The mixture was heated with stirring in a boiling water bath for one hour, and then continuously stirred at room temperature overnight to ensure that all the starch granules were completely dissolved. The solution was centrifuged at 16873 ×g for 10 min, and the supernatant was filtered through a 0.45 μm filter before measuring the molecular weight of the starch samples. The separation of starch granules with different molecular weights was carried out using a series connection of two chromatography columns: PSS GRAM 100 (8 × 300 mm, 100 Å; separation range 300–60,000 Da) and PSS GRAM 10000 (8 × 300 mm, 10,000 Å; separation range 10,000–50,000,000 Da), with the column temperature maintained at 80 °C. The mobile phase consisted of a 7 mg/mL LiBr/DMSO solution (previously filtered through a 0.2 μm nylon membrane), and the flow rate was set at 0.3 mL/min. The detector temperature was set at 25 °C. The data were collected and analyzed using the ASTRA software (Wyatt Corp., CA, USA). The Mw was calculated by the second-order Berry method, and the specific RI increment value (dn/dc) of starch in DMSO was 0.074 mL/g.

2.11. Apparent amylose content (AAC)

The AAC of isolated starch was measured using the iodine reagent method. A 20 mg starch sample (dry weight basis) was mixed with 10 mL of 0.5 mol/L KOH and thoroughly vortexed to ensure dispersion. The mixture was transferred to a 100 mL volumetric flask, followed by the addition of 5 mL of 1 mol/L HCl and 0.5 mL of iodine reagent. The solution was diluted to 100 mL and allowed to react for 20 min. Absorbance was then measured at 620 nm using a spectrophotometer (SP − 756P, Spectrum Instruments, Shanghai, China). AAC was calculated using a standard curve prepared from blends of amylose and amylopectin derived from potato starch (Sigma–Aldrich, Co., St. Louis, MO, USA) at varying ratios.

2.12. Statistical analysis

The data were statistically analyzed using the SPSS 20.0 software (IBM, USA) and presented as the mean ± standard deviation (±SD). The graphics were drawn using Origin 9.0 software (Origin Lab Inc., USA).

3. Results and discussion

3.1. Changes in the granular structure of starch

During cereal fermentation, the granular structure of starch is typically disrupted, resulting in alterations to both morphological characteristics and dimensional parameters of the starch granules. In this study, the scanning electron microscopy (SEM) and laser particle size analysis were employed to investigate the surface morphology and particle size distribution of wheat starch during the high-temperature Daqu making process.

As shown in Fig. 2, the native wheat starch exhibited smooth surface characteristic and intact granular structure, with the starch granules demonstrating a bimodal size distribution comprising larger particles (Type A-granules) ranging from 10 to 40 μm in diameter and smaller particles (Type B-granules) measuring less than 10 μm. The shapes of A-granules are mainly oblate, and elliptical, whereas B-granules are spherical and irregular in shape. The morphological characteristic of the starch before fermentation (Fig. 2B) were essentially consistent with the native wheat starch. After 7–9 days of natural fermentation, the surface morphology of some wheat starch granules in Fir-F undergoes significant changes, with the granule surfaces becoming rough and exhibiting distinct pore structures (Fig. 2C). As the fermentation time extended, the surface of the starch granules became increasingly rough, and more large and deep pores appeared. Interestingly, SEM results showed a significant decrease in the number of small-sized B-type starch granules during high-temperature Daqu fermentation process. The particle size of starch granules during the preparation of high-temperature Daqu was further analyzed using a laser particle size analyzer, with the results presented in Fig. 2G, Table 1 and Supplementary Table S1. The average starch particle size of the fermented sample significantly increased compared to before fermentation (from 15.19 μm to 23.97 μm). A marked decrease was observed in Type B starch granules (0–10 μm), with the volume percentage declining from 21.99 % (before fermentation) to 7.90 % (Fir—F). Notably, Type B-granules accounted for merely 3.08 % of the total volume composition after fermentation. Although the volume percentage of B-granule significantly decreased from 21.99 % to 3.08 %, the value of D50 increased (from 14.62 μm to 23.35 μm, Supplementary Table S1). Combined with the fermentation process of high-temperature Daqu, the increase in D50 during fermentation can be attributed to the partial hydration and structural expansion of surviving A-type particles (> 10 μm) under high temperature (60–70 °C) and high humidity (40–50 % initial moisture) fermentation conditions. However, there was no significant change in the morphological characteristics and size distribution of wheat starch during storage. The above results demonstrated that the granular structure of wheat starch was significant changed during high-temperature Daqu fermentation, and the small-sized starch granules were more easily utilized.

Fig. 2.

Fig. 2

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The scanning electron micrographs (A: Native wheat starch, B: Before fermentation, C: Fir—F, D: Sec—F, E: After fermentation, F: Daqu; 1 is low magnification image and 2 is high magnification image) and particle size distributions (G) of wheat starch during the high-temperature Daqu making process.

Table 1.

Mean particle size (d), relative crystallinity (RC), the short-range ordered structure (A1047/A1022), weight-average molecular weight (Mw), polydispersity index (PDI) and amylose content of wheat starch during high-temperature Daqu making process.

Samples d (μm) RC (%) A-type (%) V-type (%) A1047/A1022 Mw (×107 g/mol) PDI (Mw/Mn) Amylose content (%)
Native wheat starch 17.88 ± 0.02d 21.37 ± 0.24c 20.32 ± 0.19bc 1.05 ± 0.06b 0.894 ± 0.003c 6.39 ± 0.20a 2.45 ± 0.13a 23.09 ± 0.30a
Before fermentaition 15.19 ± 0.01e 21.84 ± 0.06b 20.76 ± 0.02a 1.08 ± 0.07b 0.886 ± 0.002d 5.23 ± 0.20b 2.19 ± 0.10b 23.39 ± 1.33a
Fir-F 19.64 ± 0.06c 22.48 ± 0.30a 20.12 ± 0.30bc 2.36 ± 0.02a 0.889 ± 0.006cd 2.62 ± 0.07d 1.78 ± 0.12c 23.64 ± 0.38a
Sec-F 20.63 ± 0.02b 22.69 ± 0.17a 20.45 ± 0.14ab 2.24 ± 0.03a 0.892 ± 0.003cd 3.27 ± 0.12c 2.19 ± 0.04b 20.71 ± 0.72b
After fermentation 23.57 ± 0.04a 22.72 ± 0.08a 20.46 ± 0.10ab 2.26 ± 0.07a 0.917 ± 0.008a 0.94 ± 0.03f 2.35 ± 0.03a 20.61 ± 0.66b
Daqu 23.64 ± 0.05a 22.40 ± 0.08a 20.00 ± 0.10c 2.40 ± 0.10a 0.911 ± 0.006b 1.36 ± 0.04e 2.37 ± 0.03a 19.85 ± 0.50b
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Values are the mean ± SD. Values with different letters in the same column are significantly different (P < 0.05).

3.2. Changes in crystalline structure of starch

The diffraction patterns of wheat starch during the high-temperature Daqu making process are shown in Fig. 2. The native wheat starch, and the samples during the high-temperature Daqu making process also showed typical A + V hybrid crystalline structures with strong diffraction peaks at 2θ around 15°, 17°, 18° and 23° (A-type) and 20° (V-type), which was in accordance with the previously reported findings ((Li et al., 2020; Zhang et al., 2013).

Based on a previous method (Zuo et al., 2024), the crystallinities of wheat starch during the high-temperature Daqu making process were calculated, and the results were listed in Table 1. The crystallinity of native wheat starch is 21.37 %. After fermentation, the starch crystallinity increased to 22.72 %, which can be ascribed to the high temperature and humidity during the fermentation process of high-temperature Daqu, resulting in the annealing of wheat starch. The starch molecular chains are arranged in a more orderly manner, forming a more stable crystalline structure. In addition, a significant reduction in amylose content is observed in the fermentation process (Table 1). This may be due to the preparation of high-temperature daqu, where starch does not undergo gelatinization, thereby maintaining its structural integrity. Consequently, microorganisms predominantly exploit the amorphous regions of starch as a carbon source for their metabolic activities, resulting in an elevated relative proportion of the starch crystalline region and an increase in crystallinity. However, the decrease in starch crystallinity observed in the finished Daqu after 6 months of storage, as compared to before storage, may be attributed to the action of amylase or other enzymes present in the Daqu, which act on starch molecules during storage, decomposing the starch chains and thereby affecting their crystalline structure. Interestingly, while the XRD patterns of all samples retained the hybrid A + V-type crystalline structure, the diffraction peaks at approximately 20° for the starch after fermentation showed significant enhancement compared to the starch before fermentation (Fig. 3), and the relative proportion of V-type crystallinity increased significantly during fermentation (Table 1). These results indicated the possible formation of V-type starch structures composed of amylose and lipid compounds during the fermentation process of high-temperature Daqu (Zhou et al., 2024). Correlative research indicates that the complex of amylose and lipid substances has an impact on the gelatinization and digestibility of starch (Wang et al., 2020). These complexes are more resistant to enzymatic hydrolysis, and the slow fermentation is more conducive to the action of esterification enzymes and the synthesis and accumulation of ester substances, which are crucial to the Baijiu flavor.

Fig. 3.

Fig. 3

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The XRD diffraction patterns of wheat starch during high-temperature Daqu making process.

The short-range ordered structures of native wheat starch and fermented starch during the high-temperature Daqu making process was analyzed through spectral changes in the region of 400–4000 cm−1 by FT-IR, and the results were shown in Supplementary Fig. S2. The absorption peaks around 3415 cm−1 and 2927 cm−1 were attributed to the –OH stretching vibration and the asymmetric stretching vibration of –CH2, respectively (Kong et al., 2020). The band near 1638 cm−1 is related to the absorption of water in the amorphous region of starch (Kong et al., 2020). There is almost no significant change in the infrared spectra of all samples, indicating that no chemical bonds appeared or disappeared in the wheat starch during the high-temperature Daqu making process.

The absorption peak at 1047 cm−1 and 1022 cm−1 in the starch infrared spectrum are related to the ordered crystalline structure and the amorphous structure of starch, respectively (Sevenou et al., 2002). The ratio of the absorption peak intensities at 1047 cm−1 to 1022 cm−1 (A1047/A1022) has been widely used to analyze the short-range ordered structure on the surface of starch (Zuo et al., 2024). The A1047/A1022 calculation results were listed in Table 1. The degree of order on the starch surface showed a certain upward trend during the high-temperature Daqu fermentation process, while no significant change was observed during the storage process. The results of the change in the degree of order of the short-range structure of starch surface were consistent with the increased starch crystallinity observed in XRD during the high-temperature Daqu fermentation process.

3.3. Changes in molecular structure of starch

The weight-average molecular weight of native wheat starch and fermented starches during high-temperature Daqu making process are shown in Table 1, and the molecular weight distribution of starch at each node was shown in Fig. 4. The Mw of starch in wheat and before fermentation were 6.39 × 107 g/mol and 5.23 × 107 g/mol, respectively, and the minimum Mw were higher than 1 × 107 g/mol. During the fermentation process, the α-1,4-glycosidic bonds and α-1,6-glycosidic bonds in starch were hydrolyzed by amylase and glucoamylase, resulting in the gradual hydrolysis of starch molecules. Hence, the Mw of starch was decreased during high-temperature Daqu fermentation process, and the proportion of low molecular weight (< 1 × 107 Da) starch gradually increases. Among them, the changes of Mw from before fermentation to Fir—F, and from Sec-F to after fermentation were both significant. After fermentation, the Mw of starch was significantly reduced to 0.94 × 107 g/mol, with starch molecules smaller than 1 × 107 Da accounting for 75.82 %. However, the Mw of starch in the Daqu increased after 6 months of storage, and the proportion of low molecular weight (< 1 × 107 Da) starch was decreased. The above results indicate that wheat starch was gradually hydrolyzed during high-temperature Daqu fermentation, and further decomposed into small molecule carbohydrates by amylase and glucoamylase for microbial growth and metabolism. However, the hydrolysis of starch was relatively slow during storage, and small molecule carbohydrates are utilized by microorganisms, resulting in an increase in molecular weight.

Fig. 4.

Fig. 4

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The molar mass distribution of wheat starch during high-temperature Daqu making process.

The polydispersity index (PDI) in starch molecular weight data is a crucial parameter that quantifies the width of molecular weight distribution within a population of starch molecules. The PDI of native wheat starch and fermented starches during high-temperature Daqu making process were shown in Table 1. The DPI during the high-temperature fermentation process of Daqu showed a trend of first decreasing and then increasing, with the lowest DPI value of 1.78 in the Fir-F sample, indicating its narrowest component distribution. The “first decrease and then increase” of PDI in starch fermentation is essentially the result of the combined action of enzymatic hydrolysis kinetics and microbial metabolism. In the early stage of fermentation, amylase (such as alpha amylase) preferentially attacked the amorphous region and long branched chains of starch, rapidly degrading large molecular starch into medium-sized dextrins, leading to a decrease in PDI. As fermentation progresses, starch was further enzymatically hydrolyzed to produce a large amount of low molecular weight carbohydrates (such as glucose, maltose, etc.), while some crystalline or highly branched regions of starch were difficult to be enzymatically hydrolyzed, forming high molecular weight residues. The system contains both polymer residues and small molecule sugars, resulting in a wider molecular weight distribution and an increase in PDI. After fermentation, the DPI value increased to 2.35. However, there was no significant change during storage, the DPI value of starch in Daqu after storage was 2.37.

3.4. Changes in thermal and paste properties of starch

DSC is the most widely used thermal analytical technique in food research to measure energy changes in a sample subjected to heating and cooling. The DSC thermograms for the native wheat starch and the fermented starch during high-temperature Daqu making process were shown in Fig. 5, and the corresponding parameters were shown in Table 2. All samples exhibited an obvious endothermic peak in the range of 65–74 °C, which is attributed to the gelatinization of amylopectin (Xie et al., 2010). The DSC curves of the native wheat starch and before fermentation sample showed no significant differences. However, the DSC curves of wheat starch after fermentation shifted significantly to the right compared to before fermentation, with a marked increase in the peak gelatinization temperature. Especially during the period from before fermentation to Fir—F, the increase in gelatinization temperature is most significant, which may be related to the changes in the starch crystal structure and starch particle size during the fermentation process. Related studies have shown that V-type starch requires a higher temperature to gelatinize (Goderis et al., 2014). In this study, XRD results indicated that the relative crystallinity of V-type starch increased from before fermentation to Fir—F, leading to an increase in gelatinization temperature. On the other hand, other studies have also shown that the larger the starch particle size, the smaller its specific surface area, resulting in a relatively reduced contact area between starch particles and water molecules (Ao & Jane, 2007; Tao et al., 2016). Therefore, the increase in gelatinization temperature in this study is also related to the increase in starch particle size. However, the gelatinization temperature of starch during the storage process changed slightly. The gelatinization enthalpy (ΔH) represents the energy required to disrupt the ordered structure of starch during the gelatinization process (Zheng et al., 2021). A high gelatinization enthalpy value of starch indicates a compact structure, necessitating more energy for complete gelatinization. The gelatinization enthalpy (ΔH) of the native wheat starch and the starch prior to fermentation were 7.92 and 7.62 J/g, respectively. In the Fir-F sample, the ΔH value of the starch granules increased to 8.39 J/g, which might be associated with the rapid hydrolysis of the amorphous regions and the increase in crystallinity of the starch granules during the early stage of fermentation. As fermentation progressed, the ΔH decreased, which is attributed to the reduction in crystallinity and the dissociation of starch molecule helices. During storage, there was almost no significant change in the ΔH of the starch.

Fig. 5.

Fig. 5

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The DSC patterns of wheat starch during high-temperature Daqu making process.

Table 2.

The thermal and pasting parameters of wheat starch during high-temperature Daqu making process.

Samples To (°C) Tp (°C) Tc (°C) △H (J/g) Peak viscosity
(cP)		 Trough
(cP)		 Breakdown
(cP)		 Final viscosity
(cP)		 Setback
(cP)
Native wheat starch 61.51 ± 0.12d 65.93 ± 0.08d 71.11 ± 0.25e 7.92 ± 0.08b 2544.3 ± 95.8b 1261.0 ± 19.1b 1283.3 ± 79.3b 2972.3 ± 65.9b 1711.3 ± 54.0b
Before fermentation 61.76 ± 0.09d 65.79 ± 0.28d 70.78 ± 0.52e 7.62 ± 0.06c 2666.3 ± 10.3a 1316.0 ± 3.6a 1350.3 ± 11.1a 3110.3 ± 7.3a 1794.3 ± 10.9a
Fir-F 67.61 ± 0.11c 71.40 ± 0.04c 74.60 ± 0.09d 8.39 ± 0.10a 1519.7 ± 15.8c 836.7 ± 12.2c 683.0 ± 27.8c 1936.0 ± 16.4c 1099.3 ± 28.5c
Sec-F 68.70 ± 0.14a 72.45 ± 0.05b 75.53 ± 0.10c 7.68 ± 0.28c 1055.7 ± 3.1d 447.0 ± 4.1d 608.7 ± 5.4d 1427.3 ± 6.2d 980.3 ± 7.8d
After fermentation 68.02 ± 0.02b 72.40 ± 0.21b 76.15 ± 0.13b 7.63 ± 0.06c 504.3 ± 2.1e 127.0 ± 5.1e 377.3 ± 3.3e 530.0 ± 5.7e 403.0 ± 1.4e
Daqu 68.58 ± 0.28a 73.46 ± 0.27a 78.03 ± 0.21a 7.66 ± 0.14c 448.0 ± 1.6e 123.0 ± 2.9e 325.0 ± 1.4e 438.3 ± 3.3f 315.3 ± 2.1f
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Values are the mean ± SD. Values with different letters in the same column are significantly different (P < 0.05).

The pasting properties refers to the changes in viscosity of starch suspension during heating, which can be used to characterize the changes in the physicochemical structure of starch (Balet et al., 2019). The pasting characteristics and parameters of the wheat starch during high-temperature Daqu making process were shown in Fig.6 and Table 2. The peak viscosity, trough viscosity, breakdown, final viscosity, and setback of all fermented starches were lower than those of the native wheat starch and the starch before fermentation. And during the production process of high-temperature Daqu, these parameters gradually decreased as fermentation progresses. Viscosity is an indirect indicator of starch hydration. Starch with more branched amylopectin absorbs and retains more water and exhibits a higher peak viscosity (Li et al., 2017). The decrease in viscosity was related to the molecular structure changes of starch during the fermentation process. In this study, the molecular weight of wheat starch significantly decreased during the fermentation process of high-temperature Daqu, and its molecular structure underwent significant changes, which may be the reason for the decrease in viscosity. A similar phenomenon was observed by Zhao et al. (2019) in natural fermentation of wheat starch.

Fig. 6.

Fig. 6

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The pasting characteristics of wheat starch during high-temperature Daqu making process.

3.5. Discussion on the changes in multi-scale structures of starch and the relation to physicochemical properties

The overall results indicate that the changes in starch structure and physicochemical properties are significantly greater during the fermentation stage of high-temperature Daqu preparation compared to the storage stage. This can be attributed to the lower moisture content in the samples during storage, which leads to slower microbial metabolic activity and relatively lower enzyme activity (Cruz-Paredes et al., 2021). During the fermentation process of high-temperature Daqu, extracellular enzymes secreted by microorganisms in the environment (eg. Bacillus spp. produce thermostable α-amylases) hydrolyze and degrade starch molecules, causing erosion of the surface of wheat starch molecules and forming distinct pore structures as time progresses (Tu et al., 2021). These pore structures create channels for enzymes to enter the interior of starch granules, exposing more active sites for enzyme attack. This facilitates the hydrolysis and degradation of starch during fermentation, resulting in a significant decrease in the molecular weight of fermented starch compared to the original wheat starch. Nearly three-quarters of the starch molecules have a molecular weight below 1 × 107 g/mol. The unique pore structure of starch and dextrin with small molecular weight in Daqu play an important role in the production of Jiangxiangxing Baijiu in the early rounds (a total of 7 rounds a year). Furthermore, the results of SEM and laser particle size analyzer reveal a gradual decrease in the proportion of small starch particles with particle size bellow 10 μm and a concurrent increase in the average particle size during the high-temperature Daqu fermentation process, indicating that smaller starch particles are preferentially utilized during fermentation. This can be attributed to the higher specific surface area of small starch particles, which readily expose more active sites and facilitate enzymatic hydrolysis and degradation of starch (Jung et al., 2013; Noda et al., 2005). Recent microbiome studies of high-temperature Daqu identify Bacillus (e.g., B. licheniformis), Aspergillus, and Rhizopus as key amylolytic genera (Ali et al., 2024; Huang et al., 2024). These taxa may be more prone to starch degradation when the starch particle size is smaller and more active sites are exposed. Regarding the crystalline structure of starch, the relative crystallinity of starch increases during the high-temperature Daqu fermentation process, primarily due to two factors: 1) Annealing effect: The high temperature and humidity conditions during fermentation (reaching up to 60–70 °C, with initial moisture content of 40–50 % in the samples) promote annealing of starch molecules. This leads to a denser structure of wheat starch, resulting in an increased relative crystallinity (Tester et al., 1998; Tester & Debon, 2000). 2) Selective degradation: Since the starch is not gelatinized during the raw material fermentation process, the double helix structure of amylopectin remains intact and is less prone to enzymatic degradation. However, the amylose in the amorphous region is more easily decomposed and utilized by β-amylase and glucoamylase, which are also important components of the enzyme system in high-temperature Daqu. This selective degradation leads to an increased proportion of the crystalline region and subsequently higher relative crystallinity. The structural changes in starch inevitably lead to alterations in its physicochemical properties. Due to the increased relative crystallinity, more energy is required for starch gelatinization, resulting in elevated gelatinization temperature and enthalpy during the high-temperature Daqu fermentation process. Regarding paste properties, the observed decrease in viscosity during fermentation is directly related to the significant reduction in molecular weight of starch molecules after enzymatic hydrolysis. Lower viscosity of starch in Daqu is crucial for Jiangxiangxing Baijiu fermentation, primarily manifested in two aspects: facilitating enzyme diffusion and enhancing microbial accessibility. Facilitating enzyme diffusion: reduced gel rigidity enables amylases (e.g., from Bacillus and Aspergillus spp. in Daqu) to penetrate the starch matrix more efficiently, accelerating dextrinization and saccharification. Enhances microbial accessibility: A less viscous slurry improves nutrient mobility, allowing yeast and bacteria to access glucose/maltose more readily.

4. Conclusion

This study provides novel insights into the structural and functional evolution of wheat starch during high-temperature Daqu production. The fermentation stage induces substantially more pronounced changes of wheat starch than the storage stage. Key findings reveal that microbial activity erodes starch granule surfaces, generating distinct porous architectures. These structural modifications facilitate enzymatic penetration into granules, accelerating starch degradation and significantly reducing molecular weight. Concurrently, high-temperature annealing during fermentation elevates starch relative crystallinity and promotes the formation of V-type crystalline complexes (amylose-lipid). These multi-scale structural transformations collectively drive significant changes in starch functionality, particularly enhancing thermal stability and altering pasting properties. The changes of starch structure and properties during the preparation of high temperature Daqu directly influence saccharification efficiency and flavor precursor availability in Jiangxiangxing Baijiu production. Notably, a characteristic selective degradation pattern was observed: microorganisms preferentially utilize small-sized granules (<10 μm) and amylose from amorphous regions. These mechanistic insights into starch modification elucidate critical biochemical processes underpinning Daqu functionality, offering a scientific foundation for optimizing Jiangxiangxing Baijiu production and enhancing final product quality through targeted control of fermentation parameters.

CRediT authorship contribution statement

Yang Xu: Writing – original draft, Investigation, Formal analysis, Data curation. Bohan Zhang: Investigation, Formal analysis, Data curation. Derang Ni: Methodology, Investigation. Yubo Yang: Writing – review & editing, Supervision. Fan Yang: Writing – review & editing, Project administration. Xiangli Kong: Writing – review & editing, Resources. Huabin Tu: Supervision, Project administration.

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.

Acknowledgment

This research was funded by the Institute of Science and Technology, Moutai Group (No. MTGF2021019), China.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2025.102824.

Contributor Information

Xiangli Kong, Email: xlkong@zju.edu.cn.

Huabin Tu, Email: tumoutai2023@163.com.

Appendix A. Supplementary data

Supplementary material

mmc1.docx (323.3KB, docx)

Data availability

Data will be made available on request.

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Supplementary Materials

Supplementary material

mmc1.docx (323.3KB, docx)

Data Availability Statement

Data will be made available on request.

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