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. 2026 Jan 29;12:101324. doi: 10.1016/j.crfs.2026.101324

Dendritic crystal pattern from a drying droplet: A distinctive feature of feng-flavor Baijiu

Haoran Fu a,1, Yang Zhong a,1, Lu Cai a,1, Tieyuan Cheng b, Yulin Xia b, XiaoHong Jian a, Jun Liu a, Zaixin Li a, Huibo Luo a, Ziqing Wu c, ChengYong Jin d,e, Yongming Liu c,⁎, Siqi Yuan a,f,⁎⁎, Zhi Zhang a,⁎⁎⁎
PMCID: PMC12891798  PMID: 41684707

Abstract

Visual evaluation techniques for Chinese Baijiu remain an active research area. We identified a novel dendritic crystal pattern formed after the droplet evaporation of Feng-flavor Baijiu. Interestingly, this pattern showed an obvious correlation with aging in Jiuhai container (storage vessels for aging) (R2 = 0.904, p < 0.01), suggesting its potential as a visual quality indicator of Feng-flavor Baijiu. During droplet evaporation, the local solution preferentially reaches a supersaturated state at the three-phase contact line, initiating nucleation and crystallization that ultimately assemble into a dendritic crystal pattern with a coffee-ring structure. Further analysis revealed that the crystals are mainly composed of metal ions such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+), which primarily originate from a unique aging container known as the Jiuhai container. Furthermore, we observed that trace components such as acids and esters can significantly inhibit or promote the formation of dendritic crystals, contributing to distinct dendritic crystal patterns across various quality grades (Ⅰ to Ⅴ) of Feng-flavor Baijiu samples. More interestingly, crystallization simulation experiments showed that dendritic crystal patterns formed in blended Feng-flavor Baijiu but not in other blended-flavor Baijiu samples. Consequently, the characteristic dendritic crystal patterns generated by droplet evaporation could serve as potential visual indicators for evaluating the quality of Feng-flavor Baijiu.

Keywords: Feng-flavor Baijiu, Droplet evaporation, Dendritic crystal, Metal ions, Trace components, Jiuhai container

Graphical abstract

Image 1

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Highlights

  • •

    Distinctive dendritic crystals leave behind after droplet evaporation of Feng-flavor Baijiu.

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    Dendritic crystal can serve as a valuable indicator for Feng-flavor Baijiu.

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    Unique Jiuhai container is the key for the formation of dendritic crystals.

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    Trace components in Baijiu significantly affect the morphology of dendritic crystal.

1. Introduction

As one of the six major distilled liquors in the world, Chinese Baijiu is widely favored in East Asia for its strong and unique flavor. Based on production technology and flavor profile, Baijiu is classified into 12 flavor types, namely Strong-flavor, Sauce-flavor, Light-flavor, Rice-flavor, Feng-flavor, Te-flavor, Dong-flavor, Miscellaneous-flavor, Sesame-flavor, Chi-flavor, Fuyu-flavor and Laobaigan-flavor Baijiu (Zheng and Han, 2016; Liu and Sun, 2018). Among these, Feng-flavor Baijiu is primarily produced in Shaanxi Province, China, with a history spanning over 3000 years (Hu et al., 2023). Due to unique aroma, Feng-flavor Baijiu is widely favored by the local populace. Feng-flavor Baijiu is produced using sorghum as the primary raw material, which is crushed and mixed with medium temperature Daqu (fermentation starter) made from barley and peas (fermented at 50–60 °C). The mixture is then fermented in the Jiaochi (also called mud pit for grain fermentation) for 11–15 days. The brewing process utilizes continuous distillation of grains, combined with mixed steaming and burning. Subsequently, the fresh Feng-flavor Baijiu is typically stored in Jiuhai containers for aging (Shen, 2022). The Jiuhai is woven from vines and coated with hemp paper, its surface is treated with a mixture of animal blood and lime, followed by egg white, rapeseed oil, and beeswax after drying (Shen, 2022). Due to the complex production process and raw materials used, the Jiuhai contributes numerous trace components, including metal ions and flavor compounds, which form characteristic aromas such as honey and Jiuhai aromas during the aging process (Jia et al., 2021a, 2021b, 2022; Liu et al., 2022).

Droplet evaporation is a common natural phenomenon, occurring in rain, fog, dew, etc. Evaporation of a particle-laden or saline droplet on a substrate typically leaves behind either a ring (Deegan et al., 1997), a uniform deposit, or a condensed deposit. When the saline droplet evaporates, crystallization occurs during the process or after drying, resulting in unique crystal deposition. However, the evaporation of saline droplets is complex. Multiple factors affect crystal formation and deposition patterns, such as the nature of the saline (Zang et al., 2019), surfactants (Shao et al., 2020, 2021), and substrates (Samantha et al., 2018; Noushine et al., 2008; Zhang et al., 2023), among others (Maryam et al., 2018). During evaporation, solutes are transported by internal flows within the droplet, such as Marangoni flow and capillary flow. Ultimately, crystallization occurs and contributes to deposition pattern formation (Marina et al., 2020). In an ethanol-water system containing NaCl, droplet evaporation forms a ring-like crystal deposition pattern at the contact line (Cécile et al., 2023). However, when the NaCl is added to system containing surfactants, pinning and depinning phenomena occur, resulting in the concentric circles formed by multiple crystal rings (Kaya et al., 2010). Additionally, different metal ions form distinct crystal morphologies, such as cubic or dendritic NaCl crystals (Dewangan et al., 2021; Mai et al., 2016), or calcite, vaterite and aragonite appearances for CaCO3 (Sand et al., 2012). Therefore, the presence of metal salt ions, surfactants and other components in the system leads to complex and diverse deposition patterns after droplet evaporation.

Currently, quality assessment of Baijiu relies primarily on manual sensory evaluation and instrumental techniques such as gas chromatography–mass spectrometry (GC–MS). These methods typically require extensive personnel training or involve costly equipment, so there is a pressing need to develop rapid, visual auxiliary detection methods. Baijiu is a complex ethanol–water system because it contains numerous trace components, and evaporation of Baijiu droplets produces diverse deposition patterns on substrates (Zhang et al., 2025). However, the relationship between characteristic deposition patterns and Baijiu quality assessment remains unclear. In this study, significant crystallization phenomena and crystal patterns are observed in tested Baijiu samples (1 μL droplet on indium tin oxide (ITO) glass substrate surface) through droplet evaporation. Notably, Feng-flavor Baijiu demonstrate a unique dendritic crystal pattern with a coffee-ring structure. Importantly, the crystal pattern and morphology are determined by both metal ions and flavor components in Baijiu, and the crystal structure shows a potential correlation with changes in the content of these metal salt ions and trace components across five different quality grades of Baijiu (No. I to V).

2. Materials and methods

2.1. Material and Reagents

All chemicals were obtained from commercial sources and used as received unless otherwise stated. Methanol (99.9 %, Sigma-Aldrich, #106007), ethanol (99 %, HPLC, Macklin, #E809064), potassium dihydrogen phosphate (Sigma-Aldrich, #137039), ethyl lactate (98.0 %, Sigma-Aldrich, #69799), ethyl acetate (99.5 %, Sigma-Aldrich, #319902), lactic acid (85.0 %, Sigma-Aldrich, #W261106), ethyl hexanoate (98.0 %, Sigma-Aldrich, #W243906), ethyl butyrate (99.0 %, Sigma-Aldrich, #E15701), hexanoic acid (99.0 %, Sigma-Aldrich, #153745), butyric acid (99.0 %, Sigma-Aldrich, #B103500), acetic acid (99.5 %, Sigma-Aldrich, #W200611), CaCO3 (99 %, Sigma-Aldrich, #239216), Mg(CH3COO)2 (98 %, Macklin, #M833330), Ca(CH3COO)2 (99 %, Macklin, #C850055), CH3COONa(99 %, Macklin, #767496), CH3COOK (99 %, Macklin, #P816004), NaCl (99 %, Sigma-Aldrich, #S9888), KCl (99 %, Sigma-Aldrich, #P3911), MgCl2 (99 %, Sigma-Aldrich, #M8266), CaCl2 (97 %, Sigma-Aldrich, #C4901), Ca(OH)2 (95 %, Sigma-Aldrich, #239232) were used as received.

Twelve flavor types of Baijiu samples (Table S1) were commercially obtained from China Alcoholic Drinks Association (Standard Baijiu for sommelier tasting). Additionally, Feng-flavor Baijiu samples, including samples of quality grade samples (No. I to V, three batches of 15 Baijiu samples), finished Baijiu samples (stored in Jiuhai, 12 samples), and fresh Baijiu samples (not stored in Jiuhai, 12 samples), were obtained from Shaanxi Xifeng Liquor Co. Ltd. Fifteen samples of three different brands of Feng-flavor Baijiu products were purchased from the local factories.

Brandy (VSOP, 40 % ABV) and whisky samples (France, 44 % ABV) were purchased from WAL-MART (China) Investment Co., Ltd. Superior White Rum (Puerto Rico, 40 % ABV) was acquired through commercial purchase, whereas the vodka (Sweden, 40 % ABV) was generously donated by colleagues. All samples were stored at room temperature (RT).

2.2. Observation of crystal patterns

ITO glass slides, purchased from Wuxi Juxin Technology Co., Ltd., were used for droplet evaporation. The ITO substrates were sequentially submerged in sonicated toluene, acetone, ethanol, and ultrapure water for 10 min each, followed by air drying before testing. The contact angle of ultrapure water on surface of ITO slides was measured at 85° ± 1°.

The evaporation-crystallization experiments were conducted under controlled conditions of constant temperature and humidity (20 °C ± 2 °C, 65 % ± 2 % relative humidity, RH). The ambient environment of the entire laboratory was regulated using a constant-temperature, constant-humidity intelligent precision air conditioner (HFW-75, Beijing ZKSH TEC. CO., LTD.). A 1 μL droplet of Baijiu was pipetted onto an ITO glass slide placed under an acrylic enclosure (30 cm × 15 cm × 20 cm) to isolate it from environmental fluctuations. After allowing the droplets to dry and crystallize completely for 48 h, the samples were photographed using a dark-field microscope. The crystallization of Feng-flavor Baijiu requires at least 12–36 h for complete growth. Samples were photographed using a dark field microscope (MMAF-3590, E3CMOS, Shanghai Wumo Optical Instrument Co., Ltd.) at magnifications of 40 × , 200 × , and 400 × . Each experiment was repeated ten times. Before SEM scanning, the samples were coated with a 5–20 nm platinum layer using an ion sputter coater (Model ISC150, Shenzhen Supu Instruments Co., Ltd.). The crystals on the ITO substrates were examined and observed by scanning electron microscopy (SEM, SEM5000, Field Emission Scanning Electron Microscope, National Instruments Quantum Instrument Co., Ltd., China).

2.3. Determination of drying models of droplets evaporation

Drying models of droplet evaporation were analyzed using a Theta Lite Optical Tensiometer (Theta Lite, Biolin Scientific AB, Sweden), under a controlled environment of constant temperature and humidity (20 ± 2 °C, 65 % ± 2 % RH). A 1.0 μL droplet of Feng-flavor Baijiu was placed on ITO substrates. Parameters such as contact angle, contact radius and droplet height of sessile droplet were measured or recorded over 300 s during evaporation. The measurement stopped automatically when the droplet height or contact angle became undetectable. This process was repeated three times for each sample, and results were reported as the average of the three runs.

2.4. Observation of internal flows of Baijiu droplet during the evaporation

To visualize bulk fluid motion, the fluorescent microparticles (0.1 μm, Fluoro-max R0100, Thermo Fisher Scientific Inc.) were added to Feng-flavor Baijiu samples at a final concentration of 0.01 wt %. A 1.0 μL droplet of Baijiu was placed on a cleaned ITO substrate. The movement and trajectory of fluorescent microparticles were observed and recorded under ultraviolet excitation light using a digital camera (MMAF-3590, Shanghai Wumo Optical Instrument Co., Ltd.) mounted on an upright microscope at 40 × magnification.

2.5. X-ray photoelectron spectroscopy analysis

A sample of appropriate size (typically 5 × 5 mm) was cut and affixed to the sample tray. It was then positioned in the sample chamber of the X-ray Photoelectron Spectroscopy (XPS, Thermo Scientific K-Alpha). When the chamber pressure fell below 2.0 × 10−7 mBar, the sample was transferred to the analysis chamber, maintaining a spot size of 400 μm. The working voltage and filament current were set at 12 kV and 6 mA, respectively. Full spectrum scans used a pass energy of 150 eV and a step size of 1 eV. Following correction with Thermo Avantage Software (Thermo Fisher Scientific Inc.), a full spectrum analysis of the detected elements was performed.

2.6. Inductively coupled plasma mass spectrometry analysis

The sample was dissolved and diluted to an appropriate concentration with using a 3 % nitric acid solution for inductively coupled plasma mass spectrometry (ICP-MS, iCAP™, Thermo Scientific™) analysis. Before analysis, the ICP-MS was calibrated with a 1 μg/L tuning solution to ensure the necessary sensitivity, background levels, oxide concentrations, double charge ratios, and meet the required standards. ICP-MS operating parameters were as follows: RF power 1400 W, nebulizer gas flow 0.94 L/min, auxiliary gas flow 0.29 L/min, plasma gas flow 15 L/min, and spray chamber temperature at 2 °C.

2.7. Gas chromatography-mass spectrometry analysis

Gas chromatography - mass spectrometry (GC-MS) analysis was performed on 1 mL aliquots of five standard Feng-flavor Baijiu samples, each with different quality grades (I to V), using a Triple Quadrupole GC-MS/MS system (Thermo Scientific™ TSQ™ 8000 Evo). Samples were injected directly using the split mode (split ratio 10:1) onto a DB-WAX column (length: 30 m; ID: 0.25 mm; film: 0.25 μm, Agilent Technologies, CA, USA), respectively.

The GC oven temperature program was as follows: initially hold at 40 °C for 2 min, then raised to 80 °C at a rate of 3 °C/min. Next, raise to 180 °C at a rate of 5 °C/min, followed by a ramp to 220 °C at a rate of 10 °C/min, which was maintained for 5 min. High-purity helium (99.999 %) was used as the carrier gas with a constant flow rate of 1.5 mL/min. The mass spectrum parameters were set as follows: electron ionization (EI) source, ionization energy 70 eV, ion source temperature 230 °C, injection port temperature 250 °C, and scanning range m/z 35.0–450 (Chen et al., 2022). All tests were performed in triplicate.

2.8. Determination of lactic acid using High Performance Liquid Chromatography

A 1 mL Baijiu sample was filtered through a 0.22 μm organic filter membrane before High Performance Liquid Chromatography (HPLC, Wukong HPLC K2025, Haineng Future Technology Group Co., Ltd, China) analysis. The mobile phases were prepared as follows: (1) Mobile phase A, 5 % methanol in water; (2) Mobile phase B, 95 % aqueous potassium dihydrogen phosphate solution, prepared by dissolving 1.36 g KH2PO4 in 1000 mL ultra-pure water, adjusting the pH to 2.3 with phosphoric acid, followed by suction filtration and sonication. Lactic acid was detected using HPLC equipped with an Agilent ZORBAX SB-C18 column (4.6 mm × 150 mm, 5 μm). The detection wavelength was 215 nm, column temperature 30 °C, flow rate 1 mL/min, injection volume of 10 μL.

2.9. X-ray diffraction analysis

Dendritic crystals from Feng-flavor Baijiu and standard salt samples (including Ca(CH3COO)2, Mg(CH3COO)2, CH3COONa, CH3COOK, NaCl, KCl, CaCO3, Ca(OH)2, CaCl2, and MgCl2, without further treatment) were analyzed using X-ray diffraction (XRD) with a Bruker D8 X-ray diffractometer (Bruker AXS GmbH), employing a copper anode. The parameters were set as follows: generator voltage 30 kV, generator current 10 mA, wavelength (λ) 1.541 Å, 2θ range from 5° to 60°, and a step size 0.02°.

2.10. Crystallization experiments

2.10.1. Standard salts crystallization

A 55 % ABV blended Feng-flavor Baijiu was prepared as described previously (Shen, 2022; Wang et al., 2014). Salt KCl was added to the blended Baijiu at a concentration of 50 mg/L for control variables and facilitate observation. A 1 μL droplet was placed on an ITO substrate for droplet evaporation and crystallization under controlled conditions (20 ± 2 °C and 65 % ± 2 % RH). The morphology and structure of the dendritic crystals were examined by optical microscopy (200 × ) and scanning electron microscopy (SEM; VEGA 3 SBU, TESCAN Orsay Holding A.S.), respectively. Crystallization processes for Mg(CH3COO)2, CaCO3, CaCl2, Ca(OH)2, MgCl2, and NaCl in the blended Feng-flavor Baijiu samples followed the same procedure as for KCl.

2.10.2. Crystallization of mixed salts

Based on ICP-MS detection results of metal elements in Feng-flavor Baijiu (Table 1), solutions of KCl (25 mg/L), CaCl2 (25 mg/L), Mg(CH3COO)2 (50 mg/L), and CaCO3 (100 mg/L) were dissolved in 55 % ABV blended Feng-flavor Baijiu. The solution was transferred to a 500 mL beaker and heated for 2 h, during which crystals formed. The liquid was then poured into a culture dish to allow further crystallization, and the resulting crystals were collected and subjected to XRD testing.

Table 1.

Metal ion composition of crystals determined by ICP-MS.

Elements Content (μg/mg) Elements Content (μg/mg)
Li 0.22 × 10−3 Mn 0.25 × 10−2
Na 42.54 Fe 0.0825
Mg 45.5 Se 0.825 × 10−3
Al 0.135 × 10−2 Rh 0.25 × 10−2
K 37.5 In 0.3 × 10−2
Sc 0.235 × 10−2 Sn 0.2225 × 10−4
Pb 0.1325 × 10−2 Lu 0.475 × 10−2
Bi 0.725 × 10−2 Ca 118.71
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2.10.3. Observation of Mg(CH3COO)2 crystallization in different ethanol-water binary systems

To investigate the influence of trace components on Mg(CH3COO)2 crystallization, various flavor compounds were added to a 55 % ABV ethanol–water binary system (Table S2), after which Mg(CH3COO)2 was added to reach a final concentration of 50 mg L−1. To observe Mg(CH3COO)2 crystallization in blended liquors, various trace compounds were added to a 53 % ABV ethanol–water binary system and mixed to form different blended liquors (Table S3) as described previously (Zhang et al., 2025); Mg(CH3COO)2 was then added to reach a final concentration of 50 mg L−1. Finally, a 1 μL droplet of each solution was placed on an ITO substrate to dry and crystallize. Crystal morphology and structure were analyzed by optical microscopy and/or SEM.

2.11. Sensory evaluation analysis

Sensory evaluation and description analysis were performed in accordance with the Chinese national standards "Guidelines for Sensory Evaluation of Baijiu" (GB/T 33404-2016) and "Terminology of Baijiu Sensory Evaluation" (GB/T 33405-2016). The tasting panel comprised four males and one female (average 40 years old), all certified as national and provincial Baijiu judges and national first-class tasters with at least two years of Baijiu tasting experience. Before evaluation, all judges were briefed on the experiment's purpose and provided signed informed consent.

Sensory evaluation was conducted in a dedicated sensory evaluation laboratory at the room temperature (20–25 °C). Panelists rated each sample on sensory attributes (expressed as percentages) and unique aroma/flavor characteristics (using a five-point scale), following established protocols (Sun et al., 2021; Zhao et al., 2018). Each Baijiu sample (25 mL) was poured into a national standard tasting cup and coded. Five samples representing different quality grades of Feng-flavor Baijiu (Grade Ⅰ to Ⅴ) were evaluated. All judges evaluated each sample three times. Scoring standards were: "93–100″ indicates excellent, "88–93" (excluding 93) indicates the first grade, and "82–88" (excluding 88) indicates the second grade. Characteristic aroma/flavor descriptors for Feng-flavor Baijiu included "aging-aroma", "Jiuhai-aroma", "alcoholic", "typical Feng-flavor", "aftertaste", and "honey fragrance (aroma) ". Aroma intensity was rated on a 5-point scale ("0″ means no odor, "5″ means the strongest aroma). The average value was used as the final result. The sensory evaluation was approved by the SUSE ethics committee (No. 2025LLSC002).

2.12. Correlation analysis

The relative dendritic crystal area in the deposition pattern was quantified using Image Pro Plus (version 6.0; Media Cybernetics, USA). First, images were calibrated to establish the pixel-to-actual-size ratio. Crystal regions were then selected manually, and the area ratio (proportional area) was calculated automatically by the software. Correlation analysis was performed with GraphPad Prism 6.0 (La Jolla, CA, USA). The Spearman rank correlation coefficient was used to assess the relationship between dendritic crystal area and sensory evaluation scores for quality grades (Ⅰ–Ⅴ) of Feng-flavor Baijiu samples. The p values < 0.05 and <0.01 were considered to indicate statistically significant and highly significant correlations, respectively. Quantitative data are presented as mean ± standard error of the mean (SEM) from the indicated number of experiments.

3. Results

3.1. Crystal pattern after droplet evaporation of feng-flavor Baijiu

A 1 μL droplet of various distilled spirits was placed on an ITO glass surface and dried under controlled conditions (20 °C ± 2 °C, 65 % ± 2 % RH). After 48 h, the deposition patterns were observed and recorded using a dark-field microscope. Fig. 1c and d shows crystals and crystal patterns formed after drying Baijiu droplets on the ITO substrate. Notably, these droplets of Feng-flavor Baijiu form dendritic crystals resembling pine needles upon drying (Fig. 1b), a pattern distinct from other Baijiu types (Fig. 1c–d, Fig. S1). It is noteworthy that "feng" means "phoenix" in Chinese. Surprisingly, the crystals from Feng-flavor Baijiu form patterns resembling phoenix feathers, a characteristic feature absents in other types of distilled spirits, including Brandy, Vodka, Rum, and Whisky (Fig. S1 and Table S4). To further validate this, SEM was employed to examine the deposit microstructure. Consistent with optical microscopy, the crystals exhibit a fractal structure at the microscopic level (Fig. 1e).

Fig. 1.

Fig. 1

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Crystals and crystal patterns formed after drying Baijiu droplets on an ITO substrate. (a) Schematic diagram illustrating droplet evaporation of Baijiu samples. The deposition pattern shown is from standard Baijiu samples obtained from the China Alcoholic Drinks Association. (b–d) Crystals in the deposition patterns of standard Baijiu: (b) Feng-flavor, (c) Sauce-flavor, and (d) Strong-flavor. (e–g) Corresponding SEM images for panels (b–d), respectively. (h–j) Crystal patterns of Feng-flavor Baijiu samples from three different manufacturers. (k–m) Corresponding SEM images for panels (h–j), respectively.

To further investigate the consistency of dendritic crystal patterns in Feng-flavor Baijiu, samples from three different manufacturers were subjected to droplet evaporation. All samples formed dendritic crystal deposits, though the deposition patterns varied (Fig. 1h–j). These results demonstrated that dendritic crystals were reproducible across all tested Feng-flavor Baijiu samples and could serve as a characteristic marker. Previous studies on droplet evaporation of various Baijiu samples reported distinct deposition (Zhang et al., 2025). Since these observations occurred within 3 h of evaporation, it is possible that Feng-flavor Baijiu droplets had not fully evaporated at that time, which may explain the absence of earlier crystal depositions. Recently, the evaporation of whisky and ouzo droplets has been investigated most extensively(Adam et al., 2020; Tan et al., 2016). Upon complete drying, whisky droplets leave a distinctive "whisky web" deposit on the substrate, which can be used for whisky sample analysis and product identification(Adam et al., 2020). For the evaporation of ouzo, ultimately, a large oil droplet is left behind on a surface (Tan et al., 2016). To the best of our knowledge, similar dendritic crystallization phenomena have not been reported in other distilled spirits.

3.2. Composition of dendritic crystal in feng-flavor Baijiu

Dendritic crystals composition was determined using XPS and ICP-MS. XPS results revealed elements including magnesium (Mg), calcium (Ca), potassium (K), oxygen (O), carbon (C), chlorine (Cl), and sodium (Na) (Fig. 2b, Table S5). Subsequently, ICP-MS quantified the content of metal ions in the crystals (Table 1), revealing that the dendritic crystals were composed of Mg, Ca, Na, K, and other elements, with Mg content reaching 45.5 μg/mg and Ca content reaching 118.71 μg/mg, significantly higher than other metal elements. The crystal also contained 37.5 μg/mg K and 42.54 μg/mg Na.

Fig. 2.

Fig. 2

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Composition analysis of Feng-flavor Baijiu crystals. (a) Schematic diagram of XPS scanning points. (b) XPS full-spectrum scan of these points showing survey spectrum and core-level peaks. (c) XRD patterns of dendritic crystals of Feng-flavor Baijiu compared to potential standard salts (untreated). Crystal micrographs of KCl (d), CaCO3 (e), Mg(CH3COO)2 (f), and a salt mixture (KCl: CaCl2: Mg(CH3COO)2: CaCO3, (g) formed within dried droplets of blended Feng-flavor Baijiu.

To identify plausible crystalline phases, we selected ten reference salts based on the XPS/ICP–MS results and the acetate-dominated organic profile (GC–MS; see Fig. S6), and compared their powder X-ray diffraction (PXRD) patterns with that of the dendritic deposits. As shown in Fig. 2c, the dendrite PXRD pattern is broadly consistent with a multicomponent salt mixture, and several reflections can be tentatively assigned to Mg(CH3COO)2, KCl, CaCO3, and minor Ca(OH)2. The match is not exact, which is expected for a complex evaporative residue: peak positions and intensities may be influenced by phase coexistence and peak overlap, preferred orientation, crystallite size/strain, variable hydration states, residual organics, and drying-induced transformations (e.g., partial carbonation of Ca(OH)2 to CaCO3).

Notably, although the ICP-MS analysis detected a relatively high content of Na (Table 1), the characteristic reflections of NaCl are absent or negligible in the dendrite pattern. Instead, reflections consistent with CH3COONa are observed (with minor deviations), suggesting that Na is predominantly associated with acetate rather than chloride in the crystalline fraction. Likewise, despite the high Ca content, no reflections attributable to Ca(CH3COO)2 are detected, indicating that calcium acetate is unlikely to be the dominant crystalline phase. These results collectively imply that dendrite formation is not simply dictated by the most abundant ions, but by phase-specific crystallization thermodynamics and kinetics.

To further evaluate which candidate phases can reproduce dendrite-like morphologies under Baijiu drying conditions, we performed controlled evaporation experiments by dissolving each reference salt in blended Feng-flavor Baijiu and allowing 1 μL droplets to dry on ITO substrates under identical conditions. Among the tested salts, only Mg(CH3COO)2 and CaCO3 formed crystals morphological similar to those in Feng-flavor Baijiu (Fig. 2e and f), while KCl (Fig. 2d), MgCl2, CaCl2, Ca(OH)2, NaCl and CH3COONa did not (Fig. S2). Consistent with this, SEM results confirmed the crystal morphology of Mg(CH3COO)2 and CaCO3 closely resembled the dendritic crystals (Fig. 2e and f). Therefore, Mg(CH3COO)2 and CaCO3 may be the main components, alongside KCl and minor Ca(OH)2; these compounds provide the metal ions (Mg2+, Ca2+, Na+, K+) necessary for dendritic crystal formation.

As supplementary evidence, we conducted experiments to reproduce dendritic crystals by dissolving a mixture of candidate compounds in blended Feng-flavor Baijiu. Given the difficulty in accurately determining the Ca(OH)2-to-CaCO3 ratio in the dendritic crystals found in Feng-flavor Baijiu, CaCO3 was chosen as the primary calcium source and combined with Mg(CH3COO)2 and KCl to reproduce dendritic crystals in blended Feng-flavor Baijiu. NaCl was omitted because it did not reproduce dendritic morphologies under our conditions and its characteristic reflections were absent (or negligible) in the dendrite PXRD. The crystal morphology of the mixture (KCl: CaCl2: Mg(CH3COO)2: CaCO3) was shown in Fig. 2g, demonstrating a similar to those shown in Fig. 1e. Furthermore, the XRD pattern of the mixture overlapped with those of the dendritic crystals (Fig. 2c) suggesting that these substances were likely incorporated into the dendritic crystals in similar proportions.

From a mechanistic perspective, crystallization from solution is driven by supersaturation. However, supersaturation is necessary but not always sufficient for precipitation because nucleation can be delayed within the metastable zone. In multicomponent solutions, the phase that first reaches a sufficient supersaturation to overcome the nucleation barrier typically precipitates first, followed by other phases as their respective supersaturation thresholds are exceeded (De Yoreo and Vekilov, 2003; Eugster, 1980). Importantly, mixed solvents (water-ethanol) can substantially alter electrolyte non-ideality and activity coefficients, potentially changing the precipitation sequence and even reversing which salt crystallizes first (Jeffries, 1931; Nefeli et al., 2023). This provides a plausible explanation for why Mg(CH3COO)2 crystallizes whereas Ca(CH3COO)2 does not under identical drying conditions.

It should be noted that crystallization in Feng-flavor Baijiu is highly complex, involving coupled effects of nonvolatile organics, multiple inorganic ions, and drying-induced transformations. Therefore, the present results are best interpreted as evidence for plausible crystalline constituents and a likely precipitation pathway, rather than an exhaustive identification of all phases.

3.3. "Jiuhai" storage container: the key to crystal formation

The unique dendritic crystal pattern in Feng-flavor Baijiu is attributed to its high content of Ca, Mg, K, and Na (Table S6), which are likely related to its distinct production process. Unlike other distilled spirits, this spirit is aged in a special container called Jiuhai. Jiuhai is made of hemp paper, pork blood, lime, egg white, edible oil, and beeswax (Fig. 3) (Shen, 2022). This intricate mixture creates a porous, yet protective environment that allows subtle interactions between the Feng-flavor Baijiu and its surroundings. We hypothesized that the dendritic crystal components mainly originate from Jiuhai aging process. To verify this speculation, we compared deposition patterns of fresh Feng-flavor Baijiu (not stored in Jiuhai) with those of finished Feng-flavor Baijiu (stored in Jiuhai or Jiuhai-aging) using droplet evaporation. As shown in Fig. 3, no dendritic crystals were found in the fresh Feng-flavor Baijiu, while numerous dendritic crystals appeared in finished Feng-flavor Baijiu pattern (Fig. 3b, Fig. S3), demonstrating the essential role of Jiuhai container in dendritic crystal formation. The ICP-MS analysis, combined with prior research, confirmed elevated concentrations of metal elements such as Mg, Ca, Na, and K in stored Feng-flavor Baijiu compared to fresh samples (Table S6). This finding elucidates why dendritic crystals form exclusively in Jiuhai-aged samples.

Fig. 3.

Fig. 3

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Observation of crystallization capability of fresh (a) and aged (b) Feng-flavor Baijiu. Upper panel: Schematic diagrams illustrating the fresh and Jiuhai-aged Feng-flavor Baijiu. Middle panel: Crystals and deposition patterns of fresh (left) and Jiuhai-aged (right) Feng-flavor Baijiu. Lower panel: SEM images of local deposits for fresh (left) and Jiuhai-aged (right) Feng-flavor Baijiu.

The fresh Feng-flavor Baijiu, once stored in the intricately designed Jiuhai container, is meticulously blended into finished Baijiu products and samples of various quality grades. The Jiuhai container is renowned for its unique craftsmanship (Fig. 3b) and its ability to enhance the aroma of Feng-flavor Baijiu, playing a vital role in the aging process. Simultaneously, the complex materials used in its construction contribute substantial metal ions to the Baijiu, such as sodium ions from pig's blood and calcium and magnesium ions from lime (Wei et al., 2005; Liu and Yan, 2020). Consequently, the quality grade of Feng-flavor Baijiu is determined by the intensity and elegance of the Jiuhai aroma, which is characterized by a harmonious blend of ethyl acetate and ethyl hexanoate, featuring a complex fragrance that combines freshness and richness, sweet and smooth taste, long-lasting aftertaste, and a hint of floral and honey aroma. Generally, the quality grades of Feng-flavor Baijiu are divided into five levels-Grade I, Grade II, Grade III, Grade IV, and Grade V-which exhibit different Jiuhai aroma scores determined by sensory analysis (Fig. S4, Table S7).

3.4. Correlation between crystal patterns and quality of feng-flavor Baijiu

To investigate the relationship between dendritic crystals and Feng-flavor Baijiu quality, fifteen standard samples representing quality grades I to V (all Jiuhai-aged Baijiu samples) were analyzed by droplet evaporation. These samples exhibited decreasing flavor scores with lower grades. As shown in Fig. 4a and b, grades I and II had a high density of dendritic crystals with uniform pattern. SEM further scanning indicated these dendritic crystals were mainly composed of flaky crystals. Grades III and IV also formed dendritic crystals with a coffee-ring appearance, but their crystal abundance was significantly lower than that of grades I and II. In contrast, the low-quality grade V sample showed no distinct dendritic crystals; instead, it featured regularly shaped cubic crystals arranged along the contact line (Fig. 4b). A detailed description of the crystal characteristics for five standard samples of Feng-flavor Baijiu is shown in Table S8. Interestingly, a linear regression analysis confirmed a significant correlation between the area proportion of crystals and the sensory score of Feng-flavor Baijiu (R2 = 0.904, p < 0.01; Fig. 6c).

Fig. 4.

Fig. 4

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Correlation between dendritic crystals and Feng-flavor Baijiu quality. (a) Crystallization patterns after droplet drying of five standards Feng-flavor Baijiu samples (Grades I to V). (b) SEM images of crystals. (c) Correlation analysis between the area proportion of crystals in the deposition pattern and the sensory score in five standard Feng-flavor Baijiu samples (Grades I to V). (d) Metal ion levels in five standard Feng-flavor Baijiu samples (Grades I to V) by ICP-MS. (e) Heat map of trace components in five standard Feng-flavor Baijiu samples (Grades I to V) by GC-MS.

Fig. 6.

Fig. 6

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Results of Mg(CH3COO)2 crystallization in different solutions. SEM of deposits for droplet evaporation of Mg(CH3COO)2 in different ethanol-water solutions, including in ethyl acetate-ethanol-water solution (a), in ethyl hexanoate-ethanol-water solution (b), in ethyl lactate-ethanol-water solution (c), in acetic acid-ethanol-water solution (d), in hexanoic acid-ethanol-water solution (e), in lactic acid-ethanol-water solution (f), in blended Feng-flavor Baijiu (g), in blended strong-flavor Baijiu (h), in blended sauce-flavor Baijiu (i), in blended rice-flavor Baijiu (j), in blended light-flavor Baijiu (k), and in blended sesame-flavor Baijiu (l).

The variations in crystal characteristics may be attributed to differences in metal contents across different quality grades of Feng-flavor Baijiu (Fig. 4d–Table S6). Grades I and II exhibited higher concentrations of Ca, Mg, K, and Na, as determined by ICP-MS, compared to grades III, IV, and V (Fig. 4d–Table S6), which aligns with the observed crystal features (Fig. 4b). Unexpectedly, grades III, IV, and V displayed similar metal abundance (Ca 29 mg/L, 33 mg/L, and 26 mg/L, respectively), despite notable differences in their crystal characteristics. This suggests that other trace components play a significant impact role in the formation of dendritic crystal.

Subsequently, we analyzed the five standard-grade samples of Feng-flavor Baijiu (grades I to V) by GC-MS. Seventy-six trace flavor components were identified, including 31 esters, 27 alcohols, 9 acids, and 9 other compounds (Fig. 4e–Table S9). Characteristic marker flavor components for Feng-flavor Baijiu, such as ethyl hexanoate, ethyl acetate, ethyl lactate, acetic acid, lactic acid, and caproic acid, were present at relatively high in all samples. However, the total ester content (ethyl hexanoate and ethyl lactate) was higher in grades I and II, whereas the total acid content (acetic acid and lactic acid) was elevated in grades III, IV, and V (Table S10). These variations contributed to the differences in quality grades.

3.5. Evaporation model and crystal pattern

As mentioned, the evaporation model and solute transport dynamics are essential factors influencing crystallization and pattern formation. The evaporation model of sessile Feng-flavor Baijiu droplets was studied by monitoring the contact angle, contact radius, and droplet height during evaporation. As shown in Fig. 5a and b, the Feng-flavor Baijiu droplet undergoes a constant contact radius (CCR) evaporation process due to the pinning of the three-phase contact line (TPCL) caused by substrate morphological or/and chemical heterogeneity (Liu et al., 2020). In CCR evaporation mode, the contact angle gradually decreased while the radius remains stable, and the liquid evaporating from the droplet edge was compensated by liquid from the bulk, leading to an outward capillary flow that carried the solute to the periphery, forming a typical "coffee ring" pattern (Samantha et al., 2018). In this study, crystals grew from the periphery toward the center, forming a "thorny crown" pattern (Movie S1).

Fig. 5.

Fig. 5

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Drying models, evaporation-driven flows, and crystal growth of Feng-flavor Baijiu droplet. (a) Evolution of contact angle (CA), height (H) and contact radius (CR) of Feng-flavor Baijiu sessile droplet during the evaporation. (b) Schematic diagram of CCR evaporation model of droplet. (c) Visualization of fluid flow using fluorescent particles during the droplet evaporation of Feng-flavor Baijiu. (d) Schematic side views corresponding to the flow fields above. White arrows indicate the flow pattern. Flow regimes are as follows: multiple vortices (I), a circulatory flow (II), and radial outwards flow (III). (e) Observation of crystal growth process of Feng-flavor Baijiu at TPCL (200 × ). Red arrows indicate the crystal nucleation site, and dashed white line marks TPCL.

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Crystal nucleation and growth involve solute transport. To visualize solute transport during evaporation, 0.1 μm fluorescent particles were added to trace the internal flow within the Feng-flavor Baijiu droplet. The internal flow evolution resembled that observed in sauce-flavor Baijiu droplet (Zhang et al., 2025). Stage Ⅰ (t < 50 s, Fig. 5c and d, Movie S2) was dominated by alcohol evaporation, featuring multiple vortexes. Stage Ⅱ (50 s < t < 209s, Fig. 5c and d, Movie S2) involved Marangoni convection driven by solute concentration gradients at the liquid-vapor interface. Stage III (209 s < t < 528s, Fig. 5c and d, Movie S2) was the water evaporation stage after ethanol evaporation, where capillary radial flow or compensation flow became more pronounced.

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In Stage III, faster evaporation at TPCL driven the capillary radial flow or compensation flow, transporting solutes towards the TPCL (Christy et al., 2011; Efstratiou et al., 2020). About 6 h into evaporation, the solute concentration at the TPCL locally exceeded its solubility, reaching supersaturation, and initiating nucleation and crystal growth. (Fig. 5e, Movie S3). Fig. 5e illustrates the process of crystal branching growth at TPCL, and SEM analysis further revealed that the crystals consist of needle- and flake-shaped forms that assemble into a dendrite pattern (Fig. 1). Therefore, solute transport in this stage critically impacted crystal nucleation and growth.

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3.6. Effects of trace components on dendritic crystal formation

To further investigate the effect of trace components, representative acids and esters were individually added to the Mg(CH3COO)2-ethanol-water ternary system for crystallization experiments. Surprisingly, dendritic crystals did not form when Mg(CH3COO)2 was introduced to the water or ethanol-water systems (Fig. S5a and S5b). Interestingly, esters, especially ethyl lactate, obviously promoted the formation of Mg(CH3COO)2 dendritic crystal (Fig. 6a–c), while acids, particularly lactic acid, always inhibited it (Fig. 6d–f). These finding indicate that trace components, along with metals, crucially influence dendritic crystal formation and morphology, aligning with the acid and ester content across different quality grades of Feng-flavor Baijiu. Given the substantial variation in flavor compounds in Feng-flavor Baijiu, such as ethyl hexanoate and ethyl acetate (Shen, 2022; Wang et al., 2014), corresponding variations are observed in the morphology and deposition patterns of metal salt crystals.

Finally, the crystallization capability of Mg(CH3COO)2 was evaluated in blended liquors representing different flavor types of Baijiu. More interestingly, the crystal morphology was found to be flavor-dependent. Dendritic crystals formed in the blended Feng-flavor Baijiu (Fig. 6g), whereas only needle-like crystals were observed in the Sauce- and Strong-flavor types (Fig. 6h and i), and amorphous solids were found in the others (Fig. 6j–l). These results further confirmed the novel dendritic morphology that develops upon droplet evaporation of Feng-flavor Baijiu. Consequently, we propose the development of a novel visual method for evaluating and identifying Feng-flavor Baijiu, utilizing its dendritic crystal structure.

As pointed out in the text, the formation of dendritic crystals involves the interaction of multiple metal ions and trace components. In general, esters promote crystal formation while acids inhibit it. However, the specific roles that acids and esters play in concert with metal ions during crystallization require further study. The crystal composition and formation process are complex. Due to limitations in current experimental conditions, it is difficult to qualitatively or quantitatively observe the underlying chemical and physical interactions, making the exact mechanism of dendritic crystal formation unclear.

4. Conclusion

In summary, this study investigated the evaporation of sessile Baijiu droplets on an ITO surface under controlled conditions, and revealed the distinctive formation of dendritic crystal patterns with a coffee-ring structure, especially in Feng-flavor Baijiu. The pinning of the TPCL, coupled with localized rapid evaporation at the TPCL, and the capillary radial flows towards the TPCL during the final evaporation stage (Stage III) led to supersaturation near the TPCL, which initiated crystal nucleation and subsequent dendritic growth extending toward the droplet center, ultimately forming the observed dendritic pattern within a ring.

The findings demonstrated that the unique Jiuhai-aged container is essential for formation of dendritic crystals, primarily due to the release of specific metal ions (e.g., Ca2+, Mg2+, K+, Na+). In addition to these ions, trace components, such as hexanoic acid, lactic acid, ethyl hexanoate, and ethyl lactate, also play a critical role in crystal development. Although the overall salt composition may be similar across samples, variations in these trace compounds significantly influence the morphology and deposition patterns of dendritic crystals. Notably, the dendritic crystal patterns show a statistically significant correlation with the quality of Feng-flavor Baijiu (p < 0.01). Therefore, the characteristic dendritic patterns generated by droplet evaporation hold promise as visual markers for quality assessing and identifying of Feng-flavor Baijiu.

There are several limitations to this study. Given variability introduced by factors such as storage duration, production environment, and aging in Jiuhai container, the dendritic crystal deposition patterns in Feng-flavor Baijiu may differ. Consequently, a larger sample size is required to confirm these observations; however, procuring extensive standard samples within a short timeframe is challenging due to the complexity of the brewing process and variability in storage and aging. The primary objective of this study was to characterize a unique dendritic crystal deposition pattern observed during droplet evaporation of Feng-flavor Baijiu. Establishing a standardized protocol based on a large sample set and consistent deposition patterns, combined with artificial-intelligence algorithms, could enable development of a rapid, intuitive auxiliary detection method.

Authorship contribution

Haoran Fu: Data curation, Formal analysis, Investigation, Methodology, Validation, Writing - original draft, Writing - review & editing. Yang Zhong: Data curation, Formal analysis, Investigation, Methodology. Lu Cai: Data curation, Formal analysis, Methodology, Investigation. Tieyuan Cheng: Formal analysis, Investigation, Resources. Yulin Xia: Formal analysis, Investigation, Resources. XiaoHong Jian: Resources, Supervision, Validation. Jun Liu: Resources, Software, Validation. Zaixin Li: Resources, Supervision, Validation. Huibo Luo: Methodology, Resources. Ziqing Wu: Formal analysis, Funding acquisition, Writing - review & editing. Yongming Liu: Conceptualization, Methodology, Validation, Supervision, Resources, Funding acquisition, Writing – original draft, Writing - review & editing. Siqi Yuan: Conceptualization, Methodology, Investigation, Supervision, Funding acquisition, Writing – review & editing. Zhi Zhang: Conceptualization, Funding acquisition, Supervision, Validation, Project administration, Resources, Writing - original draft, Writing - review & editing.

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 research was funded by the Sichuan Province Science and Technology Support Program (Grant numbers 2026NSFSC0150 and 2023YFS0447), the Research Support Project of Sanming University (Grant number 23YG14S, 23YG15S), the Key Research Program of Sichuan Province (Grant no. 2024YFFK0424), the Education and Research Project for Young and Middle-aged Teachers in Fujian Province (Grant number JAT231112, JAT241136), and the Key Laboratory of Brewing Biotechnology and Intelligent Manufacturing of China National Light Industry Council (Grant number 2023–04).

Handling Editor: Professor Alejandro G.Marangoni

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.crfs.2026.101324.

Contributor Information

Yongming Liu, Email: auliuym@mail.nwpu.edu.cn.

Siqi Yuan, Email: yuansiqi@suse.edu.cn.

Zhi Zhang, Email: zhangzhi@suse.edu.cn.

Appendix A. Supplementary data

The following are the Supplementary data to this article.

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