Physicochemical Characterization of Soluble and Insoluble Dietary Fiber Fractions from Apple Pomace Using Green Pre-treatment: Toward Carbon Reduction and Circular Resource Valorization

1. Introduction

Apple processing generates large volumes of apple pomace (AP), a by-product remaining after juice and cider production (Costa et al., 2022; Thomas et al., 2025). Improperly managed landfilling of AP and other biodegradable fruit-processing residues can generate methane emissions and leachate (Czubaszek et al., 2022; Putra et al., 2023). In South Korea, apple production remains substantial, with recent estimates placing national output at roughly 0.43 million metric tons and orchard area at around 33,900 ha (USDA Foreign Agricultural Service (FAS), 2023; Korea Agro–Fisheries & Food Trade Corporation (aT), 2023). From an environmental-science perspective, upcycling—converting low-value byproducts into higher-value products—can reduce disposal impacts and potentially lower waste–management–related greenhouse-gas emissions (Ampese et al., 2023; Azarakhsh et al., 2024), while enabling the recovery of bioactive compounds and biopolymer-rich fractions from agri-food by-products (Ben-Othman et al., 2020; Pascoalino et al., 2025).

AP is generated in large quantities but is often underutilized because its high moisture content and compositional heterogeneity accelerate spoilage and complicate collection, storage, and transport; therefore, rapid stabilization and careful management are required (Vandorou et al., 2024; Thomas et al., 2025). Nevertheless, systematic reviews and meta-analyses support incorporating AP for food fortification across diverse food categories, underscoring its potential as a value-added valorization pathway aligned with waste-management objectives (Antonic et al., 2020; Lyu et al., 2020; Zaky et al., 2024). In this context, fractionating AP into standardized, functionality-oriented ingredient streams—such as soluble and insoluble dietary-fiber fractions (SDF and IDF)—can enhance value capture and broaden application potential by providing more consistent, fit-for-purpose materials (Buljeta et al., 2023; Thomas et al., 2025).

AP is increasingly recognized as a fiber-rich by-product for value-added utilization because it contains abundant cell-wall polysaccharides such as cellulose, hemicellulose, and pectin; however, its high moisture content and perishability necessitate prompt stabilization (e.g., drying) to enable storage and downstream processing (Vandorou et al., 2024; Thomas et al., 2025). To obtain standardized dietary-fiber fractions—particularly soluble dietary fiber (SDF) for incorporation into food matrices—enhancing partial solubilization of the cell-wall matrix is a key step (Buljeta et al., 2023; Al Faruq et al., 2025). Acid-assisted extraction (e.g., HCl) can promote solubilization, but conventional acid– and base-assisted routes may generate chemical wastes and acidic or alkaline effluents requiring neutralization and treatment prior to discharge and may contribute to equipment corrosion, increasing operational and environmental burdens (Riyamol et al., 2023; Pattarapisitporn and Noma, 2025). Therefore, low-chemical pre-treatment strategies may offer carbon- and resource-efficiency benefits (Riyamol et al., 2023; Pattarapisitporn and Noma, 2025); workflow selection and optimization for targeted SDF and IDF recovery are largely governed by extraction methods and key processing conditions (temperature, pH, solvent, time, and liquid–to–solid ratio) (Buljeta et al., 2023; Kaur et al., 2025).

A range of pre-treatment and modification strategies (chemical and enzymatic treatments, thermomechanical processing, and microbial fermentation) has been explored to enrich SDF and improve fiber functionality, as these routes can reshape fiber microstructure and physicochemical properties, with increasing interest in more sustainable processing (Liu et al., 2024; Kumari et al., 2025; Zhao et al., 2025). In particular, sustainability-oriented “green” approaches can improve SDF yield and functionality (Al Faruq et al., 2025; Zhao et al., 2025). Within this framework, fermentation-based pre-treatment is a promising option because it leverages microbial enzymes to partially disassemble the cell-wall matrix, thereby promoting SDF release (Wang et al., 2024a; Al Faruq et al., 2025). However, several emerging green technologies—including microwave, ultrasound, and high-pressure homogenisation—remain costly and challenging to scale up (Al Faruq et al., 2025).

As a food-grade koji mold rich in carbohydrate-active enzymes, Aspergillus luchuensis 74-5 can partially deconstruct plant cell-wall polysaccharides to increase extractable soluble fractions, potentially supporting SDF enrichment during downstream fractionation (Yamashita, 2021; Futagami, 2022; Li et al., 2024). However, AP valorization studies often focus on isolated unit operations, with limited benchmarking of end-to-end workflows or clear links between process selection and the physicochemical performance of the resulting SDF and IDF ingredients (Asif et al., 2024; Thomas et al., 2025).

This gap highlights the need for a practical pre-treatment-to-fractionation strategy that (i) increases SDF recovery to improve processability and incorporation in aqueous formulations (Al Faruq et al., 2025; Kumari et al., 2025), (ii) preserves IDF yield to retain overall fiber mass and co-product potential (Ma et al., 2025a; Mammolenti et al., 2025), and (iii) reduces chemical inputs and downstream treatment requirements in support of circular-resource valorization (Al Faruq et al., 2025; Pascoalino et al., 2025).

Accordingly, this study compares a green workflow (A. luchuensis 74-5 fermentation → hot-water extraction → ethanol precipitation) with an untreated control and an HCl-assisted extraction route. Outcomes assessed include SDF and IDF yields, hydration and dispersion properties (solubility, water absorption, dispersibility), and structural signatures (FT-IR, GPC, XRD, SEM, TEM). Collectively, these comparisons clarify how process selection drives fraction recovery and functional performance, informing sustainable, circular valorization of apple pomace.

2. Materials and Methods

2.4. Extraction and fractionation

A schematic overview of the extraction and fractionation into SDF and IDF is shown in Fig. 1. SDF and IDF fractions were separated from the pre-treated, dried AP following Szymańska-Chargot et al.(2017) with minor modifications. Briefly, dried AP (150 g) was extracted at 85℃ for 30 min per cycle (two cycles total) using distilled water for the Control and 74-5 groups (1.5 L per cycle) or hydrochloric acid solutions for the HCl group (1 M for 30 min followed by 0.5 M for 30 min). After each extraction cycle, the slurry was vacuum-filtered through Whatman Grade 4 filter paper (20-25 μm particle retention), and the retained residue was collected as the IDF fraction. Filtrates from both cycles were pooled and mixed with ethanol at a filtrate:ethanol ratio of 1:4 (v/v), followed by precipitation at 4℃ for 24 h. The precipitate was recovered by vacuum filtration as the SDF fraction. Both SDF and IDF fractions were hot-air dried at 50℃ to constant weight, ground into powders, and used for subsequent analyses.

Fig. 1.

(A) Schematic workflow for SDF and IDF fractionation from apple pomace highlighting the green (74-5) process, and (B) summary of steps applied in the Control, 74-5, and HCl treatments. (A) provides a schematic of the green (A. luchuensis 74-5) process for SDF and IDF fractionation: IDF was collected as the filter-retained residue after vacuum filtration, and SDF was recovered from pooled filtrates by ethanol precipitation followed by vacuum filtration. (B) summarizes the processing steps applied to each treatment group (Control, 74-5, and HCl); ○, applied; ×, not applied; RT, room temperature (20–25℃).

3. Results and Discussion

3.1. Effects of pre-treatment on SDF and IDF yields

Treatment significantly altered the partitioning of dietary fiber into soluble and insoluble fractions (Table 1). The control showed a low SDF yield (1.51%), whereas A. luchuensis 74-5 pre-treatment and the HCl-assisted process increased SDF yield to 10.17% and 12.00%, respectively (p < 0.05). Notably, the 74-5 workflow retained a higher IDF yield than the HCl condition (50.47% vs. 30.95%; p < 0.05). These trends are consistent with reports that fermentation-based pre-treatment can promote partial solubilization of fiber-rich matrices via microbial enzyme activities, thereby increasing extractable soluble fractions (Wang et al., 2024b: Al Faruq et al., 2025). Nevertheless, fermentation effects on soluble cell–wall polysaccharides are strain- and process-dependent, and changes in SDF may reflect a balance between matrix loosening and solubilization and enzymatic depolymerization (Tuly and Ma, 2024; Zhao et al., 2025; Meng et al., 2025). In one study, Aspergillus oryzae solid-state fermentation of apple peel was reported to decrease SDF while increasing IDF, suggesting that enzymatic depolymerization may offset solubilization under certain conditions (Li et al., 2024). Accordingly, the increased SDF observed in the present 74-5 workflow suggests that matrix loosening and solubilization predominated over enzymatic depolymerization under our fractionation conditions.

Table 1.

Comparison of SDF and IDF yields and physicochemical properties in AP: Control, HCl, and 74-5 (A. luchuensis )

3.3. FT-IR spectra of SDF and IDF fractions

In the SDF spectra (Fig. 2A, red-highlighted), bands at 1730-1745 cm⁻¹ and 1600-1630 cm⁻¹ were observed across treatments and are attributable to pectic ester carbonyl (C=O) and carboxylate (COO⁻) groups, respectively (Jarrín-Chacón et al., 2023; Santiago-Gómez et al., 2025). Because the relative intensity of these bands is commonly associated with the methyl-esterification state of pectic carboxyl groups, the observed profiles suggest pre–treatment-dependent differences in pectic functional–group signatures. Notably, the HCl–derived SDF showed a relatively more pronounced band near 1730-1745 cm⁻¹ than the control and 74-5 SDFs, consistent with an acid-associated shift in the balance between ester carbonyl and carboxylate contributions under the present conditions. By contrast, the IDF spectra (Fig. 2B, blue-highlighted) were dominated by the aliphatic C-H stretching region at 2950-2850 cm⁻¹, with distinct bands near 2920 and 2890 cm⁻¹ across treatments, consistent with plant cell-wall materials (e.g., cellulose-rich fibers) (Meraj et al., 2024; Kaya, 2024).

Fig. 2.

FT-IR spectra of soluble (A) and insoluble (B) dietary fiber fractions isolated from AP after pre-treatment with either HCl or A. luchuensis 74-5.

3.4. GPC chromatograms of SDF and IDF fractions

GPC chromatograms showed that pre-treatment altered polymer populations in both SDF and IDF fractions by shifting the molecular-weight distribution (MWD) (Fig. 3). These changes were reflected in differences in dominant peak-top molecular weights (Mp) and dispersity (Mw/Mn) (PEG/PEO-equivalent; RI-based). For the SDF fraction (Fig. 3A), Control_SDF exhibited a broad, multi-peaked distribution (Mw/Mn = 114). The 74–5 pre-treatment shifted the distribution toward a dominant low-kDa population while retaining a high-MW tail (Mw/Mn = 33.4), consistent with reports that fermentation can remodel pectin populations during subsequent aqueous extraction (Xu et al., 2022; Muslu Can et al., 2024). In contrast, HCl_SDF showed the narrowest distribution centered around a mid-kDa population (Mw/Mn = 12.7), consistent with acid-catalyzed depolymerization and peak “focusing” under heated acidic extraction (Ma et al., 2025b; Pattarapisitporn and Noma, 2025). For the IDF fraction (Fig. 3B), Control_IDF was dominated by low-kDa components with high dispersity (Mw/Mn = 70.7). The 74-5 pre-treatment increased the relative contribution of a low-kDa population while retaining a minor high-MW tail (Mw/Mn = 56.5), whereas HCl_IDF displayed a simplified two-population profile with reduced dispersity (Mw/Mn = 25.4), consistent with acid hydrolysis-driven redistribution of polymer size classes (Ma et al., 2025b; Pattarapisitporn and Noma, 2025).

Fig. 3.

GPC chromatograms of soluble (A) and insoluble (B) dietary fiber fractions isolated from AP after pre-treatment with either HCl or A. luchuensis 74-5.

3.5. XRD patterns of SDF and IDF fractions

In the SDF patterns (Fig. 4A), all treatments showed a broad halo in the 13-22° (2θ) region (red boxes) with no prominent crystalline reflections, indicating predominantly amorphous polysaccharide matrices (Sayed et al., 2022). By contrast, the IDF patterns (Fig. 4B) showed cellulose-associated diffraction peaks, including a strong peak at 2θ≈22° (blue boxes) and a weak shoulder feature near 16-17° (blue circles), indicating crystalline cellulose domains in the insoluble fraction.

Fig. 4.

XRD patterns of soluble (A) and insoluble (B) dietary fiber fractions isolated from AP after pre-treatment with either HCl or A. luchuensis 74-5.

3.6. Microstructure and morphology assessed by SEM and TEM

3.6.1. SEM

In the SDF SEM images (Fig. 5A), 74-5_SDF exhibited a rough, microvoid-rich surface compared with the more compact, sheet-like appearance of Control_SDF and HCl_SDF, consistent with reports that processing can alter the porosity and surface texture of dietary-fiber/pectin-rich materials (Wu et al., 2022; Qin et al., 2024). In the IDF SEM images (Fig. 5B), all treatments largely retained a plate-like morphology, although surface roughness and particulate deposits differed among treatments. Overall, the more open surface texture of 74-5_SDF could facilitate wetting and water penetration, qualitatively consistent with its higher solubility and lower water absorption in Table 1.

Fig. 5.

SEM images of soluble (A) and insoluble (B) dietary fiber fractions isolated from AP after pre-treatment with either HCl or A. luchuensis 74-5.

3.6.2. TEM

In the SDF TEM images (Fig. 6A), Control_SDF showed a largely uniform background with only occasional electron-dense particulates. By contrast, 74-5_SDF exhibited more frequent, finely dispersed electron-dense nanoscale particulates, whereas HCl_SDF showed fewer but larger electron-dense clusters. Together, these patterns suggest that, even with an identical ethanol-precipitation step, upstream pre–treatment and extraction conditions may influence the nanoscale association of the recovered SDF, with 74-5 pre-treatment tending toward finer dispersion and the HCl-assisted route tending toward larger clusters (Guo et al., 2017; Chen et al., 2021). In the IDF TEM images (Fig. 6B), Control_IDF displayed a compact electron-dense aggregate, while 74-5_IDF showed smaller and more dispersed particulates across the field. In contrast, HCl_IDF exhibited an extensive, interconnected web-like fibrillar network. Such network-like nanofibrillar morphologies have been reported for fibrous substrates after chemical or enzyme-assisted processing, supporting the interpretation that pre-treatment altered the nanoscale organization of the insoluble matrix (Chen et al., 2015; Xie et al., 2016).

Fig. 6.

TEM images of soluble (A) and insoluble (B) dietary fiber fractions isolated from AP after pre-treatment with either HCl or A. luchuensis 74-5.

4. Conclusions

This study compared a fermentation–based pre-treatment (A. luchuensis 74–5) with an HCl-assisted route for fractionating apple pomace into SDF and IDF. The 74-5 workflow increased SDF yield from 1.51% to 10.17%, approaching the HCl-assisted process (12.00%), while retaining a higher IDF yield than HCl (50.47% vs. 30.95%; p < 0.05). The 74-5-derived SDF exhibited markedly higher solubility (93.19%) and lower water absorption (0.80 g/g) than the HCl-derived SDF (49.39% and 2.60 g/g), with comparable dispersibility. FT-IR, GPC, XRD, SEM, and TEM analyses supported workflow-dependent differences in structural signatures and microstructure across treatments.

Overall, the 74-5-based workflow (fermentation pre-treatment-hot-water extraction-ethanol precipitation) represents a reduced-chemical option for producing functionality-oriented SDF and IDF fractions from apple pomace. By avoiding strong–acid extraction, this workflow may reduce chemical inputs as well as downstream neutralization and wastewater-treatment requirements. Moreover, diverting apple pomace from anaerobic disposal pathways such as landfilling may offer potential to mitigate methane-related emissions and the associated energy and carbon burdens compared with acid-based processing.

Acknowledgments

This research was funded by the Research Program for Agriculture Science and Technology Development (Project No. PJ01733201), Rural Development Administration (RDA), Republic of Korea, and by the 2024–2026 RDA Postdoctoral Fellowship Program of the National Institute of Crop and Food Science (NICS), RDA.

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