Nanocellulose (NC) is regarded as a valuable biomaterial due to its high mechanical properties, renewability, and biodegradability. NC is generally obtained from micron-scale cellulose (referring to the purified cellulose used as the precursor for NC fabrication) and is classified into cellulose nanocrystals (CNC) and cellulose nanofibers (CNFs) based on difference in morphology, size, crystallinity, and various physicochemical and structural properties [1]. CNCs are rigid, rod-shaped particles with lengths of approximately 50–500 nm and diameters of 2–50 nm, produced by removing most of the amorphous regions from micron-scale cellulose [2]. In contrast, CNFs are bundled, flexible fibers that retain both crystalline and amorphous regions, typically measuring a few micrometers in length and 2–20 nm in diameter. When NC is added to a matrix to form a composite, it can enhance the composite's mechanical strength, biocompatibility, bioavailability, biodegradability, and thermal properties, even with very small amounts, due to its high surface-to-volume ratio [3], [4], [5]. Moreover, NC contains abundant hydroxyl groups, making it suitable for surface modification [6], [7]. These properties make NC applicable to a wide range of areas, including low-calorie food, smart and eco-friendly packaging system, cosmetics, environmental purification, wound healing systems, and drug encapsulation [4], [5], [7], [8], [9], [10].

Although NC is considered a promising biomaterial with superior physicochemical properties, eco-friendliness, and sustainability, current manufacturing methods have raised environmental concerns. CNC is typically produced via sulfuric acid hydrolysis, which selectively removes amorphous regions from micron-scale cellulose [11]. CNF is commonly obtained through TEMPO-mediated oxidation, which introduces carboxyl groups to facilitate fibrillation [12], [13], [14]. Chemical treatments using sulfuric acid and sodium periodate are effective but generate large volumes of acidic wastewater and hazardous by-products, leading to equipment corrosion, environmental pollution, and consumer concerns which are not sustainable [15]. In addition, these methods pose challenges in process control and may not be well-suited for large-scale production. Their low yields (approximately 13.3–26.1%) also result in high production costs, limiting their applicability in various industrial fields [11]. These limitations highlight the need for more sustainable and efficient production methods for NC.

To address these concerns, several environmentally friendly approaches have been investigated in recent years, including mechanical fibrillation methods such as high-pressure homogenization, milling, and ultrasonication, as well as enzymatic hydrolysis and deep eutectic solvent (DES) treatment [16], [17], [18]. While each method offers specific advantages–such as avoiding harsh chemicals and operating under mild conditions–they also present technical challenges. For instance, mechanical fibrillation processes including high-pressure homogenization, typically require high energy input and often require multiple passes to achieve uniform nanoscale fibers, which limits their scalability and increases production costs [19]. Enzymatic hydrolysis is highly selective but limited by low yields; in most studies, the yield of nanocellulose remains below 20% [20], [21], [22]. In addition, DES treatment alone often fails to achieve complete fibrillation, as bundles of microfibrils have been observed even after subsequent mechanical processing [23]. In response to these challenges, our research team has been developing a novel, eco-friendly, and sustainable method for manufacturing NC.

In this context, several studies have reported on the use of subcritical water extraction (SWE) system to produce NC. SWE operates with water at 100–374 °C under high pressure to maintain its liquid state, allowing the hydrolysis of amorphous regions in micron-scale cellulose without the need for strong acids [24], [25], [26], [27]. Under subcritical water conditions, water serves not just as a solvent but also participates in reactions—promoting partial breakdown of hemicellulose and amorphous cellulose [28], [29]. However, despite eliminating the use of harmful reagents, SWE-derived NC often suffers from limited selectivity, lower yields (approximately 4–6% based on initial biomass) compared to conventional chemical methods, and the generation of degradation by-products, which constrain its industrial applicability [24], [30]. It is crucial to note that these limitations are characteristic of processes focused on the liquid extract.

Unlike conventional SWE methods that focus on obtaining NC from micron-scale cellulose, our approach—subcritical water treatment (SWT)—targets the remaining solid residues in the treatment cell in the system (Fig. 1). We term this method SWT to distinguish it from extraction-oriented processes previously reported [24], [25]. In this study, by utilizing the solid cellulosic fraction, we obtained NC with a yield of approximately 9–13% based on the initial dry biomass. The resulting material showed better physicochemical properties than those typically reported for NC produced by conventional SWE or chemical treatments. Unlike most SWE-based approaches that utilize the liquid extract, this method targets the solid cellulose-rich fraction, thereby preserving structural integrity and minimizing cellulose degradation. This solid fraction is produced through subcritical water treatment (SWT), which selectively removes non-cellulosic components while preserving the crystalline structure. The NC obtained from this process is referred to as subcritical water-treated cellulose (SWTC). SWT-C exhibited improved crystallinity, enhanced thermal stability, stable dispersion behavior, and uniform morphology. In addition, the SWT process avoids the use of corrosive chemicals and simplifies the overall procedure, contributing to higher recovery efficiency and better suitability for scale-up. Taken together, SWT-C offers a practical alternative to NC produced by conventional methods.

This study aims to (1) investigate the physical, chemical, and structural properties of SWT-C produced under various SWT conditions, and (2) compare its characteristics with those of commercially obtained NCs (CNC and CNF). The analysis includes morphology, crystallinity, chemical structure changes, thermal stability, pigment formation, and dispersion behavior in distilled water. Based on these results, the potential of SWT-C is evaluated as a feasible alternative for applications requiring environmentally friendly processing and material consistency.