Abstract
Curcumin (Cur) is a hydrophobic nutraceutical with poor water solubility and low oral bioavailability. This study elucidates how plant protein molecular traits shape the assembly, structure, and digestive fate of Cur-loaded nanoparticles. Using a pH-shifting strategy, zein was employed as a hydrophobic core, while soluble legume protein fractions (Sup) or whey protein isolate (WPI, W) served as shell components. The resulting core–shell nanoparticles exhibited tunable particle sizes (40–130 nm), high uniformity (PDI < 0.18), and excellent loading capacity (up to 316.3 μg/mgprotein). Core–shell composition and ratio determined particle size and compactness, while shell protein identity modulated stability and encapsulation efficiency. Proteins with high hydrophobicity and flexible structures promoted stronger interactions with zein, yielding nanoparticles with superior Cur retention during storage (> 93 %) and re-dispersibility after freeze-drying (up to 86.8 %). Under simulated gastric conditions, zein–shell protein co-assemblies protected cleavage sites, reducing proteolysis and enabling delayed Cur release; the complex of zein with mung bean soluble protein at a 1: 4 ratio (4MsupZC) retained 75.1 % of Cur after 1 h of gastric digestion, outperforming other zein–shell protein composites. Peptidomic profiling confirmed that protein interactions modulated both shell and core digestibility. Correlation analysis supported these findings, revealing that zein incorporation attenuated most structure–particle and particle–digestion linkages, consistent with an interaction-masking effect that decouples shell-protein structure from functional outcomes. Collectively, this work establishes a multi-level structure–function relationship between protein molecular traits and nanoparticle performance, providing a mechanistic basis for designing scalable protein-based delivery systems for poorly soluble bioactives.
Highlights
- Core–shell composition and ratio control nanoparticle size and compactness.
- 9SupZC achieves the highest Cur loading (316.3 μg/mg) and uniformity (PDI < 0.18).
- Zein–shell co-assembly shields protease sites and delays gastric Cur release.
- Correlation analysis shows zein decouples shell proteins’ structure from digestion outcomes.
Introduction
Curcumin (Cur, C), a natural polyphenol derived from turmeric, is well known for its potent antioxidant, anti-inflammatory, and anticancer activities [1]. However, its practical application in food and pharmaceutical products remains limited due to the poor water solubility, chemical instability, and low oral bioavailability [1]. Accordingly, a variety of carriers, including emulsions [2], hydrogels [3,4], liposomes [5], and protein-based or composite nanoparticles [3,4,[6], [7], [8]], have been explored in recent years. Among these, protein-based nanoparticles have emerged as a particularly promising platform, which offer greater flexibility, scalability, and economic viability. They can be readily dispersed in beverages, incorporated into functional foods, or processed into powders for solid formulations. Moreover, their biodegradability, biocompatibility, and compatibility with clean-label formulations make protein nanoparticles highly attractive for nutraceutical applications [9].
Among various protein sources, plant-derived proteins are not only abundant and renewable, but also align with the growing demand for vegetarian and environmentally friendly food ingredients [10]. However, it also leads to heterogeneous composition, posing challenges for achieving uniform and stable protein-based nanoparticle systems [11]. The pH-shifting method unfolds proteins and induces their reassembly into core–shell nanoparticles without using organic solvents, offering a simple, low-cost, and scalable approach. Under highly acidic or alkaline conditions, proteins partially unfold into molten-globule-like structures, and subsequent neutralization promotes refolding and aggregation, yielding stable nanoparticles [12]. As demonstrated in our previous work, the pH-shifting strategy enables the assembly of protein-based nanoparticles with tunable structural characteristics. By adjusting the composition and ratio of core–shell proteins, particularly using zein (Z) as a hydrophobic core component, precise control over particle size and markedly improved uniformity can be achieved [12]. Notably, Cur, a highly hydrophobic compound, has been reported to partition into the hydrophobic cores of protein-based nanoparticles during pH-shifting process, making this system especially suitable for its encapsulation [13,14]. Building upon these findings, we propose that strategic modulation of the core–shell protein composition enables the fabrication of nanoparticles with tunable size, improved uniformity, and enhanced loading capacity for hydrophobic compounds such as Cur.
Another critical challenge in developing protein-based carriers for Cur lies in controlling its release behavior in the gastrointestinal tract. During gastric digestion, the protein matrix may undergo structural disruption or enzymatic degradation, leading to premature release of Cur. Once released, Cur is susceptible to acidic degradation and rapid metabolism, ultimately resulting in very low bioavailability [15]. To address this challenge, various strategies have been employed, including incorporation of polysaccharides to strengthen the carrier matrix, chemical or enzymatic cross-linking to enhance structural stability, and co-assembly with other biopolymers to modulate degradation behavior [[16], [17], [18]]. However, these modifications must be carefully optimized, as inappropriate protein cross-linking or the use of inappropriate agents may impair the digestibility of the carrier matrix, potentially limiting Cur release in the intestine [19,20]. Similarly, overloading the system with polysaccharides can alter the microstructure, reduce porosity, and undesirably delay release kinetics [21]. Both scenarios may compromise Cur’s delivery to its primary absorption site, the small intestine, ultimately reducing its bioavailability. In addition to these functional limitations, such strategies often pose challenges for industrial translation due to their complexity, cost, and lack of formulation robustness across production batches.
To overcome the limitations of complex multi-component formulations, simplifying delivery systems has become essential for scalable and sustainable production. A promising approach is to exploit the intrinsic structural and digestive properties of proteins—without additional matrices or cross-linkers—to achieve targeted gastrointestinal delivery. Protein-only nanoparticles not only facilitate efficient manufacturability, but also minimize side reactions such as glycation or polysaccharide–protein cross-linking that can compromise digestibility and nutrient release [22]. Protein-specific structural and digestive traits influence both their interaction with hydrophobic compounds like Cur and their behavior under gastrointestinal conditions, such as pH-induced unfolding and susceptibility to enzymatic degradation [7,8]. These properties directly affect encapsulation efficiency, release kinetics, and site-specific delivery. Additionally, particle size might further modulate these outcomes: smaller nanoparticles with higher surface area are generally more prone to enzymatic breakdown and faster release, while larger or more compact structures provide enhanced protection but may hinder intestinal absorption. By rationally selecting and engineering plant protein carriers with desirable properties, it is possible to finely tune the release behavior of Cur and enhance its bioavailability in a structure-informed and industrially viable manner. Nevertheless, the complex interplay between protein structure, and nanoparticle characteristics, digestive fate remains insufficiently understood, highlighting the need for systematic studies to support rational design of protein-only delivery systems.
In this study, a protein-only pH-shifting strategy was employed to encapsulate Cur and elucidate how intrinsic protein structures and digestive behaviors govern delivery performance. The selected legume proteins—faba bean (FPI), mung bean (MPI), pea (PPI), and soy (SPI) isolates—possess distinct molecular weights, amino acid compositions, and conformational compactness that determine their solubility and flexibility under pH variation [23]. Whey protein isolate (WPI) was included as a benchmark due to its high solubility and moderate hydrophobicity, which favor interactions with hydrophobic compounds such as Cur (Pan et al., 2014). By systematically evaluating nanoparticle formation, encapsulation efficiency, and physicochemical stability during storage and simulated digestion, this work identifies the key structural determinants of Cur protection and controlled release. The protein-only design eliminates auxiliary matrices or stabilizers, enabling direct correlation between protein attributes (surface hydrophobicity, solubility, enzymatic resistance) and delivery performance. These insights provide a mechanistic and practical foundation for developing sustainable, scalable, and efficient protein-based delivery systems for Cur and other poorly soluble bioactives.
Continue reading here
Xiang Lin, Xiaoting Zhai, Wenting Huang, Yuhong Mao, Plant protein–curcumin nanoparticles: How protein traits shape particle properties, International Journal of Biological Macromolecules, 2025, 149583, ISSN 0141-8130, https://doi.org/10.1016/j.ijbiomac.2025.149583.
Read also our introduction article on Proteins & Amino Acids here:
