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# Curcumin-modified gelatin nanocomplexes as novel food foaming Agents: Impact on foamability and interfacial properties

Jilong Li, Yi Zhang, Corinne Nardin

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Jilong Li, Yi Zhang, Corinne Nardin. Curcumin-modified gelatin nanocomplexes as novel food foaming Agents: Impact on foamability and interfacial properties. Food Hydrocolloids, 2026, 172, pp.111856. 10.1016/j.foodhyd.2025.111856. hal-05233534

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https://univ-pau.hal.science/hal-05233534v1

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# Curcumin-modified gelatin nanocomplexes as novel food foaming Agents: Impact on foamability and interfacial properties

Jilong Li<sup>a, ∗</sup>, Yi Zhang<sup>b, †</sup>, Corinne Nardin<sup>a, ‡</sup>

ARTICLE INFO

Keywords:

Curcumin

Gelatin

Interfacial properties

Dilatational rheological properties

Polyphenol modification

Foam stability

A B S T R A C T

Although polyphenol modification can enhance the foaming properties of hydrophilic gelatin, the mechanisms by which polyphenols affect gelatin interfacial and foaming behavior are not well understood. In this study, hydrophobic curcumin was used to modify gelatin (Type A), forming a nanocomplex designed as a novel foaming agent. In this study, we investigated the adsorption of curcumin/gelatin nanocomplexes at the air/water interface and the resulting foaming properties. Nanocomplexes were prepared at curcumin to gelatin mass ratios of 0, 0.05, 0.1, 0.15, and 0.2. Their bulk behavior was assessed based on particle size, zeta potential, complexation efficiency, and surface hydrophobicity. As the mass ratio of curcumin to gelatin increased from 0:1 to 0.2:1, the diameter of the nanocomplexes gradually increased from 32 nm to 279 nm, whereas their negative surface charge decreased from -3.4 mV to -1.3 mV. Additionally, surface hydrophobicity increased with curcumin addition, as indicated by the amount of bound bromophenol blue sodium, which rose from 11 µg to 30 µg. The interfacial properties were evaluated using dynamic surface pressure and dilatational rheology. The presence of curcumin increased the nanocomplex surface pressure, leading to enhanced foamability. The interfacial behavior was predominantly elastic; whereas, as the curcumin concentration increased, the elastic modulus did not improve significantly, resulting in no significant enhancement in foam stability. These findings suggest that curcumin/gelatin nanocomplexes are potential excellent foaming agents to be used in food, pharmaceutical, and other industries.

1. Introduction

Foam is a biphasic system in which air bubbles are dispersed in an aqueous continuous phase (Zhan et al., 2022). Various foods, including beer, bread, whipped cream, sour drinks, mousse, and marshmallows, contain substantial amounts of foam (Sun et al., 2024; Wang et al., 2024; Zhan et al., 2018), and the foam contributes to their specific structure, texture, and taste (Zhan et al., 2020). On the other hand, foam is a thermodynamically unstable system, of high free energy, low density, and large interfacial area (Wang et al., 2024). In practice, surfactants are commonly added to enhance the kinetic stabilization of the air bubbles (Zhang et al., 2017), by reducing the interfacial energy, facilitating the formation of elastic films, inducing Marangoni effects, controlling gas exchange, preventing coalescence, and thickening of the liquid phase (Simonsen et al., 2023; Yu et al., 2021; Zhang et al., 2017).

Among foaming agents in foods, proteins, considered as polymeric

surfactants with multiple anchoring sites at the interface, are extensively
used. At the air/water interface, adsorbed proteins undergo rearrange-
ment into interfacial cross-linkable conformations, forming highly
viscoelastic films around the bubbles (Xie et al., 2023). Proteins also
contribute to the stabilization of the interfacial layer through the
unfolding of adsorbed protein molecules (Zhan et al., 2018). Gelatin is a
widely used foaming agent in various food products, such as marsh-
mallows, mousses, and foam-templated oleogels (Abdollahi et al., 2020;
Briceno-Ahumada et al., 2022), due to its notable surface-active prop-
erties (Abedinia et al., 2020), which result from the presence of hy-
drophobic segments along the peptide chains (Zhang et al., 2022). These
properties enable gelatin to absorb at the air/water interface, thereby
reducing surface tension. Consequently, gelatin can stabilize air bubbles
and impart smooth textures to food products. However, its foaming
property is still inferior to those of casein and whey proteins because
gelatin is of higher hydrophilicity owing to the higher abundance of

<footnote><sup>a</sup> Université de Pau et des Pays de l'Adour, CNRS, IPREM, Pau, 64000, France</footnote>

<footnote><sup>b</sup> Department of Food Science, The Pennsylvania State University, University Park, PA, 16802, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: yjz5549@psu.edu (Y. Zhang), corinne.nardin@univ-pau.fr (C. Nardin).</footnote>
hydrophilic groups (e.g., amino groups, hydroxyl groups, and sulfhydryl groups) (Hajlaoui et al., 2024; Zhang et al., 2018). Therefore, modifying gelatin is essential to enhance its foaming property and enable it to compete with more effective food foaming agents. Various modification methods have been investigated to enhance the hydrophobicity of gelatin, including functionalization (e.g., glycosylation, phosphorylation, and succinylation), conjugation with other surfactants, enzymatic modification using transglutaminase, microwave-induced depolymerization, and polyphenol modification (Aewsiri et al., 2009; Dai et al., 2024; Feng et al., 2021; Zhang et al., 2020; Zhao et al., 2022). Compared with the above modification strategies, polyphenol modification of gelatin provides food-grade compatibility for improving the foaming properties of gelatin (Demircan et al., 2024), because polyphenols can selectively bind to specific amino acid residues on gelatin through non-covalent or covalent bonds (Demircan et al., 2024; Poungchawanwong et al., 2020), thus enhancing surface hydrophobicity ($S_0$) without significantly altering the overall structure of gelatin (Sahraeian et al., 2024). Polyphenol-rich plant extracts have been used to modify gelatin to enhance its foaming properties (Demircan et al., 2024; Rahayu et al., 2015). However, the mechanism remains unclear as to how polyphenol modification improves the gas/liquid interfacial behavior and foaming property of gelatin. This study focuses on exploring the mechanism of polyphenol-modified gelatin for foaming property enhancement by investigating the dynamic interfacial rheological properties, hydrophobicity changes, and gelatin aggregation behavior.

Among various polyphenols, curcumin (CUR) stands out for its strong hydrophobicity and well-defined modes of interaction with proteins (Li et al., 2024; Mohammed et al., 2017). Unlike complex polyphenol mixtures derived from plant extracts, curcumin is a chemically well-characterized compound, enabling controlled and reproducible modifications. Furthermore, curcumin is considered a safe polyphenol and is widely used in functional foods due to its health-promoting properties, including antioxidant, anti-tumor, and anti-inflammatory effects (Mohammadian et al., 2024). Adjusting the solution's pH from high to neutral enables curcumin to bind with gelatin via a series of non-covalent interactions, including hydrogen bonds, π-alkyl interaction, and π-anion interaction forces, forming water-insoluble nanocomplexes (Kan et al., 2024). These nanocomplexes can increase gelatin's surface tension and reduce its dynamic interfacial tension. So far, curcumin-modified gelatin nanocomplexes have been studied as emulsifiers (Kan et al., 2023), but not as foam stabilizing agents. We hypothesize that curcumin can reduce the dynamic interfacial tension and enhance the hydrophobicity of gelatin to promote its foamability.

Despite the growing interest in polyphenol-modified gelatin, its potential in foaming systems remains insufficiently explored. Moreover, the mechanisms by which hydrophobic polyphenols influence the air/water interface and the foaming behavior of gelatin are not fully understood. This study addresses these gaps by systematically investigating the physicochemical, interfacial, and foaming properties of CUR/GA nanocomplexes, with the aim of elucidating their structure-function relationships.

In this work, novel food foaming agent nanocomplexes were formed using gelatin type A (GA) complexed with curcumin through a pH-driven method. To understand the mechanism of curcumin-modified gelatin for potential foaming properties enhancement, the behaviors of nanocomplexes in bulk and at interfaces were investigated. The interaction between curcumin and gelatin in the bulk phase was analyzed using dynamic light scattering (DLS). The complexation efficiency and $S_0$ of the nanocomplexes were determined. Surface rheological measurements were conducted to investigate the dilatational rheological properties of CUR/GA nanocomplexes at the air/water interface. Subsequently, their foaming properties were assessed. The correlation between bulk properties, interfacial behaviors, and the associated functional properties of CUR/GA nanocomplexes was confirmed. Additionally, a potential foaming mechanism is proposed to elucidate the impact of hydrophobic polyphenol modification on the interfacial

properties of gelatin.

## 2. Materials and methods

### 2.1. Materials

Gelatin type A (from porcine skin) and curcumin (approximately 70%, HPLC grade, product number C1386, batch number MKCF7577) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Other chemicals were of analytical grade.

### 2.2. Preparation of CUR/GA nanocomplexes

CUR/GA nanocomplexes were prepared using our reported pH-driven method with slight modifications (Li et al., 2024). Briefly, gelatin (0.12 g) was dissolved in 20 mL of deionized water to prepare a gelatin solution. The pH of the solution was then increased to 12 by adding 1 mol/L NaOH. After stirring at 400 rpm for 5 min, varying amounts of curcumin (0, 6, 12, 18, and 24 mg) were added. Subsequently, the pH was adjusted to 7.0 using 1 mol/L HCl. The mixtures were then continuously stirred for 4 h to produce CUR/GA nanocomplex solutions of different CUR/GA mass ratios (0, 0.05, 0.1, 0.15, and 0.2).

### 2.3. Fourier transform infrared (FT-IR) spectroscopy

The dialyzed CUR/GA nanocomplex solutions were subsequently freeze-dried to obtain powdered samples. The spectra of curcumin, gelatin, and CUR/GA nanocomplexes were obtained using a Nicolet® iS 50 FT-IR spectrometer (Thermo Fisher Scientific, United States) in the attenuated total reflectance (ATR) mode. The spectral data was recorded in the range of 400–4000 cm⁻¹.

### 2.4. Particle size and zeta potential

The particle size and zeta potential of CUR/GA nanocomplexes at various gelatin to curcumin mass ratios were measured using a Zetasizer Nano-ZS (Malvern, UK) at 25 °C (wavelength, $λ = 632$ nm). The particle size and zeta potential measurements for each nanocomplex were measured five times after a twenty-fold dilution in a Tris-HCl solution (50 mM, pH 7.0).

### 2.5. Surface hydrophobicity ($S_0$)

The quantity of bound hydrophobic bromophenol blue sodium (BPB) serves as an index of gelatin's $S_0$ and was measured following the method of Chelh et al. (2006). Phosphate buffer (0.2 mol/L, pH 7.0) was used to prepare 1 mg/mL CUR/GA nanocomplex solutions at different mass ratios. For 1 mL of nanocomplex solution, 200 µL of BPB solution (0.1 mg/mL) was added and thoroughly mixed. For the control (without CUR/GA nanocomplex), 200 µL of BPB solution was added to 1 mL of phosphate buffer (0.2 mol/L, pH 7.0). Samples and the control were stirred at room temperature for 10 min and subsequently centrifuged at 10,000 rpm for 15 min. Then, the absorbance of the supernatant was measured at 595 nm against a blank phosphate buffer with a UV-Vis spectrophotometer (UV-2450, Shimadzu, Japan). Since curcumin slightly absorbs at 595 nm, the absorbance of CUR/GA nanocomplex samples at 595 nm was also measured. The corrected amount of BPB bound was calculated according to Eq. (1):

$$ \text{Bound BPB (µg)} = 200 \times (A_c - A_s - A_{CUR}) / A_c \quad (1) $$

where $A_c$, $A_s$, and $A_{CUR}$ represent the absorbance of the control, samples, and curcumin at 595 nm, respectively.
## 2.6. Determination of curcumin and percentage complexed to gelatin

The complexation efficiency (CE) is defined as the percentage of the amount of complexed curcumin to the total amount initially added, and the bound percentage (BP), indicates the percentage of curcumin combined with gelatin. To determine the curcumin content in the nanocomplexes (Mangolim et al., 2014; Zhan et al., 2018), 0.5 mL of the CUR/GA nanocomplex solution was centrifuged at 10,000 rpm for 15 min. After discarding the supernatant, 9 mL of anhydrous ethanol was added. The mixture was then ultrasonicated for 2 h and centrifuged at 10,000 rpm for 15 min. The curcumin content in the supernatant was measured at 426 nm by UV-Vis spectroscopy. CE and BP were calculated using Eqs. (2-3):

$$CE (\%) = M_C/M_T \qquad (2)$$

$$BP (\%) = (M_T \times CE) / M_G \qquad (3)$$

where $M_C$ is the mass of complexed curcumin, $M_T$ is the total mass of curcumin added initially, and $M_G$ is the mass of gelatin.

## 2.7. Measurement of interfacial properties at the air/water interface

A Tracker instrument (Teclis, France) was used to evaluate the interfacial properties of 3 mg/mL CUR/GA nanocomplex solutions at 25 °C. Before measurement, an airdrop (constant volume of 5 µL) was formed in a rectangular cuvette partially filled with the CUR/GA nanocomplex solution. Images of the drop were recorded using a CCD camera.

The dynamic interfacial behavior of CUR/GA nanocomplex solutions at the air/water interface was monitored for 5000 s. The surface tension ($\gamma$) was calculated from the drop's profile using the Young-Laplace equation (Chen et al., 2006). The surface pressure ($\pi$) is calculated as Eq. (4):

$$\pi = \gamma_0 - \gamma \qquad (4)$$

where $\gamma_0$ is the water interfacial tension at 25 °C and $\gamma$ is the surface tension of the CUR/GA nanocomplex solution at adsorption time.

Sinusoidal interfacial compression and extension tests were performed to obtain the surface dilatational rheology parameters, including the complex viscoelastic modulus, elastic modulus, viscous modulus, and phase angles. An angular frequency of 0.1 Hz and a drop volume at 10 % deformation amplitude were selected (Zhan et al., 2020). The experiments consist of 5 cycles of oscillations, alternating with 5 cycles at rest.

## 2.8. Foaming properties

A previously reported method was used to measure foam rupture rate, foamability (FA), and foam stability (FS) (Peng et al., 2017). Briefly, 5 mL of CUR/GA nanocomplex solutions (mass ratios of CUR: GA = 0, 0.05, 0.1, 0.15, 0.2: 1) were filled into 15 mL graduated centrifuge tubes. The solutions were then foamed at 30 Hz for 5 min using a TopMix FB15024 (Thermo Fisher Scientific, United States). The foam rupture rate was obtained by comparing the change in foam volume over 24 h for the different samples. The foam volumes of the solutions were determined at 0 and 60 min to evaluate FA and FS values using Eqs. (5-6):

$$FA (\%) = V_T/5 \qquad (5)$$

$$FS (\%) = V_T/V_T \qquad (6)$$

where $V_T$ is the foam volume at 0 min and $V_T$ is the foam volume at 60 min.

The foam was imaged using a Leica DLM optical microscope (Leica, Wetzlar, Germany) at 2, 10, 30, and 60 min. The obtained images were

analyzed using the Nano Measurer 1.2 (Department of Chemistry, Fudan University, China).

## 2.9. Statistical analysis

The analyses were carried out in triplicate and the obtained data are presented as (mean ± standard deviation (SD)). Statistical analyses were conducted using the Student's t-test in SPSS 26 to compare two groups, with a significance level set at $p < 0.05$. For comparisons involving more than two groups, one-way analysis of variance (ANOVA) was utilized, with differences considered statistically significant at a p-value below 0.05.

# 3. Results and discussion

## 3.1. Interactions and structure of gelatin induced by curcumin

CUR/GA nanocomplexes were prepared by adjusting the pH of the gelatin-curcumin solution from strongly alkaline to neutral (Fig. 1A). This process involves gelatin secondary structure changes and non-covalent interactions between gelatin and curcumin, including hydrogen bonding, $\pi$-alkyl interactions, $\pi$-$\pi$ interactions, and van der Waals forces, as illustrated in Fig. 1B (Tang, 2020). The molecular interactions between gelatin and curcumin were revealed by FT-IR. As shown in Fig. 1C, gelatin exhibited characteristic absorption peaks at $1631 cm^{-1}$ (amide I, C=O stretching), $1530 cm^{-1}$ (amide II, N-H bending), and $1236 cm^{-1}$ (amide III, involving in plane vibrations C-N and N-H groups or CH₂ groups of side chains) (Li et al., 2024). When gelatin complexed with curcumin, a shoulder peak for curcumin appeared at $1513 cm^{-1}$, indicating successful complexation of curcumin with gelatin. Additionally, a shift of amide II (from $1530 cm^{-1}$ to $1544 cm^{-1}$) was observed due to the formation of hydrogen bonds (Kan et al., 2024). The amide I region ($1700-1600 cm^{-1}$) was isolated to analyze changes in the secondary structure of gelatin induced by curcumin. As shown in Fig. 1D, the second-derivative FT-IR spectra were used to determine the centers of Gaussians. Each Gaussian contour represents a specific type of the secondary structure (Fig. S1 and Table S1). After pH-driven self-assembly, the signals of the antiparallel $\beta$-sheet ($1616 cm^{-1}$) and parallel or loose $\beta$-sheet ($1632 cm^{-1}$) structures merged into a single minimum at $1625 cm^{-1}$ in all CUR/GA nanocomplexes. The resulting secondary structure content are shown in Fig. 1E. The antiparallel $\beta$-sheet content in all CUR/GA nanocomplexes was close to 0 %, likely due to structural rearrangements induced by pH changes (Liu et al., 2014). With the addition of curcumin, $\beta$-sheet content decreased from 36 % to 24–30 %, whereas the $\beta$-turn/triple helix/random coil content increased from 64 % to 69–76 %. This shift is attributed to curcumin acting as a hydrogen bond receptor during the pH-drive process, reorganizing part of the triple-helical structure of gelatin (Li et al., 2024).

The interactions between curcumin and gelatin in bulk were analyzed by dynamic light scattering (DLS). Fig. 1F shows the intensity size distribution of gelatin and the nanocomplexes formed at different curcumin concentrations. The gelatin solution exhibited a multimodal distribution, characterized by a high intensity peak at 30 nm and a weaker intensity peak at 200 nm. This distribution indicated the multiscale organization of gelatin within the solution, encompassing both smaller structures and larger molecular clusters (Nogami et al., 2021).

As shown in Fig. 1G, the gelatin solution shows a single volume distribution peak, indicating that the gelatin retained its monomer conformation (Zhan et al., 2018). With increasing CUR/GA ratio to 0.05, a single-peak distribution was observed at 233 nm. As the curcumin concentration was further increased, a minor peak of growing intensity appeared at 5000 nm. Furthermore, an additional minor peak was observed at 1280 nm when the CUR/GA ratio was increased to 0.2. These minor peaks can be attributed to the aggregation of gelatin molecules, caused by alterations in the protein's secondary structure driven
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Photographs of CUR/GA nanocomplexes (A); schematic illustration of interactions between gelatin and curcumin (B); FT-IR spectra (C), second-derivative FT-IR spectra (D), and secondary structure content (E) of gelatin and CUR/GA nanocomplexes; intensity (F), volume size distribution (G), and particle diameter and zeta-potential (H) of CUR/GA nanocomplexes.</figcaption></figure>

by pH acidification (Wang et al., 2022), and non-covalent binding facilitated by curcumin (Tang, 2020). The diameter increased from (32 ± 1) nm to (279 ± 9) nm with rising curcumin to gelatin ratio (Fig. 1H). This trend further highlights the aggregation behavior. The appearance of aggregation peaks suggests that the CUR/GA nanocomplexes were nearing aggregation thresholds. To ensure system stability, the CUR/GA mass ratio was limited to a maximum of 0.2:1 to prevent further aggregate formation.

Additionally, all nanocomplexes have a zeta potential close to 0 mV, indicating that pH 7 is close to the isoelectric point (pI) of gelatin (Alipal et al., 2021). Moreover, as the curcumin ratio increased, the zeta potential rose from (−3.4 ± 0.3) mV to (−1.3 ± 0.1) mV. This increase in zeta potential may be attributed to the aggregation of gelatin molecules under the influence of curcumin, which reduces the exposure of ions and functional groups on the surface of the nanocomplexes. Overall, the zeta potential of CUR/GA nanocomplexes is very low, resulting in weak electrostatic repulsion and facilitating partial flocculation (Peng et al., 2017). This low zeta potential value would promote the formation of a denser particle layer at the air/water interface, in comparison to the water/air interface stabilized by materials of high zeta potential, thus enhancing its mechanical stability (Peng et al., 2017). This stability is particularly important for foam systems, as it prevents the merging and rupture of bubbles, thereby improving the overall stability of the system.

## 3.2. Effects of complexation on the S₀

The S₀ of the protein is directly related to its functional properties including foaming property, as it reflects the protein's conformation, structure, and distribution of hydrophobic amino acid residues on its surface (Kang et al., 2022; Zhan et al., 2018). In this study, BPB was

selected as a hydrophobicity probe due to its high aqueous solubility and sensitivity to hydrophobic microenvironments, making it suitable for assessing protein S₀ in the bulk phase (Bertsch et al., 2003). In addition, since the zeta potentials of the CUR/GA nanocomplexes were close to 0 mV, the contribution of electrostatic interactions to BPB binding is likely minimal (Yu et al., 2023). This suggests that the observed binding predominantly reflects hydrophobic interactions. As shown in Fig. 2A, the introduction of curcumin significantly increased the S₀ of the nanocomplexes (p < 0.05). In the absence of curcumin, the mass of bound BPB in the sample was (11 ± 1) µg. Upon reaching a CUR/GA mass ratio of 0.05, the bound BPB increased to (30 ± 2) µg. As curcumin is a non-polar polyphenol (Mohammed et al., 2017), the increase of the gelatin S₀ can be attributed to the incorporation of more non-polar groups within the nanocomplexes. This can be confirmed by the increased percentage of curcumin bound to gelatin from 3 % to 14 % with increasing CUR/GA mass ratio (Fig. 2B). Additionally, the ability of curcumin to enhance the exposure of non-polar groups on proteins contributes to an increase in the protein's S₀ (Mohammadian et al., 2019). For CUR/GA mass ratios of 0.05 and 0.1, no significant difference in CE was observed (p > 0.05). Meanwhile, the percentage of curcumin combined with gelatin increased from 3 % to 6 %. With further increase of the mass ratio, although the bound percentage increased, no significant change in the bound BPB of the nanocomplexes was observed (p > 0.05), stabilizing at approximately 35 µg. These results align with the particle size observations (Fig. 1C), indicating that the introduction of curcumin led to gelatin molecular aggregation, consequently reducing S₀. Curcumin enhances the S₀ of nanocomplexes by introducing nonpolar groups, exposing the nonpolar regions of gelatin, and promoting gelatin aggregation. This increase in S₀ improves the adsorption capacity of CUR/GA nanocomplexes at the air/water interface.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Surface hydrophobicity of CUR/GA nanocomplexes, measured by the amount of BPB bound to gelatin (A); complexation efficiency of CUR/GA nanocomplexes (B) (* indicates a significant difference between groups, *p* < 0.05).</figcaption></figure>

### 3.3. Interfacial properties of CUR/GA nanocomplexes

Fig. 3A presents the time evolution of surface pressure ($\pi$) during the adsorption of CUR/GA nanocomplexes at the air/water interface. A rapid increase in $\pi$ can be observed during the first 1000 s. Especially for the samples containing curcumin, $\pi$ increased from approximately 16 mN/m to about 23 mN/m. Subsequently, $\pi$ nearly stabilized, indicating that 3 mg/mL of CUR/GA nanocomplexes was sufficient to saturate the interface. Compared to the sample without curcumin, both the initial and stable adsorption state $\pi$ of the other nanocomplexes significantly increased. This indicates that the addition of curcumin enhances the adsorption capacity of GA at the interface. The increased $S_0$ as described in section 3.2 suggests that these nanocomplexes are tightly packed and stabilized at the air/water interface, effectively reducing interfacial tension and increasing $\pi$, thereby decreasing the adsorption barrier (Zhan et al., 2020). The similar large $S_0$ of the CUR/GA nanocomplexes could also explain their similar $\pi$.

The elastic modulus (E') and surface dilatational viscoelastic modulus (E) as functions of oscillation time are shown in Fig. 3B and C. The increase in E' and E with adsorption time is associated with the adsorption of CUR/GA nanocomplexes at the air/water interface. It was observed that E' and E for all nanocomplexes increased from about 10 mN/m to 30 mN/m between 0 and 5000 s, indicating an increase in the adsorption of surface-active substances at the interface. Meanwhile, this

results in a higher interfacial density because of the rearrangement of these components at the interface, leading to increased E' and E (Zhan et al., 2018). For different mass ratios of CUR/GA nanocomplexes, E' was close to E, while the viscous modulus (E') remained significantly lower than E' (Fig. S2), indicating that air/water interface exhibited predominantly elastic behavior (Peng et al., 2017). Compared to the sample without curcumin, the nanocomplexes exhibited slightly higher E and E'. This indicates that curcumin has a limited effect on enhancing the viscoelasticity of gelatin. This observation is in line with the results obtained for sodium caseinate in the presence of tannic acid at the air/water interface (Zhan et al., 2018).

The E versus $\pi$ curve was used to investigate the interaction and interfacial adsorption capacity of CUR/GA nanocomplexes. For an ideal gas, the slope of the E versus $\pi$ plot is 1 (Zhan et al., 2020). Therefore, a slope greater than 1 indicates the presence of surface-active components engaged in interactions (Wan et al., 2014). The evolution of E with $\pi$ is presented in Fig. 3D, all CUR/GA nanocomplexes exhibit surface-activity. For all samples, E increased with increasing $\pi$, indicating that CUR/GA nanocomplexes adsorbed from the bulk to the interface. Additionally, the slopes were significantly higher than 1, suggesting strong molecular interactions between the adsorbed CUR/GA nanocomplexes (Zhan et al., 2018). The other samples (CUR/GA mass ratios of 0.05, 0.1, 0.15, and 0.2) reveal smaller slopes compared to those without curcumin, indicating that curcumin promotes the interaction with the adsorbed gelatin layer. Notably, as the CUR/GA ratio decreased, the slope gradually approached 1, indicating better interaction between the components of the adsorbed layer at a low CUR/GA mass ratio.

### 3.4. Foaming properties of CUR/GA nanocomplexes

The CUR/GA nanocomplexes prepared using a pH-drive method were used to stabilize foams. As described above, they exhibited small size, uniform distribution, low zeta potential, and high $S_0$. As shown in Fig. 4A, the rupture dynamics of foam stabilized with CUR/GA nanocomplexes were monitored over 24 h. With all CUR/GA nanocomplexes, similar foam rupture rates were observed. Notably, the foam volume remained at approximately 3 mL even after 24 h. To further analyze the foaming properties, the influence of the CUR/GA mass ratio on FA and FS was investigated, and the results are shown in Fig. 4B. FA refers to the capacity of a solution to entrap gas, whereas FS describes the ability of the formed foam to retain its structure for 60 min (Chang et al., 2020). In this study, FA and FS were used to evaluate the initial formation and structural persistence of foams produced by CUR/GA nanocomplexes, respectively. Both are critical parameters in food applications that require sustained foam integrity. A significant increase in FA ($p < 0.05$) was observed when the CUR/GA mass ratio was increased from 0 to 0.05, rising from (92 ± 11) % to (120 ± 7) %. However, no significant difference in FA was observed as the ratio increased further ($p > 0.05$). Moreover, a similar FS was observed (see Fig. 4B, $p > 0.05$).

Further insights into FA and FS were obtained by measuring temporal changes in the microstructures of the nanocomplex-stabilized foams (Fig. 4C). Initially, all samples were constituted of nearly spherical and relatively small air bubbles. However, variations in the sizes of air bubbles were observed during the foaming process as the mass ratio of curcumin to gelatin changed. As shown in Table 1, the largest bubbles formed in foams prepared using nanocomplexes without curcumin. Conversely, the average size of foams prepared with curcumin-containing nanocomplexes was nearly half that of those without curcumin. This indicates that curcumin enhances the capacity to form smaller bubbles during the foaming process.

Over time, the sizes of air bubbles in all samples increased (Fig. 4C), attributable to a combination of Ostwald ripening and coalescence effects (Chen et al., 2019). Additionally, a change in bubble shape from spherical to polyhedral was observed, attributable to film drainage effects (Hill & Eastoe, 2017). In CUR/GA nanocomplex-stabilized bubbles,
<figure><img src="image_6.png" /><figcaption>Fig. 3. Time evolution of the surface pressure (π) (A), elastic modulus (E') (B), and surface dilatational modulus (E) (C) for the adsorption of CUR/GA nanocomplexes at the air/water interface; E as a function of π (D).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Foam rupture dynamics over time (A), foamability (B), foam stability (C), and time evolution of bubbles within foams (D) of CUR/GA nanocomplex solutions (* indicates a significant difference between groups, p < 0.05).</figcaption></figure>
<caption>**Table 1**</caption>

The diameter of foams stabilized with CUR/GA nanocomplexes of different CUR/GA mass ratios after resting for 2 and 60 min.

<table><thead><tr><th rowspan="2">Mass ratios of CUR/GA nanocomplexes</th><th colspan="3">Diameter after 2 min (µm)</th><th colspan="3">Diameter after 60 min (µm)</th></tr><tr><th>Max</th><th>Min</th><th>Average</th><th>Max</th><th>Min</th><th>Average</th></tr></thead><tbody><tr><td>0</td><td>631</td><td>51</td><td>244 ± 144</td><td>905</td><td>10</td><td>305 ± 190</td></tr><tr><td>0.05</td><td>412</td><td>20</td><td>111 ± 80</td><td>842</td><td>7</td><td>290 ± 183</td></tr><tr><td>0.1</td><td>589</td><td>8</td><td>133 ± 130</td><td>640</td><td>9</td><td>239 ± 166</td></tr><tr><td>0.15</td><td>564</td><td>5</td><td>127 ± 101</td><td>845</td><td>28</td><td>252 ± 184</td></tr><tr><td>0.2</td><td>403</td><td>7</td><td>96 ± 65</td><td>755</td><td>26</td><td>224 ± 177</td></tr></tbody></table>

minor coalescence was observed after 10 min of storage, evolving to significant coalescence after 30 min. Moreover, large bubbles with average diameters ranging from 224 to 305 µm were observed after 60 min (Table 1). Overall, these results indicate that CUR/GA nanocomplexes could serve as foaming agents in food and other products.

In conclusion, curcumin enhanced FA of gelatin but had minimal effect on FS. Notably, the trend in FA was consistent with $S_0$ of CUR/GA nanocomplexes, supporting previous findings by Zhan et al. (2018), who reported a positive correlation between $S_0$ and FA. Additionally, $π$ plays a critical role in foam formation. As discussed in Section 3.3, the presence of curcumin accelerated the initial adsorption of nanocomplexes at the air/water interface, resulting in tighter interfacial packing and improved FA. Following adsorption, the CUR/GA nanocomplexes interacted and gradually fused into viscoelastic interfacial films, which is important for FS (Maldonado-Valderrama & Patino, 2010). Coalescence, a primary mechanism of foam destabilization, is influenced by the interfacial film E' (Langevin, 2023). Although curcumin decreased the zeta potential of gelatin, the zeta potentials of all CUR/GA nanocomplexes remained close to 0 mV. This resulted in comparable interfacial packing behavior, leading to the formation of interfacial films with similar E' (Peng et al., 2017). Consequently, the FS remained consistent across all samples.

### 3.5. Comparison with other polyphenol-modified protein foaming systems

To evaluate the potential of curcumin-modified gelatin nanocomplexes as food foaming agents, their performance was compared with other reported polyphenol-modified protein systems. The mechanism of polyphenol modification on the foaming properties of gelatin is complex. As shown in Table 2, the modification of gelatin with oxidized phenolic extracts significantly enhanced its FA and FS. However, when the mass ratio of extracts to gelatin increased from 1 % to 3 %, the effects of modification vary depending on the type of extract. Gelatin modified with pomegranate peel waste extracts showed no significant changes in FA and FS, with FA stabilizing at approximately 42 % and FS at 20 %. In contrast, gelatin modified with green tea waste extracts exhibited notable improvements, with both FA and FS increasing by approximately 10 %. Conversely, gelatin modified with black tea waste extracts demonstrated a decreasing trend in both FA and FS as the proportion of extracts increased. Unlike complex polyphenol extracts with undefined and variable compositions, curcumin is a single and well-characterized compound. It is thus more suitable for industrial protein modification because of the greater reproducibility and control. Previous studies have shown that curcumin modification of various proteins, such as soy protein isolate, glutaminase-modified soy protein isolate, and *Lepidium sativum* protein hydrolysate, did not significantly improve foaming properties and in some cases reduced them. This may be due to a decrease in $S_0$, especially in some hydrophobic proteins. In contrast, this study demonstrated that curcumin modification increased the FA of hydrophilic gelatin without negatively impacting FS, indicating that this system is more suitable as a food foaming agent.

<caption>**Table 2**</caption>

Foaming performance comparison.

<table><thead><tr><th>Protein</th><th>Polyphenol</th><th>Foamability</th><th>Foam stability</th><th>References</th></tr></thead><tbody><tr><td>Bovine gelatin</td><td>Pomegranate peel waste extracts</td><td>No significant change</td><td>No significant change</td><td>(Demircan, Oral, Toker, & Palabiyik, 2024)</td></tr><tr><td>Bovine gelatin</td><td>Green tea waste extracts</td><td>Increased</td><td>Increased</td><td>(Demircan, Oral, Toker, & Palabiyik, 2024)</td></tr><tr><td>Bovine gelatin</td><td>Black tea waste extracts</td><td>Decreased</td><td>Decreased</td><td>(Demircan, Oral, Toker, & Palabiyik, 2024)</td></tr><tr><td>Soy protein isolate</td><td>Curcumin</td><td>No significant change</td><td>–</td><td>Tapal and Tiku (2012)</td></tr><tr><td>Glutaminase-modified soy protein isolate</td><td>Curcumin</td><td>Decreased</td><td>Decreased</td><td>Xiang et al. (2018)</td></tr><tr><td><i>Lepidium sativum</i> protein hydrolysate</td><td>Curcumin</td><td>No significant change</td><td>–</td><td>Kadam et al. (2019)</td></tr><tr><td>Gelatin type A</td><td>Curcumin</td><td>Increased</td><td>No significant change</td><td>This study</td></tr></tbody></table>

–, No report.

### 3.6. Potential foaming mechanism of CUR/GA nanocomplexes

Curcumin enhances $S_0$ of the nanocomplexes by introducing nonpolar groups, which expose the hydrophobic regions of gelatin and promote its aggregation. The increased $S_0$ improves the adsorption capacity of CUR/GA nanocomplexes at the air/water interface. The enhanced adsorption facilitates the formation of tightly packed and stabilized interfacial structures, effectively lowering interfacial tension and increasing $π$, thereby reducing the adsorption energy barrier. These interfacial changes enable the formation of smaller bubbles during foaming, leading to an improved FA.

All nanocomplexes exhibit near-neutral zeta potentials, which promotes the formation of dense particle layers at the interface and contributes to mechanical stability. The resulting interfacial films display predominantly elastic behavior. Although curcumin slightly reduces the zeta potential, the effect is minimal and does not substantially enhance the viscoelastic properties of gelatin at the interface. Consequently, coalescence is not significantly suppressed, and all CUR/GA nanocomplexes display comparable FS.

In summary, curcumin complexation improves the FA of gelatin without negative impact on FS.

## 4. Conclusion

CUR/GA nanocomplexes obtained by modifying hydrophilic proteins with hydrophobic polyphenols exhibited small size (32–279 nm), uniform distribution, low zeta potential (−3.4 to −1.3 mV), and high $S_0$ (with bound BPB mass ranging from 11 to 35 µg). These physicochemical properties enable the nanocomplexes to diffuse rapidly and adsorb at the water/air interface. Rheological studies indicated that CUR/GA nanocomplexes effectively increased $π$ of the system and reduced the adsorption barrier. Curcumin promotes strong molecular interactions between the adsorbed gelatin layers, and nanocomplexes with a CUR/GA mass ratio of 0.05 exhibiting the strongest molecular interactions. These characteristics led to an increase in the foamability of the system with the introduction of curcumin. Additionally, the interfacial behavior was predominantly elastic. However, curcumin had little effect on the
zeta potential and the system's elastic modulus, resulting in similar foam stability for all samples. The results confirmed that a low concentration of curcumin could effectively promote the formation of CUR/GA nano-complexes, reduce their zeta potential, increase S₀, and improve the interfacial behavior of the system, ultimately enhancing FA. The optimal CUR/GA mass ratio was determined to be 0.05, as it significantly enhanced FA, whereas higher mass ratios did not provide additional benefits. Overall, curcumin addition significantly improved the FA of gelatin while maintaining FS, making this system a highly promising foaming agent for applications in food, pharmaceutical, and other industries. Although curcumin demonstrates significant potential for enhancing the FS of gelatin, its practical application may be constrained by factors such as high production cost, intense coloration that can affect the visual appeal of food products, and limited stability under light and alkaline conditions. To address these limitations, future studies should investigate alternative polyphenols that are cost-effective, colorless, and stable, while maintaining or enhancing interfacial performance.

### CRediT authorship contribution statement

**Jilong Li:** Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Yi Zhang:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. **Corinne Nardin:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization.

### 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.

### Acknowledgments

The work was funded by I-SITE E2S with Aid award agreement No. ANR-16-IDEX-0002. We also appreciate the support by the USDA National Institute of Food and Agriculture and Hatch Appropriations under Accession number 7005076. This work was also financially supported by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie programme (grant agreement No 823883). J. Li acknowledges a scholarship from the China Scholarship Council (CSC) (202208220048).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.foodhyd.2025.111856.

### Data availability

Data will be made available on request.

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