Licorice extract/gellan gum aerated gels: Insights into structure and properties

Abstract

The incorporation of dispersed air in gel-type foods can meet the new textural and functional demands. The present study was therefore conducted to create aerated gels using gellan gum (GelG; 0.7, 1, and 1.3 % w/v) and licorice root extract (LRE; 0.4, 0.7, and 1 % w/v) through cold-induced ionic gelation. The gels were characterized using Fourier transforms infrared spectroscopy and X-ray diffraction to confirm the interaction between the components. The aerated gels had an overrun of up to 156.04 %, porosity of up to 60.81 %, and a water holding capacity of approximately 100 %. Additionally, the elastic characteristics (G′) were more prominent than the viscous ones (G′′), suggesting that the gels exhibit solid-like behavior. The calcium release from the aerated gels significantly increased by approximately 16 % as a function of GelG concentrations. The aerated gels with high overrun, stability, and viscoelasticity can be used to design novel low-calorie and healthy aerated foods.

Highlights

• •Dispersed air in gels meets new textural and functional demands.
• •Aerated gels were created using gellan gum and licorice root extract.
• •Gels showed an overrun of up to 156.04 % and 60.81 % porosity.
• •Elastic properties were superior to viscous properties in the gels.
• •Aerated gels can help design novel low-calorie, healthy foods.

1. Introduction

Air bubbles play a crucial role in the structure of various foods, including bread, cakes, meringues, aerated chocolate bars, whipped cream, mousses, and milkshakes (Zúñiga & Aguilera, 2008). Numerous studies have demonstrated that incorporating air into food products can effectively lower their overall caloric density. This, in turn, can lead to a reduction in energy intake, as well as slower gastric emptying and an overall increase in satiety. Additionally, incorporating a gas phase into the food matrix can also increase the surface area and modify digestibility (Jakubczyk et al., 2019). The multi-scale structure of food matrices can greatly influence their digestive and physicochemical properties (B. Li et al., 2024; Qin et al., 2025; Zhao et al., 2024). Aerated gels are used for a variety of purposes, including the production of new dietetic foods, the inclusion of aromatic substances, and as carriers for nutraceuticals (Zúñiga & Aguilera, 2008).

Foods that have been aerated are prone to processes that can destabilize them, such as coalescence and disproportionation. Additionally, these types of foods are often thermodynamically unstable, which can further contribute to their instability (Cox et al., 2009; Deng et al., 2019; Zúñiga & Aguilera, 2008). To slow down the destabilization processes that can occur in food foams, it is common to modify the gas-liquid interface. This can be achieved through a variety of methods, such as increasing the viscosity of the continuous phase or introducing surface-active molecules like proteins, emulsifiers, or solid particles such as fat crystals. By modifying the interface in this way, it is possible to slow down the destabilization processes and maintain the desired structure and texture of the food foam (Cox et al., 2009; Zúñiga & Aguilera, 2008). One effective method to enhance the viscosity of the continuous phase in aerated foods and gel production is by utilizing food gums.

Gellan gum (GelG) is an anionic polysaccharide that is composed of repeating tetrasaccharide units featuring glucose, glucuronic acid, and rhamnose residues in a ratio of 2:1:1 (Matricardi et al., 2009). Gellan gum is a versatile ingredient that is capable of withstanding high temperatures and acidic conditions (Ambebila et al., 2019; Nag et al., 2011). It is widely accepted that at elevated temperatures, gellan gum polysaccharide exists as random coils in solution, resulting in low viscosity. As the temperature drops, the gellan gum begins to cross-link by forming double helices, leading to a significant increase in viscosity and the formation of a gel (Lavaei et al., 2022). By using gellan gum in these types of products, it is possible to stabilize the aerated network and create a desirable texture and structure that is both stable and long-lasting.

The selection of suitable foaming agents is critical for minimizing phase separation in aerated gels. Owing to health-related concerns associated with synthetic surfactants, naturally occurring surface-active compounds—such as proteins—are generally favored in the food industry. However, their use is constrained by limited availability and susceptibility to variations in pH, temperature, and storage conditions (Gonzalez & Sörensen, 2020; Sabeghi et al., 2024). In a study conducted by Orrego and co-workers (2015), it was found that the use of thermal treatment led to a decrease in gas hold-up and mean diameter of air bubbles in aerated gels made with whey protein isolate dispersions (Orrego et al., 2015). As a result, the food industry is actively seeking out alternative natural emulsifiers and foaming agents that can address these concerns and help to create healthier, more eco-friendly food products (Gonzalez & Sörensen, 2020).

Saponins represent a diverse category of stable non-ionic surfactants found in numerous plant species. Many saponins can create a highly viscoelastic layer at the interfaces of air and water, as well as oil and water. This layer exhibits significant viscoelasticity due to the strong interactions among saponin molecules (Ma et al., 2019; Sabeghi et al., 2024). Licorice (Glycyrrhiza glabra) root is often harvested for its medicinal properties, including its antioxidant activity and antimicrobial effects. Glycyrrhizin, which is also known as glycyrrhizic acid, is a triterpenoid saponin and the primary active ingredient in licorice root. It can be used as a natural foaming agent that is capable of withstanding high temperatures and acidic conditions, making it an ideal alternative to traditional foaming agents in the food industry (Nooshkam et al., 2022). Glycyrrhizic acid has a unique amphiphilic structure and supramolecular chirality that allows it to exhibit hierarchical self-assembly in water. It can additionally create multilayer structures at both oil-water and air-water boundaries, thanks to robust interfibrillar interactions stemming from hydrogen bonding (Ma et al., 2019). Moreover, glycyrrhizic acid is low in calories and contributes the distinct sweetness associated with licorice root (Nooshkam et al., 2023). Sabeghi et al. (2024) found that a foamulsion gel with excellent physical stability can be achieved using gellan gum and an extract from Acanthophyllum glandulosum, which is high in saponins, without any release of water or oil.

Given that still not much is known about the aerated gels, especially those based on non-protein foaming and stabilizing agents, the aim of this study was to investigate the ability to create a stable aerated gel using GelG and licorice root extract (LRE). The resulting gel would have unique textural and physicochemical properties, a low-calorie content, and the potential for additional health benefits.

2. Materials and methods

2.1. Materials

Zagros Licorice Co. (Kermanshah, Iran) donated the powder of LRE, while the low acyl gellan gum was acquired from Amstel Products BV Co. (Burg. Stramanweg 63, Netherlands). Other chemicals were obtained from Sigma-Aldrich Co. (St Louis, MO, USA) or Merck Co. (Darmstadt, Germany) and were of analytical grade.

2.2. Aerated gel preparation

To prepare the aerated gel, GelG was dissolved in distilled water (pH 7) at concentrations of 0.7, 1.0, and 1.3 % (w/v) under continuous stirring at 500 rpm and 65 °C. Subsequently, CaCl₂ (0.04 % w/v) was incorporated and stirred until fully dissolved. Thereafter, LRE was added at concentrations of 0.4, 0.7, and 1.0 % (w/v), followed by thorough mixing. The resulting GelG/CaCl₂/LRE solution (100 mL) was cooled to 40 °C and transferred to a beaker. Aeration was achieved by whipping the mixture for 1 min using a 5-speed hand mixer (Ecovitta, EM-707C, 200 W, Singapore) equipped with a wire whisk. Finally, the GelG/CaCl₂/LRE solution was left at an ice bath for 15 min to form the aerated gels.

2.3. Physicochemical analyses

2.3.1. Gel density and overrun

To calculate the density and overrun of the aerated gel, a method similar to the one outlined by Nooshkam et al. (2022) was used with a few modifications. First, the newly prepared aerated gel was carefully poured into a 25 mL plastic cup. Then, the surface of the cup was smoothed with a spatula, and the cup was weighed at room temperature. The density and overrun of the aerated gel were determined using the following formula:

$$\text{Density}\left({\rho }_g;\mathrm{g}{\mathrm{cm}}^{-3}\right)=\frac{W_g}{V_l}$$ (1)

$$\text{Overrun}\left(\%\right)=\frac{\left(\frac{1}{{\rho }_g}-\frac{1}{{\rho }_l}\right)}{\frac{1}{{\rho }_l}}\times 100$$ (2)

In this equation, W_g represents the weight of the gel, V_l represents the volume of the liquid (a mixture of GelG and LRE, with a total volume of 25 mL), ρ_g represents the density of the gel, and ρ_l represents the density of the liquid mixture (GelG/LRE).

2.3.2. Gel porosity

The porosity, which is also known as gas-hold up or air-volume fraction, of the aerated gels was calculated by comparing the density of the aerated gel (ρ_g) with the density of the liquid (ρ_l), using the following formula as described by Nooshkam et al. (2022):

$$\text{Porosity}\left(\%\right)=\left(1-\frac{{\rho }_g}{{\rho }_l}\right)\times 100=\left(\frac{\%\text{Overrun}\times 100}{\%\text{Overrun}+100}\right)$$ (3)

2.3.3. Syneresis

To determine syneresis, fresh aerated gels in 25 mL plastic cups were kept upside down at room temperature for 48 h, and the weight of the expelled water was measured. Syneresis was then calculated using the equation developed by Zand-Rajabi and Madadlou (2016):

$$\text{Syneresis}\left(\%\right)=\frac{\text{Expelled water}\left(\mathrm{g}\right)}{\mathrm{Gel}\text{weight}\left(\mathrm{g}\right)}\times 100$$ (4)

2.3.4. Water holding capacity (WHC)

To determine the WHC of aerated gels, a centrifugation method was employed. In this method, 25 mL of aerated gel was centrifuged at 1500 ×g for 10 min at room temperature. After centrifugation, the separated water was weighed to measure the WHC (Alavi et al., 2018):

$$\mathrm{WHC}\left(\%\right)=\frac{W_t-W_r}{W_t}\times 100$$ (5)

where, W_t is the gram of water in gels before centrifugation and W_r is the gram of water released from gels.

2.3.5. Color

The chromameter (Konica Minolta, CR-410, Japan) was used to measure the L*, a*, and b* color coordinates of the aerated gel samples. The measurements were taken using a C-illuminant, at 10° observer, and through reflectance mode, following the method of Alkobeisi et al. (2022). To calibrate the instrument, a white standard tile was used. The whiteness index (WI) and browning index (BI) of the aerated gels were then determined using the equation given below:

$$\mathrm{WI}=100-{\left[{\left(100-{\mathrm{L}}^{\ast }\right)}^2+{\mathrm{a}}^{\ast 2}+{\mathrm{b}}^{\ast 2}\right]}^{0.5}$$ (6)

$$\text{Browning index}\left(\%\right)=\left(\frac{\mathrm{x}-0.31}{0.172}\right)\times 100$$ (7)

$$\mathrm{x}=\frac{\left({\mathrm{a}}^{\ast }+1.75{\mathrm{L}}^{\ast }\right)}{\left(5.645{\mathrm{L}}^{\ast }+{\mathrm{a}}^{\ast }-3.012{\mathrm{b}}^{\ast }\right)}$$ (8)

2.4. Structural analyses

2.4.1. XRD analysis

Following the aeration process, the gels were freeze-dried at −50 °C and 0.07 mbar. The X-ray diffractometer (Explorer, GNR, Italy) was used to record the X-ray diffractogram of GelG, LRE, and freeze-dried GelG/LRE gels. The Cu K-radiation was generated at 40 kV and 35 mA in the differential angle (2) range of 5–75° (Karthika & Vishalakshi, 2015). Relative crystallinity (%) was determined by calculating the ratio of the area of crystalline peaks to the total area under the XRD spectrum curve, then multiplying the result by 100. The area calculations were conducted using Origin Pro software, version 9 (Ramos et al., 2024).

2.4.2. Fourier transform infrared (FTIR) spectroscopy

The freeze-dried samples were combined with KBr and compressed into tablets. The resulting spectra were collected in transmittance mode using a Thermo Nicolet AVATAR-370 FTIR instrument, covering a range from 4000 to 400 cm⁻¹ (Nooshkam et al., 2022).

2.5. Gel texture

A texture analyzer (Stable Microsystems, TA.XT Plus, UK) was employed to perform a penetration test on cylindrical samples measuring 3 × 3 cm. A cylindrical probe with a diameter of 25 mm was used, descending at a pre-test speed of 1 mm s⁻¹ and a post-test speed of 3 mm s⁻¹ until reaching 50 % of the sample's original height. The load cell capacity was set to 5 kg. The test measured the firmness of the sample.

2.6. Gel rheology

To evaluate the rheological properties of the aerated gels, a rheometer (MCR301, Anton Paar, Austria) was used, equipped with a 20 mm parallel plate geometry with a 1.0 mm set gap. The strain sweep test was conducted at a frequency of 1.0 Hz, varying the shear strain from 0.01 % to 100 % to identify the linear viscoelastic region. Following this, a frequency sweep test was executed from 0.01 to 10 Hz within this region (with a strain of 0.1 %) to measure the elastic modulus (G′) and the viscous modulus (G′′). All measurements were performed at room temperature. The relationship between the elastic and viscous moduli of the aerated gels and angular frequency was described using a power-law model:

$$G^{\prime }=K^{\prime }{\omega }^{n\text{'}}$$ (9)

$$G^{\text{'}\text{'}}={K^{\text{'}\text{'}}}^{}{\omega }^{n^{\text{'}\text{'}}}$$ (10)

where ω represents the angular frequency (in Hz); K′ and n' denote the consistency index and flow behavior index for the elastic modulus (G′), respectively; and K′′ and n'′ refer to the consistency index and flow behavior index for the viscous modulus (G′′), respectively (Nooshkam et al., 2022).

2.7. Gel microstructure

The microstructure of GelG/LRE aerated gels was examined using a scanning electron microscope (SEM; LEO 1450 VP, Germany). The process involved quick freezing of the aerated gels in liquid nitrogen, followed by freeze drying. The samples were then mounted on aluminum holders and coated with gold using a sputter coating technique. Finally, the microstructure of the samples was assessed at various magnifications (Kazemi-Taskooh & Varidi, 2021).

2.8. In vitro gastric digestion

A 20 mL portion of the aerated gel was combined with 20 mL of simulated gastric fluid, which was made by adding 1.75 mL of concentrated HCl to 250 mL of deionized water and incorporating 0.064 g of pepsin. The pH of the combined solution was then adjusted to 2.5. This mixture was incubated at 37 °C for a period of 2 h, during which it was agitated at a speed of 100 rpm (Nooshkam & Varidi, 2021). The atomic absorption spectrometer (PG990, PG instrument, Australia) was used to quantify the amount of free calcium present (Amat et al., 2024).

2.9. Statistical analysis

The experiments were carried out in triplicate, resulting in a total of nine experiments based on a completely randomized factorial design. The data obtained from these experiments were analyzed using Minitab 16 software. To determine significant differences between the mean values, a Tukey test was performed at a 95 % confidence level (p < 0.05). The Pearson correlation between responses was analyzed using Origin Pro software (version 9), and a correlation heat map was generated using ChiPlot (https://www.chiplot.online/) (accessed on July 16, 2025).

3. Results and discussion

3.1. Density and overrun

The data presented in Table 1 provides information on the porosity, density, and overrun characteristics of the aerated gel. It was observed that the concentration of GelG had a significant impact on the density, porosity, and overrun of the gels. As the GelG concentration increased, there was a decrease in the overrun (∼ 76 %) and porosity (∼ 56 %), while density increased (∼ 86 %) (p < 0.05). In addition, the LRE content had a noticeable effect on the responses. The increase in LRE level from 0.4 % to 1 % resulted in a decrease of around 20 % (p > 0.05) and 16 % (p < 0.05) in the overrun and porosity, respectively, while the gel density increased by 14 % (p < 0.05). Overall, the greatest overrun (156.04 %) and porosity (60.81 %) were recorded in aerated gels composed of 0.7 % GelG and 0.7 % LRE, whereas the lowest values were noted in the combinations of 1.3 % GelG and 0.7 % LRE.

Table 1.

Physicochemical properties of GelG/LRE aerated gels.

GelG (%)	LRE (%)	Overrun (%)	Density (g cm−3)	Porosity (%)	WHC (%)	WI	BI (%)	Firmness (g)
0.7	0.4	142.02 ± 24.61ab	0.41 ± 0.04cd	58.47 ± 4.22ab	53.18 ± 4.39de	62.49 ± 0.15a	37.08 ± 0.81f	572.95 ± 13.01c
	0.7	156.04 ± 21.47a	0.39 ± 0.03d	60.81 ± 3.29a	47.00 ± 8.52e	60.60 ± 0.73ab	41.49 ± 0.78ef	470.08 ± 25.69c
	1	145.19 ± 11.84ab	0.41 ± 0.02d	59.17 ± 1.97ab	42.98 ± 2.50e	47.78 ± 1.67f	66.35 ± 2.86a	445.64 ± 33.24c
1	0.4	93.94 ± 1.57cd	0.53 ± 0.01bcd	48.44 ± 0.42abc	89.85 ± 0.41ab	61.38 ± 0.24a	38.21 ± 0.18f	944.86 ± 48.91bc
	0.7	101.11 ± 0.09bc	0.57 ± 0.10bc	42.97 ± 10.31bc	76.42 ± 3.81bc	58.22 ± 0.27b	45.30 ± 0.80de	908.64 ± 83.72bc
	1	94.01 ± 1.09de	0.67 ± 0.01ab	32.89 ± 0.48cd	68.49 ± 2.19cd	49.23 ± 2.40ef	59.10 ± 4.54b	814.27 ± 72.12bc
1.3	0.4	48.11 ± 7.58de	0.68 ± 0.02ab	32.40 ± 3.46cd	99.99 ± 0.00a	52.68 ± 1.18cd	51.29 ± 2.72c	1712.12 ± 33.16a
	0.7	26.88 ± 0.79e	0.80 ± 0.01a	21.18 ± 0.49d	96.29 ± 5.24a	50.79 ± 0.68de	49.68 ± 0.68cd	1823.13 ± 360.42a
	1	32.76 ± 7.39e	0.76 ± 0.04a	24.56 ± 4.19d	87.59 ± 1.17ab	53.87 ± 0.11c	47.48 ± 0.18cd	1057.05 ± 79.08b

Different letters indicate significant differences between samples at p < 0.05.

GelG, gellan gum; LRE, licorice root extract; WHC, water holding capacity; WI, whiteness index; BI, browning index.

These observations contradict previous studies that reported an increase in the overrun values of foam or ice cream systems with licorice extract addition (Hayoğlu et al., 2017). The decrease in the aerated gel properties can be attributed to the increase in the viscosity of the system as the concentration of GelG and LRE increased. High viscosity limits air incorporation and inhibits mobility of surface-active molecules, resulting in lower overrun and air phase, as well as prolonging the lamella thinning period (Dabestani & Yeganehzad, 2019). It is important to highlight that highly aerated systems are defined as those exceeding 150 % overrun or having an air-volume fraction greater than 60 % (indicated as gel porosity in Table 1). Examples include sponge cakes, bread loaves, and various dairy products—which typically contain about 80 %, 75 %, and 15–60 % air-volume fractions, respectively (Nooshkam et al., 2022). As a result, GelG/LRE aerated gels with air-volume fractions ranging from 21.18 % to 60.81 % hold potential for the development of innovative low-calorie aerated functional foods.

3.2. Syneresis and WHC

It is noteworthy that the syneresis experiment did not demonstrate any water release from the aerated gels (data not shown), which suggests that the components possess the capability to create aerated gels with excellent physical stability.

Table 1 displays the impact of GelG and LRE levels on the WHC of aerated gel. The WHC of the samples varied, measuring 42.98 % in the 0.7 % GelG/1 % LRE formulation and reaching up to 99.99 % in the 1.3 % GelG/0.4 % LRE systems. The concentration of both GelG and LRE had a significant effect on WHC. When the GelG level was increased from 0.7 % to 1.3 %, the WHC increased from 47.72 % to 94.62 % (p < 0.05). This increase in WHC could be attributed to the presence of hydrophilic groups in polysaccharides such as GelG, which have a high affinity for binding water (Babaei et al., 2019). The higher WHC also coincided with an increase in gel firmness. The denser network structure resulting from the addition of gum led to more water being trapped in the gel matrix, which increased the WHC of the aerated gel (Mi et al., 2021). This finding is consistent with previous studies, where GelG was observed to increase the WHC of hydrogels (Zhou et al., 2024).

However, the WHC decreased from 81.01 % to 66.35 % when the LRE concentration was increased from 0.4 % to 1 % (p < 0.05). The lower WHC value for aerated gels with higher LRE concentration was likely due to the poor gel network. A gel characterized by a more porous structure generally tends to be softer compared to one with a denser configuration. Furthermore, a relatively weak gel with an open structure will exhibit lower WHC (Weel et al., 2002). This reduced WHC may also stem from an excess of surfactant molecules present at the lamella, which amplify the gravitational effects on the drainage of the aerated gel. This leads to continuous liquid loss from the film that forms between neighboring bubbles (Majeed et al., 2020).

3.3. Color

The marketability of food products is greatly influenced by their color, which is a crucial quality characteristic (Sarabi-Aghdam et al., 2021). Table 1 displays the BI and WI parameters of aerated gels. The results indicate that as the GelG and LRE percentages in aerated gels increased, the WI decreased significantly, by around 8 % and 14 %, respectively. Additionally, the aerated gel made with 1 % LRE and 1.3 % GelG had notably higher BI values (up to 37 % and 5 %, respectively) compared to other gels. Overall, the WI of the samples demonstrated significant fluctuations, with a value of 47.78 % for the 0.7 % GelG/1 % LRE formulation and peaking at 62.49 % in the 0.7 % GelG/0.4 % LRE formulation. In terms of the BI, the highest reading occurred in the 0.7 % GelG/1 % LRE at 66.35 %, while the lowest was noted in the 0.7 % GelG/0.4 % LRE at 37.08 %. The dark color of the product could be attributed to the high content of phenolic and flavonoid compounds in LRE. These phenolic compounds have the ability to absorb light at shorter wavelengths, which leads to a decrease in WI and produces gels that appear more brownish and yellowish, along with elevated BI values (Cruz-Tirado et al., 2020). Moreover, the added GelG content increased the viscosity of the samples, which could reduce the overrun of the system and ultimately lead to a lower brightness (Sabeghi et al., 2024). In a similar study, it has been reported that gel-like foams with high levels of LRE had the lowest WI and highest BI values (Nooshkam et al., 2022). The visual observations of the aerated gels are provided in Fig. 1.

Fig. 1.

Visual observation of GelG/LRE aerated gels. GelG, gellan gum; LRE, licorice root extract.

3.4. FTIR

When polymers of varying types are compatible, they engage in intermolecular interactions. This compatibility alters the FTIR spectrum of the blend, differentiating it from the spectra of the individual, pure polymers. This alteration is beneficial for the investigation of polymer compatibility (Xiao et al., 2001).

In the GelG spectrum, bands appearing at 1033 and 1414 cm⁻¹ are indicative of the C—O stretching vibrations and CH₂ bending vibrations, respectively (Fig. 2). The signal at 1616 cm⁻¹ is associated with the stretching vibrations of C O, while the band at 3419 cm⁻¹ signifies the presence of hydroxyl groups that are involved in hydrogen bonding (Kazemi-Taskooh & Varidi, 2021). In the LRE spectrum, distinct bands are identified at various wavelengths: 3378 cm⁻¹ corresponding to the hydroxyl (OH) stretching vibration, 1611 cm⁻¹ indicating the stretching of alkenes (-C=C-), and 1422 cm⁻¹ related to the carbon‑carbon (C—C) stretch in aromatic compounds. Additionally, there are bands within the 1076–1045 cm⁻¹ range that represent the C—O stretch from carboxylic acids, alcohols, esters, or aliphatic amines, as well as C–OH bending and C—C stretching in licoflavones, glycyrrhizins, and saccharides (Corciova et al., 2019). The absorption peak at 1515–1513 cm⁻¹ corresponds to the C C bond in aromatic rings, which is a distinctive absorption feature of licoflavones in LRE (Wang et al., 2014).

Fig. 2.

FTIR of gellan gum (GelG), licorice root extract (LRE), and GelG/LRE aerated gels.

The marked increase in the intensity of the C O stretching vibration peaks observed in the GelG/LRE mixtures, compared to pure GelG, may be indicative of strong interactions between LRE and the GelG matrix (Fig. 2). This observation implies that the addition of Ca influenced both the intensity and the wavenumber region of the C O absorption bands (Ellerbrock & Gerke, 2021). A wide peak was detected between 3600 and 3000 cm⁻¹, which is linked to the overlapping hydroxyl group peaks from GelG and LRE present in the aerated gels. An increase in the spectrum intensity in this region indicates an increase in hydrogen bonding (Kamer et al., 2021). Furthermore, the observed spectral shift from 1049 cm⁻¹ in LRE to approximately 1040 cm⁻¹ in the GelG/LRE mixtures indicates the formation of hydrogen bonding interactions between unbound LRE and GelG (Nooshkam et al., 2022).

3.5. XRD analysis

In Fig. 3, the XRD patterns of GelG, LRE, and aerated gels are presented. The XRD test was employed to determine the degree of crystallinity and investigate the presence of amorphous characteristics versus the crystalline structure. Sharp peaks in XRD are associated with crystalline characteristics, while broadened peaks suggest an amorphous structure (You et al., 2018). The diffractogram of GelG displayed a broad feature at lower values of the diffraction angle (2θ), which is indicative of an amorphous structure. Nevertheless, the observation of a peak at 20° suggests the existence of crystalline phases within the material. The relative crystallinity of GelG was calculated to be 44.94 %. Similar results were reported for XRD patterns of GelG (Ribas Fonseca et al., 2020; Xu et al., 2007). Furthermore, the LRE XRD pattern indicates that at a diffraction angle of 19.05°, the intensity of the vibration peaked at 239, with the calculated relative crystallinity being 28.49 %. There were many diffraction peaks with small intensity, which could be due to licorice flavonoids and glycyrrhizic acid (Zu et al., 2014).

Fig. 3.

XRD of GelG/LRE aerated gels. GelG, gellan gum; LRE, licorice root extract.

The diffraction peaks corresponding to GelG and LRE at 19.05° and GelG at 20.7° were lowered in all samples of aerated gels. The aerated gels with 1.3 % GG/1 % LRE and 0.7 % GG/0.4 % LRE had the highest intensity (41.44 at 19.05°) and lowest intensity (16 at 19.05°), respectively. Zhu et al. (2014) state that when the crystalline and non-crystalline elements of a composite material are effectively mixed, the overall crystallinity of the composite is reduced compared to that of the separate crystalline parts. The alterations observed in the diffraction patterns indicated that intermolecular hydrogen bonding interactions took place between LRE and GelG, which disrupted the crystalline structures of both LRE and GelG. The relative crystallinity indices for the aerated gels composed of 0.7 % GelG/0.4 % LRE, 0.7 % GelG/1 % LRE, 1.3 % GelG/0.4 % LRE, and 1.3 % GelG/1 % LRE were determined to be 3.27 %, 9.55 %, 14.92 %, and 15.19 %, respectively. FTIR results were consistent with these observations, as an increase in the spectrum intensity of hydroxyl and C O stretching vibrations flexural were observed in all aerated gel samples, indicating the formation of more hydrogen bonds.

3.6. Gel firmness

Table 1 presents the results of the firmness of the aerated gels. The data indicates that with an increase in GelG concentrations, there is a significant enhancement in the firmness of the aerated gels (p < 0.05). The sample with 1.3 % GelG exhibited the lowest overrun and the highest aerated gel density, which aligns with the texture results of the aerated gels. This suggests that an increase in aerated gel density, indicative of a reduction in the air phase, leads to an increase in the firmness of the aerated gel. As the concentration of GelG in the aerated gels increases, a denser network is formed due to the high levels of interpenetration and entanglement of polymer chains (Oliveira Cardoso et al., 2017). This observation is consistent with previous reports that the firmness of gellan/gelatin mixed gels increased with the proportion of gellan (Lau et al., 2000). However, an increase in the level of LRE negatively impacts the firmness significantly (p < 0.05). In summary, the samples exhibited the greatest firmness at 1823.13 g for the 1.3 % GelG/0.7 % LRE combination, while the lowest firmness recorded was 445.64 g for the 0.7 % GelG/1 % LRE mixture. It has been noted that glycyrrhizic acid, the primary saponin and active ingredient in LRE, carries negative charges, and GelG is a linear negatively charged exopolysaccharide (Nooshkam, Varidi and Alkobeisi, 2022, Nooshkam, Varidi and Alkobeisi, 2023; Nooshkam & Varidi, 2021). Hence, high concentrations of LRE can induce static and spatial repulsion between the polymer chains of GelG, leading to an increase in structural inhomogeneity, which is not conducive to the formation of a robust gel (Babaei et al., 2019). This finding is in agreement with previous reports that an increase in the LRE level in foam-gel results in a decrease in the hardness and cohesiveness of the gel (Mardani et al., 2022).

3.7. Rheology

The role of rheological behavior is pivotal in the creation of food products, as it greatly affects the stability of aerated systems (Mardani et al., 2022). Fig. 4 shows how G′ and G′′, which represent the elastic and viscous properties of materials (Fan et al., 2022), change with angular frequency in aerated gels. For both 0.7 % GelG/1 % LRE and 1.3 % GelG/1 % LRE, an increase in angular frequency led to a rise in both G′ and G′′, suggesting the presence of a network structure. Over the range of frequencies tested, these parameters primarily showed solid-like behavior. For 0.7 % GelG/0.4 % LRE and 1.3 % GelG/0.4 % LRE, G′′ also rose with increasing angular frequency. Interestingly, for 0.7 % GelG/0.4 % LRE and 1.3 % GelG/0.4 % LRE, G′ initially rose and then slightly fell with increasing angular frequency. A crossover point was observed for 0.7 % GelG/0.4 % LRE where G′ was greater than G′′ at lower frequencies, indicating a predominance of solid-like properties until the crossover frequency or gel point (G′ = G′′) was reached. After this point, G′′ became dominant. High aeration and low system concentration can lead to a crossover in gel samples (Jabarkhyl, Barigou, Badve, & Zhu, 2020; Salahi & Mohebbi, 2021). The corresponding decrease in the storage modulus can be reasonably attributed to inertial effects, indicating a predominance of the liquid-like property at high frequencies and that the bubbles and capillary network are significantly strained and forced to rearrange.

Fig. 4.

Elastic (G′) and viscous (G′′) moduli of GelG/LRE aerated gels. GelG, gellan gum; LRE, licorice root extract.

As the concentrations of LRE in the systems increased, so did the G′ and G′′ values for the aerated gels. This implies that the elasticity of the structure increases with the amount of air it contains (Mardani et al., 2022; Okesanjo et al., 2020). However, as the concentrations of GelG increased, the G′ values for the aerated gels decreased, but the G′′ value remained unaffected by the presence of GelG. This could be due to the decrease in aeration caused by the increase in gum concentration, which was also observed in the porosity results. It has been noted that the elastic and solid behavior of food foams increases with air incorporation (Nooshkam et al., 2022). Notably, the aerated gels based on 0.7 % GelG/1 % LRE had significantly higher G′ and G′′ values compared to other samples. The ratio of viscous modulus to elastic modulus is tan δ, and if the value is less than 1, the substance is considered more elastic, and if the value is more than 1, it is considered more viscous (Mekala et al., 2022). In this regard, all the aerated gels, except 0.7 % GelG/0.4 % LRE, had a higher G′ than G′′ (tan δ < 1), indicating a predominance of elastic behavior over viscous properties. However, at higher frequencies, the 0.7 % GelG/0.4 % LRE had a higher G′′ than G′ (tan δ > 1), indicating a predominance of viscous behavior over elastic properties.

The structure of aerated gels can be inferred from the frequency dependence of G′ and G′′. To quantitatively analyze the degree of frequency dependence of G′ and G′′, the Power-law model was applied to the frequency sweep data. The aerated gels exhibited pseudoplastic behavior (K > 0 and 0 < n < 1) (Table 2). As the GelG level in the system increased from 0.7 % to 1.3 %, the K′ value of the aerated gels significantly decreased by an average of 0.8-fold (p < 0.05), but the K′′ value remained unchanged. Conversely, an increase in LRE concentration resulted in a 3.6-fold increase in K′ and a 3-fold increase in K′′ (p < 0.05). This index, representing the slope of the Power-law equation, suggests that an increase in its value reflects a transition of the material properties towards solid-like or liquid-like behaviors, respectively (Abebe & Ronda, 2014). Although the changes in K′′ were occasionally larger than those in K′, the aerated gels demonstrated solid elastic properties, as illustrated in Fig. 4. The increase in GelG and LRE content did not significantly affect the n' value (p > 0.05), but it did result in a significant decrease in n'′ (0.8-fold) of the aerated gel systems (p < 0.05). This suggests that the aerated gels were becoming more elastic-like. The larger K and smaller n values of the aerated gel produced by 0.7 % GelG and 1 % LRE could be due to the amphiphilicity and chirality of glycyrrhizic acid molecules. These molecules demonstrate anisotropic self-assembly in water, leading to the creation of elongated nanostructures such as nanofibrils and nanotubes. As the concentration rises, they can further combine and intertwine to produce a supramolecular hydrogel characterized by a hydrogen-bonded three-dimensional network. The robust interfibrillar interactions resulting from hydrogen bonding enable glycyrrhizic acid nanofibrils to generate multilayered fibril shells at the interfaces, as well as viscoelastic hydrogel networks within the continuous phase (Q. Li et al., 2022). Similarly, it was found that foam-gels exhibited more elastic behavior at higher concentrations of licorice root extract powder (Mardani et al., 2022). Overall, the significant viscoelasticity of the aerated system may contribute to its rigidity, making it well-suited for various food applications.

Table 2.

Rheological parameters of GelG/LRE aerated gels.

GelG (%)	LRE (%)	Storage modulus (G′)		Loss modulus (G″)
		K′ (Pa.sn)	n'	K″ (Pa.sn)	n″
0.7	0.4	532.65 ± 52.70d	0.0747 ± 0.0074a	89.70 ± 8.88d	0.2024 ± 0.0200a
	1	4494.10 ± 254.00a	0.0496 ± 0.0028b	523.29 ± 29.60a	0.1224 ± 0.0069b
1.3	0.4	1453.60 ± 123.30c	0.0483 ± 0.0041b	207.67 ± 17.60c	0.1269 ± 0.0107b
	1	2707.60 ± 191.00b	0.0627 ± 0.0044ab	367.17 ± 26.00b	0.1299 ± 0.0092b

Different letters indicate significant differences between samples at p < 0.05.

GelG, gellan gum; LRE, licorice root extract.

3.8. Microstructure

To gain a deeper understanding of the microstructure, SEM images of aerated gels were taken (Fig. 5a). LRE and GelG, acting as a surfactant and thickening agent respectively, created a robust network that trapped water and air. It is hypothesized that the surfactant, through its hydroxyl and carbonyl groups, interacted with the hydroxyl groups of gellan via hydrogen bonding, contributing to the structure of the aerated gels. Generally, an increase in GelG concentration resulted in a significant reduction in the porosity of the aerated gels, a trend also observed in the overrun results. The high viscosity of the liquid would inhibit the entrapment of air during whipping or mechanical mixing (Abd Karim & Wai, 1999b). From these findings, it can be inferred that there is an optimal concentration for gel aeration, which in this case is 0.7 %. At lower concentrations, there was a greater number of small air bubbles in the aerated gel. Conversely, as the concentration of GG rose, the occurrence of larger air bubbles also increased proportionately. This is clearly visible in the microstructure of the 1.3 % GelG/0.4 % LRE gel. The slower kinetics of surface tension resulting from a higher GelG content, up to a certain point, led to the formation of larger bubbles as the surfactant diffused more slowly to the air-water interfaces (Jabarkhyl, Barigou, Zhu, et al., 2020). It can also be noticed that certain air bubbles lost their spherical shape and became elongated. This may have been influenced by the pressure from adjacent bubbles, as well as the possibility of excess air being trapped within the bubble, exceeding what the interfacial film can support (Abd Karim & Wai, 1999a).

Fig. 5.

(a) Microstructure and (b) release behaviors of GelG/LRE aerated gels. GelG, gellan gum; LRE, licorice root extract. Different letters indicate significant differences between samples at p < 0.05.

Furthermore, it appears that increasing the concentration of LRE resulted in the destruction of the film between the bubbles and the formation of thin, hair-like strands. This may be due to an excess of surfactant molecules at the lamella. It has been indicated that an abundance of surfactant molecules intensifies the influence of gravity on foam drainage. This causes a continuous flow of liquid from the film that exists between neighboring bubbles, ultimately causing the foam film to break down (Majeed et al., 2020).

3.9. In vitro release behaviors

Medications that are readily absorbed from the GIT and have short half-lives are quickly removed from the systemic circulation. To maintain effective therapeutic activity, these drugs need to be administered frequently. To overcome this challenge, oral sustained-controlled release formulations have been developed. These formulations are designed to gradually release the drug into the GIT and sustain a therapeutic concentration in the bloodstream for an extended duration. Upon oral intake, this drug delivery system would stay in the stomach, releasing the drug at a controlled rate, which ensures a steady supply to its absorption sites within the GIT. Consequently, it is essential to create a system that prolongs gastric residence time, thereby enhancing the window for drug absorption in the small intestine (Prajapati et al., 2013). Calcium is an essential mineral for bone development and is mainly absorbed in the duodenum, where active calcium-binding protein (CaBP) absorption sites are located in the upper GIT. Numerous factors affecting calcium absorption have been explored in existing literature. Considering these distinct absorption properties, it is important to prolong the gastric residence time of calcium-loaded dosage forms to ensure that calcium can reach the active absorption site (Bajpai & Tankhiwale, 2008).

Fig. 5b presents the in vitro release of calcium from aerated gels in the GIT (stomach). The calcium release from the aerated gels significantly increased by approximately 16 % as a function of GelG concentrations in the stomach environment (p < 0.05). However, LRE did not significantly affect the calcium release from the aerated gels (p > 0.05). The calcium release from the aerated gels was correlated with the overrun results (Fig. 6). The aerated gel with 1.3 % GelG/1 % LRE demonstrated the highest rate of calcium release, and as the aeration in gels increased, the release of calcium decreased. The process of gelation and aggregation of GelG involves chemical bonding between calcium and the carboxylic groups present in the gellan chains. Calcium, being a strong electrophile, electrostatically interacts with the carboxylate group of GelG. These interactions are prone to hydration and can therefore be broken under aqueous conditions (Verma & Pandit, 2011). The gels with 0.7 % GelG/0.4 % LRE and 0.7 % GelG/1 % LRE had a greater number of bubbles, and the release of calcium could only occur when water infiltrated the interior of the gels. As the bubbles were intact and air-filled, it would require time for the gels to become wet. Moreover, the presence of air bubbles lengthens the diffusion path, which should aid in extending the release (Svagan et al., 2016). Consequently, less calcium will be destroyed in the stomach medium, and a substantial amount of calcium will be accessible in the intestine. This suggests that the aerated gel could serve as a matrix for the slow release of various active ingredients.

Fig. 6.

Heatmap illustrating the Pearson correlation coefficients among response variables.

3.10. Correlation analysis

The categories and associated properties analyzed in this study encompassed foaming characteristics (including porosity, density, and overrun), color metrics (WI and BI), firmness, water holding capacity (WHC), rheological parameters (K and n), crystallinity, and release properties, which were evaluated for correlation analysis utilizing a mathematical model grounded in Spearman's correlation coefficient (Fig. 6).

Consistent with expectations, a substantial positive correlation was observed between overrun and porosity (R = 0.959), as well as a significant negative correlation between overrun and density (R = −0.965). This indicates that increased air incorporation correlates with heightened overrun and porosity, while concurrently resulting in decreased density (Nooshkam et al., 2022). There is a strong positive relationship between WHC and firmness, with a correlation coefficient of 0.90. This indicates that gels that retain more water tend to be firmer, likely due to the stabilizing role of water within the gellan gum structure (Ge et al., 2022; Mi et al., 2021). Nonetheless, there was a strong negative correlation between firmness (R = −0.89) and overrun. An increase in overrun indicates a larger proportion of a compressible dispersion phase, which leads to diminished resistance against an applied force and, consequently, a lower perception of firmness when overrun values are higher (Roy et al., 2021). Additionally, overrun and release were negatively correlated at −0.73, suggesting that more aerated systems can impede the release of compounds, potentially because of lower diffusion coefficient in aerated matrices (Zúñiga & Aguilera, 2008). Conversely, the WI and BI share a significant negative correlation of −0.94, highlighting that as browning intensifies, whiteness diminishes (Sarkar et al., 2024). It is important to highlight that an inverse relationship was observed between porosity and crystallinity (R = −0.87). This indicates that as the crystallinity of the material rises, its porosity tends to decline, with the amorphous phase exhibiting the greatest surface area (El Koulali et al., 2024).

These relationships underscore the complex interplay among structural, textural, and release characteristics, where factors such as air incorporation, water retention, and crystallinity significantly affect the gel's functionality. The underlying mechanisms likely involve gellan gum's capacity to create networks, with licorice extract influencing these attributes through interactions with the polymer.

4. Conclusions

The study findings suggest that the concentrations of GelG and LRE have a significant impact on the characteristics of aerated gels. By controlling the levels of GelG and LRE, it is possible to generate stable aerated gels with GelG-LRE interactions. Among the variants evaluated, the aerated gel made from 0.7 % GelG and 0.7 % LRE stood out with the most impressive attributes, showcasing the highest overrun (156.04 %), porosity (60.81 %), and ideal viscoelastic stability. This formulation also demonstrated remarkable water-holding capacity (WHC) along with minimal syneresis, signifying strong structural integrity. In contrast, using greater concentrations of GelG (1.3 %) and LRE (1 %) resulted in decreased aeration and increased density, although these concentrations offered enhanced firmness and improved calcium release properties, making them more suitable for controlled-release purposes. Moreover, the elevated G′ value of the gels signifies solid and elastic properties, which play a crucial role in preventing liquid drainage from the gels. This approach has potential applications in dairy-based gels like yogurt or mousses, where aeration can enhance their texture and reduce calories by volume.

CRediT authorship contribution statement

Mehdi Varidi: Writing – review & editing, Visualization, Validation, Supervision, Methodology, Funding acquisition, Conceptualization. Fatemeh Alkobeisi: Writing – original draft, Software, Resources, Methodology, Investigation, Data curation. Majid Nooshkam: Writing – review & editing, Writing – original draft, Software, Resources, Methodology, Investigation.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used “Grammarly” in order to paraphrase and grammatically check the sentences. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the published article.

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.

Data availability

Data will be made available on request.