Clarification of Clove Basil Extract Using Spinel Hollow Fiber Membranes

Abstract

This study investigates the application of spinel (MgAl₂O₄) hollow fiber membranes for clarification of clove basil (Ocimum gratissimum L.) aqueous extract, a rich source of bioactive compounds. The membranes were produced using a phase-inversion and sintering method at 1350 °C, combining alumina and dolomite as raw materials. The calcination of the powder materials at 1350 °C resulted in the spinel phase formation, as indicated by the XRD analyses. The spinel hollow fiber membrane presented a hydrophilic surface (water contact angle of 74°), moderate roughness (144.31 ± 12.93 nm), and suitable mechanical strength. The ceramic membrane demonstrated a water permeability of 35.28 ± 2.46 L h⁻¹ m⁻² bar⁻¹ and a final permeate flux of 9.22 ± 1.64 L h⁻¹ m⁻² for filtration of clove basil extract at 1.0 bar. Fouling analysis identified cake formation as the dominant mechanism for flux decline. The membrane retained 44% of the total phenolic compounds and reduced turbidity by 60%, while preserving significant antioxidant capacity in the permeate. The results highlight the potential of spinel-based hollow fiber membranes as a cost-effective and efficient solution for clarifying bioactive plant extracts, offering enhanced mechanical properties and lower sintering temperatures compared to conventional alumina membranes.

1. Introduction

Membrane technology has gained significant attention in various industrial applications, including wastewater treatment, solvent distillation, water desalination, juice filtration, and gas separation. Among the available membrane types, ceramic membranes stand out due to their superior thermal and chemical stability, stability under harsh conditions, and ease of cleaning. In particular, hollow fiber geometry offers the highest surface area-to-volume ratio compared to flat sheet, disc, and tubular membranes, making hollow fiber membranes highly efficient for industrial applications [1]. Despite their advantages, ceramic membranes face challenges like brittleness and susceptibility to cracking under high operating pressures [2]. In general, high temperatures are required during the sintering process to impart mechanical strength to ceramic membranes, which affects production costs, making the development of alternative materials and optimized sintering conditions essential for cost-effective membrane fabrication.

Alumina (Al₂O₃), a precursor with outstanding mechanical and thermal properties, is widely used to produce ceramics. However, the elevated sintering temperature required for processing poses economic challenges. To mitigate these drawbacks, alternative low-cost ceramic materials such as attapulgite, kaolin, clay, dolomite, niobium pentoxide, fly ashes, and raw clays have been explored [3,4]. Spinel is a class of crystalline materials with the general formula AB₂O₄, where A is a divalent cation (such as Mg²⁺, Fe²⁺, Zn²⁺) and B is a trivalent cation (e.g., Al³⁺, Fe³⁺, Cr³⁺) [5]. Spinel materials exhibit outstanding properties such as high thermal and chemical stability, mechanical strength, and resistance to harsh operating conditions, and some low-cost materials have been evaluated for producing spinel-based ceramic membranes. The pore size, pore size distribution, and porosity of the membrane are critical for determining its separation capabilities. Membranes with tailored pore structures can achieve permeation fluxes and favorable mechanical and surface properties. While the spinel structure provides a robust framework, the membrane performance in aqueous separations is primarily dictated by its ability to selectively allow certain molecules to pass through its pores, which is mainly governed by the pore size distribution and surface interactions.

Chen et al. [6] prepared NiAl₂O₄ spinel hollow fiber ceramic membranes using a mixture of nickel-laden wastewater sludge and bauxite. The resulting spinel-based membranes, which featured a variety of microvoids across the structure, presented 99.5% of oil rejection and a high permeate flux of 7473 L h⁻¹ m⁻² bar⁻¹. Aziz et al. [7] developed low-cost alumina-spinel composite hollow fiber membranes using aluminum dross waste as the starting material. The spinel hollow fiber sintered at 1275 °C exhibited superior performance in removing oil droplets, with an oil rejection of 92.41%. Jiang et al. [8] tailored copper/cobalt-rich ferrite spinel-based ceramic membranes by integrating gold mine tailings. The membranes were fabricated at sintering temperatures of 1100 °C, 1200 °C, and 1300 °C, achieving an oil rejection of 97.9%. Bessa et al. [9] applied mixtures of dolomite and alumina to produce spinel (MgAl₂O₄) ceramic hollow fiber membranes and investigated the effect of the calcination temperature on the characteristics of the produced membranes. The membranes sintered at 1350 °C exhibited a mechanical resistance of 54.88 ± 4.25 MPa, a 49% increase compared to pure alumina membranes sintered at the same temperature [9]. These spinel hollow fiber membranes were successfully applied for oily wastewater treatment, achieving an oil rejection rate of 94.5%. Paraíso et al. [1] showed that the filtration through spinel hollow fiber membranes was efficient in reducing tea cream formation in hibiscus extracts.

Membrane separation technology has attracted significant interest in the processing of natural extracts, with ceramic membranes being particularly favored for their uniform pore size distribution, exceptional chemical and thermal resistance, reduced tendency for fouling, and extended lifespan. Despite their advantages, ceramic membranes face significant barriers for bioactive compound purification applications, with high production costs being a primary limiting factor [10]. Thus, novel membrane material and fabrication processes should be proposed to enhance the economic viability of ceramic membranes in the processing of bioactive compounds.

This study aims to investigate the performance of spinel-based ceramic hollow fiber membranes in the filtration of crude clove basil extract, a medicinal plant that presents significant phytochemicals with high antioxidant capacity. The impact of the membrane characteristics on the filtration performance will be fully assessed, and fouling occurrences during the filtration of the clove basil extract will be elucidated. This work introduces an innovative approach by exploring the potential of spinel-based membranes in the filtration of plant extracts, providing valuable insights into their application for bioactive compound isolation. This investigation extends the application of spinel hollow fiber membranes [9] to the concentration of bioactive compounds from clove basil extract. The ceramic powder mixture was composed of alumina and dolomite in a defined proportion to promote spinel phase formation after sintering at 1350 °C. In addition, the phase-inversion parameters during extrusion (such as ceramic suspension and bore fluid flow rates, and air gap distance) were carefully controlled to produce a hollow fiber morphology suitable for microfiltration. These structural features support selective retention of suspended solids and larger biomolecules while allowing the passage of clarified permeate with preserved antioxidant activity.

2. Materials and Methods

2.1. Material

The following reagents were used to produce the ceramic hollow fibers: alumina (Al₂O₃, α-phase, 99.9% purity, Alfa Aesar, Ward Hill, MA, USA), dimethyl sulfoxide (DMSO, Êxodo Científica, Sumaré, SP, Brazil), dolomite (HMX Chemical Specialties, Uberlândia, MG, Brazil), polyethersulfone (PES, Veradel 3600P, Solvay, Houston, TX, USA), and polyvinylpyrrolidone (PVP, Sigma Aldrich, St. Louis, MA, USA). The following reagents were used for the physicochemical analyses: 2,2-diphenyl-1-picrylhydrazyl (DPPH·, Sigma-Aldrich, USA), 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox, Sigma-Aldrich, USA), Folin-Ciocalteu reagent (Cromoline, Diadema, SP, Brazil), gallic acid (Dinâmica Química, Indaiatuba, SP, Brazil), methanol (CH₃OH, Isofar, Duque de Caxias, RJ, Brazil), and sodium carbonate (Na₂CO₃, Dinâmica Química, Indaiatuba, SP, Brazil).

2.2. Fabrication of Spinel-Based Hollow Fiber Membranes

Production of the hollow fiber membranes followed the phase-inversion technique described by Bessa et al. [9] with modifications. The starting material was composed of alumina and dolomite. Polyethersulfone was used as the polymer binder, dimethyl sulfoxide was used as the aprotic solvent, and polyvinylpyrrolidone was used as the additive.

At first, PES (5.7 wt.%) was completely dissolved in DMSO (36.8 wt.%) under magnetic stirring. Then, the ceramic powder composed of alumina (42.8 wt.%) and dolomite (14.3 wt.%) was slowly mixed into the previous solution. The ceramic suspension was mixed under ball milling for 48 h. At last, PVP (0.4 wt.%) was added to the mixture, and the ball milling process was continued for a further 48 h. The material proportions suggested by Bessa et al. [9] were slightly adjusted to achieve an appropriate visual viscosity for the extrusion process. Air bubbles were removed from the ceramic suspension by a degassing process in a vacuum pump at 700 mmHg.

The extrusion of the ceramic suspension into a coagulation bath was performed in a tube-in-orifice spinneret (3.0 mm of outer diameter and 1.2 mm of inner diameter). Water was used as both the external and internal coagulant. The air-gap distance from the spinneret tip to the water bath was 5 cm. Two syringe pumps (Harvard Apparatus XHF, Holliston, MA, USA) were used to control the flow of the ceramic suspension at 10 mL min⁻¹ and the flow of the internal coagulant at 15 mL min⁻¹. The phase-inversion process occurred when the ceramic suspension, containing PES polymer dissolved in DMSO, was in contact with a non-solvent (water). During extrusion, the exchange between the solvent and non-solvent could be observed as the solvent diffused from the green ceramic hollow fiber into the water bath.

Following extrusion, the fiber precursors were washed and cut into segments measuring approximately 10 cm in length. Prior to calcination, the membrane precursors were gently straightened by using rectangular rulers, taking care to avoid mechanical deformation. After drying at room temperature (~25 °C) for 24 h, the fibers were placed in ceramic holders and calcinated in a tubular furnace (Carbolite TZF 16/610, Hope, UK) at 1350 °C, following a three-stage temperature ramp. First, the temperature was raised from 30 °C to 300 °C at a 2 °C min⁻¹ rate. After achieving 300 °C, the temperature was raised to 600 °C at a 1 °C min⁻¹ rate, with a dwelling of 1 h. At this sintering step, the polymer phase is eliminated. Then, the temperature was raised to 1350 °C at a 9 °C min⁻¹ rate, with a dwelling of 5 h. The decreasing temperature ramp was targeted at 30 °C at a 2 °C min⁻¹ rate.

The crystalline structures of the ceramic mixture were characterized by X-ray diffraction (XRD) using a diffractometer (Shimadzu XRD-6000, Kyoto, Japan) coupled with Cu-Kα radiation (λ = 0.1541 nm), at a scan rate of 1° min⁻¹ and a 2θ range from 5° to 85°. The morphology of the spinel-based hollow fiber was investigated by scanning electron microscopy (SEM, Zeiss EVO MA10, Oberkochen, Germany) after a thin gold layer deposition on the sample surface (Leica EM SCD050, Vienna, Austria). The mean outer surface roughness was verified by tip-scanning atomic force microscopy (AFM, Shimadzu SPM-9600, Kyoto, Japan) at an image size of 15×15 µm. The water contact angle measurement was performed in a drop-shape analyzer (Krüss DSA25E, Hamburg, Germany) using a sessile drop and the ellipse fitting method.

2.3. Membrane Filtration of the Clove Basil Extract

Clove basil leaves were processed according to Dorneles et al. [11]. The aqueous clove basil extract was prepared with a concentration of 10 g L⁻¹ (dried leaves to distilled water) at 75 °C under magnetic stirring (IKA C-MAG HS 7, Staufen im Breisgau, Germany) for 120 min. Extracts were pretreated by filtration through a 10-mesh stainless-steel strainer and a quantitative grade disc filter paper (particle retention: 20–25 µm, Unifil, Londrina, PR, Brazil), followed by a centrifugation step (Beckman Coulter Avanti J25, Brea, CA, USA) at 8000 rpm for 20 min.

A single spinel hollow fiber with a filtration area of 3.83 × 10⁻⁴ m² and open edges was assembled in a homemade permeation module. The membrane was compacted by filtering ultrapure water at 4.0 bar for 1 h. The hydraulic permeance of the hollow fiber was assessed by measuring the water flux at different transmembrane pressure values. Filtrations of clove basil extract were carried out in a crossflow system at room temperature (~25 °C), a transmembrane pressure of 1.0 bar, and a crossflow velocity of 0.5 m s⁻¹. The filtration assembly included a progressing cavity pump (Netzsch NEMO, Pomerode, SC, Brazil) coupled to a frequency inverter at 40 Hz, a pressure gauge, and a needle valve to adjust pressure in the system. The permeate mass flow rate was recorded using an analytical balance.

To verify fouling occurrences, the flux decline data from the filtration of clove basil extract was fit to the model presented by Field and Wu [12], as indicated in Equation (1):

$$-{\displaystyle \frac{dJ}{dt}}=k_n\left(J-J_s\right)J^{2-n}$$

(1)

where J is the permeate flux over time (L h⁻¹ m⁻²), J_s is the final permeate flux (L h⁻¹ m⁻²), t is the filtration time (h), k is the fouling constant for crossflow operation (m⁽ⁿ⁻²⁾ h^(1-n)), and the n index was fixed for each fouling mechanism occurring during filtration (n = 0 for cake filtration; n = 1 for partial pore blocking; n = 1.5 for internal pore blocking; n = 2 for complete pore blocking).

Data fitting for each “n” value was performed using the fourth-order Runge-Kutta method and the Levenberg-Marquardt algorithm. The coefficient of determination (R²) and the residual sum of squares (RSS) were pondered to indicate the main fouling mechanisms.

The fouling index (FI) was calculated using Equation (2):

$$FI\left(\%\right)={\displaystyle \frac{J_{wd}}{J_{wc}}}\times 100$$

(2)

where J_wd is the water flux through the dirty membrane (L h⁻¹ m⁻²) and J_wc is the water flux through the clean membrane (L h⁻¹ m⁻²).

2.4. Physicochemical Analyses

The size distribution of clove basil extract was performed using the dynamic light scattering (DLS) technique with a particle size analyzer (Malvern Zetasizer Pro, Malvern, UK). Measurements were conducted in polystyrene cuvettes at 25 °C.

Analyses for total phenolic compounds content (TPC), antioxidant capacity by a stable radical method, turbidity, soluble solids, and color were performed in the feed and permeate extracts. A portable turbidimeter (MS Tecnopon TB-1000, Piracicaba, SP, Brazil) was used for turbidity measurements. A colorimeter (Konica Minolta CR-5, Tokyo, Japan) in transmittance mode and CIELAB color space was used for color measurements. A portable digital refractometer (Hanna Instruments HI96801, Nusfalau, Romania) was used to determine the soluble solids content.

Phenolic content and antioxidant capacity were estimated according to the methods described by Dorneles et al. [11]. For TPC, 150 µL of the extract, 150 µL of Folin-Ciocalteu reagent (50% v/v), and 2700 µL of a sodium carbonate aqueous solution (0.264 mol L⁻¹) were mixed and remained in the dark for 30 min to react. Absorbance values of the reaction were verified in a spectrophotometer (Shimadzu UV 1280, Kyoto, Japan) at 725 nm. Results are presented in gallic acid equivalents (mg_GAE g⁻¹). For the DPPH· radical scavenging assay, 150 µL of the extract and 2850 µL of a DPPH· working solution were mixed and remained in the dark for 1 h. Absorbance values were verified in a spectrophotometer at 515 nm. Results are expressed in scavenging activity (SA, %) and Trolox equivalents (µmol_TE g⁻¹).

The analyses were carried out in triplicate, and the results were expressed as mean and standard deviation. When applicable, results were submitted to the variance and Tukey tests for the minimum significant difference (p < 0.05) between mean values.

3. Results and Discussion

3.1. Characteristics of the Produced Spinel Hollow Fibers

Figure 1 presents the X-ray diffraction spectra of pure alumina, pure dolomite, and the alumina-dolomite mixture at a mass ratio of 3:1, all sintered at 1350 °C. The XRD patterns of pure alumina exhibited characteristic diffraction peaks at 25.6°, 35.1°, 37.8°, 43.4°, 52.5°, 57.5°, 66.5°, and 68.2° [13]. These peaks confirm the high purity and crystalline nature of the alumina raw material, which is critical for achieving mechanical stability in membranes. Meanwhile, pure dolomite material displayed a strong diffraction peak near 31° [14]. After calcination of the alumina-dolomite mixture at 1350 °C, the XRD spectrum reveals the formation of spinel (MgAl₂O₄), evidenced by peaks matching the spinel phase. The absence of residual alumina or dolomite peaks suggests complete reaction, yielding a homogeneous spinel structure. The spinel phase is advantageous due to its high thermal/chemical stability and mechanical strength, which are superior to pure alumina at lower sintering temperatures.

Figure 2 presents the SEM images (cross-section and outer surface) of the spinel hollow fiber membranes before and after calcination. The spinel hollow fiber presented a regular contour with outer and inner diameters of 2.638 ± 0.038 mm and 2.061 ± 0.007 mm, respectively, before calcination (Figure 2a). Following calcination at 1350 °C, dimensional shrinkage occurred, resulting in reduced outer and inner diameters of 1.816 ± 0.003 mm and 1.402 ± 0.004 mm, respectively (Figure 2b). This corresponds to linear shrinkage of approximately 30% for both outer and inner diameters, which is probably associated with phase transformations, as also evidenced by Bessa et al. [9]. Notably, while significant length reduction occurred during thermal treatment, the membrane structural integrity was not compromised, with no observable cracking or deformation in the SEM images. As evidenced in Figure 2c,d, the fiber presented a multi-pore size distribution that will promote permeation through the membrane. Figure 2c,d indicate a reduction in the fiber porosity following calcination, confirming the expected densification behavior at elevated temperatures (1350 °C). The microstructural evolution shows improved particle packing density and grain boundary formation characteristic of sintering processes. The spinel hollow fiber also presented a packed pore network on its outer surface, also identified as a sponge-like layer (Figure 2e,f). The ceramic particles are more interconnected on the fiber outer surface prior to calcination (Figure 2e) compared to the post-calcination state (Figure 2f). This difference likely stems from the presence of organic binders/polymers in the green fibers, which temporarily enhance particle cohesion but are subsequently removed during thermal treatment.

Figure 3 presents the AFM image of the hollow fiber outer surface, where the topographic analysis revealed an average roughness of 144.31 ± 12.93 nm. This value is comparable to that reported by Dias et al. [15] for alumina hollow fibers (121.3 ± 3.63 nm). Surface roughness is a critical parameter for microfiltration membranes, as it influences permeability, fouling occurrences, and the overall membrane performance. The peaks and valleys observed in the AFM image (Figure 3) are likely to contribute to cake formation during the filtration of the extract.

The water contact angle of 74° measured for the produced spinel hollow fiber membranes confirms their hydrophilic character (contact angle <90°). Aziz et al. [7] also reported the hydrophilic behavior of spinel-based ceramic membranes. Chen et al. [6] reported a water contact angle of 46° for spinel-based hollow fiber membranes.

The structural and mechanical properties of the spinel hollow fiber membranes were verified in a previous study [9], revealing key features for membrane performance, as presented in

Table 1. Main properties of spinel hollow fiber membranes.

Property	Value	Implications	Reference
Average roughness of the outer surface	144.31 ± 12.93 nm	Moderate roughness enhances permeability while maintaining fouling resistance	This work
Water contact angle	74°	The hydrophilic nature improves water permeability and reduces organic fouling	This work
Pore size in the sponge-like layer	0.16 µm	Provides mechanical support while contributing to selective separation	[9]
Pore size of the microchannels	5.29 µm	Facilitates high permeate flux by reducing hydraulic resistance	[9]
Porosity	43%	Enhances permeability for filtration	[9]
Bending Strength	54.88 ± 4.25 MPa	Ensures durability under operational pressures (critical for industrial use)	[9]

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While alumina hollow fiber membranes exhibit similar asymmetric pore structures [15], their mechanical strength (~40 MPa) is significantly lower than the spinel membranes developed in this work (54.88 ± 4.25 MPa). The enhanced bending strength of spinel-based hollow fibers, combined with their moderate calcination temperature (1350 °C vs. >1600 °C for alumina), demonstrates both superior suitability for high-pressure applications and greater cost-effectiveness for industrial-scale production.

3.2. Filtration of Clove Basil Extract Through the Spinel Hollow Fibers

Figure 4 presents the experimental data of water flux through the produced spinel hollow fiber according to the transmembrane pressure. The linear fit, based on Darcy’s law, resulted in a water permeability of 35.28 ± 2.46 L h⁻¹ m⁻² bar⁻¹. This value is lower than some of those reported in the literature for ceramic membranes [6,9,16,17]. The permeability value is probably associated with the fiber densification during the calcination process, caused by the shrinkage and rearrangement of spinel grains at high temperatures. Jiang et al. [8] investigated the production of spinel-incorporated ceramic membranes from gold mine tailings, and the membranes sintered at 1200 °C and 1300 °C presented water permeability of 32.61 L h⁻¹ m⁻² bar⁻¹ and 10.64 L h⁻¹ m⁻² bar⁻¹, respectively.

The experimental permeate flux decline data for filtering the clove basil extract through the spinel hollow fiber are presented in Figure 5. The initial permeate flux was 23.98 ± 1.67 L h⁻¹ m⁻², which is close to the flux of pure water through the membrane. After 150 min of filtration, the achieved final flux was 9.22 ± 1.64 L h⁻¹ m⁻². A more pronounced flux decline was verified in the first 30 min of filtration.

Experimental permeate flux values were fitted to the model proposed by Field and Wu [12] to evaluate fouling mechanisms in crossflow filtration. The analysis considered four potential fouling modes: cake formation (n = 0, Figure 5a), partial pore blockage (n = 1, Figure 5b), internal pore blockage (n = 1.5, Figure 5c), and total pore blockage (n = 2, Figure 5d). The best fit was obtained for n = 0, indicating that cake formation is the dominant fouling mechanism in the filtration process. This suggests that flow resistance primarily results from the accumulation of solid materials on the membrane surface rather than from pore blocking or other fouling phenomena. The complexity of the crude clove basil extract probably contributed to cake formation during filtration. Additionally, the coalescence of spinel particles on the membrane outer surface after the calcination process, as evidenced in the SEM image (Figure 2), further reduced permeate flux and intensified cake layer formation. Filtration was conducted at a crossflow velocity of 0.5 m s⁻¹, and increasing the crossflow velocity is recommended to mitigate cake formation. Chen et al. [6] also observed predominant cake layer formation at low crossflow velocities (0.56 and 1.12 m s⁻¹) during the filtration of oily wastewater using spinel-based ceramic membranes.

Additionally, the fouling index (FI) for the microfiltration of clove basil extract was 66.72%. Paraíso et al. [1] reported an FI of 70.69% for the clarification of a centrifuged hibiscus (Hibiscus sabdariffa L.) extract, while Castro-Muñoz et al. [18] reported an FI of 77.90% for the processing of xoconostle (Opuntia joconostle) fruit extract by ultrafiltration.

3.3. Physicochemical Characteristics of Feed and Permeate

Dynamic light scattering (DLS) analysis revealed particle size distributions of crude and centrifuged clove basil extracts (Figure 6). The crude extract exhibited a bimodal distribution with two prominent populations: a primary peak centered at approximately 1000 nm, and a secondary population around 5000 nm. Centrifugation pretreatment reduced the volume fraction of larger particles, while maintaining the smaller nanoparticle population. The crude extract presented a Z-average size of 514.1 ± 18.7 nm and a polydispersity index (PI) of 0.66 ± 0.02, which indicates a moderate-dispersed sample. After the centrifugation pretreatment, the Z-average size and the PI decreased to 285.9 ± 3.8 nm and 0.43 ± 0.01, respectively, suggesting the removal of larger particles.

The evaluated physicochemical characteristics of crude, centrifuged, and permeate samples of the clove basil extract are presented in

Table 2. Characteristics of aqueous clove basil extract before and after filtration.

Parameter	Clove Basil Extracts
	Crude	Centrifuged	Permeate
TPC (mgGAE g−1)	232.95 a ± 5.86	229.29 a ± 1.21	128.59 b ± 0.34
Antioxidant capacity (DPPH, µmolTE g−1)	1923.37 a ± 16.50	1024.72 b ± 3.91	510.94 c ± 1.05
Turbidity (NTU)	58.3 a ± 1.5	16.2 b ± 0.3	6.5 c ± 0.3
Soluble solids (%Bx)	0.40 a ± 0.00	0.35 a,b ± 0.07	0.20 b ± 0.00
Color coordinate L*	88.47 c ± 0.01	90.24 b ± 0.08	92.06 a ± 0.01
Color coordinate a*	−1.00 a ± 0.01	−1.22 c ± 0.01	−1.17 b ± 0.00
Color coordinate b*	16.16 a ± 0.01	12.27 b ± 0.01	9.38 c ± 0.01

Statistical differences (p < 0.05) are indicated by distinct superscript letters in the same line.

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A substantial number of phenolic compounds were obtained in the clove basil extract. Centrifugation pretreatment showed no significant effect (p > 0.05) on total phenolic content. Anusmitha et al. [19] found a range of phenolic content from 229.8 to 246.2 mg_GAE g⁻¹ in an ultrasound-assisted aqueous extraction of Ocimum species, under a concentration of 100 g L⁻¹ and an extraction time of 60 min. Ibrahim et al. [20] detected a TPC content of 146.31 ± 0.04 mg_GAE g⁻¹ in an aqueous extract of Ocimum basilicum, under a concentration of 133 g L⁻¹ and 300 min. Since the solid-liquid ratio used in this work was lower (10 g L⁻¹), the findings are not only comparable to the literature but superior in terms of phytochemicals obtained by the extraction process. Anusmitha et al. [19] described the presence of rosmarinic, sinapic, and chlorogenic acids, as well as flavonoids, in Ocimum species. A previous work identified seven phenolic acids, including the ones cited above, in the aqueous extract of clove basil [11]. It is worth noting that location and harvest conditions impact the characteristics of the plant and the produced extract.

The clove basil extract presented a considerable concentration of antioxidant compounds, as expressed by the antioxidant capacity in terms of the DPPH· method. The centrifugation process reduced this activity by approximately 50%, likely due to the sedimentation of antioxidant compounds bound to larger macromolecular complexes. While the soluble solids content showed only marginal reduction, a pronounced 72% decrease in turbidity confirmed efficient removal of colloidal particles. These changes correlated with measurable optical modifications: increased luminosity (color L* coordinate) and reduced color a* and b* coordinates, suggesting centrifugation preferentially eliminated light-scattering macromolecules and chromophoric complexes while preserving low-molecular-weight antioxidants in the solution.

Filtration with the spinel hollow fiber reduced turbidity by approximately 60% and retained almost 44% of the TPC. The permeate stream presented almost 50% of the antioxidant potential expressed by Trolox equivalents, as also indicated in the literature for the microfiltration process [21]. In a previous work, the use of a 10 kDa polymeric membrane to clarify the clove basil extract was able to retain 77% of the TPC and reduce turbidity by 84% [11]. Even so, the permeate stream showed turbidity values compatible with those suggested in the literature for ready-to-drink beverages. Reig-Valor et al. [22] reported an average rejection of phenolic compounds of 54.2 ± 3.0% for the filtration of wine lees in a tubular ceramic membrane. Retention of polyphenols occurs partially due to interaction with other compounds, as they may stay trapped in proteins and polysaccharides, impacting their separation [1,22]. After filtration, there is a slight increase in luminosity and a reduction in the contribution of yellow shade, but both feed and permeate kept the yellowish to brownish appearance.

In comparison with some results reported in the literature, Cimini and Moresi [23] applied an alumina hollow fiber system to clarify pale lager beer, achieving high permeation fluxes for high transmembrane pressure values (e.g., 173 ± 7 L h⁻¹ m⁻² at 3.56 bar). Mora et al. [24] conducted the clarification of grape marc using a commercial mono-tubular ceramic membrane and reported low rejection of sugars and almost complete removal of suspended solids. For this system, the final permeate flux was around 15 L h⁻¹ m⁻² at 2.25 bar. Tomczak and Gryta [25] applied a membrane composed of titanium deposited over alumina hollow fiber to clarify a 1,3-propanediol fermentation broth, obtaining a final flux of 4.25 L h⁻¹ m⁻² at 1.0 bar and attesting that high permeate fluxes were achieved for transmembrane pressure values over 4.0 bar.

4. Conclusion

The asymmetric spinel hollow fiber membranes developed in this study demonstrated significant potential for industrial applications in bioactive plant extract processing. By combining alumina and dolomite precursors and sintering at 1350°C, this work achieved membranes with remarkable mechanical strength and hydrophilic properties. The calcination process enabled the formation of the spinel phase that provided a unique pore distribution in the hollow fiber membrane. The membranes achieved effective turbidity reduction (60%) and retained a substantial portion of phenolic compounds (44%), while maintaining the antioxidant capacity of the permeate. The membrane retention was associated with the membrane morphology and pore size distribution. Fouling analysis revealed that cake formation was the primary mechanism, suggesting that optimizing crossflow velocity could further improve the filtration performance. The dense outer membrane surface probably contributed to the cake layer formation. Compared to traditional alumina membranes, the spinel-based membranes offer a cost-effective alternative due to their lower sintering temperature and superior mechanical properties. This work underscores the potential of spinel hollow fiber membranes for industrial-scale applications in the processing of bioactive plant extracts, combining efficiency, durability, and economic viability. Future research could explore modifications to enhance fouling resistance and selectivity for specific phytochemicals.