Impact of Nanomaterials on Wine Quality: A Focus of Siliceous, Aluminosiliceous, and Carbon-Based Nanomaterials on the Phenolic Fraction of Wine

Abstract

Nanomaterials represent reduced scale versions of conventional materials, their nanometric structures possessing totally different and unexpected properties in contrast to the same substance at the macroscopic level. Nanomaterials present crystalline structure and manifest high resistance at high temperatures and acidic pH. Due to these properties, nanomaterials have widespread applications in different areas, such as pharmaceutics and wine industry (the degradation or removal of pollutants, the immobilization or vectorization of yeast, the increasing content of bioactive compounds). Nowadays, consumer demanding is centralized on wines with interesting sensory profile and increased nutritional value. Phenolic compounds play pivotal roles in shaping the sensory attributes of wines. The integration of nanomaterials can contribute to augmenting the extraction of phenolic compounds, depending on the specific type of nanomaterial employed, its concentration, and the particular winemaking technology applied. This chapter is focused on the impact of siliceous and aluminosiliceous porous and carbon-based materials on the phenolic profile of wines. In accordance with the existing studies, phenolic profile of wines is selectively modified by nanomaterials, but a more-deep comprehension of the nuanced interaction between nanomaterials and phenolic compounds is anticipated, offering insights that may underpin innovative strategies aimed at enhancing the overall quality of wines.

1. Introduction

Nanoscience focuses on studying innovative materials and technologies applied at the nanoscale, typically with particle sizes ranging from 1 to 100 nm. Nanoparticles can be used in viticulture as fertilizers, pesticides, and fungicides in the management of diseases (anthracnose, downy mildew, and powdery mildew) and vine pests (flea beetles, bugs, thrips, red mites, fungi, hyacinths, and stem borers). Conventional fertilizers can be replaced by nano-fertilizers (metallic nanoparticles; metal oxide nanoparticles; nanocomposites from nitrogen, phosphorus, potassium; micronutrients; hydroxyapatite; double layered zinc-aluminum hydroxide; and zeolites,) which show high efficiency, having the ability to reduce the frequency of application to increase the availability of nutrients and stress tolerance toward crops [1]. Moreover, nanomaterials could contribute to the optimization of wine technologies to enhance stability and improve both structure and composition [2]. Nanoporous materials, characterized by their substantial adsorption capacity, are driven by high specific surface area, pore volume, nanopore size distribution, and precisely ordered structure and also exhibit notable selectivity. This selectivity arises from a narrow pore size distribution and specific interactions with pore walls. Additionally, their favorable adsorption kinetics, stemming from a regular pore structure, dimensional characteristics, and pore sizes, ensure stability, durability, and the reversibility of adsorption and desorption processes. Notably, these materials demonstrate commendable mechanical properties in resisting abrasion and compression [3].

Nanomaterials have been used in wine quality analysis equipment. This allowed for improving the performance of the instruments by simplifying the working methodology, reducing working time, using a small sample volume, and reducing or even eliminating the pre-processing stage of samples. Some nanoparticles have been studied as part of detectors to analyze the total polyphenol content of must and wine (gold, silver, and zinc dioxide nanoparticles or multi-walled carbon nanotubes). Nanoparticles with magnetic properties have been used to create high sensitivity detection methods to analyze the content of ocratoxin A and histamine in beverages. In consequence, analysis methodologies using gold nanoparticle biosensors have been developed to allow the detection of low concentrations of ochratoxin A (up to 0.068 ng/mL). Also, electrodes functionalized with a nanostructured sensing surface that includes various nanoparticles can not only be used to detect polyphenols, sulfur dioxide, and glycerol but also some unwanted yeasts, such as Brettanomyces bruxellensis [4, 5].

In the wine industry, the treatment of different nanomaterials has been studied to reduce the concentration of various pollutants and the oxidative processes, thus inducing better stability and safety [6, 7, 8, 9, 10]. Phenolic compounds are natural components found in grapes, and while they contribute to the flavor and color of wine, excessive amounts can negatively impact its quality. Therefore, the removal of excessive phenolic compounds is a desirable process in winemaking. The focus of this chapter is that of reducing the content of phenolic compounds responsible for oxidative processes in wine, thus ensuring better stability and improved sensory characteristics. These treatments were also carried out in order to follow their selectivity in the retention of different phenolic compounds from the wine.

2. Key-phenolic compounds in wine

Wine represents a complex system characterized by a diverse array of phenolic compounds, playing a pivotal role in determining the wine’s quality, its sensory attributes, and imparting valuable antioxidant properties [11]. Among the phenolic compounds derived from grapes, anthocyanins, flavan-3-ols, proanthocyanidins, flavonols, phenolic acids, and stilbenes can be mentioned. These molecules come from different parts of the grape clusters, being extracted during winemaking. In grapes, phenolic acids are primarily present as glycosidic compounds, which can undergo decomposition through acid hydrolysis or as esters (tannins, gallic, and ellagic acids), which are released through alkaline hydrolysis. Free forms, particularly prevalent in red wine, result from the hydrolysis of phenolic acid combinations and the breakdown of more complex molecules (anthocyanins). Phenolic acids, although colorless in alcoholic solutions, may turn yellow after oxidation reactions (gallic acid, protocatechuic acid, caffeic acid, ferulic acid, and so on.) [12]. The main phenolic compounds existent in wines are presented in Figure 1.

Figure 1.

Key-phenolic compounds from wine.

Despite lacking distinctive taste or odor, these compounds serve as precursors for certain volatile phenolic compounds that may emerge due to interactions with specific microorganisms in wine. When the wines are aged in new oak barrels, the wood used in the manufacture of the barrels leads to the formation of new compounds (ethyl phenol, vinyl phenol, guaiacol, methyl guaiacol, vinyl guaiacol, propyl guaiacol, etc.) with a smoky and burned smell. The phenolic profile of grapes and wine is strongly affected by different factors such as varietal variability, cultivation technology, climate and soil conditions, pathogen attacks, winemaking practices, etc. [12]. For example, the application of mechanized harvesting practices can induce a substantial increase in the content of polyphenols in wine. The concentration of phenolic compounds transferred from the raw material can result in concentrations of up to approximately 4 g/L gallic acid in red wines and 350-500 mg/L in white wine samples [11]. Table 1 presents the medium levels of phenolic compounds in wines.

Compound name	White wines (mg/L)	Red wines (mg/L)
Benzoic acids	1–5	50–100
Cinnamic acids	50–200	50–200
Flavonols	Traces	15
Anthocyanin	0	20–50
Monomeric flavanols	Traces	150–200
Procyanidins	<100	1500–5000

Table 1.

Phenolic compound levels in white and red wines [13].

Hydroxycinnamic acids, including caftaric, coutaric, and fertaric, often show widely varying concentrations, their level usually being below the detection threshold. When combined with other wine components, these non-flavodoids significantly impact the organoleptic perception, particularly in relation to the alcoholic strength. For example, caftaric acid has been noted to enhance bitterness perception, while p-coumaric acid may imprint nuances of myrrh and cinnamon, etc. Moreover, these compounds have bactericidal, choleretic, and diuretic properties in the human body [12].

Beyond phenolic acids, musts and wines contain other crucial compounds for assessing their organoleptic quality, known as stilbenes. The predominant stilbenes in wines are resveratrol, tyrosol, methoxyrosol, and tryptofol. For example, the production of tyrosol by yeast metabolism can contribute to the imprinting of the bitter taste, particularly notable in sparkling wines. The level of this compound considerably rises during the second fermentation in the bottle. Even in white wines, tyrosol can impart bitterness at levels of 25 mg/L [11].

Volatile phenols, derived from phenol, play a major role in defining the aromatic profile of wine [14]. About 20 such compounds have been identified in wine, some of them being presented in Table 2.

Compound name	Detection limit (μg/L)	Origin	Aroma descriptors	References
Phenol	30	Lignin degradation	Chemical	[15]
Guaiacol	23		Smoke, sweet	[11]
4-Methyl guaiacol	21		Smoke, ash	[16]
Syringol	57		Smoke, drugs	[11]
Eugenol	6		Cloves, spicy note	[16]
Vanillin	200		Vanilla	[11]
M-cresol	20		Leather	[16]
4-Ehylphenol	440	Brattanomyces spp.	Leather, manure	[11]
4-Vinylphenol	180		Medicinal, phenolic, tobacco	[11]
4-Ethylguaiacol	33		Spicy, cloves	[11]
4-Vinyl guaiacol	40	Saccharomyces spp.	Smoke, phenolic	[16]

Table 2.

Key volatile phenols in wine.

Tanning substances, primarily synthesized by plants, can be categorized into hydrolyzable and condensed types. Hydrolyzable tannins, based on nonflavonoid phenols, are found as esters and are susceptible to degradation or hydrolysis. Condensed tannins (procyanidins) cannot be easily broken down by hydrolysis. Wine tannins consist of polymers of leucoanthocyanidins and catechins. White wines contain small amounts of catechins (3-flavanol) and leucoanthocyanins (3,4-flavandiol), contributing to the wine’s structure and body [11]. Tannins form blue-colored complexes upon reaction with Fe³⁺ and interact with proteins, imprinting astringency. They can originate from all the solid grape parts. On average, 58.5% of condensed tannins are found in seeds, 21% in stems, 16.5% in leaves, and the remaining 4% in berry skins across different grape varieties. During aging, tannin concentrations in wines decrease significantly as a result of oxidation and protein precipitation [17]. These substances are of particular importance in defining the quality of wines, their stability, and evolution (they have important antioxidant action). From an organoleptic point of view, tannins give hardness, astringency, as well as bitter and sour notes, proportional to their concentration [14]. Bitterness perception is mediated by taste buds on the tongue, and various phenolic compounds activate distinct combinations of bitter taste receptors. Astringency, on the other hand, involves dryness and puckering, and it is linked to the interaction of tannins with salivary proteins, influenced by factors such as tannin structure, wine matrix composition, and saliva characteristics. Seed proanthocyanidins were perceived as more astringent than skin proanthocyanidins, and specific subunits of tannins impact bitterness sensation. The amount of saliva also affects the perception of astringency with a linear correlation found between protein concentration and tannin-binding affinity. Salivary proteins, including the PRP family, α-amylase, statherin, histatins, and mucins, exhibit diverse abilities to interact with tannins, with some proteins specifically binding to astringents [12].

The coloring substances in wine are mainly represented by anthocyanins (in red wines) and flavones (in white wines). Anthocyanins exist as glycosides and are often 0linked or acylated with acetic, caffeic, or p-coumaric acids. Their color and intensity are not only given by the number of hydroxyl groups that bind to the benzene nucleus but also by the pH of the medium. They are primarily located in the skin and occasionally in the pulp. They also exist in significant quantities in the leaves toward the end of the vegetation period. Structurally, anthocyanins feature a flavylium cation structure, consisting of two benzene rings connected by a positively charged unsaturated oxygen heterocycle derived from a 2-phenyl-benzopyrylium nucleus. These molecules are more stable when found in the glycosidic form (anthocyanins) compared to the aglycone form (anthocyanidins). The color of these pigments is determined by environmental conditions (pH, SO₂) and molecular structure. On the one hand, the substitution at the lateral benzene nucleus leads to a bathochromic change of the wavelength with maximum absorption (to violet). Moreover, glucose fixation and acylation change the color to orange. Predominantly located in the cells of seed skin, these pigment molecules form a concentration gradient from the interior to the exterior. In the presence of other polyphenols (phenolic acids, flavonoids, etc.), these molecules exist in solution, influencing their color. These factors contribute to the diverse range of colors observed in red grape varieties. While all grape varieties share a fundamental structure for anthocyanidin, with minimal compositional variations, the malvidin molecule is dominant across all grape varieties. Malvidin-monoglucoside plays a crucial role in determining the color of red wines, with its concentration varying based on the specific grape variety [18].

In white wines, flavones are the main responsible for their yellow-brown color. From a chemical point of view, the structure of these compounds is similar to that of anthocyanins. The main flavonic compounds that can be separated in wine are represented by quercitin, kaempferol, and myricetin [19].

2.1 Phenolic oxidation in wines

Changes in the phenolic profile can occur due to the participation of these compounds in various chemical reactions (copigmentation, cycloaddition, polymerization, and oxidation). Copigmentation (additional colorless flavonoids known as cofactors or “copigments” enhance the pigmentation caused by anthocyanidins) generally gives a purple tint to wine. These reactions begin immediately after the grapes are crushed and continue during the fermentation and aging period, helping to define the sensory properties of the wines, in particular color and astringency. Iron plays a role in this process by reducing oxygen to form the hydroperoxyl radical (often known as hydrogen superoxide, which is a protonated type of superoxide with the chemical formula HOO• with a crucial role in cell biology). Phenolics, particularly those with specific hydroxyl group configurations, exhibit high reactivity toward these radicals, forming stabilized semiquinone radicals (Figure 2). Examples of phenolics prone to oxidation include caffeic acid, catechin, and quercetin. Monophenols and non-catechol phenolics are less susceptible to oxidation. The primary wine anthocyanin, malvidin-3-glucoside, shows a good resistance to oxidation processes. The reactivity of oligomeric and polymeric phenolics, such as procyanidins and condensed tannins, is similar to monomeric vicinal dihydroxy phenolics in the presence of reactive oxygen species. Overall, understanding the oxidation dynamics of these phenolic compounds is crucial for preserving the quality of wines [20].

Figure 2.

Oxidation of phenolic compound [20].

Understanding the relationship between the quality of a given wine and its phenolic composition remains one of the major challenges in oenological research [21].

3. Wine conditioning and stabilization using nanostructured materials

Preserving the distinctive qualities, taste, color, and aroma is a crucial aspect for all types of wines, including red, rosé, and white varieties. The inherent oxidative processes in wine can lead to taste deterioration, diminished vibrancy, and the development of oxidation tones, ultimately compromising the wine’s quality. To address this challenge, various technological methods are employed throughout the winemaking process. This includes careful selection of temperature regimes and the utilization of specific oenological products designed for clarification and stabilization. The wine clarification process is dedicated to eliminating suspended and colloidal particles that contribute to turbidity. Simultaneously, it aims to remove unstable proteins and other macromolecules that might undergo denaturation or aggregation, resulting in a cloudy appearance post-bottling. Stabilization is achieved through adsorption processes, employing substances such as bentonite, chitosan, fish glue, and albumin, and polymeric compounds such as PVP (Polyvinylpyrrolidone), PAA (Polyacrylic acid), and PEO (Polyethylene oxide). These materials effectively target polyphenols, proteins, and crystalline compounds, ensuring stability. Additionally, membrane filtration is employed to contribute to stabilization [22].

3.1 Bentonite treatment

Bentonites are hydrated aluminosilicates, mainly composed of montmorillonite that exhibit distinctive physicochemical properties influenced by their geographical origin. While German or North African bentonite is rich in Ca²⁺ ions, the variety from the United States contains Na⁺ ions and is widely recognized as highly effective in wine treatment. The colloidal properties of bentonite stem from its layered structure, enabling significant swelling in aqueous environments, boasting a large adsorption surface, and carrying a substantial negative charge. The crystal-chemical structure of clay minerals, specifically montmorillonite, involves layered arrangements of tetrahedral and octahedral layers, forming 1:1 or 2:1 type packages. Montmorillonite, the primary component of bentonite, is not abundantly found in its pure state in nature. However, its cation exchange capacity can range from 70 to 100 meq/100 g of clay, with specific surface areas up to 120 m²/g and effective pore sizes characteristic of montmorillonite. Recommended for wine treatment, bentonites undergo activation with sulfuric acid or alkaline salts. Their high ionic exchange capacity allows loading with H⁺, Na⁺, or Ca²⁺ ions, resulting in acidic, calcium, or sodium forms of bentonite. The sodium form is particularly favored, exhibiting extensive swelling in wine, high adsorption capacity for proteins, and maintaining a stable colloidal character. This treatment effectively removes natural proteins, resulting in a clear liquid and protecting the wine from copper case. Bentonite treatment stands out as the prevalent method for removing excess proteins, especially in red wines with elevated concentrations of colored colloids generated through heating or harsh mechanical treatments applied to grapes. While effective in stabilizing wine, this treatment comes with the trade-off of a noticeable loss of color [23].

3.2 Ion-exchange materials

The treatment with ion-exchange materials involves the use of insoluble polymeric resins (formed by styrene and divinyl benzene) activated with various functional groups in ion-exchange reactions (sulfonic group or carboxyl group for cation exchange and the quaternary ammonium ion or the salt of a tertiary amine for the anion exchange) [24]. The resin presents a three-dimensional (0.3-1.2 mm diameter) and porous matrix that supports the ion-exchanging groups and consists of a synthetic resin that is obtained by condensation and polymerization. The appearance of the resins can be observed in Figure 3. Ion-exchange resins are usually activated by treatment with an acid of mineral origin (sulfuric acid), incorporating H⁺ ions. The activated resin exchanges its H⁺ ions with cations (e.g., K⁺, Ca²⁺, and among others) present in grape must and wines. The exchange is stoichiometric and generates a reduction in the pH value and an increase in acidity, organic acids being released. Cationic resins follow the laws of affinity. Thus, higher valence cations are preferred to be exchanged over lower valence ones (Al³⁺ > Ca²⁺ > K⁺). Also, the divalent cations (Mg²⁺ and Ca²⁺) of the must and wines are fixed in the cationic resin in preference to the monovalent ions (like Na⁺ and K⁺). However, if two cations have the same valency, the preference is for the higher atomic number [26].

Figure 3.

Examples of SEM images with the resin before (a) and after (b) Cd²⁺ adsorption [25].

3.3 Carbon-based materials

Carbon nanotubes are cylindrical nanostructures with exceptional thermal and electrical conductivity and high mechanical strength and rigidity. One advantage of using carbon nanotubes is that they can be regenerated and reused, making process economically viable. Consistent with Mamvura et al. [27], carbon-based nanomaterials can be an effective alternative as artificial flocculants to improve the flocculation capacity of brewer’s yeast. Used as filter membranes, carbon nanotubes remove multiple components of heavy hydrocarbons from oil, while in water, they remove bacteria, such as Escherichia coli and 25 nm nano-sized polyviruses [6, 28]. Considered the thinnest material in existence [29], graphene is the two-dimensional version of graphite, consisting of a two-dimensional arrangement of carbon atoms arranged in a hexagonal lattice. Also, this material is the best-known conductor of electricity and heat. Some typical appearances of carbon-based nanomaterials can be analyzed in Figure 4. Graphene and reduced graphene oxide, which is graphene oxide that has been reductively processed by various methods to reduce its oxygen content, has been used to fabricate biosystems, consisting of nucleic acids, peptides, proteins, and enzymes [30], active adsorbent materials that remove heavy metal ions, pesticides, and natural dyes from water [31, 32]. Colibaba et al. [33] obtained an important increase of proline and threonine in wines treated with graphene oxide, while phenylalanine, arginine, and tyrosine level were decreased. Also, carbon nanotubes generated the diminishing of shikimic and fumaric acids.

Figure 4.

Typical SEM images of graphene nanopowder (a), graphene oxide (b), and carbon nanotube powder (c) [29].

3.4 Microporous zeolitic materials

Zeolites, whether of natural or synthetic provenance, are crystalline aluminosilicates hydrated with alkaline and alkaline metals such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Sr²⁺, and Ba²⁺, with pore diameters less than 2 nm (Figure 5). These materials are considered safe for human consumption, being used not only in water and soil decontamination, in agriculture (in soil corrections and carriers for fertilizer) but also in food and beverage processing. In the latter context, zeolites find application in tasks such as tartrate and protein stabilization, prevention of light-struck taste, removal of off-flavors and metals, waste management, and cell immobilization. Moreover, zeolites are used as catalysts, detergents, adsorbents, molecular sieves, and ion exchangers in diverse chemical processes. The foundational structure of zeolites is three dimensional, arising from the interconnection of tetrahedral TO₄ units (SiO₄ and AlO₄) through oxygen bridges. Within standard conditions, the cavities and/or channels are occupied by metallic cations (Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺) or organic cations required to compensate for the excess negative charge of the tetrahedra (AlO₄), a consequence of the presence of Al(III) and water molecules. The mobility determined by both cations and water molecules facilitates reversible ionic exchange and dehydration processes. Among the most used zeolites in the oenological processes are Charazite, Clinoptilolite, Edingtonite, Faujasite, Mordenite, Phillipsite, and Linde Type A [35].

Figure 5.

Examples of SEM images for zeolite [34].

3.5 Mesoporous materials

Mesoporous materials are obtained by combining specific proportions of inorganic compounds (sodium silicate solution, aerosol) or organometallic compounds (such as alkylated silica) with the organic molecules of surfactant substances acting as structure-directing agents (templates). These materials usually present cylindrical pores with 2-50 nm diameter and large surface area (700-1500 m²/g). Silicon-based mesoporous materials can be formed by pure (MCM, SBA, HMS) or modified silicates (which include transition metal oxides and nonmetallic oxides). Mesoporous siliceous oxide has multiple industrial applications, including adsorption, ion exchange, catalysis, photocatalysis, chemical and electrochemical sensors, permselective membranes, electronic relays, zeolitic batteries, rapid ion conductors, semiconductors, thin films, materials for data and image storage, molecular-sized electronic, optical, and magnetic devices, and controlled drug release systems, among others [36].

Mesoporous silica nanomaterials (MSN) are often used in the pharmaceutical field due to their association with numerous benefits to human health. The group of M41S-type mesoporous materials comprises three other types, namely MCM-41 (hexagonal), MCM-48 (cubic), and MCM-50 (lamellar). Of these, the M41S variant has a pore size between 2 and 20 nm and a specific surface area of up to 1000 m²/g. In addition, this type of material has distinct adsorption properties associated with its pore volume (around 0.9 cm³/g) [10]. Various mesoporous silica materials, including MCM (Mobil Composition of Matter), SBA (Santa Barbara Amorphous), FDU (Fudan University), and KIT (Korean Institute of Science and Technology), have been successfully synthesized using different templating methods. KIT-6 silica, in particular, exhibits a bicontinuous cubic structure with Ia3d symmetry and features interpenetrating cylindrical pores, making it well-suited for serving as a hard template and catalyst support. The synthesis process allows for the precise control of mesoporous silica’s pore size by adjusting factors such as reaction temperature, surfactant type or concentration, and swelling agent concentration. This control over pore size is crucial in adapting KIT-6 for the production of specific target materials with enhanced properties [37].

SBA-15 demonstrates compelling textural characteristics, notably large specific surface areas exceeding 1000 m²/g, uniformly sized pores ranging from 4 to 30 nm, substantial framework walls, small crystallite size of primary particles, and complementary textural porosity. The utilization of SBA-15 as a support offers additional benefits, including a high surface-to-volume ratio, versatile framework configurations, and elevated thermal stability [38].

These materials are efficient as potential supports in catalysis or for the adsorption of molecules with large molecular sizes and can be synthesized by various methods such as sol-gel processing, template-assisted techniques, microwave-assisted techniques, and chemical etching techniques [36]. Figure 6 illustrates some SEM images of different types of mesoporous materials.

Figure 6.

SEM images of calcinated samples of Al-MCM-41 (a), SBA-15 (b), MCM-41 (c), and KIT-6 (d) (original).

Some of these materials were used by Dumitriu et al. [8] in the production of some wine samples. The authors presented that MCM-41 and SBA-15 can reduce significantly turbidity units of wine, while KIT-6 had low effects on the final protein level.

The focus on developing and investigating novel polymeric materials and adsorbents, primarily derived from bio-renewable plant sources, provides insights into their potential targeted application for producing high-quality wines.

4. Synthesis of siliceous, aluminosiliceous, and carbon-based nanostructured materials for beverage manufacturing usage

Carbon nanotubes exemplify innovative nanostructures derived from chemical synthesis approaches. Despite their simple chemical composition and atomic bonding configuration, nanotubes showcase remarkable diversity and richness among nanomaterials in terms of structures and structure-property relationships. Metallic and semiconducting nanotubes have been identified across materials synthesized through arc discharge, laser ablation, and chemical vapor deposition methods [39]. Elemental carbon in sp² hybridization can create various remarkable structures. In addition to the well-known graphite, carbon can construct closed and open cages with a honeycomb atomic arrangement. Nanotubes consist of up to several tens of graphitic shells, known as multi-walled carbon nanotubes, with adjacent shell separation of 0.34 nm, diameters of 1 nm, and a high length-to-diameter ratio. Generally, carbon nanotubes have diameters ranging from less than 1 nm up to 50 nm as a group. While their lengths are typically several microns, recent advancements have extended the nanotubes to much longer lengths, measured in centimeters. The choice of carbon nanotubes for phenolic compound adsorption should be guided by experimental testing and optimization. Factors, such as the type of phenolic compounds, concentration, and desired properties of the final product, should be considered in the selection process. The literature mentions many strategies for manufacturing large amounts of carbon nanotubes: arc discharge, laser ablation, chemical vapor deposition, silane solution method, and flame synthesis method. The electric arc discharge technique uses high temperatures above 1700°C between water-cooled graphite electrodes in a helium-filled chamber at subatmospheric pressure. Different gases, such as helium, methane, and hydrogen, lead to variations in the final structure. The existing papers indicate that organic molecular atmospheres contribute to higher multi-walled carbon nanotube yields, and the pressure also plays a role. Maintaining the desired distance between electrodes during the growth process, typically between 1 and 4 mm, is crucial for achieving a high yield and stable arc discharge growth process. The electrode feed is essential to ensure a constant gap and facilitate efficient multi-walled carbon nanotube synthesis [40].

Activated carbon is a versatile and highly porous material with a wide range of applications, including water purification, air filtration, gas adsorption, and more. The synthesis of activated carbon involves the carbonization of carbon-rich raw materials followed by activation processes to enhance its porosity. Common raw materials include coconut shells, wood, peat, coal, sawdust, and agricultural residues. The selected raw material should have a high carbon content to ensure a more effective activation process. The raw material undergoes carbonization in the absence of air or in a controlled environment with limited oxygen. This process involves heating the material to high temperatures (typically 600-900°C) to remove volatile components and convert the material into a carbon-rich structure. Activation is a crucial step that introduces porosity to the carbon structure and involves impregnating the carbonized material with chemical agents, such as potassium hydroxide or phosphoric acid. The impregnated material is then heated to activate and create pores. To ensure the physical activation of the carbon, high temperatures, and gases are needed, such as steam or carbon dioxide. The temperature during the activation process influences the properties of the activated carbon. Higher temperatures generally result in increased porosity. The activated carbon is cooled and then thoroughly washed to remove any residual impurities or chemicals used during activation. The final activated carbon product is dried to remove excess water. The activated carbon may undergo grinding and sizing processes to achieve the desired particle size for specific applications. Activated carbon is characterized based on parameters such as surface area, pore size distribution, and adsorption capacity [41].

Jiříčková et al. [42] proposes a method to obtain graphene oxide. In this, a mixture of phosphoric acid and sulfuric acid in a ratio of 1:9 is mixed with potassium permanganate and graphite in a ratio of 6:1, all in an ice bath. The mixture is heated to 50°C and stirred for about 12 hours. After cooling, the solution is poured onto ice and 30% H₂O₂ is added to remove excess potassium permanganate. More modern approaches to oxidizing graphite to prepare graphene oxide include using potassium chromate with perchloric or nitric acid. Alternatively, potassium ferrate, considered to be less toxic in sulfuric acid, has been proposed. Oxidation of graphite can also occur in water with H₂O₂ at 50°C by Fe(VI) or at 110°C by benzoyl peroxide. It is worth noting that chemically prepared graphene oxide often exhibits a highly damaged structure due to harsh acidic conditions and the presence of impurities, making it less suitable for electronic applications. While chemical methods, particularly the chlorate and permanganate processes, produce graphene oxide with suboptimal electrical properties, ongoing research aims to improve these methods. Electrochemical production of graphene oxide is considered more environmentally friendly than chemical methods due to the reuse of electrolytes and minimal washing of utensils. The use of aqueous electrolytes and the absence of oxidizing agents contribute to the superior quality of electrochemical graphene oxide compared to standard procedures. Also, the use of biological systems, such as Acidithiobacillus ferrooxidans or Pseudomonas, has been explored to oxidize graphitic materials in an environmentally friendly manner. Studies have shown that, however, after microbial cultivation, graphite oxidation may not be homogeneous.

The procedure for synthesizing mesoporous silica material SBA-15 is proposed by Zhao et al. [43] and by Luchian et al. [10]. The hydrothermal synthesis of the material can be achieved using a reaction system with the following molar composition: 1SiO₂: 0.017 P123: 5.87 HCl: 194 H₂O.

The process of obtaining solid SBA-15 powder consists of dissolving 4 g of P123 in 150 mL of 2 M acidic HCl solution. Drops of tetraethylorthosilicate are added under continuous stirring (9.6 mL). The mixed solution is kept at 45°C for 8 h, and finally, the sol–gel suspension is heated to 80°C for 5 h in a conventional oven. Next, the white solid is filtered, which is washed several times with deionized water and dried at room temperature and finally by calcination at 550°C for 6 h (heating rate of 10 C/min).

The synthesis of mesoporous KIT-6 silica can be achieved following the proposal of Xiaoying et al. [44]. For this, 5 g of Pluronic® P123 (amphiphilic nonionic triblock copolymer) is dissolved in 180 g of distilled water and 9.9 g of HCl solution (35%) under vigorous stirring at 35°C. After complete dissolution, add 5 g of n-butanol (99.4%). After additional stirring for 1 h, immediately add 10.75 g of tetraethylorthosilicate. The mixture was stirred at 35°C for 24 h and then transferred to an autoclave, which, in turn, was sealed and kept at 100°C for 24 hours. The resulting solid product was filtered and dried at 100°C overnight. After a brief ethanol/HCl wash, dry the final sample at 70°C and calcine at 550°C for 6 hours in air.

For the synthesis of the Al-MCM-41 mesoporous material, the method presented by Stein & Holland [45] is proposed. Thus, 22.3 mL of tetraethylorthosilicate was mixed with 0.68 g of aluminum isopropoxide. The resulting solution is stirred for 30 minutes at 250 rpm, after which tetraethylammonium hydroxide solution (10% water) is added with continuous stirring for another 30 min, at a speed of 250 rpm until the gel is formed (pH = 11). A total of 7.2 g (0.2 mol) of cetyltrimethylammonium bromide (30 mL/h) was added dropwise. The gel becomes a suspension. After additional stirring for 1 h, transfer to a Teflon steel autoclave and heat at 150°C for 48 h. After cooling, the sample is recovered by filtration. The obtained solid is washed with distilled water and ethanol, then air-dried at 70°C for 1 hour, and finally calcined at 540°C for 6 hours.

5. Impact of siliceous, aluminosiliceous, and carbon-based nanomaterials on wine phenolic profile

As research in nanomaterial applications continues to evolve, exploring the synergy between carbon nanotubes and winemaking processes opens new opportunities for innovation and quality improvement in the wine industry. The adsorption process involves the attachment of phenolic compounds to the surface of carbon nanotubes through Van der Waals forces, π-π interactions, and hydrogen bonding. Several papers published by our team presented the impact of carbon-based materials on wine phenolic profile (Figure 7). Codreanu et al. [6] investigated the impact of carbon-based materials on the phenolic profile of wines from Romania. Different nanomaterials (carbon nanotubes, graphene, and oxide graphene) have been introduced in both the pre- and post-alcoholic fermentation stages of Cabernet Sauvignon wines (Figures 8 and 9). The authors obtained an important decrease in phenolic content when carbon-based materials were used. Also, the results were influenced by the moment of application. So, when the materials are applied before alcoholic fermentation, the total content of phenolic compounds suffered a slight decrease after the use of mentioned materials. Thus, the activated carbon determined the greatest decrease in the concentration of the main phenolic compounds analyzed (gallic acid, protocatechuic acid, gentisic acid, vanillic acid, caffeic acid, m-hydroxybenzoic acid), followed by samples treated with carbon nanotubes. When the materials were added after alcoholic fermentation, the content of protocatechuic acid exhibited variations based on the applied treatments: graphene and graphene oxide resulted in an increase in concentration, while treatments with nanotubes and activated carbon led to a decrease in the level of protocatechuic acid.

Figure 7.

Total phenolic compounds in wines (Folin-Ciocâlteu method) treated with nanomaterials in pre-fermentation and post-fermentation (b) stages (mg/L gallic acid). M—Untreated sample, G—Graphene, GO—Graphene oxide, CNT—Carbon nanotubes, and AC—Activated carbon.

Figure 8.

The impact of nanomaterials on the concentration of phenolic compounds when applied in the pre-fermentation phase (mg/L) gallic acid (a), protocatechuic acid (b), gentisic acid (c), catechin (d), vanilic acid (e), syringic acid (f), caffeic acid (g);,m-hydroxybenzoic acid (h). M—Untreated sample, G—Graphene, GO—Graphene oxide, CNT—Carbon nanotubes, and AC—Activated carbon.

Figure 9.

The impact of nanomaterials on the concentration of phenolic compounds when applied in the post-fermentation phase (mg/L) gallic acid (a), protocatechuic acid (b), gentisic acid (c), catechin (d), vanillic acid (e), syringic acid (f), caffeic acid (g), m-hydroxybenzoic acid (h). M—Untreated sample, G—Graphene, GO—Graphene oxide, CNT—Carbon nanotubes, and AC—Activated carbon.

The concentration of p-hydroxybenzoic acid increased following the treatment of wines with carbon-based materials, with the highest amount observed in the sample treated with graphene oxide. Among these materials, it was noted that graphene oxide had an impact on the wine composition by elevating the concentrations of gallic acid, protocatechuic acid, p-hydroxybenzoic acid, gentisic acid, vanillic acid, m-hydroxybenzoic acid, p-coumaric acid, trans-resveratrol, and rutin. Wines treated with carbon nanotubes recorded lower values for gallic acid and m-hydroxybenzoic acid. In contrast, activated carbon proved to be the most effective in reducing the levels of protocatechuic acid, gentisic acid, catechin, syringic acid, caffeic acid, m-hydroxybenzoic acid, p-coumaric acid, trans-resveratrol, and rutin in wine.

The addition of carbon-based materials in the post-alcoholic fermentation stage significantly influenced the phenolic content of wines.

According to Filipe-Ribeiro et al. [46], activated carbon treatment can be efficient for reducing undesirable odors arising from volatile phenols, such as 4-ethylphenol and 4-ethylguaiacol in wines contaminated with Dekkera and Brettanomyces. The effectiveness of this treatment relies on the surface area and micropore volume of the activated carbon. It has been observed that higher mesopore surface area and total pore volume negatively impact anthocyanins and color intensity, while a greater surface area and micropore volume are crucial for the removal of phenolic acids. The successful reduction of volatile phenolics plays a pivotal role in enhancing the positive perception of the fruity attribute in wines. Through a careful selection of the physicochemical characteristics of activated carbon, it becomes feasible to efficiently eliminate volatile phenols without compromising the sensory quality of the wine. This optimal selection ensures that the activated carbon treatment positively contributes to improving the overall quality of the wine without adversely affecting its sensory attributes.

Cotea et al. [47] chose to evaluate the influence of mesoporous silica material SBA-15 on polyphenols content in Cabernet Sauvignon samples. The authors presented SBA-15 as an efficient alternative for the extraction of phenolic compounds such as quercitin and cis- and trans-resveratrol from red wine. On this line, similar results were presented for KIT-6 materials in Fetească neagră wines [7]. Dumitriu et al. [2] confirm the already mentioned findings and demonstrate that the use of SBA-15 in winemaking leads to significant decrease in the total polyphenol index. In another study, Luchian et al. [10] obtained a significant rise in concentration of caffeic acids in Cabernet Sauvignon samples treated with KIT-6 (double value compared to the control sample) after 5 months of maturation. Also, the results present a notable increase of rutin content in Merlot samples, when SBA-15 and MCM-41 materials were used. Dumitriu et al. [48] presented a higher retention of phenolic compounds on MCM-41, compared to KIT-6 and SBA-15 in Muscat Ottonel wines. Even if studies on the effect of nanostructured material on phenolic compounds in wine are limited, there are several papers that refer to water and present some essential conclusions. Thus, Pattanaik et al. [49] postulated that higher MCM-41 dosage can lead to better adsorption of phenols while increasing pH values usually generates contrary results. For this study, pH = 5 was the optimum value. In accordance with these results, Kalash et al. [50] confirmed that MCM-41 can be a great alternative in removing 67% of different phenols from contaminated water in different experimental conditions (pH = 4-9, mixing rate = 200 rpm, at room temperature). Also, the authors that the adsorption mechanism fits better with Langmuir isotherm.

The successful application of mesoporous material SBA-15 for the adsorption of phenolic compounds from wine was also proposed by Niculescu et al. [51]. In this study, catechin has the highest retention, followed by rutin, trans-cinnamic acid, trans-resveratrol, and gallic acid. The authors attributed this adsorption to the dipole moment of these molecules. Moreover, the presence of the electrons in phenyl rings and their availability may increase the interaction with Si-OH groups for the adsorption. This material is proposed as a viable alternative for efficiently extracting trans-resveratrol and other significant compounds. According to Anbia & Amirmahmoodi [52], the adsorption isotherm for SBA-15 fits well with the Freundlich equilibrium model.

6. Conclusions

In this chapter, an attempt has been made to cover information regarding phenolic adsorption and its derivatives using various nanocomposites. Minimizing the polyphenolic content in red wines prevents the oxidative browning process while preserving the wine’s color. The polyphenolic extract, with its antioxidant properties, could be of interest as pharmaconutrients or as potential food sources, given its positive impact on human health. The utilization of siliceous, aluminosiliceous, and carbon-based materials in wine treatment supports the partial extraction of phenolic compounds and prevents browning and precipitation, thereby enhancing overall stability. Phenolic compounds play a crucial role in wine quality and sensory attributes, and the selective adsorption facilitated by carbon nanotubes could contribute to refining and enhancing the overall characteristics of wines.

Conflict of interest

The authors declare no conflict of interest.