Introduction

Halloysite is a naturally occurring clay mineral that consists of tiny tubular structures known as nanotubes. Halloysite is a clay mineral represented by a bilayer aluminosilicate with a chemical structure similar to kaolinite (Al2Si2O5(OH)4) (Cherednichenko et al., 2022). HNTs have a high aspect ratio and a unique structure that makes them attractive for use in various applications, including as additives in food packaging materials. HNTs are known for their excellent mechanical, thermal, and chemical properties, which make them suitable for improving the performance of packaging materials (Darie-Niţă & Vasile, 2017; Li et al., 2021). In terms of mechanical properties, HNTs can improve the strength and durability of packaging materials. This is because the tubular structure of HNTs provides a reinforcing effect, which can increase the stiffness and toughness of the packaging material (Lee et al., 2018). Additionally, HNTs can improve the thermal properties of packaging materials by providing better insulation and reducing heat and mass transfer.

HNTs can also provide significant benefits in terms of gas barrier properties. HNTs can act as a barrier to gases, such as oxygen and carbon dioxide, which can affect the quality and shelf life of food products. The tubular structure of HNTs allows them to form a compact network that can prevent gas molecules from passing through the packaging material. This can help to maintain the freshness and quality of food products, extending their shelf life (Abdullah et al., 2019a). Another benefit of using HNTs as functional additives in food packaging materials is their antimicrobial properties. HNTs have been shown to have antimicrobial properties against a wide range of microorganisms, including bacteria, fungi, and viruses. This can be particularly useful in preventing the growth of microorganisms that can cause spoilage and reduce the shelf life of food products (Alkan Tas et al., 2019).

Active food packaging refers to a type of packaging technology that actively interacts with the food inside the package to improve their safety and quality and also help to prolong the shelf life of perishable food products by controlling the rate of oxidation and microbial growth (Deshmukh et al., 2022; Kumar et al., 2021, 2022a, b, c). Furthermore, the use of HNTs in active food packaging can provide an alternative to traditional synthetic materials, such as plastics. HNTs are a natural and sustainable material, and their use can help to reduce the environmental impact of food packaging. HNTs can also be easily incorporated into biodegradable packaging materials, further reducing their environmental impact (Abdullah & Dong, 2019). Using HNTs in food packaging can also help address emerging food safety concerns. For example, the COVID-19 pandemic has highlighted the need for packaging materials that can provide enhanced antimicrobial properties to prevent the transmission of the virus (Gopal & Muthu, 2023). The use of HNTs as functional additives in food packaging materials could help to address this need. A functional additive is a substance that is added to a material to enhance its properties and/or provide additional functionality. In the context of food packaging, functional additives are used to extend the shelf life of food products, maintain freshness, and prevent spoilage (Perera et al., 2023). In addition to active food packaging, HNTs have other potential applications in the food industry. For example, HNTs can be used as a carrier for antimicrobial and antioxidant agents and food additives, such as vitamins, flavors, and colors. HNTs can also be used as a coating for food products to provide a protective layer that can help to prevent spoilage and extend shelf life (Zemljič et al., 2022). Figure 1 displays the visual representation of HNT production, modification, and application in active food packaging industries.

Fig. 1
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Visual representation of HNT production, modification, and application in active food packaging industries

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This review focuses on the use of HNTs as functional additives in active food packaging applications as it has significant potential to improve the performance and functionality of food packaging materials. HNTs can provide benefits such as improved mechanical and thermal properties, enhanced gas barrier properties, antimicrobial activity, and even controlled release of active encapsulated compounds. Different surface modifications, such as physical, chemical, biological, and electrostatic methods, were discussed briefly. Furthermore, the use of HNTs in food packaging can provide an alternative to traditional synthetic materials and help to address emerging food safety concerns discussed.

Treatments for HNT to Enhance Its Functionality

Halloysite nanotubes (HNTs) are naturally occurring clay minerals that have recently received significant attention in the field of science and technology due to their unique structural properties and potential applications. HNTs have a tubular structure with an inner diameter of 10–15 nm and an outer diameter of 40–50 nm, making them ideal for a wide range of applications such as drug delivery, catalysis, encapsulation, and nanocomposites (Massaro et al., 2020; Wong et al., 2021). However, the pristine HNTs have limited solubility, low surface area, and poor dispersibility in water and other solvents, which restricts their practical application. Therefore, surface modification of HNTs has become an important research area to enhance their properties and compatibility with various matrices (Moghari et al., 2021a). Various types of HNT modification methods, such as physical, chemical, biological, and electrostatic, are depicted in Fig. 2.

Fig. 2
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Different methods employed for the modification of HNT as a functional additive: a electrostatic method, b chemical method, c physical method, d biological method

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Physical Surface Modification

Physical surface modification involves altering the surface of HNTs using physical methods such as milling, sonication, high-temperature calcination, plasma, and thermal treatment. These methods can improve the dispersion of HNTs in water or other solvents, enhancing composite material’s mechanical properties. However, the physical modification does not change the chemical composition of the HNTs, and the modifications may not be stable over long periods of time (Tharmavaram et al., 2018). Milling and sonication can reduce the particle size of HNTs, leading to increased surface area and improved dispersion in solvents. The most used physical method for HNT modification is sonication. Sonication involves using high-frequency sound waves to agitate the HNTs in a liquid medium. This process leads to the breakage of weak bonds on the surface of HNTs, exposing new surface sites for subsequent chemical reactions. The duration and intensity of sonication can be controlled to achieve the desired surface modification (Moghari et al., 2021b). For instance, Shi et al. (2011) used sonication to modify HNTs for drug delivery applications. The modified HNTs exhibited enhanced drug loading capacity and sustained drug release properties (Shi et al., 2011). In another study, the authors demonstrated the layer-by-layer encapsulation of spherical and tubular cores of HNT with sonication-assisted non-washing techniques, which resulted in high colloidal stability and sustained release as drug nanocarriers at 2–3 mg/mL in serums (Shutava et al., 2014). Another physical method for surface modification of halloysite nanotubes is high-temperature calcination. This involves heating the HNTs to a high temperature (usually above 500 °C) in an inert atmosphere to remove any organic impurities and induce changes in the surface structure. Calcination can lead to the formation of new surface functional groups, such as hydroxyl groups, which can enhance the surface reactivity of HNTs (Ouyang et al., 2014; Voronin et al., 2020; Yuan et al., 2013).

Several studies have reported using high-temperature calcination for surface modification of HNTs. Du and Zheng (2014) prepared TiO2-HNT nanocomposite with calcination treatment at 100–500℃, and they observed that the highest adsorption of methylene blue in the shortest time (29.64 mg/g after 12 h), and it was lower than the adsorption of methylene blue over HNT due to high-temperature calcination treatment (Du & Zheng, 2014). In another study, researchers reported that the raw HNT has a surface area of 56.7 m2/g; after heat treatment, the tube wall transformed into an amorphous structure during dehydration at 450℃. The morphology of the HNT was maintained up to 1100 ℃ for 6 h, and the specific area enlarged up to 65.7 m2/g (Ouyang et al., 2014).

Chemical Surface Modification

The chemical surface modification involves introducing functional groups onto the surface of HNTs, which can significantly enhance their properties and expand their applications. Various chemical modification methods have been reported, such as silanization, amidation, esterification, and grafting (Tan et al., 2016). Among these methods, silanization is the most widely used method, which involves the reaction of HNTs with silane coupling agents, such as 3-aminopropyltrimethoxysilane (APTMS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). Silanization can improve the dispersibility, hydrophilicity, and surface charge of HNTs, which makes them suitable for various applications such as encapsulation, drug delivery, gas adsorption, and catalysis (Carli et al., 2014; Liu et al., 2008; Yuan et al., 2008). The lumen surface was modified with organophosphonic acid (ODP) by dissolving it in the aqueous–ethanol mixture, and HNT powder was added to the solution. The dispersion is then transferred to the vacuum jar and evacuated within three cycles for maximum loading of ODP into the HNT lumen (Yah et al., 2012). Figure 3 depicts transmission electron microscope images of raw HNT and alkali-treated HNT; alkaline chemical treatment of raw HNT enhanced pore size (Gaikwad et al., 2018). In another study, HNT was treated with a low concentration of sodium hydroxide. The resultant formation of the hydroxyl group on the HNT surface elevated the HNT dispersion in the water, organic solvents, and epoxy matrix (Zeng et al., 2014).

Fig. 3
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Transmission electron microscope (TEM) images of raw halloysite nanotubes (left) and alkali-treated halloysite nanotubes (right) (Gaikwad et al., 2018)

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Biological Surface Modification

Biological surface modification involves using biological molecules such as proteins, enzymes, and DNA to modify the surface of HNTs. This method can improve the biocompatibility and bioactivity of HNTs, making them suitable for various biomedical applications such as drug delivery, tissue engineering, and biosensors. However, the biological molecules used for modification are often expensive and unstable, limiting their widespread applications (Lee et al., 2013). Several studies have reported the use of biological surface modification for enhancing the biocompatibility and bioactivity of HNTs. For instance, Xu et al. (2018) reported the immobilization of horseradish peroxidase (HRP) on the surface of HNTs through physical adsorption. The modified HNTs exhibited improved stability and catalytic activity of HRP and were used as a biosensor for detecting hydrogen peroxide (Zhang & Cai, 2018). Lee et al. (2013) reported that DNA was applied for wrapping the surface of HNTs to improve the water dispersibility in the application of drug delivery for doxorubicin (DOX) inside the cells (Lee et al., 2013). Biological surface modification of halloysite nanotubes is a promising approach for improving their biocompatibility and bioactivity in various biomedical applications. The selection of biomolecules and modification methods should be based on the desired surface properties and application requirements.

Electrostatic Assembly

Electrostatic assembly involves using the electrostatic interaction between charged HNTs and oppositely charged molecules to modify the surface of HNTs. This method can be used to introduce various functional groups onto the surface of HNTs, such as polyelectrolytes, nanoparticles, and biomolecules. Electrostatic assembly can improve the dispersibility, stability, and functionality of HNTs, which makes them suitable for various applications such as drug delivery, gene therapy, and biosensors (Li et al., 2022). In another study, Wang and Rhim (2017) reported the electrostatic modification of HNTs with sodium carboxymethyl cellulose (CMC) to prepare HNT/CMC nanocomposites. The modified HNTs exhibited improved dispersion and compatibility with the CMC matrix, resulting in improved mechanical and thermal properties of the nanocomposites. Hybrid HNT/sodium alkenoates were prepared where the inner cavity of HNT was selectively modified via the electrostatic method. The HNT and sodium alkenoate interaction resulted in the exfoliated lumen, improved loaded substance, and increased total net charge, rendering stable nano-clay dispersion (Cavallaro et al., 2014). Anionic and cationic surfactants employed for exfoliating the different charges in the inner and outer surfaces showed that the adsorption of anionic surfactant into the HNTs lumen increases the net negative charge of the nanotubes enhancing the electrostatic repulsions and, consequently, the dispersion stability (Cavallaro et al., 2012). Overall, electrostatic surface modification of HNTs provides a simple and effective approach for preparing various functional materials with improved properties for different applications.

HNT as Functional Material in Packaging

HNT is a non-toxic, low-cost, and biocompatible naturally occurring nano-clay material with a unique tubular structure and can be incorporated into packaging material for multifunctional properties. Due to its high aspect ratio, and unique morphology, it has attracted much attention as a functional additive in food packaging applications. Incorporating HNT into packaging material can exhibit additional active properties. One of the main applications of HNT is as a reinforcing nanofiller in the polymer matrix. It can improve the mechanical properties and high-temperature endurance of packaging materials. HNT can also be utilized as a nanocarrier of active antioxidant and antimicrobial agents and ethylene gas absorbing in food packaging applications. The schematic diagram of HNT being utilized as a functional additive for active food packaging applications is shown in Fig. 4. Overall, HNT has excellent potential as a functional additive in food packaging applications (Gaikwad et al., 2019; Singhi et al., 2023).

Fig. 4
figure 4

Schematic representation of HNT, a functional additive used as a nanofiller; b encapsulating nano-particle, antioxidant, and antimicrobial agents; c ethylene gas absorber for active food packaging application

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Encapsulation of Antioxidant and Antimicrobial Agents

cHNT, hollow nanotubular-shaped aluminosilicate clay, is widely used as a carrier for active antioxidant and antimicrobial agents. These agents are incorporated inside porous HNT clay with the help of vacuum or freeze-induced loading techniques and active agent-loaded HNT known as nano-capsule (Saadat et al., 2022b; Voronin et al., 2021). Thus, HNT loaded with active compounds will act as a protective casing for highly sensitive polyphenolic and phytochemical compounds in antioxidants and antimicrobial agents (Fakhrullina et al., 2019; Stavitskaya et al., 2020). This casing protects active compounds against external biological, physicochemical, chemical, and thermal degradation and enhances bioavailability and shelf life (Đorđević et al., 2014). Active agents can be in the form of nanoparticles, phenolic acid, anthocyanins, and essential oil from plant material. Essential oils are highly volatile in nature and not appropriate to use in their free form in food packaging applications. Encapsulation of essential oils into HNT clay stabilizes from their liquid state and also controls their release rate over a longer period (Cherednichenko et al., 2021; Stavitskaya et al., 2022).

The controlled release study of HNT loaded with rosemary essential oil was carried out by Gorrasi (2015). The same amount of essential oil was directly dispersed into a pectin polymer film as the control sample. The UV spectrometry analysis shows that all the rosmarinic acid was released from the pectin film (control sample) in 4 h. Only 25% of rosmarinic acid was released from rosemary oil-loaded HNT-based pectin film at the same time, and 90% of rosmarinic acid was released after about 28 days. The rosemary essential oil contains several active compounds, such as rosmarinic acid, betulinic acid, ursolic acid, and rosmanol (Rahbardar & Hosseinzadeh, 2020). These active ingredients help in exhibiting antimicrobial activities against several microorganisms. Nanocomposite film based on HNT loaded with rosemary essential oil has shown antimicrobial activity against mold formation for 3 months, at 25–30 ℃ and 60% RH (Gorrasi, 2015). In another research, the release rate of thyme essential oil as an antioxidant agent was investigated by Lee and Park (2015). The active substance, thyme essential oil (TO), was encapsulated into HNT, a nanocarrier, by vacuum process, and the amount of thyme oil released into absolute ethanol was calculated over time. The study has concluded that liquid-state thyme oil has been released 79.73% in 12 h and completely evaporated within 24 h. On the other hand, TO loaded into HNT has maintained its release rate, and only 61.76% of TO was released in 24 h. Overall, the release of TO from essential oil-loaded HNT was sustained for 96 h. The thyme oil-loaded HNT nano-capsule has also exhibited DPPH radical scavenging of around 67% for the concentration of 40 mg/mL (Lee & Park, 2015).

Blagojević et al. (2022) synthesized HNT nano-capsules loaded with dried ethanol–water-based extract of blackthorn (Prunus spinosa L.) fruit and investigated the release rate of phenolic acid and anthocyanin compound. 3-O-caffeoylquinic acid was the most dominant phenolic acid, and cyanidin 3-O-rutinoside, peonidin 3-O-rutinoside, and cyanidin 3-O-glucoside were derivative of cyanidin and peonidin as anthocyanins (Blagojević et al., 2022). As mentioned, the outer surface of HNT is negatively charged, and positively charged anthocyanin compounds are attached to the outer surface. This interaction between HNT and positively charged anthocyanin slows the release of anthocyanin from nano-capsule. A study has shown that anthocyanin and polyphenolic compounds of blackthorn fruit exhibit antioxidant and antimicrobial activities (Popović et al., 2020; Sabatini et al., 2020). In addition, Saadat et al. (2022a) investigated the antioxidant and antimicrobial activities of chitosan-based film containing ajwain (Trachyspermum ammi) seed oil loaded in the lumen of NaOH-treated HNT nano-clay for food packaging applications. Ajwain seed oil contains flavonoids, carotenoids, thymol, terpinene, and cymene compounds responsible for strong antioxidant and antimicrobial activities against microorganisms (Chahal et al., 2017). HNT being a nanocontainer for the controlled release of essential oil, the active film has shown the highest DPPH radical scavenging activity of 56.5%. The active film also inhibited the growth of bacterial strains (Bacillus mojavensis and Escherichia coli) and fungal strains (Aspergillus flavus and Aspergillus niger) (Saadat et al., 2022a). The antimicrobial activity of chitosan and polyvinyl alcohol-based composite film incorporated with ZnO nanoparticles encapsulated into HNT clay was assessed by Giannakas et al. (2022). The film sample has shown a clear, effective inhibition zone against food-pathogenic bacteria such as Escherichia coli, Salmonella enterica, Staphylococcus aureus, and Listeria monocytogenes (Giannakas et al., 2022).

HNT as Ethylene Scavenger

HNT, considered to be a “green” material, is mined from natural deposits and falls into the category of generally recognized as safe (GRAS) for food packaging applications (Gaikwad et al., 2018). HNT has a high aspect ratio, non-toxic nature, and low cost, and it can be used as an adsorbent and nanofiller in the polymeric matrix. The high performance and low cost of HNT are prime factors for replacing traditional ethylene scavengers in packaging applications. Further, the pore size and aspect ratio of HNT can also be increased by acid- or alkaline-based treatments. These treatments can selectively etch the inner surface wall of HNT, which will enlarge the lumen area, pore volume, and overall surface area of HNT (Gaaz et al., 2016; Surya et al., 2021). Gaikwad et al. (2018) investigated the ethylene adsorption rate of alkaline-treated and alkaline-untreated HNTs and reported that alkaline-treated HNTs have a higher surface area and ethylene adsorption capability than untreated ones. Due to their high aspect ratio, porous structure, surface adsorption capability, suitability as a nanomaterial for polymer matrix, and approval for food contact application, HNTs are an ideal ethylene gas absorber material for fresh produce packaging (Gaikwad et al., 2018, 2019).

The ethylene scavenging activity of pine needle-based active paper containing micro-fibrillated cellulose and HNT was evaluated by Kumar et al., (2022a, b, c). Active paper loaded with 30 wt.% HNT has shown the highest scavenging efficiency compared to the control sample. After 23 dyas of study, the active paper has reduced 79.9% of ethylene gas in sealed glass vials from 20 to 4.01 \(\mathrm{\mu L}\) (Kumar et al., 2022a, b, c). Tas et al. (2017) evaluated the ethylene gas-adsorbing capacity of HNT and polyethylene (PE)/HNT-based nanocomposite film as an active packaging material by static sorption experiment. Results demonstrated that at 1 bar pressure, 1 g of HNTs can adsorb 6.36 mL of ethylene gas, showing a larger adsorbing capacity than palladium-based ethylene scavenger (4.16 mL/g) and Sensitech Ryan® (3 mL/g). The HNT (5 wt.%)-loaded PE nanocomposite film has exhibited the ethylene adsorption capacity. It was observed that, at 1 bar pressure, 1 g of prepared nanocomposite film sample had adsorbed 0.56 mL of ethylene gas. In general, most climacteric fruits and vegetables release ethylene gas in the range of 0.1–100 \(\upmu\) L/ kg h, so the fabricated PE/HNT (5 wt.%) has the ability to adsorb the naturally produced ethylene gas during the ripening of fruits, and vegetables, and prolong the shelf life of fresh produce (Saltveit, 1999; Erdinc Tas et al., 2017).

Yu et al. (2023) prepared superhydrophobic paper coated with halloysite nano-clay as an active agent and evaluated its ethylene scavenging potential. The active, prepared paper could uptake up to 0.38 mL/g of ethylene gas at 25 ℃ and 100 kPa, which shows that HNT-loaded super-hydrophobic paper has excellent scavenging activities against naturally produced ethylene gas (Yu et al., 2023). In another study, ethylene scavenging activities of chitosan-based active film loaded with HNT (5 wt.%) were investigated by Wang et al. (2022). At 100 kPa of pressure, 1 g of active film absorbed 0.60 mL of ethylene gas injected in a sealed steel reactor containing an active film sample (Wang et al., 2022).

HNT as a Reinforcing Agent for Barrier Properties

Food packaging films having high water vapor, and oxygen gas barrier properties are desirable as they retard the transmission rate of moisture or gas molecules between the external environment and food products, which enhances the quality and shelf life of food products. HNT, an inorganic nano-clay, is widely used as a nanofiller in the film-forming matrix. It has a high aspect ratio and low tube-to-tube interaction, which ensures a high degree of nanofiller dispersion. HNT also has low hydroxyl groups on the surface, making it moderately hydrophobic (Yuan et al., 2015). The dispersed, impermeable nano-clay particles create a tortuous path during the migration of permeant molecules through the polymeric film, which increases the overall transmission time and barrier against water and gas molecules. At present, the Cussler and Nielsen models are widely used for predicting permeability through nanocomposite film containing HNT-like hollow nanofillers. These models consider the random and regular dispersion of HNT particles for analyzing the water and gas permeability through nanocomposite films (Takahashi et al., 2006; Tan & Thomas, 2016).

Abdullah et al. (2019a, b) investigated the effect of halloysite nano-clay when incorporated into polyvinyl alcohol and starch-based bio-composite. The results show that film samples containing 5 wt.% of HNT have the highest water, oxygen, and gas barrier properties. This film sample has exhibited the lowest oxygen permeability of around 0.8 × 10−8 m × m3/m2 × min × kPa and air permeability of 0.6 × 10−8 m.m3/m2.min.kPa. The water vapor permeability (WVP) was also the lowest for 5 wt.% of the HNT film sample. At 50% relative humidity (RH), this film has shown a wvp of 0.9 × 10−3 g × m/h × m2 × kPa (Abdullah et al., 2019b). Another study has shown that incorporating HNT into chitosan-based composite film has significantly improved the water and oxygen barrier properties. Chitosan film containing 5 wt.% of HNT has shown oxygen permeability of around 4 cm3/m2 × 24 h × 0.1 MPa, 52.94% less than neat chitosan film. Chitosan film containing 3 wt.% of HNT has shown significantly lower water vapor permeability of 7 × 10−11 g × m/m2 × Pa × s compared to neat chitosan film (Wang et al., 2022). Polyethylene-based active packaging incorporated with HNT has also created a tortuous path during water and oxygen molecule transmission. The film sample loaded with 1wt.% of HNT has shown a significantly lower oxygen transmission rate of 900 mL/m2 × day and water vapor transmission of 10.5 mL/m2 × day (Erdinc Tas et al., 2017).

Tham et al. (2016) has fabricated polylactic acid/halloysite nanotube-based nanocomposite films via melt compounding and compression molding. The oxygen permeability of these film samples was quantified by the oxygen permeability coefficient, defined as the amount of oxygen permeate per unit of time and unit area of the film sample. Nanocomposite film with 6 wt.% of HNT has shown the lowest oxygen permeability coefficient of 1.804 × 10−4 cm3 × m/m2 × day × kPa compared to neat and rest of the film containing HNT as a filler (Tham et al., 2016). In another study, regenerated cellulose-based nanocomposite films loaded with halloysite nanotubes were prepared via solution casting. The film sample containing 8 wt.% of HNT has created the longest tortuous path during oxygen transmission, showing the lowest oxygen permeability of 2.40 × 10−18 m3 × m/m2 × Pa × s (Soheilmoghaddam). Bidsorkhi et al. (2015) investigated the oxygen permeability of ethylene vinyl acetate (EVA)-based film containing halloysite nanotube (HNT) as a nanofiller. EVA/HNT3 containing 3 wt.% of nanotubes has shown the lowest oxygen permeability of 2.342 × 10−10 cm3 × cm/cm2 × s × cm × Hg (Bidsorkhi et al., 2015).

HNT as a Reinforcing Agent for Thermal and Mechanical Properties

HNT is the most preferable nanofiller for polymeric materials compared to the rest of the inorganic clay. Its unique layered tube-shaped structure contains a low density of hydroxyl groups. The inner hydroxyl groups are situated between nanotube layers, and others are at the nanotube’s surface (Lei et al., 2011). The surface polarity and low hydroxyl group density permit smooth diffusion and dispersion in the polymer matrix (Chen et al., 2012; Pasbakhsh et al., 2009). HNT nanofiller effectively interacts with the host polymer, forming a closely compacted microstructure that greatly enhances thermal stability and mechanical properties such as ultimate tensile strength and elongation at break (Gaaz et al., 2017).

Chitosan-based nanocomposite film loaded with HNT clay was fabricated by Singhi et al. (2023), and HNT’s impact on thermal and mechanical properties was analyzed. Incorporating HNT into a chitosan polymer matrix enhanced the crystalline nature of the film, thus improving the mechanical properties of the developed film. The tensile strength and elongation at a break of 0.25 wt.% HNT-added nanocomposite film were 14.47 MPa and 32.98%, respectively. The thermogravimetric and derivative thermogravimetric analysis indicated that adding HNT nano-clay improved the film’s thermal stability (Singhi et al., 2023). Another research by Sharma et al. (2019) has concluded that blending HNT as a reinforcing agent affects the chain interaction between polylactic acid (PLA) molecules and improves the film’s thermal stability (Sharma et al., 2019).

Suppiah et al. (2019) prepared the carboxymethyl cellulose (CMC)-based nanocomposite films loaded with HNT (10 wt.% and 20 wt.%) via the solution casting method. The thermal stability was analyzed by the yield of reside char during the thermogravimetric analysis of film samples. The neat CMC film has 12.3% of residue, while CMC/HNT-10 and CMC/HNT-20 films have a higher residue content of 16.3% and 19.2%, respectively. The high residue suggested that increasing the concentration of HNT delayed the mass transport during the thermal degradation process (Suppiah et al., 2019). Abdullah and Dong (2018) have prepared a nanocomposite film of polyvinyl alcohol (PVA) and starch (ST) with different concentrations of HNT as a nanofiller. Incorporating 0.5 wt.% of HNT has increased the tensile strength by 20% compared to neat PVA/ST film. HNT, a hollow tubular nano-clay, entraps the volatile particles and acts as a barrier to mass and heat transfer, enhancing the thermal stability of nanocomposite film (Abdullah & Dong, 2018).

Active Food Packaging Application

Food packaging protects food products by providing a protective layer around them through film or coating. Incorporating HNT into packaging material can impart additional properties and enhance the packaging performance. HNT, a multifunctional nanotube with a high aspect ratio, can act as a filler to enhance the thermal stability and mechanical properties of packaging. It can also work as a nanocarrier for active antioxidant and antimicrobial agents, which will release the active agents into a polymer matrix in a controlled manner. HNT can also absorb ethylene gas produced during the ripening of climacteric fruits and vegetables. All these aspects ultimately enhance the performance of packaging materials, which prolong the shelf life of food products (Biddeci et al., 2016).

Various research have been conducted to measure the impact of HNT as a functional material for active food packaging applications. The nanocomposite of chitosan polymer incorporated with HNT (2 wt.%) and tea polyphenols was prepared by 3D printing technology. Blueberries were stored inside a container made of the printed nanocomposite. Blueberries stored in an active 3D-printed container maintained their firmness with the least change in their appearance. Tea polyphenols reduced the organic acid consumption during the metabolism process and inhibited the enzymatic oxidation of blueberries (Lan et al., 2019; Y. Liu et al., 2021). Another study by Abdullah et al., (2019a, b) has concluded that bio-composite film fabricated from polyvinyl alcohol (PVA), starch (ST), and HNT (1 wt.%) has prolonged the shelf life of peaches and avocados. The fresh peaches and avocados were stored at 8℃ temperature and relative humidity of 85%, and after 14 days, the fruits packed with active film showed no color change and fungi growth as well. This active film also helped to maintain the weight of fruit products. All these results indicated that active film has high water and oxygen barrier properties, which maintained the freshness of packed fruits (Abdullah et al., 2019b).

Polyethylene (PE)-based active polymeric film comprising HNT as nanofiller for food packaging application was prepared by melt extrusion method. The fabricated nanocomposite film is expected to adsorb naturally produced ethylene gas, enhancing the shelf life of climacteric fruits such as bananas and tomatoes. Ripening of banana fruits due to ethylene gas was controlled by active PE-based packaging film containing 5 wt.% of HNT. At room temperature, the banana stored in this film retained its green color and was free from brown spots after the 8 days of storage period. The tomato’s firmness indicates the degree of ripening, and PE film having 5wt.% of HNT has significantly maintained the firmness. Tomatoes wrapped with neat PE film have lost 72% of firmness, but tomatoes packed with active PE film have shown only a 16% reduction in firmness after 10 days of storage experiment. Here, naturally produced ethylene gas was absorbed by the HNT nanofiller, which reduces the aging, ripening, and softening of bananas and tomatoes (Erdinc Tas et al., 2017).

The ethylene absorbing potential of active paper based on pine needle waste containing halloysite nanotube was investigated by Kumar et al. (2022a, b, c). The senescence process and ripening of banana fruits were delayed when stored in a packaging bag containing PN/MFC/HNT-30% based active ethylene scavenging paper (Kumar et al., 2022a, b, c). Wang et al. (2022) has prepared the chitosan-based active ethylene scavenging film by incorporating HNT into the film-forming matrix. The 5 wt.% films have shown 4.07 times higher scavenging activities than neat chitosan film, maintaining the freshness of cherry tomatoes and bananas for 7 days of storage period (Wang et al., 2022). The superhydrophobic paper loaded with HNT nano-clay was used to study the ripening of cherry tomatoes. The ripening process is accelerated due to high ethylene production, and firmness is a major indicator of their maturity level. After 7 days of the storage period, this active paper has significantly maintained the firmness of cherry tomatoes by absorbing ethylene gas available in the headspace of the packaging. The firmness of tomatoes stored in the active paper was reduced by 33.6%, while the firmness of control samples was reduced to 65.6% (Yu et al., 2023).

HNT for Advanced Food Packaging Applications

Phase change materials (PCMs) are wax, fatty acid, or mineral-based advanced category of materials that can maintain the temperature of food products in a cold supply chain by absorbing or releasing latent heat while simultaneously changing their physical state from solid to liquid and vice versa (Singh et al., 2018). Incorporating PCMs into HNT increases the overall heat release or gain time of the prepared PCM/HNT-based nano-capsules. Cuneyt Erdinc Tas and Unal (2021) have prepared PE-based nanocomposite packaging film containing two PCMs (PEG-400 and PEG-600) impregnated into HNT. Both PCMs were incorporated into HNT by solvent-assisted vacuum treatment, and the nanocomposite film was prepared via melt compounding with polyethylene polymer. These films were used to analyze the thawing of meatball samples at room temperature. Meatballs wrapped inside neat PE reached 4℃ in just 26 min, while the thawing of meatballs was retarded to 44 min by nanocomposite film containing PCM and HNT. The PCM-embedded HNT nanohybrid-based nanocomposite film absorbs the latent heat over a broad time interval, providing a lag of 18 min for the meatball to attain the defrost temperature (Cuneyt Erdinc Tas & Unal, 2021).

Challenges

The utilization of HNT as a multifunctional filler has been accelerated due to its high aspect ratio, dispersion tendency, chemical stability, and active agent carrier for active food packaging applications. This emerging application of HNTs as functional material has gained more attention from researchers due to their nano-scale dimension. Still, many concerns about food, consumer, and environmental safety remain unanswered, which motivates researchers to bridge the gap between academia and industry (Dimitrijevic et al., 2015). The science and technology of functional nanofillers are still in their infancy. However, they impart beneficial properties to nanocomposites, such as thermal stability with enhanced mechanical, moisture, and gas barrier properties. The utilization of HNT nanofiller has been exploited in recent years, but the migration of HNT from food packaging film to food or the environment has not been explored extensively (Kushwaha et al., 2021; Pasbakhsh et al., 2016).

Nanomaterials have higher surface reactivity and nanotoxicity due to their high aspect ratio and unique structure. They can migrate from primary packaging to food products or the environment via dissolution, diffusion, and abrasion phenomena. Food products containing nano-scale material can harm humans as nano-scale particles can penetrate the human cell membrane and even react with genomic sequence. Further, these nano-toxic particles can directly migrate into environments such as air, water, and soil, thus entering the human body through dermal contact or inhalation. Several prime factors like shape, size, aspect ratio, crystallinity, diffusivity, and purity of nanomaterial and pH value of food product, nano-clay, and polymer interaction should be considered for the possible risk assessment of nano-scale materials for food packaging application (Enescu et al., 2019; Savolainen et al., 2010). HNT migration is an alarming concern due to nanotoxicity; researchers must characterize and detect the migration level of nanoparticles from primary active packaging to food products for consumer safety (Deshmukh et al., 2023).

To optimize HNT properties for specific applications, they can be modified using physical, chemical, biological, and electrostatic methods (Pasbakhsh et al. 2016). However, excessive physical treatment can damage the HNTs and alter their structure, affecting their properties. Also, controlling the degree of modification is challenging, as it depends on several factors, such as the duration and intensity of the mechanical treatment, the concentration of the HNTs, and the nature of the solvent (Deng et al., 2009). The extent of chemical modification can also be challenging to control, as it depends on the concentration of the reagents, reaction time, temperature, and pH (Massaro et al., 2018). Biological modification of HNTs requires a thorough understanding of the interaction between the biomolecules and the HNTs to prevent any undesired effects on their properties. Attaching biomolecules onto the surface of HNTs can also be challenging, as it requires specific functional groups on both the HNTs and the biomolecules (Tardani et al., 2020). Controlling the degree of electrostatic modification can be challenging, as it depends on the concentration and type of the charged species, pH, and ionic strength of the solution (Lun et al., 2014).

The active antioxidant and antimicrobial agents can be encapsulated into the HNT for their release in a controlled manner. The enlargement of the lumen area can enhance the loading capacity of active agents into the HNT through selective etching processes. This modification of HNT by the etching process can change the overall dispersion behavior of nanofillers, which can affect the thermal stability, mechanical properties, and morphology of polymeric films. It presents a challenge for their use in active food packaging applications (Pasbakhsh et al., 2016). Research has shown that the encapsulation efficiency and payload value during the HNT loading process (vacuum method) are almost 15%. The release study data of volatile compounds encapsulated inside HNT has shown an initial burst of active compounds, which might be due to free volatile compounds that were not encapsulated, and these were available in free form on the outer surface of HNT or diffused into the polymeric matrix (Lee & Park, 2015).

Conclusion

In conclusion, HNTs have shown caliber as a functional additive for active food packaging applications. HNTs are natural clay nanotubes with a unique hollow structure that provides a large surface area and high aspect ratio, making them ideal for improving the properties of food packaging materials. HNTs can be modified to enhance their functionality in food packaging applications by incorporating antimicrobial agents, antioxidants, or other bioactive compounds. The modification of HNTs using physical, chemical, biological, or electrostatic methods can also improve their compatibility with different packaging materials, such as polymers and biopolymers. Each method has its own limitations and requires a thorough understanding of the interaction between the HNTs and the modifying agents. HNT can be utilized as a carrier of active agents, an ethylene scavenger, and a nanofiller to enhance food packaging material’s thermal and mechanical properties. Therefore, carefully considering the modification method and optimizing the parameters involved are crucial to obtain the desired properties of HNTs for active food packaging applications.