Polyvinyl alcohol-based materials encapsulating carvacrol obtained by solvent casting and electrospinning
Graphical abstract
Introduction
Nowadays, we are increasingly witnessing a higher consumer demand for safer foods. A modern strategy that provides an increase in food quality and safety is the use of “smart” active packaging that interacts with the food and its environment [1,2]. These active packaging materials could control the permeability for gases and/or moisture, scavenge oxygen (to prevent foodstuff oxidation) or limit microbial growth, safely extending product shelf life. Also, the packaging industry is actively changing direction towards environmentally-friendly biodegradable components, stimulating the creation of new kinds of coatings and multilayer packaging materials for the food industry [1]. Nevertheless, currently the great majority of food-preserving packaging is based on oil-derived synthetic plastics, which, despite having multiple advantages (such as low cost, versatility, good barrier qualities), are not biodegradable, thus generating a significant environmental impact [3]. Then, the new packaging concept should be based on biodegradable materials in order to address these concerns.
Polyvinyl alcohol (PVA) is a synthetic polymer that is widely used for different applications in various branches of the industry, medicine and food sectors, obtained easily through the polymerization of vinyl acetate followed by the hydrolysis of the acetate groups [4]. It is water soluble, completely biodegradable, colourless and odourless, with good mechanical properties, high oxygen barrier capacity and biocompatibility. PVA also has the potential to act as a carrier matrix for the incorporation of active compounds [5].
Regarding the active compounds, natural substances could be used, such as plant extracts and essential oils, for the purposes of replacing the synthetic food preservatives. The essential oils (EO) are a complex blend of volatile and semi volatile substances, mostly made up of terpenes, terpenoids and aromatic compounds of low molecular weight. Different EO have proven to exhibit antimicrobial properties against bacteria [6,7], or fungi [8], and have been recognized as safe compounds (GRAS) to be used in the food industry by legal organisms, such as the Food and Drug Administration in the USA. Nevertheless, despite their advantages, the concentration levels of EO that would exert the desired effects could also introduce unwanted flavours and potential fitotoxicity into the foodstuffs they are required to protect. This is the case of carvacrol (CA), a monoterpenoid phenol that can be found in high concentrations in the EO of oregano or thyme [9], with proven antimicrobial and antioxidant effects. It is considered a good preservative for a wide range of foodstuffs [10], but its direct application is limited because of its strong flavour, high volatility, low water solubility or its potential reactions with certain food components that might alter the food properties and limit its efficacy as a preservative [11,12]. The encapsulation of the active compounds may favour their stability, their preservation efficiency while allowing for a controlled release towards the foodstuffs [13].
One encapsulation method that is gaining interest in the food packaging sector is the electrospinning (ES) technique [14,15]. It has a simple working principle, using an electric field to stretch a polymeric solution, forming structures (mainly fibres) with high specific surface that can be used to coat support materials useful in the development of multilayer active packaging [16]. Some successful applications have been reported, such as antioxidant multilayers constituted by polylactide (PLA) cast films coated by ES with PLA mats encapsulating gallic acid [17], antimicrobial multilayer systems composed of a polyhydroxyalkanoate (PHA) support coated with an electrospun layer of PHA enriched with silver nanoparticles [18], and antibacterial multilayers with reduced water barrier properties developed from gelatine sheets coated by ES with poly-ε-caprolactone loaded with black pepper oleoresin [19].
In the development of active multilayer materials, the extension of a polymer solution containing the active compound on a polymeric support can also be used to obtain coated films with an active layer, where the active compound would be encapsulated into the polymer matrix of the cast layer. However, the effective extension of a determined active-polymer solution will be greatly affected by the solutions' wettability and spreadability on the supporting polymer layer, which in turn, depend on the contact angle of the polymer-solution system and surface tension of the solutions. All these factors play a crucial role in the coating thickness and effectiveness. Additionally, the concept of multilayer films involves the assembly of polar and non-polar polymers to take advantage of their respective complementary barrier properties to gases and water vapour. In this sense, the effective extension of polymer polar solutions on non-polar polymers or vice versa has the problems associated with the lack of chemical affinity. In contrast, the ES technique allows for the electrodeposition of the solvent-free polymer on the supporting film surface, thus avoiding the problems of the extension of the solution. However, developing active multilayer films requires knowledge of the encapsulation efficiency of the polymer involved in the active sheet of laminate, and by testing different encapsulating technologies, the most appropriate method is selected in order to ensure the best functional properties of the active multilayer systems.
The electrodeposition of polyvinyl alcohol has been studied by several authors [20,21] by analysing the influence of different parameters, such as molecular weight and concentration on the ES process. The electrospun fibre architecture depended on the PVA molecular weight (Mw) and its concentration in the solution; beads and spindle-like formations being more present at lower concentrations or lower Mw than fibres. The diameter of fibres became larger as Mw or concentration increased, and a broader fibre distribution could be obtained. The presence of salts (such as NaCl) in the solution could disrupt the fibrous assembly for a low Mw polymer, however for the higher Mw the salt presence leads to thinner fibre diameters. The formation of PVA fibres was not affected by the pH variation, but an increase in voltage and salt concentration was not favourable to the ES process [21]. The use of the electrospun PVA matrix as a carrier for different active substrates, such as silver particles, medicinal drugs, enzymes, bifidobacteria, or plant essential oils have been reported [14]. Electrospun PVA nano-mats with uniform fibres containing cinnamon essential oil entrapped in β-cyclodextrin have been successfully obtained [13]. The authors demonstrated that the ES process is favourable to maintaining the volatile active compound in the electrodeposited mat, which also presents thermal stability, due to the molecular interactions between the PVA, the essential oil and the β-cyclodextrin.
The purpose of this study was to analyse the ability of PVA to encapsulate carvacrol, incorporating or not surfactant, by means of the ES technique, in comparison with the casting method. The characterization of the obtained materials in terms of the encapsulation efficiency, microstructure and thermal behaviour was carried out.
Section snippets
Materials and reagents
Polyvinyl alcohol (PVA) (Mw 13,000-23,000; 87–89% hydrolyzed), polyoxyethylene sorbitan trioleate Tween 85 (T85), carvacrol (CA) and phosphorous pentoxide (P2O5) were acquired from Sigma-Aldrich (Sigma–Aldrich Chemie, Steinheim, Germany). Purified water (resistivity of 18.2 MΩ cm) was prepared using a MilliQ Advantage A10 equipment from Millipore S.A.S., Molsheim, France. Absolute ethanol (UV grade) used for extraction was obtained from Panreac AppliChem (Panreac Química S.L.U, Barcelona,
Properties of the liquid systems
Rheological behaviour, conductivity, surface tension and the emulsion droplet size (in the case of systems containing CA) of the liquid systems can affect their behaviour in ES processing as well as the microstructure of the electrospun material. So, these parameters were characterized in order to better understand differences between samples.
Fig. 1 shows the droplet size distribution of the emulsions containing carvacrol with and without surfactant. The presence of the surfactant gave rise to
Conclusions
PVA aqueous solutions (15% w/w) containing CA (15 g/100 g polymer) exhibited good electrospinning behaviour, leading to loaded matts with fibres and spherical beds, that retained up to 83% of the active, giving rise to matrices with up to 12 g carvacrol/100 g polymer. The greatest encapsulation efficiency occurred in the blend without surfactant (83%) and the formulations with surfactant exhibited similar encapsulation efficiency to that obtained in the casting process (75–77%). Two fractions
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.
Declaration of Competing Interest
None.
Acknowledgements
The authors thank the Ministerio de Economia y Competitividad (MINECO) of Spain, for the financial support provided for this study as part of the project AGL2016-76699-R. The author A. Tampau also thanks MINECO for the pre-doctoral research grant #BES-2014-068100.
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2022, Composites Part A: Applied Science and ManufacturingCitation Excerpt :In recent times, PVA has attracted much attention in biomedical applications owing to its unique properties such as water solubility, promising biodegradability and excellent biocompatibility [36]. Among all the potential applications in this field, controlled drug release is one of the most widely studied [30,37-43]. Moreover, during electrospinning, PVA can form nanofibrous mats with good mechanical, biological and chemical properties.