Natural Cork Research Articles

Promising honeycombed cork activated carbon for high desalination performance brackish water treatment

Cork activated carbon (CAC), derived from natural cork, was successfully prepared through two steps of carbonation and activation, and used for desalinating brackish water. The CAC presents a uniform honeycomb structure composed of porous carbon nanoflakes (200–300 nm) and large specific surface area (SSA) (2680 m2 g−1), offering transport channels and a large number of adsorption sites to NaCl solution. The CAC showed excellent capacitance (116 F g−1), small internal resistance and better charge-discharge performance. Moreover, the optimized CAC showed high desalting capacity (9.44 mg g−1), charge efficiency (46.72 %), and lower energy consumption (54.19 Wh m−3) in the desalination process, superior to that of the reported biomass-based activated carbons, which can be attributed to the stable honeycomb structure, high microporous volume (0.86 cm3·g−1) and large pore size (2.43 nm). This investigation demonstrates the potential application of cork granules-based CAC electrodes in the field of desalination.

In-plane thermal anisotropy of natural cork and its variability

This study investigates the natural cork thermal anisotropy and its variability using plane slice samples of 1.4 mm thickness, cut along the principal radial, longitudinal, and tangential directions, from bark previously treated for cork stoppers production. Heating was pointwise applied to the rear face using a copper needle coupled to a Peltier element, while an in-plane thermographic technique monitored temperature evolution on the front surfaces using a Flir SC5650 infrared camera. Thermal diffusivity measurements yielded values of 0.172 ± 0.038 mm2/s (radial), 0.161 ± 0.022 mm2/s (longitudinal), and 0.160 ± 0.027 mm2/s (tangential). Additionally, a volumetric heat capacity of 0.25 MJ/(m3·K) was determined using the Hot Disk technique. This information enabled the calculation of thermal conductivity, ranging from 0.033 to 0.048 W/(m·K). Findings align with existing literature, which reports thermal diffusivity between 0.08 and 0.24 mm2/s and thermal conductivity between 0.035 and 0.045 W/(m·K). This study contributes valuable insights into the thermal properties of natural cork and their orientation-dependent variability.

Mass insulation enhancement of cork stoppers through heat treatment: experimental measurements, modeling, and optimization

Natural cork stoppers, used as bottle sealers for high-end products such as ingredients and flavorings, belong to the highest class in terms of cork quality. Various studies have been carried out to examine the solute diffusion in cork using conventional methods. However, the investigations were limited to natural cork stoppers that had been boiled and cut in a single direction. In this study, the anisotropic cylindrical shape of cork samples was modeled, considering various cutting directions. Additionally, we subjected the stoppers to boiling then to an optimal THT cycle to improve the cork's mechanical qualities. The optimal thermal cycle is determined using TGA analysis. IR and SEM tests are employed to verify the enhancements in the cork. The apparent mass diffusion coefficient (Dapp) is determined by the calibration of the model with the experimental data acquired via the conductometric method. The coefficient values are optimized using the Bat algorithm with a relative error of 10–5. The Dapp was found to be better for the treated cork grown in the Jijel region than for the cork Medea, which is better than of Skikda cork. Regarding the impact of the cutting direction on the Jijel's cork, the results indicate that the mass insulation is higher when the treated stoppers are cut in the tangential-longitudinal direction (DJTLt=4,12.10-13m2s-1) compared to the tangential-radial direction (DJTRt=7,75.10-13m2s-1) and the longitudinal-radial direction (DJLRt=8,53.10-13m2s-1). The treatment, which serves as a solution for the cork industry, showed an improvement in mass insulation 13-fold greater than the natural cork.

Properties of Binderless Insulating Boards Made from Canary Island Date Palm and Cork Particles

Agglomerated cork is a natural cork that has gone through a process of crushing and pressing using heat and binders. One of its applications is thermal insulator in construction. The design of these materials is becoming an essential part of building. The raw materials currently used to make insulators consume a large amount of energy, which has created the need to increase the use of renewable and ecological resources such as plant fibers to reduce the environmental problems generated. The objective of this study was to determine the different properties of experimental particleboard panels made from cork and Canary Island date palms without using any binder at minimum energy consumption. The produced cork–palm boards (density of 850 kg/m3, reached a MOR 8.83 N/mm2, MOE 794.5 N/mm2, and IB 0.38 N/mm2) are higher values than the traditional cork particleboards with UF made from cork. The thermal conductivity values obtained 0.069 to 0.096 W/m·K are higher than cork boards with UF. Ecological boards that can be used as rigid thermal insulators in the construction industry have been achieved to improve the mechanical properties of the traditional agglomerated cork.

The role of coatings on the sealing of cork stoppers

Natural cork stoppers, renowned for their sealing efficacy in wine bottles, face challenges in gas-tightness due to cork surface irregularities. This study investigates the impact of lubricant coatings (silicone or paraffin emulsions), on cork stopper gas-tightness. These coatings facilitate cork conformity to glass, minimizing gaps at the interface and significantly improving sealing capacity. A small amount of lubricant leads to a two-order-of-magnitude reduction in gas flow. However, careful control of coating is crucial to ensure reliable sealing. Additionally, water absorption by cork is proposed to contribute to sealing by generating expansion forces and softening the cork, enhancing conformity to glass.This study emphasizes that natural cork stoppers should be viewed as both sealants and membranes. These findings shed light on the importance of surface treatments and water interaction in achieving reliable sealing in cork stoppers, ultimately ensuring the preservation and quality of bottled wine.

Responsible Humidity Sensor by Direct Laser Writing on Cork Substrate

AbstractA novel laser‐induced graphene (LIG) resistive relative humidity (RH) sensor is successfully fabricated by direct laser writing on a common natural cork sheet. In this work, a straightforward fabrication is presented where LIG, porous multilayer graphene, is generated by a laser photothermal process on a carbon‐based substrate, in this case, an agglomerate cork sheet. The formation of LIG material is revealed by structural and morphological characterization using various analyses, including Raman spectroscopy, X‐ray Photoelectron spectroscopy (XPS), and Scanning Electron Microscopy (SEM). The electrical analyses are conducted to measure the resulting sheet resistance, getting results as low as 31 Ω sq−1. With the laser parameter optimized, a LIG resistive humidity transducer is manufactured and characterized in a climate chamber. This biodegradable sensor shows good linearity with a sensitivity of 0.015%/%RH from 40 to 80% RH. Additionally, the influence of temperature is studied and demonstrated a low impact on the sensor's response toward RH. Furthermore, a proof of concept is successfully implemented by integrating the transducer onto a cork stopper. Considering all, the prospect of creating an easy, fast, scalable, and cost‐effective transducer combined with the use of natural and abundant materials, leads the way for future large‐scale production of sustainable sensors.

Plasticizing effect of depolymerized suberin derivatives from natural cork and potato periderm in poly(lactic acid) (PLA) for improved toughness and processability

In this study, the inherent brittleness of biodegradable poly(lactic acid) (PLA) was mitigated through the utilization of a carbon-neutral biomass—depolymerized suberin derivatives (DSDs)—as a plasticizer. The DSDs were subjected to repolymerization under mild conditions, resulting in weight-average molecular weights of 2744 g/mol and 11546 g/mol. These polymerized DSDs (pDSDs) were subsequently blended with PLA, leading to a notable reduction in the glass transition temperature of the blends, confirmed through differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) characterizations. Remarkably, the tensile toughness exhibited a substantial increase up to 1148% from 0.56 MJ/m3 in the neat PLA to a range of 4.37–6.99 MJ/m3 for blends incorporating more than 3 wt% of pDSDs. Furthermore, the blends demonstrated a marked reduction in shear viscosity compared to neat PLA, corroborated by an increased melt flow index which shifted from 10.0 g/10 min to a range of 29.3–33.3 g/10 min. This alteration promises to be beneficial in enhancing the quality and producibility during the injection molding process. Importantly, the biodegradability of the pDSDs/PLA blends remained comparable to that of the neat PLA, showing a 90% degree of disintegration in 12 weeks from the biodisintegration test. This study can pave the way to valorize other bio-based materials with aliphatic compounds for the plasticizing modification of various polymers. Furthermore, understanding the interactions between different bio-based aliphatic compounds and various polymers could lead to tailor-made solutions for specific applications, ranging from biomedical devices to structural parts, further broadening the scope of biodegradable and sustainable materials in many engineering fields.

Investigation of synthetic and natural cork fiber laminate polymer composite bending characteristics

Investigation of synthetic and natural cork fiber laminate polymer composite bending characteristics

Degradation of Azo Dye Solutions by a Nanocrystalline Fe-Based Alloy and the Adsorption of Their By-Products by Cork.

In this study, the efficiency of mechanically alloyed Fe80Si10B10 in degrading basic red 46 azo dye is investigated. Moreover, the influences of different parameters, such as pH and time, on the elimination of the aromatic derivatives obtained as by-products of the fracture of the azo group are also analyzed. After beginning the reduction to the normal conditions of pH (4.6) and temperature, the experimental findings showed a discoloration of 97.87% after 20 min. The structure and morphology of the nanocrystalline Fe80Si10B10 powder were characterized by SEM and XRD before and after use in the degradation process. The XRD patterns of the Fe-Si-B powder after redox reaction suggest that the valent zero Fe of the alloy is the reducing agent. Powdered cork was then used as a biosorbent for the removal of the by-products generated, resulting in increasing removal percentages from pH 7 (26%) to pH 9 (62%) and a contact time of 120 min. The FTIR spectrum of the cork after adsorption shows a shift of the bands, confirming the interaction with the aromatic amines. The present findings show that metallic powders and natural cork perform well together in removing azo dye solutions and their degradation products.

Is Airborne 2,4,6-Trichloroanisole (TCA) a Threat for Bottled Wine?

Background and Aims. This work investigated the TCA permeability of twelve types of commercial bottle closures during 24 months of bottle storage in the d5-TCA-contaminated atmosphere: medium pollution (max. ∼50 ng/L of d5-TCA in the air) and high pollution (max. ∼500 ng/L of d5-TCA in the air). Methods and Results. The d5-TCA content of wine samples and bottle closures was monitored by GC-MS analysis, and the closures of one group (comprising natural corks, agglomerated stoppers, and BVS Tin Saran™ screw caps) were found to be excellent barriers against airborne d5-TCA, i.e., no contaminant was detected in wine under any storage conditions. In contrast, a second group of closures (synthetic stoppers with low OTR, BVS Saranex™, and plastic body screw caps) allowed permeation of d5-TCA, polluting the wine when air contamination was high, albeit no d5-TCA was detected in wines following storage under medium air contamination conditions. A third group of closures (synthetic stoppers with medium and medium + OTR, MCA screw caps, and glass stoppers) allowed d5-TCA to accumulate in wine under both medium and high contamination environments. Conclusions. Some commercial bottle closures were found to permeate airborne d5-TCA, thereby contaminating bottled wine under certain storage conditions. Significance of the Study. This work provides the wine industry with insight into the potential for postbottling contamination of wine by airborne TCA.

The Use of Cork and Silicone Rubber with Embedded Sensors in Sports' Smart Protections: Studies on the Impact Absorption Performance

In several sports, such as martial arts, point scoring depends on applied impact. Embedded sensors in wearable protection devices help to quantify the impact intensity. In parallel, there is a growing interest in developing products with a reduced ecological footprint, using natural materials such as cork. Considered a light, resilient, flexible, and hypo-allergenic material, cork has been proving to be a material with interesting impact absorption properties to be integrated into personal protection, as is the case with helmets and vests. Silicon rubber has also been proposed as an alternative elastomer material with adequate sealing and electrical isolation properties, while resistant to ageing and impact. Since injuries occur more frequently in lower extremities, presented work is part of a project that aims to explore the interest of replacing foams and synthetic leathers usually applied in foot protections with smaller CO2 footprint materials while increasing their impact absorption properties. At this stage, a part of a protection prototype consisting of a plate made from natural cork, coated with silicone rubber protection containing embedded force sensors, was studied. The objective is to explore the potential of such materials to replace those conventionally applied in commercial foot protections. Firstly, the adhesion between imprinted silicon and cork was confirmed through peeling tests. Then, considering the opportunity of inserting sensors within the additive manufacturing of the parts, different silicone coating configurations and sensor placements were tested regarding their impact performance. Comparison between impact absorption performance was carried out using an impact test device, assessing sandwich composite performance and force sensing reading accuracy. Silicone printability during the robocasting extrusion technique demonstrated to facilitate the sensor placement without requiring deposition path modification. It showed to be an easy way to manipulate the sensor position within the composite layers and, by that, to modify its impact performance. This is an important contribution to the main goal of the undergoing project, which is to completely replace commercial materials with this new sandwich composite.

Towards better understanding of the strain–stress curve of cork: A structure–mechanical properties approach

If the compressive strain–stress curve of cork has already been largely described, its relationship with the cork structure still requires a better understanding. In this study, quasi-static and Progressive Repeated Loading (PRL) tests in compression have been performed on natural cork with a tensile machine. The elastic modulus was measured by a quasi-static compression test on cube or parallelepiped specimens considering different strain measurements methods: crosshead displacement, extensometer, and full field measurements method (Digital Image Correlation or DIC method). Regardless of the considered strain measurement method, the elastic modulus of cork remains in the same order of magnitude. The PRL test carried out at different nominal strains (3 %, 20 %, 46 %, 66 %, 80 %) highlighted the time dependant behaviour of cork (viscous properties). Moreover, a stiffening of the material was observed when increasing the strain at each loading/unloading cycle. Finally, the full field measurement following compression revealed the heterogeneity of the strain distribution on the cork external surface, resulting in localised regions of deformation. From the perspective of compression behaviour, these results lead to a new interpretation of cork which can be considered as a composite material.

Thermal Desorption of 2,4,6-Trichloroanisole from Cork.

While extensive efforts have been made over the past two decades to understand how cork becomes contaminated by 2,4,6-trichloroanisole (TCA), the nature of its bond to cork remains unclear. A deeper understanding of this interaction is crucial in designing processes to effectively remove TCA from cork stoppers. This study presents an investigation into the thermal desorption of TCA from cork under vacuum conditions. To facilitate detection by a quadrupole mass spectrometer, samples were artificially contaminated with sufficient TCA. A calibration system was developed to determine the absolute rate of TCA released from the cork. Desorption spectra revealed two peaks at 80 °C and 170 °C. Despite the known variability of cork, repeated measurements demonstrated reasonable repeatability. The low-temperature peak decreased with time and after preheating the sample to 50 °C. It is proposed that the high-temperature peak corresponds to TCA bonded to the cork material. Experiments with naturally contaminated cork stoppers revealed a significant reduction in the amount of releasable TCA following a vacuum-heating process. This study provides an insightful discussion on the adsorption of TCA on cork and proposes an estimate for the adsorption energy. Furthermore, it discloses a process capable of removing TCA from natural cork stoppers.

Effect of Different Packaging Strategies on the Secondary Shelf Life of Young and Structured Red Wine.

When bottled wine is opened, a completely different scenario occurs that can accelerate the oxidation of the product. This is called the secondary shelf life (SSL), which is generally shorter and less predictable than the primary shelf life (PSL). In this context, the research aim was to evaluate the changes that occur in two types of red wine during two tests to evaluate the secondary shelf life as a function of the packaging systems. The variation of Total SO2 and Free SO2 and the other chemical parameters (polyphenols, anthocyanins, proanthocyanidins, color, and volatile acidity) were used to assess the oxidation rate of the packaging samples after opening during the SSL. In both tests and for the two types of stored red wine, the polymeric cap showed the best results. The other types of closure (screw cap, natural cork, crow cap, and Tetra Brik) showed a negative trend and a reduced SSL for both red wines. Finally, the sensory results confirmed that with the polymeric cap, the SSL increases considerably compared to other capping systems. These results may be due to the technical characteristics of polymeric materials, which tend to vary slightly in shape after repeated usage.

Sustainable Laser‐Induced Graphene Electrochemical Sensors from Natural Cork for Sensitive Tyrosine Detection

AbstractCork laser induced graphene (cork‐LIG) electrochemical sensors are fabricated by direct laser writing of natural cork sheets. Laser writing is performed with a hobbyist visible laser. The obtained cork‐LIG structures display graphene‐like Raman signatures, high conductivity, and fast electron‐transfer rates. After a 10 min electrochemical pretreatment, electrodes are used for detection of Tyrosine (Tyr) in the presence of uric acid, dopamine, ascorbic acid, urea, and glucose. linear detection of tyrosine is achieved in the 5–150 µm range with a limit of detection (LOD) of 3.03 µm. Calibration of dopamine detection is achieved with a LOD of 1.1 µm. Finally, the cork‐LIG electrochemical sensors show linear response of Tyr in artificial sweat in the relevant physiological range of 5–250 μm (sensitivity, 1.8×10−2 A m−1). The LOD is calculated as 3.75 μm. These results open the door to the exploitation of renewable materials such as cork for the development of high‐quality, green sensors for the monitoring of health and wellbeing parameters.

Migration of polyphenols from natural and microagglomerated cork stoppers to hydroalcoholic solutions and their sensory impact

During bottling aging, the wine comes into contact with the cork stopper due to the horizontal position of the bottle. The release of compounds, such as cork phenolic compounds, thus take place between the cork and the wine, depending on the type of cork stopper and the surface treatments applied. Many publications describe the extraction of these phenolic compounds in wine or hydroalcoholic solutions from natural corks, but few address microagglomerated corks, which are increasingly used by winemakers to seal their bottles. The aim of this study was therefore to compare the polyphenols, mainly hydrolysable tannins, transferred from natural and microagglomerated corks treated with supercritical CO2 into hydroalcoholic solutions. For this purpose, polyphenols released in macerates of natural and microagglomerated cork stoppers were identified and quantified by HPLC-DAD-ESI-QQQ. Suberic acid was also quantified. In this study, despite the high intra-“natural cork stopper” variability, significant differences were found between both types of stoppers for all polyphenols, the agglomerated corks releasing significantly less polyphenols; i.e., 25 times less. In contrast, suberic acid was extracted from both types of corks in similar concentrations; therefore, its extractability was not impacted by the type of stopper. A sensory profile was also carried out on the macerates. Macerates of natural cork stoppers were perceived with notes of “cardboard, dust, plank, wood” and “cork taint” significantly higher than supercritical CO2 treated microagglomerated cork stopper macerates. Moreover, the natural cork macerate with the highest content in polyphenol was perceived as being more bitter than that of microagglomerated cork stoppers.

Long-Term Behavior Related to Water Ingress in Mortars Which Combine Expanded and Natural Cork Lightweight Aggregates and Eco-Friendly Cements

The water ingress plays an important role in building materials’ degradation. The use of lightweight aggregates is interesting in terms of sustainability, because they reduce the density of cement-based materials, among other advantages. The development and use of new lightweight aggregates, such as cork granulates, is a current research topic. In the present work, water ingress performance of sustainable mortars which combined expanded and natural cork aggregates and cements with slag, fly ash and limestone has been studied. Mortars produced with sand and expanded clay were also prepared. Bulk density, water absorption, drying capacity and gel and capillary pores were studied. Tests were carried out at 28 days and 1 year. A good behavior has been generally observed when an addition was incorporated to the binder, especially slag or fly ash. Regarding the new non-standardized lightweight cork aggregates, mortars with natural cork showed lower water absorption and lower volume of permeable pore space in the long term than mortars with expanded cork. At one year, natural cork mortars had an adequate water absorption performance compared to those with expanded clay, which may be due to the high volume of small capillary pores (100 nm–1 µm) in natural cork mortars.

Качество и безопасность продукции в процессе хранения в системе «вино – упаковка, укупорка»

Poor packaging often affects the quality and safety indicators of wine during storage. The need to preserve and prolong wine stability during long-term storage requires a comprehensive study of processes that occur in wines packaged and capped in various ways.
 This research featured 56 samples of white and red dry wines, which were bottled in four types of packages and stopper for 18 months. The effect of synthetic and natural stopper methods involved a model system that simulated the main composition of wine in line with the EAEU standards. The contents of free SO2 and dissolved oxygen, as well as the microbiology of natural corks, were studied using standard methods. The analysis of volatile substances relied on gas chromatography, mass spectrometry, and solid phase micro-extraction.
 The study recorded critical changes in the quality of wine products, characterized by increased oxidation markers and low free sulfur dioxide, which fell below 10 mg/dm3. In polyethylene terephthalate bottles, these changes occurred after 12 months of storage, while it took them 18 months to develop in multilayer bags and combined packaging materials. The components of natural and artificial plugs of microbial and chemical origin entered the model systems after 10 days of contact. The microbiological contamination was twice as high as the permissible level. The migration of microbial and chemical substances into the wine samples spoiled their quality and safety.
 The effect of packaging on the oxidative processes in white and red dry wines during storage resulted in different concentrations of dissolved oxygen and sulfur dioxide. The results can help wine producers to choose the most appropriate packaging and stopper means depending on the wine sort and its subsequent storage conditions. Alternative types of packaging proved to have a limited shelf life.

Rapid and non-invasive estimation of total polyphenol content and antioxidant activity of natural corks by NIR spectroscopy and multivariate analysis

The use of near infrared spectroscopy (NIRS) coupled with multivariate data analysis has been evaluated for the rapid and non-invasive estimation of total polyphenol content (TPC) and antioxidant activity of natural corks. A total of 132 samples were used, using two-thirds and one-third of spectra for calibration and external validation sets, respectively. Regression models were developed by partial least squares (PLS) regression analysis. The best models were achieved by different spectral preprocessing approaches with 7 latent variables for TPC and 9 for antioxidant activity, a variance explained by the models between 64.7 % and 72.0 %, and a standard error of calibration (SEC) between 3.6 and 6.8, respectively. Good relationships between chemical values measured by conventional assays and NIRS predicted values were obtained, with standard error of prediction (SEP) of 5.0 and 5.8 for ABTS and DPPH antioxidant activity, and 5.8 for TPC. Regression lines for calibration and external validation presented good correlation coefficients (r2) from 0.80 to 0.85, and from 0.62 to 0.76, respectively. The models obtained showed that it is possible to estimate rapidly and simultaneously, the TPC and antioxidant activity of natural corks by combining the NIRS with different multivariate analysis tools.

Headspace Solid-Phase Microextraction: A Useful and Quick Tool for the Traceability and Quality Assessment of Wine Cork Stoppers

Natural cork remains a favored option for sealing high-quality wine bottles, despite its high cost for wineries. The cork industry faces the challenge of certifying the quality and traceability of these corks, with physical–chemical characterization being a valuable tool in establishing these parameters. While cork taint compounds must be absent or in low concentrations, the volatile fraction of cork contains numerous compounds that, even in small amounts, can impact the wine’s final aroma. Moreover, these volatile compounds are indicative of the geographical origin of the cork planks used to make the stoppers. In this work, a total of 68 volatile compounds (alkanes, terpenes, benzenic compounds, aldehydes, ketones, acids, esters, alcohols and furanic and pyranic compounds) from natural corks of different qualities and origins were identified, using a fast and sensitive technique: headspace solid-phase microextraction coupled to gas chromatography–mass spectrometry (HS-SPME-GC-MS). Based on these volatile compounds, it was possible to establish differences between corks of different origins, although no discernible differences were detected in the samples of differing visual qualities, as this is a subjective parameter largely dependent on the cork’s external appearance. These findings show that the analysis of the volatile composition of corks via HS-SPME-GC-MS can be used as a quick tool for tracking their traceability and selecting the most appropriate parameters at each stage of processing to minimize the increase in unwanted compounds.