Cork Planks Research Articles

Trichoderma atrobrunneum associated with yellow stain defect of cork planks and critical values of TCA for wine cork stoppers industry

Trichoderma atrobrunneum associated with yellow stain defect of cork planks and critical values of TCA for wine cork stoppers industry

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.

Analysis of cork quality and cork tree health in stands of western Spain

Good knowledge of the factors that influence cork quality and the health of forests is essential when making decisions regarding the management of productive cork oak forests. The quality of cork planks depends on: plank thickness, plank porosity and the presence of anomalies. So far, no studies have evaluated cork quality based on cork anomalies, partly due to the difficulty of obtaining adequate data. In 1985, it was developed the first Cork Quality Field Assessment Plan (CQFAP) to provide information about the quality of harvested cork by collecting data of very high quality. This work analyzes the anomalies present in 2049 cork sampling units collected from ten stands under the CQFAP for the period 1986–2012 in three consecutive cork stripping cycles. Data on diseases, pests and stripping damages are also analyzed in the 654 trees where cork sampling units were collected in the third stripping cycle. Additionally, mean monthly temperature and precipitation values for the 35 years covering the three stripping periods were calculated. A progressive deterioration of cork quality was observed in the sampled cork oak stands. It was found a positive relationship between cork thickness and temperature indicating that the cork growing period could be extended when temperatures are moderate. Sampled cork oaks were more affected by Cerambycidae attacks and Coraebus undatus indicating their prevalent nature. The presence of Coraebus undatus were higher in stands with good health status. The studied diseases were Biscogniauxia mediterranea, Phytophthora sp. and fungi of the family Botryosphaeriaceae, finding a not previously reported correlation between the presence of Crematogaster scutellaris and the three studied diseases. Stripping damages increase with smaller cork thickness but climate is not a decisive factor for a proper stripping. Although having room for improvement, the CQFAP is proving to be an effective tool for assessing the quality of cork, the health status of cork oak forests and the evolution of these forests over time. The information provided could be also used to identify places worthy of further studying. The main recommendation for improving the CQFAP would be to monitor the same trees in each consecutive sampling for properly monitoring cork thickness and the presence of defects to be used as a proxy for tree health. Regarding forest management, the main recommendations would be to extend the length of stripping cycles to enhance cork production, and to use the new tools and systems for cork stripping to reduce damage.

Phytochemical Composition of Extractives in the Inner Cork Layer of Cork Oaks with Low and Moderate Coraebus undatus Attack

The beetle Coraebus undatus, during its larval stage feeds, and excavates galleries on the cork-generating layer of Quercus suber L. trees, seriously affecting the cork quality with significant economic losses for the cork industry. This work compared the composition of the extracts present in the innermost cork layers (the belly) of cork planks from Q. suber trees with low and moderate C. undatus attack in one stand. The total extractives in the inner cork layer from trees with moderate and low C. undatus attacks were similar (on average 22% of the cork mass) with a high proportion of polar compounds (91% of the total extractives). The chemical composition of the inner cork lipophilic extractives was the same in trees infested and free of larvae, with triterpenes as the most abundant family accounting for 77% of all the compounds, predominantly friedeline. The hydrophilic extractives differed on the levels of phenolic compounds, with higher levels in the inner cork extracts of samples from trees with low attack (90.0 mg GAE g−1 vs. 59.0 mg GAE g−1 of inner cork mass) The potential toxic activity of phenolic compounds may have a role in decreasing the larval feeding.

Environmental Impact Analysis of Natural Cork Stopper Manufacturing

For both wine makers and customers, natural cork stoppers are a symbol of quality. Moreover, they are essential for maintaining the organoleptic properties of bottled wines throughout their lifespan. This research relied on the life-cycle assessment (LCA) methodology to analyze the relationship between the efficient usage of cork planks and the environmental impact of the cork stopper manufacturing industry. The goals of this research were to analyze and determine the environmental impact of producing 1 kg of natural cork stoppers. The analysis considered cork stoppers of two sizes—24 × 44 mm and 26 × 44 mm—and two manufacturing methods—punching and turning. Our findings indicated that the 24 × 44 mm cork stoppers produced with the punching method had a slightly lower environmental impact (1.36 kg CO2 eq/kg) across the ten analyzed impact categories. Conversely, 26 × 44 mm turned corks had the highest impact on the environment (1.49 kg CO2 eq/kg). Additionally, a comparison of same-sized punched and turned cork stoppers showed that the former had a lower environmental impact. This phenomenon is directly related to plank usage. In conclusion, there is a clear relationship between environmental impact and the efficient usage of raw material. In turn, an efficient usage of raw material depends on both the manufacturing method and stopper size.

Fitting and calibrating a three-level mixed effects cork growth model

Cork value is determined mainly by its thickness at the moment of debarking but is also affected by the presence of any discontinuities related to porosity characteristics and/or the presence of anomalies in the cork plank. Cork quality is closely related to the final price of the raw material, so inventories conducted prior to the cork debarking are aimed at identifying the characteristics of the cork and therefore its potential price. Besides information from these cork inventories, forest managers also need a reliable prediction of the final cork thickness to help them determine the most profitable moment to harvest. In this work, a three-level nonlinear mixed effects cork growth model was developed for Spanish cork oak forests using 1027 cork samples taken in nine regions of provenance into which Spain is divided for identification of forest reproductive material. The fitting step revealed that the region and the stand level explained much less variability than the tree level in the three-level mixed-effects models. Therefore, subject–specific predictions were obtained with the joint use of region-, stand-and tree-level random effects. The subject–specific predictions were compared with three alternative prediction options when no additional measurements are available. The precision of predictions with calibrated mixed-effects model using region-, stand-, and tree-level random effects was 0.1 mm when taking one additional measurement at any time during the rotation period. The developed model is more precise and has a wider range of application than the previous model, being suitable for predicting accumulated cork thickness in more than 90% of the cork oak distribution area in Spain. However, it should only be applied to make predictions within a given cork rotation period and not to simulate cork growth over potential successive cork rotations. In the future, it would be necessary to develop an annual cork growth model which includes site, stand and tree variables and which is capable of providing long-term predictions under different climatic and management scenarios. The model developed in this study could provide the basis for such a model because its random effects could be used to identify the main variables related to cork growth through correlation analysis.

2-Piece Cork Stoppers as Alternative for Valorization of Thin Cork Planks: Analysis by LCA Methodology.

Natural stoppers are a magnificent closure for the production of aging wines and unique wines, whose application is limited by the availability of raw materials and more specifically of cork sheets of different thickness and quality. The growing demand for quality wine bottle closures leads to the search for alternative stopper production. The two-piece stopper is an alternative since it uses non-usable plates in a conventional way for the production of quality caps. The present study has analyzed the impact of the manufacture of these two-piece stoppers using different methodologies and for different dimensions by developing an LCA (Life Cycle Assessment), concluding that the process phases of the plate, its boiling, and its stabilization, are the phases with the greatest impact. Likewise, it is detected that the impacts in all phases are relatively similar (for one kg of net cork produced), although the volumetric difference between these stoppers represents a significant difference in impacts for each unit produced.

Corky off-flavor compounds in cork planks at different storage times before processing. Influence on the quality of the final stoppers.

Cork companies store cork planks before processing them for a minimum of 6months to dry up and to stabilize their texture and chemical composition, although many companies extend this storage period up to 12 months. However, there is no information about the influence of this seasoning period on their 'corky' off flavors. For this reason, the main compounds responsible for the 'cork taint' of planks stored before processing from 6 to 12 months were investigated. Four haloanisoles and three halophenols were identified: 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), pentachloroanisole (PCA), 2,4,6-tribromoanisole (TBA), 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP), and 2,4,6-tribromophenol (TBP). All of the planks presented some haloanisole or halophenol after 6 and 9months of storage, which practically disappeared after a year of storage. These compounds were only detected in the cork stoppers made from planks with 6 and 9months of storage. Of the alkylmethoxypyrazines, 2-methoxy-3,5-dimethylpyrazine (MDMP), 3-isopropyl-2-methoxypyrazine (IPMP), and 3-isobutyl-2-methoxypyrazine (IBMP) were identified. The MDMP was detected in a larger number of planks with 6months of storage and at higher concentrations than IPMP and IBMP. However, MDMP was not detected in the cork stoppers made from planks at 6, 9, and 12 months of storage. A storage time of 6 months before processing of raw cork planks would be sufficient to obtain cork stoppers with low concentrations of corky off-flavor compounds. An increase in storage up to 9 or 12 months would result in practically 'cork taint'-free natural stoppers. © 2021 Society of Chemical Industry.

Modelling bark thickness variation in stems of cork oak in south-western Portugal

The thickness of bark (cork) is of utmost importance for its industrial processing and, therefore, for the sustainable management of cork oak (Quercus suber L.) woodlands. Cork thickness has been reported to decrease from the stem base upwards but, to our knowledge, prediction models have not yet been presented. This study intended to develop reliable models to predict stem profiles of cork thickness. We addressed the relationship between bark thickness and stem height by measuring stem diameters and cork thickness at various stem heights in 76 trees. Through a repeated measure ANOVA, we selected the bark thickness response variables, cork-ring width and cork thickness. A mixed-effect modelling approach was used to fit linear bark thickness curves with stem height. Our results showed that the equations that performed better for cork thickness were functions of the stem relative height and cross-sectional cork area increment. The decreasing trend upwards the stem might lead to underestimation of cork thickness at the base, more than overestimate it at upper stem parts, in relation to the values at breast height, which are generally used to estimate the thickness of the whole cork plank. Furthermore, cork thickness curves were tree size-related, with larger trees exhibiting a steeper reduction in cork thickness upwards the stem.

Qualitative and quantitative assessment of cork anomalies using near infrared spectroscopy (NIRS)

Abstract The aim of this study was to develop applications of near infrared spectroscopy (NIRS) to detect certain anomalies in cork used in the manufacture of stoppers for wine bottling. 326 spectra of cork planks were used in a qualitative analysis to evaluate the feasibility of NIRS to discriminate between pure cork, yellow stain and lignified cork. Classification and validation errors varied between 0 % and 11.4 %. In addition, 360 spectra of cork granulate were used in a quantitative analysis to determine the percentage of yellow stain and corkback. A root mean standard error of cross validation (RMSECV) of 1.50 % was obtained for yellow stain percentage (R² = 99.62 %). For corkback percentage RMSECV was 2.96 % (R2 = 98.53 %). These results show that NIRS technology has potential, alongside traditional methods, to determine the quality of cork used to manufacture stoppers.

Phellem Cell-Wall Components Are Discriminants of Cork Quality in Quercus suber.

Cork is a renewable, non-wood high valued forest product, with relevant ecological and economic impact in the Mediterranean-type ecosystems. Currently, cork is ranked according to its commercial quality. The most valuable planks are chosen for cork stoppers production. Cork planks with adequate thickness and porosity are classified as stoppable quality cork (SQC). The chemical composition of cork is known, but the regulation of metabolic pathways responsible of cork production and composition, hence of cork quality, is largely unknown. Here, we tested the hypothesis that post-genomic events may be responsible for the development of SQC and N-SQC (non-stoppable quality cork). Here, we show that combined proteomics and targeted metabolomics (namely soluble and cell wall bound phenolics) analyzed on recently formed phellem allows discriminate cork planks of different quality. Phellem cells of SQC and N-SQC displayed different reducing capacity, with consequential impact on both enzymatic pathways (e.g., glycolysis) and other cellular functions, including cell wall assembly and suberization. Glycolysis and respiration related proteins were abundant in both cork quality groups, whereas the level of several proteins associated to mitochondrial metabolism was higher in N-SQC. The soluble and cell wall-bound phenolics in recently formed phellem clearly discriminated SQC from N-SCQ. In our study, SQC was characterized by a high incorporation of aromatic components of the phenylpropanoid pathway in the cell wall, together with a lower content of hydrolysable tannins. Here, we propose that the level of hydrolysable tannins may represent a valuable diagnostic tool for screening recently formed phellem, and used as a proxy for the quality grade of cork plank produced by each tree.

Characterization of chemical compounds susceptible to be extracted from cork by the wine using GC-MS and 1H NMR metabolomic approaches

This work presents a metabolomics study of cork by gas chromatography-mass spectrometry (GC-MS) and 1H nuclear magnetic resonance (NMR) spectroscopy to characterize compounds susceptible to be extracted from cork by the wine in an attempt to find a relationship between the content of these compounds and the geographical origin of cork. Cork from eleven geographical regions was studied, five from Portugal and six from Spain. Unsupervised pattern recognition techniques unveiled three main clusters of regions according to their chemical similarity but not related with geographical proximity. Nineteen compounds were found to be responsible for the clusters, including terpenes (trans-squalene, friedelin, camphene, trans-3-pinanone, 1-terpinen-4-ol, two sesquiterpenes), polyphenols (vescalagin, castalagin), among others (pyrogallol, glucosan, sitost-4-en-3-one, o-cymene, quinic acid, five unknowns). These preliminary results unveiled the potential for a more efficient selection of cork planks for stoppers production based on the compounds susceptible to be extracted from cork by the wine.

Variation of cork quality for wine stoppers across the production regions in Portugal

Cork production and technological quality were characterized following a field sampling covering the whole area of cork oaks in Portugal (six regions), with one site per 20,000 ha of cork oak area (30 sites), using samples taken from the tree at the time of cork stripping (20 trees per site). The following quality parameters were determined: thickness of the raw cork planks, calliper of the cork planks after boiling, cork growth index, i.e., growth of the first 8 complete years, density and porosity. The cork growth index was 28.6 mm, with significant differences between regions and sites within regions. A production cycle of 9 years was applied to 51% of the trees with annual productivity of 0.93 kg m−2 year−1 of debarked surface, 30.3 mm raw cork plank thickness, 33.9 mm corkboard calliper, 251 kg m−3 density and a porosity coefficient of 5.8%. For all the cork quality parameters analysed, the regions and sites within regions were a significant factor of variation. This research covers the natural variability of cork production features for industrial processing, namely for the production of wine stoppers. It is the most extensive work characterizing the geographical variation of cork as an industrial raw-material. The results are useful for optimizing the production cycle and for improving management practices and regulations.

Evaluation of Parameters that Determine Cork Plank Quality (Quercus suber L.) by Near Infrared Spectroscopy

Cork quality control is one of the most important problems affecting the cork sector. The aim of this work is to develop chemometric near infrared models to ensure cork planks quality, taking into account the technological parameters and defects that affect it. The samples were scanned using a LabSpec®Pro A108310 spectrophotometer. The models developed to predict the technological parameters of the transverse section yielded satisfactory results, with r2 > 0.7 for color, r2 > 0.6 for quality, caliper, earthy cork, blown cork, and belly and r2 > 0.5 for stained cork. In terms of the standard error of cross-validation (SEVC), the coefficient of determination (r2), and the residual predictive deviation (RDP), the results are similar in the transverse and radial sections for caliper, quality, and color. Using near infrared spectroscopy technology will not replace the traditional method, but will clearly contribute to improving quality control processes in cork planks.

Correlation between porosity of cork planks before and after boiling using near infrared spectroscopy

The aim of this study was to evaluate the potential of near infrared spectroscopy (NIRS) in predicting the coefficient of porosity of raw and boiled cork samples used mainly for producing cork stoppers for wine bottling. The classification of cork planks is currently performed visually in the last stage of preprocessing after boiling in water. In this study, calibration models have been developed on 279 cork samples before and after boiling. Three models were obtained: one which predicts the coefficient of porosity of unprocessed samples, a second one that predicts porosity in boiled samples and finally a model that predicts the coefficient of porosity of boiled samples from spectra of raw samples. The models obtained showed a coefficient of determination of 0.58, 0.59 and 0.52, respectively. These results show that the modified partial least squares models based on NIRS technology can be used for screening cork porosity, not only at the two stages of processing, but also to predict boiled porosity. This suggests that NIRS technology could be used for determining the coefficient of porosity in boiled cork planks, and could also provide a useful tree selection tool.

Stand factors related to sensory defects of cork planks

Wine bottle stoppers are the main bio-technological product obtained from cork. The 2,4,6-trichloroanisole (TCA) is considered the main responsible for “cork taint” in bottled wines. This study intent to prove the relation between TCA and mancha amarela-yellow spot (MA) and to identify field factors that can be related to the cork TCA presence. Cork planks with and without MA were collected from cork oak stands in ten different regions of the country and 15 variables were analyzed. Three samples were taken from each plank for TCA quantification. The results show that TCA and MA are significantly related and that the TCA level is higher in cork samples coming from stands with shrubs in the understorey.

Prediction of tension properties of cork from its physical properties using neural networks

A tool to predict the tensile properties of cork was applied in order to be used for material and application selection. The mechanical behaviour of cork under tensile stress was determined in the tangential and axial direction. Cork planks of two commercial quality classes were used and samples were taken at three radial positions in the planks.For the construction of the predictive model, nine properties were measured: mechanical properties (Young’s modulus, fracture stress and fracture strain) and the physical properties (porosity, number of pores, density, approximation of the pores to elliptical and circular shape and distance to the nearest pore). The aim of this research work was to predict the mechanical properties from the physical properties using neural networks.Initially, the problem was approached as a regression problem, but the poor correlation coefficients obtained made the authors define a classification problem. The criterion used for the classification problem was the test error rate, training the neural network with a variety of neurons in the hidden layer until the minimum error was achieved. The influence of each individual variable was also studied in order to evaluate their importance for the prediction of the mechanical properties.The results show that the Young’s modulus and fracture stress can be predicted with an error rate in test of 10.6 and 10.2 %, respectively, being the measure of the approximation of the pores to elliptical shape avoidable. Regarding the fracture strain, its prediction from physical properties implies an excessive error.

The problem of 2,4,6-trichloroanisole in cork planks studied by attenuated total reflection infrared spectroscopy: proof of concept.

Attenuated total reflection infrared spectroscopy (ATR-IR) proved to be a promising detection technique for 2,4,6-trichloroanisole (TCA), which confers organoleptic defects to bottled alcoholic beverages, allowing the proposal of a criterion for cork plank acceptance when meant for stopper production. By analysis of a significant number of samples, it was proved that the presence of TCA, even in very low concentrations, imparts subtle changes to the cork spectra, namely, the growth of two new bands at ∼1417 (νC═C of TCA ring) and 1314 cm–1 (a shifted νCC of TCA) and an increase in the relative intensities of the bands at ∼1039 cm–1 (δCO of polysaccharides) and ∼813 cm–1 (τCH of suberin), the latter by overlapping with intense bands of TCA. These relative intensities were evaluated in comparison to a fingerprint of suberin (νasC–O–C), at 1161 cm–1. On the basis of those spectral variables, a multivariate statistics linear analysis (LDA) was performed to obtain a discriminant function that allows classifying the samples according to whether they contain or not TCA. The methodology proposed consists of a demanding acceptance criterion for cork planks destined for stopper production (with the guarantee of nonexistence of TCA) that results from combining the quantitative results with the absence of the two TCA correlated bands. ATR infrared spectroscopy is a nondestructive and easy to apply technique, both on cork planks and on stoppers, and has proven more restrictive than other techniques used in the cork industry that analyze the cleaning solutions. At the level of proof of concept, the method here proposed is appealing for high-value stopper applications.

Application of a kiln drying technique to Quercus suber L. cork planks

At present, all of the cork used to manufacture natural stoppers is air dried for a minimum of 6 months. This study evaluates the feasibility of applying a kiln drying technique to cork planks. We used a sample of 65 planks, each of which was divided into two pieces. One sub-set of planks was air dried for 6 months following the traditional procedure, while the other was kiln dried for 14 days. The evolution of moisture content was controlled, and both methods were compared by means of a statistical analysis of the thickness, quality, porosity coefficient, color and compressive properties of the cork.The reduction in moisture content was similar for both traditional air drying and kiln drying. At the end of the drying, cork thickness increased with both methods, although a larger increase was observed for the kiln drying method. Statistically significant differences were not found for the mean porosity coefficient or quality. Although statistically significant differences in the color of the cork were detected, it would be necessary to verify if such differences are detectable by the human eye. No differences were found in the compressive properties of the cork for a significance level of less than 1%.This feasibility study concludes that kiln drying is a suitable technique as it significantly reduces the drying time and holding costs of the raw material. The application of this technique would constitute an innovation in the transformation process by controlling the climatic variables that affect the drying process.

Cyclic voltammetry: A tool to quantify 2,4,6-trichloroanisole in aqueous samples from cork planks boiling industrial process

Chloroanisoles, namely 2,4,6-trichloroanisole, are pointed out as the primary responsible of the development of musty off-flavours in bottled wine, due to their migration from cork stoppers, which results in huge economical losses for wine industry. A prevention step is the detection of these compounds in cork planks before stoppers are produced. Mass spectrometry gas chromatography is the reference method used although it is far beyond economical possibilities of the majority of cork stoppers producers. In this work, a portable cyclic voltammetry approach was used to detect 2,4,6-trichloroanisole extracted from natural cork planks to the aqueous phase during the cork boiling industrial treatment process. Analyses were carried out under ambient conditions, in less than 15 min with a low use of solvent and without any sample pre-treatment. The proposed technique had detection (0.31±0.01 ng/L) and quantification (0.95±0.05 ng/L) limits lower than the human threshold detection level. For blank solutions, without 2,4,6-trichloroanisole addition, a concentration in the order of the quantification limit was estimated (1.0±0.2 ng/L), which confirms the satisfactory performance of the proposed methodology. For aqueous samples from the industrial cork planks boiling procedure, intra-day repeatabilities were lower than 3%, respectively. Also, 2,4,6-trichloroanisole contents in the aqueous samples determined by this novel approach were in good agreement with those obtained by GC-MS (correlation coefficient equal to 0.98), confirming the satisfactory accuracy of the proposed methodology. So, since this novel approach is a fast, low-cost, portable and user-friendly method, it can be an alternative and helpful tool for in-situ industrial applications, allowing accurate detection of releasable 2,4,6-trichloroanisole in an earlier phase of cork stoppers production, which may allow implementing more effective cork treatments to reduce or avoid future 2,4,6-trichloroanisole contaminations of wine.