**4.4 Cork properties as closures**

Cork has an unusual combination of properties: low density, very low permeability to liquids and gases, low conductivity, chemical stability and durability, and high compressibility with dimensional recovery.

These are properties that are fundamental for the application of cork as a sealing closure in bottles. To a cork stopper, it is requested that (a) it does not allow any leakage from the liquid content either through the stopper itself or at the interface between stopper and bottle; (b) it does not negatively alter the liquid chemical and sensory features; (c) it is durable and preserves its physical and chemical characteristics during storage; and (d) it can be removed from the bottle for consumption easily. To satisfy these requirements, namely, the first requirement of liquid sealing,

**231**

**Figure 5.**

*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

it is necessary to have an appropriate compression against the bottleneck and close contact between the stopper and the bottle surface to avoid liquid percolation as

*Typical 2D "slices" from a 3D grayscale image of a cork stopper scanned at 50 μm resolution. Different sections through the cork stopper in three orthogonal planes: Transversal, radial, and tangential. Crossing lines* 

Cork is a viscoelastic material which allows large deformations under compression without fracture, with substantial dimensional recovery when stress is relieved [36]. The stress-strain compression curve of cork is characteristic of a cellular material: it shows an elastic region up to 5% strain, followed by a wide plateau where strong dimensional reductions occur for small stress increases due to the undulation of cells and their collapse, until a densification phase with a strong stress increase

The compressive behavior of cork is anisotropic although the differences between directions are not very large: the strength in the radial direction is only slightly higher than in axial and tangential directions, which are more similar (**Figure 6**) [30]. This anisotropy in the stress distribution is also noticed in the compression of a stopper with the radial direction corresponding to the maximal

The cork stoppers have a diameter well above the inside diameter of the bottleneck, and when inserted in the bottle, they will be compressed exerting pressure against the bottle glass. The strains in bottled cork average 30%, a value that is in the plateau region of the stress-strain curve corresponding to the physical phenomenon of the buckling of the cells and to stress values in the range of 1 MPa (**Figure 6**). Industry technical guides refer that cork stopper compression must never be more than 33% of their diameter, as there is a risk that this could compromise its elasticity, with loss of part of the recovery

stress and the tangential direction to the minimum value [3].

and consequent difficulty in the correct sealing of the wine in the bottle.

well as material's impermeability to avoid diffusion through the stopper.

but without cell or cell wall fractures.

*represent the same point in the three images.*

*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

#### **Figure 5.**

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

processing in real time the surface image of the stoppers [33].

ization and identification of some defects within the cork stopper.

the lowest porosity as the visible one [21].

**4.4 Cork properties as closures**

high compressibility with dimensional recovery.

The lenticular channels, woody inclusions, and other defects that give the cork surface its typical visual heterogeneity are together considered as the porosity of cork [20, 32]. Nowadays the evaluation of cork quality is made by visual analysis of the outer surface (lateral body surface and tops) using automated image-based inspection systems with high throughput rates based on line-scan cameras and a computer embedded in an industrial sorting machine capable of acquiring and

*Image analysis frames (eight frames in the lateral surface and two tops) and the corresponding cork sections ranging from tangential (A and E) to radial (C and G) sections, with in-between sections (B, D, F, and H).*

The comparison of porosity between the two tops of a cork stopper confirms the existence of axial variation in the tree, for example, one top may have significantly lower porosity than the other top. This fact can be used in practical terms in the production of capsulated natural cork stoppers for spirits by selecting the top with

X-ray tomography was used as a nondestructive technique to acquire knowledge on the internal structure of natural cork stoppers and quantify the lenticular channels present in different classes of cork stoppers [34, 35]. Due to the relationship between X-ray absorption and material density, this technique allowed the visual-

The image resolution with a voxel size of 50 μm achieved by Oliveira et al. [34] allowed the observation of lenticular channel development and geometry (**Figure 5**). The channels are loosely filled with a tissue of rigid unsuberified cells with thick walls, showing ruptures and intercellular voids to a great extent [3]. The region bordering the lenticular channels showed a higher density than the surrounding material due to the presence of lignified and thick-walled cells at their borders.

Cork has an unusual combination of properties: low density, very low permeability to liquids and gases, low conductivity, chemical stability and durability, and

closure in bottles. To a cork stopper, it is requested that (a) it does not allow any leakage from the liquid content either through the stopper itself or at the interface between stopper and bottle; (b) it does not negatively alter the liquid chemical and sensory features; (c) it is durable and preserves its physical and chemical characteristics during storage; and (d) it can be removed from the bottle for consumption easily. To satisfy these requirements, namely, the first requirement of liquid sealing,

These are properties that are fundamental for the application of cork as a sealing

**230**

**Figure 4.**

*Typical 2D "slices" from a 3D grayscale image of a cork stopper scanned at 50 μm resolution. Different sections through the cork stopper in three orthogonal planes: Transversal, radial, and tangential. Crossing lines represent the same point in the three images.*

it is necessary to have an appropriate compression against the bottleneck and close contact between the stopper and the bottle surface to avoid liquid percolation as well as material's impermeability to avoid diffusion through the stopper.

Cork is a viscoelastic material which allows large deformations under compression without fracture, with substantial dimensional recovery when stress is relieved [36]. The stress-strain compression curve of cork is characteristic of a cellular material: it shows an elastic region up to 5% strain, followed by a wide plateau where strong dimensional reductions occur for small stress increases due to the undulation of cells and their collapse, until a densification phase with a strong stress increase but without cell or cell wall fractures.

The compressive behavior of cork is anisotropic although the differences between directions are not very large: the strength in the radial direction is only slightly higher than in axial and tangential directions, which are more similar (**Figure 6**) [30]. This anisotropy in the stress distribution is also noticed in the compression of a stopper with the radial direction corresponding to the maximal stress and the tangential direction to the minimum value [3].

The cork stoppers have a diameter well above the inside diameter of the bottleneck, and when inserted in the bottle, they will be compressed exerting pressure against the bottle glass. The strains in bottled cork average 30%, a value that is in the plateau region of the stress-strain curve corresponding to the physical phenomenon of the buckling of the cells and to stress values in the range of 1 MPa (**Figure 6**). Industry technical guides refer that cork stopper compression must never be more than 33% of their diameter, as there is a risk that this could compromise its elasticity, with loss of part of the recovery and consequent difficulty in the correct sealing of the wine in the bottle.

#### **Figure 6.**

*Average stress-strain curves for the compression of cork in the radial (R) and nonradial (A, T) directions (adapted from [3]). The arrows indicate the compression region of a stopper in different conditions: in the bottling machine, in the bottleneck of a still wine, and a champagne stopper inserted in the bottle neck.*

After insertion in the bottle, there will be a quick stress relaxation [3]. Following compression to 30%, recovery is almost total after approximately 20 days; however, the recovery rate decreases appreciably with time and increases with the degree of deformation previously imposed [37]. The standard (ISO 9727-4:2007) specifies a test method for determining the percentage of diameter recovery of cylindrical cork stoppers, after compression, and specifies that this recovery after 5 minutes shall be greater than 90%.

The extraction force applied in the longitudinal direction of the bottleneck after fixing the cork to a pulling device (usually a corkscrew) depends on the compressive stress against the bottle and on the sliding friction between cork and glass. The extraction force increases with the increase of dimensions of the stopper: a longer stopper will increase the contact surface while a larger diameter stopper will increase the compressive stress [3]. The surface treatments (silicon and paraffin coatings) can reduce the extraction force. It is considered that the extraction force of natural cork stoppers should be 20–40 decanewtons (daN). The test method for the determination of maximum extraction force is specified in ISO 9727-5:2007.

The sealing performance of a cork stopper involves two possibilities for liquid passages: leakage between the stopper and the bottleneck and diffusion through the cork material. In the first possibility, the sealing capacity is evaluated by the penetration of the liquid in the interface, and it is usually measured by applying liquid pressures over the atmospheric pressure and observing the depth of penetration of the liquid [3]. The standard (ISO 9727-6:2007) specifies a test method for determining the liquid tightness of a cylindrical cork stopper: the liquid seal capability is expressed as the maximum internal pressure that the stopper can support in a bottle (at 1.2 bar internal pressure).

On the other hand, a cork stopper absorbs water/wine, and this liquid penetration through the cork is governed by the diffusion of the liquid in contact with the surface of the stopper [3]. González-Adrados et al. [38] evaluated the magnitude and evolution in time of absorption phenomena under conditions as close to reality as possible and described the transport of liquid as a combination of liquid progression by the cork-glass interface and diffusion through cell walls.

**233**

**Figure 7.**

*(adapted from [41]).*

*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

essentially impermeable to atmospheric oxygen [46].

rate from the 3rd to the 12th month and thereafter (**Figure 7**) [49].

**bottled wine aging**

**5. Oxygen transmission rate (OTR) properties of cork stoppers during** 

The oxygen transmission rate (OTR) into the closed bottle is one important parameter for the wine cellars given its relation to the quality development of the wines [4, 39–43]. Therefore, the OTR properties of cork stoppers will define their ability as a quality sealant, also in comparison with other types of wine closures [41, 44, 45]. The OTR behavior depends on the type of wine closure. As shown by Lopes et al. [41] and presented in **Figure 7**, technical stoppers allow the lowest value of OTR into the closed bottle (1.0–1.2 mg of oxygen over 36 months), while synthetic closures present the highest value with an average oxygen ingress of 1.6 mg of oxygen in the first month. The path of oxygen ingress into the bottle was also experimentally studied by Lopes et al. [46]: the oxygen coming into the bottle and the wine during the storage period originates from the cork stopper itself, that is, from its macroscopic and cellular structure and not from an interface flow. In fact, the closed cells of cork contain air-filled lumens while lenticular channels or other tissue voids may provide additional air-filled pockets [26]. The cork itself has very low permeability to oxygen [47, 48], and correspondingly, the cork stoppers are

The kinetics of oxygen ingress into the bottle could be adjusted to logarithmic models, with an initial high ingress rate, followed by a decreasing ingress rates during the 1st month and further on, until stabilizing a low and rather constant ingress

As primarily suggested by Ribéreau-Gayon [50], oxygen ingress into bottles occurs mainly out of the cork structure due to the high internal pressure in the cork cells created when the cork stoppers are compressed into the bottleneck. Natural cork stoppers with 24 mm diameter and 45 mm length have a volume of 20.4 mL of which 80–85% is air contained in the cell lumen, implying the existence of 4.9–5.2 mg of oxygen within their structure [3]. Oliveira et al. [49] showed that, in average, 1.88 and 2.35 mg of oxygen diffuses from the natural cork stoppers that

*Kinetics of oxygen ingress through different closures (natural cork stoppers of "flor" and extra quality grade, microagglomerated cork stoppers, 1 + 1 technical stoppers, and Nomacorc synthetic stoppers) into commercial bottles stored horizontally over 36 months. Error bars represent the standard deviation of four replicates* 

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

After insertion in the bottle, there will be a quick stress relaxation [3]. Following compression to 30%, recovery is almost total after approximately 20 days; however, the recovery rate decreases appreciably with time and increases with the degree of deformation previously imposed [37]. The standard (ISO 9727-4:2007) specifies a test method for determining the percentage of diameter recovery of cylindrical cork stoppers, after compression, and specifies that this recovery after 5 minutes shall be

*Average stress-strain curves for the compression of cork in the radial (R) and nonradial (A, T) directions (adapted from [3]). The arrows indicate the compression region of a stopper in different conditions: in the bottling machine, in the bottleneck of a still wine, and a champagne stopper inserted in the bottle neck.*

The extraction force applied in the longitudinal direction of the bottleneck after fixing the cork to a pulling device (usually a corkscrew) depends on the compressive stress against the bottle and on the sliding friction between cork and glass. The extraction force increases with the increase of dimensions of the stopper: a longer stopper will increase the contact surface while a larger diameter stopper will increase the compressive stress [3]. The surface treatments (silicon and paraffin coatings) can reduce the extraction force. It is considered that the extraction force of natural cork stoppers should be 20–40 decanewtons (daN). The test method for the determination of maximum extraction force is specified

The sealing performance of a cork stopper involves two possibilities for liquid passages: leakage between the stopper and the bottleneck and diffusion through the cork material. In the first possibility, the sealing capacity is evaluated by the penetration of the liquid in the interface, and it is usually measured by applying liquid pressures over the atmospheric pressure and observing the depth of penetration of the liquid [3]. The standard (ISO 9727-6:2007) specifies a test method for determining the liquid tightness of a cylindrical cork stopper: the liquid seal capability is expressed as the maximum internal pressure that the stopper can support in a bottle

On the other hand, a cork stopper absorbs water/wine, and this liquid penetration through the cork is governed by the diffusion of the liquid in contact with the surface of the stopper [3]. González-Adrados et al. [38] evaluated the magnitude and evolution in time of absorption phenomena under conditions as close to reality as possible and described the transport of liquid as a combination of liquid progres-

sion by the cork-glass interface and diffusion through cell walls.

**232**

greater than 90%.

**Figure 6.**

in ISO 9727-5:2007.

(at 1.2 bar internal pressure).
