1. Gas measurement

A NomaSense O₂ P6000 and a Pst 3 sensor (Nomacorc, USA) were used to monitor the evolution of oxygen concentration in the tank and the barrel. The measurement range was 0-100 % O₂, and the detection limit was 0.03 % O₂ (according to the manufacturer). For the oxygen compensation measurements, the system unit was stored inside the tank in order to record the internal temperature and pressure at the same time. The probe was placed at bung height in the same place for all the experiments. The system was calibrated using CO₂ (4.5 grade, Linde AG, Germany) and atmospheric air at 20.95 % O₂. During the experiment, the measurements were automated and recorded every 10 min.

2. Hermetic tanks

Two stainless steel hermetic tanks tailored to contain a barrel were manufactured by BELLOT Company (Gradignan, France). A door at one end of the tank and sealed with elastomer joints and vacuum PTFE grease gave access to the barrel inside. A small trap door was located on the top of the tank for CO₂, O₂ and air injection. Figure 1 shows the tanks in opened and closed positions. The total volume of a tank was 1522 L (according to the manufacturer).

Figure 1. Schematic representation of the stainless-steel tanks in open (loading position) and closed (ready to assay OTR) positions.

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3. Barrel selection

A total of 10 brand-new 225 L classic Bordeaux barrels with 6 hoops and made out of French oak wood (Quercus petraea) were provided by Seguin Moreau Cooperage (Cognac, France). Each barrel had undergone a water pressure test by the manufacturer to ensure the absence of leakage. They were delivered with a silicone bung of 50 mm in diameter. Table 2 gives detailed information about the barrels as provided by the cooper. The external volume of each barrel selected for the study was calculated using parabolic formula; the volumes varied from 283 to 287 L depending on the stave thickness of the barrels. The average weight of a standard 21 mm barrel was 38.5 kg at delivery. Before each assay, the external surface of the oak barrels was cleaned with absolute ethanol in order to prevent the development of mould during the experiment in the closed tank.

4. Barrel hydration protocols

In order to evaluate the effect of hydration protocols on OTR level, a different protocol was followed for each of three barrels of identical stave thickness and toasting level: 1) The bottom of Barrel 6 was filled with 20 L of cold water (15 °C) and then closed with the bung; the barrel was stored vertically for 12 hr then turned upside down and left for another 12 hr, 2) Barrel 5 was completely filled (225 L) with cold water (15 °C) and then the bung hole was closed with a bung; the full barrel was left for 48 hr and then emptied, and 3) Barrel 7 was hydrated for 12 and 22 min using a vapour generator (Vapo-Clean from R-tech Solutions, France) capable of producing 30 kg/h of vapour; the vapour was released inside the barrel through a stainless-steel stick output. All the remaining barrels were simply filled with cold water during the desired time in order to reach different moisture contents. They were then weighed before the OTR measurement.

5. Moisture content calculation

Wood is a hygroscopic material; i.e., wood gains or loses moisture from the air and/or the liquid depending on the conditions of the surrounding environment. Water can migrate into wood in three ways: i) as a fluid through the cell lumens via capillary tension, ii) as vapour through the cell lumens, and iii) via molecular diffusion through the cell walls. The moisture content of wood can be calculated from the relationship between the mass of water in the wood and the mass of the wood without the water. As the absolute dry mass of a barrel remains unknown, we calculated a relative variation of moisture content (ΔMC; Eq.1), based on the difference between the actual barrel mass (m_h) and the lowest barrel mass (m_i).

(Equation 1)            $\Delta MC\left(\text{\%}\right)=\frac{m_h-m_i}{m_i}*100$

6. Diffusion coefficient and OTR calculation

As soon as they come into contact with liquid, extractable oak wood compounds, like lignanes, coumarines, ellagitanins and lactones, progressively dissolve in the liquid phase. As a consequence, when using an oxygen dipping probe in the liquid, the total oxygen ingress evaluation could be distorted by the consumption of oxygen by these compounds (García-Estévez et al., 2017; Pons et al., 2014; Pascual et al., 2017), as well as by the dynamic evolution of the barrel moisture content during the measurement. In the present study, this was overcome by working in the gas/gas phase for barrel characterisation, as is already widely-applied in research in oenology for the monitoring of gas/gas OTR of stoppers. A barrel-tank characterisation system equivalent to a membrane characterisation system, with two individual spaces separated by oak wood of the barrel (Figure 2), was used in this study.

Figure 2. Schematic representation of the barrel-tank characterisation system, with the oxygen concentration gradient at a given time between the barrel saturated with O2 and the vat saturated with CO2 (d = thickness of the stave).

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The initial trials were conducted with new oak barrels filled with CO₂ (< 1 % O₂) and surrounded by atmospheric air (21 % O₂), but the time required to reach a steady state to perform one measurement was 20 days and was thus not suitable to study multiple barrels. This parameter was excessively long due to the relatively low oxygen concentration gradient and to the low permeability of oak wood to oxygen. Therefore, in order to decrease the time required for each experiment, the barrels were filled with air saturated by approximately 90 % oxygen, then closed tightly with a silicone bung before storage in the middle of the tank. Once the tank was closed, it was filled with CO₂ (O₂ dropped to approximately 3 % saturation). A high oxygen gradient concentration was thereby created between the inside of the barrel and the inside of the tank, increasing the flow of oxygen through the wood and allowing the diffusion coefficient to be determined within 4 to 7 days. In this experiment, the diffusion of oxygen occurred from the inside to the outside of the barrel; it was important to measure the quantity of oxygen passing through the barrel without modifying the integrity of the barrel by passing probes through the wood.

The permeation of gases through the wood with a thickness d (m) was calculated with an equation (eq.2) based on Fick's first law (Ruiz de Adana et al., 2005; Sorz and Hietz, 2006):

(Equation 2)            $V_t\frac{d{x}_t}{dt}=\frac{A·D}{d}\left(x_{final}-x_t\right)$

where V_t . dx/dt is the volumetric flow of gas transported per unit time (m³/s) from the inside of the barrel (volume V_b) to the outside (volume V_t). This is proportional to the oxygen fraction gradient (Δx = x_final - x_t, where x_final is the oxygen fraction at equilibrium and x_t the oxygen fraction recorded over time) from the outside of the barrel to its inner surface A (m²). The constant D is the diffusion coefficient (m²/s). In such a closed system, the principle of mass conservation applies and the final concentration (at equilibrium) can be calculated as follows (eq.3):

(Equation 3)            $x_t{·V}_t+x_b·V_b=x_f\left(V_t+V_b\right)$

Equations 2 and 3 were merged (eq.4) and solved by using boundary conditions (eq.5):

(Equation 4)            ${\int }_{x_t^0}^{x_f}\frac{d{x}_t}{x_f-x_t}=\beta ·D·{\int }_0^{t}dt$

(Equation 5)            $f\left(x_t\right)=ln\left(\frac{x_t-x_f}{x_t^0-x_f}\right)=-\beta ·D·t$With $\beta =\frac{A}{d·V_t}\left(1+\frac{V_t}{V_b}\right)$

The diffusion coefficient of barrel D was obtained from the experimental data x_t = f(t) plotted according to Equation 5. This linearisation method allowed D to be calculated from the experimental data acquired in a short time.

From the obtained coefficient D, it was possible to estimate the Oxygen Transfer Rate (OTR) of oxygen passing from atmospheric air to the wine via wood of the oak barrel. The concentration of dissolved oxygen inside the barrel was assumed to be close to 0 mg/L (the oxygen consumption rate in the wine is faster than the oxygen ingress rate), and the dissolved oxygen concentration at the interface between air and wet wood to be proportional to the partial pressure of oxygen in the atmospheric air, according to Henry's law. However, the saturated concentration of oxygen in wine depends on multiple parameters, such as temperature, type of wine or alcohol content. In this study, an oxygen concentration of 7.4 mg/L was used for all the calculations (with wine at 19 °C), as used by Devatine et al. (2007). It was possible to estimate the OTR by inserting the experimental diffusion coefficient into equation 2 (eq.6):

(Equation 6)            $OTR=D·\frac{A·7.4}{d·V_b}$

1. Gas tightness test

The gas tightness of stainless-steel tanks is of primary importance for the development of a gas/gas transfer method. In this study, two tanks were submitted to a gas tightness test before use. With the main end door closed, CO₂ was injected through the top hole until the concentration of O₂ inside the tank had decreased from around 21 % to lower than 5 %. The trap door of the top hole was then closed. The concentration of O₂ was monitored for three weeks. For both tanks, the gas tightness was calculated to be lower than < 0.001 %/day. This very low transfer enabled the gas exchange in each barrel to be accurately monitored.

2. Initial diffusion coefficient of brand-new barrels

The diffusion coefficient of 10 brand new barrels - described in Table 1 - was measured using the tank system described earlier. An example of data processing is given in Figure 3, where the function of oxygen fraction in the tank (x_t) is plotted against time, according to equation 5. In this case, a high coefficient of determination (R² = 0.999) was obtained for a diffusion coefficient of 1.94x10^-9 m²/s.

Figure 3. Example of experimental data linearisation f(xt)/-β = f(t). The slope represents the diffusion coefficient D in m²/s.

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Figure 4. Example of the diffusion coefficients of 10 new oak barrels (Q. sessilis).

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The initial D values of the 10 barrels ranged from 3∙10^-10 to 2∙10^-9 m²/s (Figure 4) with a good repeatability in the two measurements for each barrel. These results show that the D-values of barrels can be very heterogenous when the barrels are used for the first time. The main factor that could explain this range is wood moisture content, as similarly reported for oak barrels by Nevares et al. (2014) and for corks by Fonseca et al. (2013). Indeed, some barrels were stored in our lab for weeks before D measurement, while others were characterised immediately at delivery. This result indicates the importance of barrel moisture content before use in order to ensure the homogeneity of the diffusion coefficient.

3. Influence of barrel preparation on diffusion coefficient

Oak wood, like many natural products, is considered to be a hygroscopic material: it naturally absorbs or releases moisture in the form of water vapour from or to the air. Empirically, an expression of this phenomenon – which is well known by wine makers – is that when empty oak wood barrels are stored for a long time, they lose their watertightness (and thus start leaking), and they thus need to be filled with water before use. Coopers deliver finished barrels as quickly as possible, and the winemakers are advised to do barrelling at their earliest convenience after reception.

Coopers usually also recommend that the brand-new barrels be soaked before being filled with wine. The main reason is to swell the wood and thus avoid any leakage of liquid by forming a tight seal. Another reason is to remove residual sawdust and the harshest of the tannins. The manufacturers recommend several ways to soak the barrels; for example, filling them with a few litres of hot water and bunging them for two hours; actively rinsing for five minutes with hot water; filling them with cold water and leaving for 24 or 48 hours; or filling with hot water and leaving for 24 hours. All these processes have different cost in terms of water consumption and labour, and their impact on the diffusion coefficient is not precisely described in literature.

In this experiment, each of three barrels (21 mm thickness) was prepared following one of the following procedures generally recommended by coopers and described earlier in the barrel hydration protocols section. For each treatment, the water consumption, the relative humidity percentage corresponding to the increase in barrel mass after the hydration process (ΔMC) and diffusion coefficient (D) were determined after each treatment, as summarised in Table 3. As can be seen, the diffusion coefficients of the three barrels were reduced by a factor of 10 compared to the corresponding control barrels. Water consumption is shown to be equivalent when comparing the vapour and 20 L hydration treatments, but with vapour treatment reducing the preparation time from 24 h to 12 min. The diffusion coefficient of the 48 hours whole barrel treatment was found to be similar to a 22 min vapour treatment, but with the latter consuming only 11 L of water instead of the 225 L consumed in the former. As a consequence, vapour treatment seems to be a great alternative for winemakers in terms of water and time saving.

4. Impact of the oak barrel moisture content on the evolution of the diffusion coefficient

During the previously described experiment, we observed that the diffusion coefficient decrease was concomitant with a relative mass increase. Therefore, in order to evaluate the relationship between the moisture content and the diffusion coefficient of a barrel more precisely, the diffusion coefficient of an oak barrel (B1) was evaluated at different levels of moisture content (Figure 5).

Figure 5. Evolution of the diffusion coefficient D of oak barrel B1 according to the relative increase in moisture content (ΔMC).

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As reported by other authors (del Alamo-Sanza and Nevares, 2014; Sorz and Hietz, 2006; Vivas et al., 2003), this trend indicates that the moisture content of wood has a fundamental role in the oxygen diffusion coefficient of a barrel.

In order to further support our observation, we extended our experiment to include nine more barrels from our selection. These were weighed and monitored at two different moisture contents for different diffusion-coefficients (initial “i” and hydrated “h”). As the absolute humidity content (not measured in this study) of the barrels would have been required to compare them, the ratio of the two diffusion-coefficients (initial and hydrated) for each of the nine barrels was calculated and plotted against the corresponding ΔMC (Figure 6).

Figure 6. Ratios of initial (Di) and hydrated (Dh) diffusion-coefficients of 9 new oak barrels ●(B1) ◇ (B2) ■ (B3) ○(B5) ✱(B4) ☐ (B6) ◆ (B8) ▲(B9) △(B10) against their corresponding ΔMC.

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Regardless of the thickness of the staves, the Dh/Di ratio of all the barrels seems to be highly correlated to ΔMC through an exponential function. Such behaviour has been observed for conifers by Sorz and Hietz (2006), even though they found a more linear relationship for Quercus wood. However, in this study, the measurement is based on an entire wood barrel, and the exponential decrease in diffusion based on moisture content can be explained by a combination of multiple phenomena occurring simultaneously. First, when a barrel is filled, a physical expansion of the wood caused by the absorption of liquid occurs. As a consequence, the pressure between the staves may increase, thus decreasing oxygen transfer (Qiu et al., 2018). In parallel, the increase in MC during hydration is due to the advance of the moisture from the inside of the barrel (which is in contact with the liquid) to the outside of the barrel (which is in contact with the air). As described by Singleton (1995), the liquid saturates the cells and vessels of the wood in a free fluid state. Progressively and towards the outside, the free liquid decreases and the gas content of the lumens increases. At a certain point, there is no more free liquid and the lumens are filled with gas and vapor only. The more waterlogged the wood is, the lower the diffusion coefficient. Ruiz de Adana et al. (2005) established a model to describe this phenomenon and calculated a diffusion coefficient of 1.05x10^-11 m²/s for the internal side of the stave and 1.33x10^-8 m²/s for the external side. These results correspond well to the measured global coefficients in the present study (from 2x10^-10 m²/s for the “hydrated” state to 3x10^-9 m²/s for the “dry” state). However, the diffusion coefficient of oxygen is known to be 2x10^-5 and 2x10^-9 m²/s in air and water respectively (Sorz and Hietz, 2006). In water only and in dry wood, the two oxygen diffusion coefficients were found to be rather similar, making it impossible to determine which one is limiting the diffusion. In terms of hydrated wood, there is a difference in an order of magnitude of 2 or 3 between the two diffusion coefficients, meaning that the interactions between cell walls and water are the main limiting diffusion factors in this case.

Figure 7. Evolution of the relative moisture content (ΔMC) over hydration time when barrel B1 was completely filled with water.

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Figure 7 shows the evolution of ΔMC in barrel B1 over hydration time when completely filled with water. The moisture content shows a high correlation with hydration time (R² = 0.986) through a derivation of the Lucas-Washburn equation (Fries & Dreyer, 2008; Zhmud et al., 2000) which originally described the penetration depth of water according to the time in porous media by capillarity. In accordance with the latest (at the time of writing) results in the literature (Roussey et al., 2021), this result confirm that barrels need around 20 to 40 days to enter into a pseudo steady state in terms of moisture content, and as a consequence, in terms of OTR.

5. Total dissolved oxygen intake

In order to evaluate the annual dissolved oxygen intake using the new method described in this study, an example is given of a calculation based on the results obtained from Barrel 1. By using the results obtained from Figures 5 and 6, it is possible to calculate the diffusion coefficient according to the time since filling. This coefficient can be used to evaluate an OTR in mg/L.day (eq. 6) through the barrel during wine aging.

Figure 8. Evolution of the barrel OTR (µg/L.day) during aging over time since filling.

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When applying the trapezoidal rule, an integration of the curve representing this OTR over time since filling (Figure 8) results in a total OTR of 11.4 mg/L per year. Moreover, if the volume of air contained in wood pores is assumed to be completely desorbed in the wine as it impregnates the barrel, the amount of desorbed oxygen per litre of wine (m_dO2/L) after barrel filling can be calculated as follows:

$m_{dO2/L}=\frac{∆MC\cdot m_{barrel}{\cdot M}_{O2}\cdot \phi \cdot P_{O2air}}{\rho \cdot V_b\cdot R\cdot T}$

where m_barrel is the mass of the barrel, φ the wood porosity, M_O2 the molar mass of molecular oxygen, P_O2air the partial pressure of oxygen in the atmosphere, ρ the density of wine, V_b the volume of wine in the barrel, R the ideal gas constant and T the temperature. Under the experimental conditions of the present study, barrel B1 (with a theoretical porosity of 0.6 and a ΔMC of up to 4 kg, and under atmospheric pressure at 15°C) desorbed a maximum of 3 mg of oxygen per litre of wine around 30 days after barrel filling. Taking into account the calculated OTR and the estimated amount of oxygen desorbed from the wood pores, the total intake of oxygen can be estimated as being 14.4 mg/L during the first year. This means that around 21 % of the oxygen is released as a result of wood porosity. Similar results have recently been observed by Prat-García et al. (2020) with an average dissolved oxygen release of 1.3 to 1.8 mg/L via wood porosity nine days after barrel filling, and an average OTR of 14.4 mg/L per year for commercial barrels, leading to a total intake of 16 mg/L of oxygen in the first year.