The maturing of Bordeaux wines in barrels

          In the traditional wine-making process, we distinguish vinification, which corresponds to the transformation of the grape into wine, from maturation, which includes the phases of work, treatment and transformation. It is a break in the life of the wine with a first phase essentially microbiological, secondarily chemical and a second phase mainly chemical and very slightly microbiological. This is an almost clear-cut split that has the advantage of being convenient. Moreover, oenology teaching and treatises make abundant use of this distinction. In reality, things are not so simple, but the adoption of this mode of presentation is justified by the basic sciences to which each stage refers.

 

          Wine maturation cannot be represented as a global and homogeneous oenological concept. It includes numerous operations and auxiliary treatments. The goals are often very different and the associated constraints are specific. When we hear the term “élevage”, we wrongly and unconsciously think of a dark and cool cellar where rows of barrels are placed. This is an image that only reflects one of the possible ways of maturing. The availability, by the wine industry, of more specific products than in the past, has allowed to greatly diversify the practices. We know the possibilities offered by micro-oxygenation; there is also the tannic reprofiling of the wine thanks to the tannins of the skins and pips, the yeast derivatives mimicking the effects of the lees and finally, the vast field of application allowed by the woodage products (pieces of wood, chips, dust, liquid and concentrate) which are regularly used in most of the world’s vineyards. Everything or almost everything has been tried to reproduce the evolution of wines during traditional maturation, in particular the cumulative effects of the temperature-aeration couple. An example is the aging of Rivesaltes which, after mutage with alcohol, undergoes a stage of conservation in glass jars, exposed outside to temperature variations and light. The heating and the air stirring of Madeira wines are part of the same postulate. We could multiply endlessly the evocation of these regional products. Currently, the stabilization and preparation of wines for consumption require operations that respect the fruity character of the wines, their freshness and clarity. We are therefore moving towards gentler transformations under the effect of practices whose intensity and duration can be controlled. Whatever the technique chosen, it always responds to three basic configurations: either a treatment under oxidative conditions, or reducing conditions, or even controlled oxidations under reducing protection.

          Depending on whether one is more interested in the general freshness or in the evolutionary aromas and woody/heated flavors, the maturation does not follow exactly the same path. In the past, the maturation had to be long to favor the evolution of the wine’s characteristics. The color had to be tiled rather than red or blue/purple and the so-called vinous aromas had to be replaced by more oxidized, cooked, and candied notes. The wines were more acidic and more tannic due to the fact that they were just ripe, sometimes early, and that some or all of the stalks were present. But this was in line with the physico-chemical constraints of the wines of the time; because, in fact, at lower pH levels, the oxidizability and evolution capacity of the wines are largely slowed down, with the direct effect of extending the duration of the maturation and conservation in bottles. Currently, we are at the opposite of this observation. Firstly, because the grapes are harvested much more ripe, therefore less rich in seed tannins, systematically destemmed and with a higher pH, the wines produced are naturally rounder and more supple. Secondly, the consumer is looking for beautiful, deep, dark colors as opposed to the evolved, faded hues of the past. Finally, aromatic clarity and in particular the smell of fresh fruit are almost mandatory. We are then in an environment where the respect of the product and its original typicity is a priority. It is easy to understand that the choices of maturation, or more generally of the work of the wines, are different. The wines are not always entrusted to the good care of the producers; sometimes they are collected and then transported to trading units or cooperatives, whose aim is to design and market branded wines. On a global scale, this is a very large volume of wine, with a relatively small number of players. One of the first and most acute concerns is transportation, which must be carried out in compliance with a few elementary rules. We know, at least by experience, that a bad transport can make lose a big part of the initial qualities of a selected wine. On arrival, or several days later, the product depreciates and no longer resembles the one initially selected. Oxygen, heat, the volume of filling of the tanks and the stirring undergone by the wine on the road are the main causes. In this case, the respect of rigorous specifications, including the measurement of dissolved oxygen, represents an indispensable procedure. On these wines, the first operations consist in homogenizing their state of stability, in particular the cloudiness and the tartaric deposits for the reds, in addition to the risks of proteinic breaks for the whites. Then, the wines are assembled to form a merchant product, which can undergo some operations of breeding or associated treatments. These wines are often poorly aerated, and may undergo colloidal additions in the form of partially lysed yeast or gum arabic, the aim being to improve the taste of the wine. Acidification, as long as it remains within the legal limits, is a valuable tool to decrease the sensitivity of the wine and increase its conservation capacity, through its effect on lowering the pH value. But it is also possible to proceed differently. In this case, each wine is brought to its optimum by suitable treatments and perfectly adapted maturation, and then the blending will take place only a few weeks before bottling. This method has the major disadvantage of not being very suitable for large production units that need flexibility and simplification of technological processes, but it is more respectful of the diversity of the wines. In the case of technological red wines from, for example, continuous work processes such as thermovinification, maturation is limited to stabilizing the wines. In these products, the colloidal coloring matter constitutes an important fraction of the color; it is very sensitive to temperature variations and can precipitate at any time, with color losses representing 15 to 30% of the color intensity of the wine. In this case, we are satisfied with a short aging, reducing to keep the maximum color and fruit aromas, and cold treatment to obtain stable products. Sometimes, to improve the roundness and suppleness of the wines, it is possible to add polysaccharides in various forms (fresh lees, partially lysed yeast or gum arabic). Among the so-called technological wines, there are those obtained by forced extraction processes during the vinification (rotating tanks, sequential pumping-over tanks, tanks incorporating devices for destructuring the cap by gas or liquid movements,…). If the process is properly applied to good grapes, the wines are full-bodied, balanced and particularly fruity. These wines are often – but not always – produced from short vatting periods (only a few days), so that their polyphenolic profiles require a stabilization phase. They are destined for a low oxidative aging but carried out as close as possible to the end of the fermentation process (alcoholic and malolactic). Sometimes, the use of commercial tannins from grape seeds favors early stabilization of the coloring matter. For certain ranges of wines, the addition of wood is a necessary complement before being put on the market. Moreover, it is curious to note that these wines are called “woody” when in reality they are almost exclusively “toasted” wines. The aroma conferred to the wines is either brought by a traditional aging in oak barrels, with all that this requires in terms of additional work, or it is the result of wood-aging processes using various wood-based treatments or more or less transformed extracts. Barrel aging is a particular way of aging, reserved for a minority of products. On the other hand, wood treatment (infusion, addition of extracts) is much more generalized and is justified by the lower selling prices of the wines. Alongside these relatively common methods of aging, there are more specific methods often represented in a particular wine region. This is the case, for example, for the maturation of sherry wines (Spain), tawny wines (Portugal) or straw wines (French Jura). They all have in common that they are oxidative maturings: either by chemical oxidation such as Portuguese tawny wines, or by microbiological oxidation such as sherry and straw wines. For these three cases, maturing in barrels is the rule, but the wood must only act through the oxidations allowed by its porous structure; any woody contribution is forbidden. In fact, having experienced this, Tawny or vintage Ports that show the slightest perceptible trace of vanilla, toast or woody notes are systematically rejected during blind tastings. This is why, for these products, the use of five year old barrels or more has become the norm.

 

 

          After several centuries, maturing in barrels is still the preferred way to perfect the qualities of a wine and ensure its stability over time. It is even for the greatest wines an obligatory step. However, aging in wood does not mean the massive use of new wood. In the past, barrels were only replaced when they became irreparable. The high percentage of new wood in the maturing process is a relatively recent practice compared to the long history of barrel use. There was an exception, however, for wines shipped in barrels to the United Kingdom, for example. This was 100% new wood, as the shipping barrels were almost never shipped back empty, as the cost of transport made no economic sense.
We deliberately use the term “barrels” rather than “casks” or “pieces”, which have a certain regional connotation and a specific capacity. Even if today, all over the world, the term “barrel” is used to refer to wine casks. For spirits and brandies, the terms used are more variable.

 

          The maturation process causes profound changes in the constitution and character of the wine. Barrels in general, and barrels in particular, represent a widely used tool in the premium wine range in many countries. In traditional wine regions, such as France, Spain, Italy and even Germany and Greece, wines are frequently aged for several months in barrels of varying ages. While in terms of volume, these wines represent only a small percentage of the world market, in economic terms, they are the majority in the world wine trade. Moreover, in the imagination of a good number of consumers, barrels and wine quality are inseparable, even if sometimes and rather contradictorily, the excessive “taste” of wood is decried.

 

          Wooden barrels are often made from American, French or Eastern European white oak, more rarely from acacia, cherry or eucalyptus. In all cases, the container acts in several ways to allow the evolution of the wine and its transformation. It is at the same time refining, stabilizing, clarifying, aromatizing, softening, integrating, revealing and stripping; but these are only the major functions because barrel aging has many other effects on the wines. To situate the aging in wood in the oenological process, we can compare the same wine at the end of the tank and after its complete aging. The exercise is very demonstrative. When the wine comes out of the vat, after a more or less long vatting period, the wine is in the process of becoming; the tasting is prospective, made of predictions, evolution and anticipation of what the wine will be. At this stage, during the tasting, even a well-informed consumer can be worried and disappointed. However, behind a wall of youth, rusticity sometimes, reduction often, hides the finished wine which impresses by its expression, its elegance, its length and its potential of aging. After a few months to a year, sometimes more, the tasting of the same wine gives an immediate pleasure: it is then ready to be drunk. Aging in general and wood aging in particular are stages that allow the wine to acquire its drinkability or, to put it simply, to pass from the stage of a beautiful potential to a consumable and commercial product. In a way, through these transformations, the wine gains qualities that it does not yet have and acquires a market value, a selling price. The issue is therefore not secondary, nor incidental. When taking charge of the wine for its maturation, one must always keep in mind the weight of the consequences that will be associated with all the technical decisions until the bottling.

         

          The use of wooden containers for maturing is essentially a choice dictated by the potential quality of the original grapes and by the possibility of selling the finished wine at a sufficiently high price. Beyond the speeches heard, it is therefore the economic spring that determines the strategy of wine maturation. We cannot omit the role of tradition, which remains for certain wine-producing regions in the world, a strong driving force, justifying the recourse to wooden containers of different capacities. In other cases, the motivation is quite different and it is a question of making a particular selection of a limited volume of wine using the barrel, both to produce an exceptional wine and to position it as a showcase for the estate and the rest of the existing range of wines. Aging in wood is therefore at least as much a technical necessity as a vector of image and communication. It is also a costly practice in terms of time and manpower. Generally speaking, the use of wood requires careful and repeated care which makes it a demanding practice. The aging under wood requires relatively long periods of evolution. Each time we want to shorten this time, it is to the detriment of the possibilities offered by this method of maturation, in particular stabilization and integration, two functions that require time to be given. Without this, the use of faster and more flexible woodworking methods is both more efficient and more satisfying.

          In the strategy of wood aging, the renewal of barrels is an important parameter to take into account: on the one hand, because the impact of the transformations induced by the barrels remains a function of their duration of use, and on the other hand, because the level of risk of depreciation increases beyond what is reasonable over a long duration of use (six years and more). This is particularly true for the dryness and hardness characteristics linked to the higher solubilization from year to year of the sulfates resulting from the repeated sulfiting of the barrels. This is also the case for deviations or the appearance of false tastes, or a loss of aromatic clarity in the wines. If the average age of the barrel in a winery becomes very high (ten years, sometimes more), the choice of this method of maturation is no longer justified. The interest of wood is only conceived as an improver and not as a container, because if it is an excellent tool for maturing, it is, on the contrary, a bad container for conservation. We must not forget that wood is not a smooth and homogeneous material, it is porous and has many dead zones where dirt and potentially contaminating micro-organisms can accumulate. Cleaning such a material is never complete. A rigorous quality control or a continuous monitoring of the batches of wines under wood is mandatory to limit the risk factor to a minimum.

 

          The wood, as an aromatization, cannot be forgotten. The barrel, especially new, inevitably brings woody notes and especially toast to the wines. Its use integrates this important aspect. However, the success of maturing in wood depends on the integration of the additional aromatic notes brought by the wood and toasting and not on the superimposition of sometimes dominant aromas. The management of aromatization in a wood aging process must rather aim at making the part of the wood and the part of the wine and/or the grapes disappear in an overall impression. The integration function, which we mentioned earlier, aims to achieve a blended sensation for which it becomes difficult to estimate the part of the barrel.

 

            

          Controlled oxidation is a very particular notion that is put forward to talk about specific oxidations during barrel aging. It corresponds to the modifications induced in the wine by the slow and continuous penetration of oxygen through the wood and all the interstices formed by the parts that make up the container (head joints, joints between staves, bungs). In the literature, reference is often made to controlled oxidation without giving a precise definition and framework; it is more a notion than a technological concept. However, it is a mode of oxidative evolution, subordinated to the maturation in wood; this is why its characterization and definition are an essential prerequisite. Compared to a maturation in vats associated with traditional aeration or the addition of oxygen by various processes, the barrel alone allows a slow but continuous penetration of oxygen by the phenomenon of gas/liquid transfer through the wood. We have shown the reality of oxygen diffusion through dry wood. On the other hand, except for the “wood grain” parameter, the humidity of the fibers limits the diffusion speed, since from 20 to 100 ppm of oxygen/h on dry wood, we pass to only a few ppm/h on wood in equilibrium with the ambient humidity. However, these values, carried over the total duration of an aging period of several weeks to several months, are very sufficient to ensure the oxidative evolution of the wine. In the same spirit, the wood grain (syn. tree growth speed) acts by increasing the diffusion speed in important proportions, for example by a factor of 2 to 4 for a tight grain (1-1.5 mm/year) compared to a medium grain (2-3 mm/year) and a coarse grain (4-6 mm/year). This is a decisive step in the understanding of oxidative equilibrium in wines under wood, since in the past, some authors have denied the role of oxygenation in barrels based on the fact that a wood sample placed in wine or water under reasonable pressure does not soak and confusing the notions of permeability and porosity, which are very different in their principles and determinisms. Beyond this aspect, the hermetically sealed barrel favors in a few days a depression of about 100 to 150 mBar (in ten to fifteen days). The mechanism is simple: ambient air enters the barrel progressively, oxygen is then consumed more or less quickly, nitrogen remains, concentrates, in proportion to the oxygen which decreases and causes a difference in composition between the internal atmosphere of the barrel (a few percent oxygen for more than 80% nitrogen and CO₂) and the external air (19% oxygen for 80% nitrogen); a depression is created which favors a forced penetration of the external air. The mechanism can be compared to a pumping effect from the outside to the inside of the barrel. The tightness of the closure, the temperature and the capacity of the wine to consume oxygen can favor the mechanism. On the other hand, repeated opening of the bung during topping up delays the formation of this depression and cancels the pumping mechanism. The atmosphere on top of the wine in the barrel is composed of nitrogen and CO₂ and can be compared to a natural inerting mode. Finally, in the barrel and more particularly in the new barrel, the controlled oxidation takes place in the presence of the ellagitannins of the wood. Their role is complex since they are both gas pedals of oxidation by consumption of oxygen and production of ethanal, and regulators by their ability to capture free radicals and various activated forms of oxygen.

          The aging in wood over a period of several months up to one or two years allows the progressive fall of the particles forming the cloudiness of young wines. This is very true for the largest particles, but sometimes the clarity of the wine is not perfect and requires additional operations to finish the clarification; this is one of the purposes of the fining of wines and the finishing filtration. The attenuated variations of seasonal temperatures improve the obtaining of a good clearness. Thus, the periods of cold winter and relative heat in summer participate in the instability of the cloudiness and its deposit. In new barrels, the clarification is generally more complete. This phenomenon is explained on the one hand, by the influence of ellagitanins and the walls of the barrels acting as a fining agent and as an adsorbent, and on the other hand, by the better decarbonication of the wine which facilitates the sedimentation of the deposits in suspension. In wines, there are different factors causing the formation of cloudiness. First of all, the fermentative phenomena, by the multiplication of yeasts of the alcoholic fermentation and bacteria of the malolactic fermentation, give important milky disorders in the new wine. Knowing that yeasts are larger than bacteria, the sedimentation capacity will be different: yeasts of a few µm deposit faster than bacteria of about 0.1 to 0.01 µm. Then, the plant debris from the handling and crushing of the harvest forms hydrated particles, more or less mucilaginous, which deposit quite easily, at least after the alcoholic fermentation has stopped, whose gaseous release is a hindrance to the deposition of lees of any kind. Then, the technological and environmental factors act either as facilitator of the sedimentation of the disorders and deposit or on the contrary by supporting the resuspension or by slowing down the rhythm of the clarification. Globally, all the manipulations of the wine favor the resuspension of a fraction of the disorders accumulated in the lees. The more recent the lees are, the easier the resuspension is, contrary to lees that are a few months old, probably more compacted, which are less sensitive to the various movements the wine can undergo. In the laboratory, the mechanism is easy to visualize. We take a sample of fresh lees and another one of four to six months old lees; we wait in the cooler for the sedimentation of the cloudiness, then on a stirring plate at minimum speed, we observe the behavior of the deposit. It appears from the beginning that the fresh lees form a milky cloud in a few minutes; while the modality containing aged lees remains almost clear over the same period of time, the cloud is slowly suspended again. The interpretation is to be sought in the composition of the lees; the younger ones contain more proteins and soluble polysaccharides and the older ones have a low soluble fraction compared to an insoluble fraction of yeast wall residues and plant tissues of variable size. The autolytic phenomenon and the solubilization in the wine of part of the macromolecules are the cause. In a vat, the temperature gradient between the top and the bottom favors thermoconvection movements. This slows down spontaneous clarification as well as vibrations or the presence of excess CO₂ in the wine (> 900 mg/L) giving it a perlite impression. Stainless steel tanks are more prone to these disadvantages because this material is a good conductor of cold and heat and also because these tanks can be of large capacity, thus increasing the chances of creating a thermal gradient. In large capacity tanks, which are taller than they are wide, the distance at which the particles fall from the turbidity is greater, and therefore the time for the deposit to form is longer. In wooden or concrete tanks of reasonable size (50 to 150 hL), spontaneous clarification is better than in stainless steel tanks. Cellars with constant and low temperature are also more favorable places to clarify the wine. Finally, accidental disorders and deposits such as colloidal coloring matter or tartaric precipitations are important factors of instability of the future or acquired clarity. Quite easy to implement in winter, a cold passage of wines facilitates and improves the level of clarification in a shorter time than in the case of a spontaneous clarification. Indeed, the cold, by reducing the agitation of the particles, allows a fast and complete fall of these. Often, a treatment at 5°C for two weeks is sufficient. However, care must be taken not to aerate the wine at this time because its saturation threshold is higher than at 15 or 20 °C. It should be noted that the presence of exocellular glucan from Botrytis cinerea makes the clarification of musts and wines more difficult on a botrytized harvest. This polysaccharide acts as a protective colloid, like gum arabic under certain conditions, opposing the spontaneous sedimentation of an important part of the particles responsible for the cloudiness, often the smaller particles. As the cloudiness becomes stubborn and resistant to gentle methods of clarification, the use of specific glucanase allows the problem to be solved.

 

          It is useless to present all the interest that the aging in barrel and in particular in new barrel has. The aromatic contributions, the gustatory modifications as well as the whole of the factors of improvement of stabilization and integration of the elements of the quality of a matured wine were the object of an abundant literature to which we contributed significantly. We will limit ourselves to considering this subject through the main principles and their technological cause.

          The sense of smell is very much in demand during tasting. Barrel aging, and in particular new barrels, has a great impact on the wine’s aroma. There are many studies on this subject, which focus on the identification of odorant molecules, their concentrations and their real impact on the aroma of wines. We can mention some particularly significant ones. Very often during tastings, for the sake of simplification, we talk about the woody impact of the barrel and the aromas of wood or oak. In reality, with rare exceptions, we mostly feel the toasting aromas, which are often more powerful and have a greater impact on the overall smell of the wine. For specific wines, a low toast seems to be the best choice. This is what happens in Priorat in Catalonia. In this case, the share of wood aromas is higher than the toasting aromas and therefore gives the wine real woody notes. In the vast majority of cases, these are toasted notes (syn. heated). Thus, toasting, through the profound modifications induced in the composition of the wood and its odorant power, brings to the wine complementary aromatic nuances that can easily become excessive and blur or erase the aromatic notes specific to the wine and the grape varieties that compose it. It is over-wooding with all the negative connotations associated with it. Generally, fresh wood has a faint odor reminiscent of coconut, very discreetly vanilla with acetic notes when it has just been cut. Moreover, on a wood yard, it is easy to notice that it is the oldest wood piles that have a more intense woody smell, so that naturally dried wood is more odorous than freshly split or sawn wood. Analytically, this is not always explained by an increase in the content of aromatic compounds, as this phenomenon is more related to the evolution of the two isomers of whisky lactone, the main odorant molecules of fresh wood. Indeed, the cis form is three to five times more odorous than the trans form; during natural drying, the trans isomer evolves slowly and spontaneously towards the cis isomer which accumulates and reinforces the fresh wood aroma. We find in figure 1 the structures of the main odorous molecules of fresh oak. In opposition to drying, toasting is at the origin of a real disruption of the wood composition. The aromatic intensity of the heated wood increases as well as the diversity of the developed nuances. The heat acts on non-odorous molecules called precursors which, during their thermo-degradation, lead to the formation of new substances, some of which have a strong odorant power. In oak, the main precursors are polysaccharides (cellulose and hemicellulose) and lignin. The final weight of the wood aromas in the wine aroma depends on the choice of the method and the moment of the maturation. The earlier the wine is put into barrels, the better the integration between wood and wine. The yeast cloudiness ensures the adsorption of a part of the odorous substances and decreases the intensity of the woody character. In the dead yeast constituting the lees, there remains a more or less residual reductase activity that ensures the reduction of aldehydes into the corresponding alcohol. Under the effect of the enzyme activity, vanillic aldehyde, for example, is reduced to odorless alcohol. By this mechanism, the aromatic impact of the wood is further reduced. A third mechanism acts more subtly; the release of soluble yeast polysaccharides fixes part of the aromas released by the wood. In terms of aromatic perception, this mechanism causes a decrease in the intensity of the woodiness or toasting, with an increase in aromatic persistence. The result is a significant decrease in barrel aromas in favor of grape and wine aromas. Concerning the quality of the woody integration, it is not recommended to carry out late tapping on clear wine, for example six to eight months after the end of the malolactic fermentation. The result is often caricatured and requires important dilutions with wines kept in vats, which is a pity in the particular context of barrel aging.

 

 

          More than anything else, when tasted, barrel aging is noticeable by the change in the wine’s color and impression. To the naked eye as well as to the analysis, the color of the wine seems darker and gains a blue-purple nuance. The coloring matter gradually transforms into complex forms combining aldehydes and proanthocyanidols, to form the new generation of wine coloring. It is a question of polymerization of the free forms of the color in more complex and especially more stable substances. Indeed, the color of a young wine is more or less what it is in the grape skins, i.e. anthocyanins sensitive to the physico-chemical conditions of the environment, to SO₂ and whose lifespan is limited in the conditions of the wine to a few months. Without any transformation, the wine loses its red color and gains a yellow-orange shade often called “le tuilé”. At the bottom of the bottle, an abundant black deposit of degraded and insoluble polyphenols accumulates. To avoid this fatal outcome, the maturation process allows the color to gain stability by integrating it into complex forms. Moreover, all the chemical complexity of this coloring matter is far from being known, our knowledge on the subject being fragmentary and schematic. The maturation, in general, increases the intensity of absorbance on the whole spectral range. The wine therefore has more color. The maximum absorbance, in the red – near 520 nm – moves to longer wavelengths (bathochrome effect) to be located around 550 nm. By this phenomenon, directly related to the structural changes of the color during aging, the impression of the color changes from red to blue. The wine is perceived as more colorful because the blue tone gives more thickness and depth to the color. Finally, the new forms of color have a higher coloring power than the anthocyanins alone: this is the hyperchromic effect. The normal evolution of wine color is characterized by a more or less slow loss of red and a progression of yellow. This is what can be observed in bottles when comparing vintages of increasing age. This is a spontaneous and inevitable evolution. However, under the effects of maturation, the color can take on variable nuances. For example, an excess of oxygen makes the color evolve prematurely as if the wine were older than it actually is. The temperature and oxygen couple also acts since the same dose of oxygen at 30°C or 15°C leads to very different colors. At 30°C, the destruction of part of the red color is accelerated, the polymerization of the coloring matter and tannins is also accelerated; the result is an abundant precipitate and a tiled color characteristic of an old wine. On the contrary, at 15°C, the slower oxidations favor the transformation of the coloring matter and limit the destruction and the precipitation; the consequence is a higher proportion of blue in the color. This also illustrates the duality in oxidative mechanisms between good and bad oxidation. On a more technological level, the factors of variation and control of color during aging are numerous. Schematically, the moment of the addition and the temperature seem to be two determining parameters. It is indeed important to take into account the moment of oxygenation to estimate its quantity. The earlier the oxygenation, the better it is for the wine. On young and cloudy wines, the yeast lees in suspension delay the possible effects of an over-oxidation. The risks of over-oxidation or the negative effects of oxidation also increase during the conservation of the wine. This is why aeration is not recommended after the first eight to twelve months of aging. Because oxygen at this stage presents the greatest risk of altering the qualities of the wine. This translates into a premature evolution of the color towards undesirable orange tones, a weakening of the intensity of the fruitiness or even a slimming of the wine. A simple rule is to concentrate 75% of the total oxygen intake during the first six months of the wine’s life. Respecting this rule significantly reduces the probability of making a mistake due to excessive oxidation. Then, the thermal rhythm and extreme values have a clear effect on the color. Temperatures that are too high (>18-19 °C) cause a rapid tilting of the wines or at least an evolution of the hue to the detriment of the freshness of the color. Temperatures that are too low (< 10°C) initiate the formation of colloidal coloring matter, by insolubilization of the acidic polysaccharides of the grape, with the consequence of an instantaneous loss of 10 to 30% of the initial color intensity. Sudden variations in temperature upwards or downwards have similar effects. Thus, regularity and average values of around 14 to 17°C constitute the best environment for color stabilization and stability.

 

 

          The use of barrels for wine making and maturing goes back several centuries. Throughout this long period, many practices have been developed to facilitate or optimize the work; the most interesting ones have been preserved and constitute today the tradition of the work and the treatment of the wine under wood. This is the case of the various methods of entonnage (hot, cold, after a strong aeration,…), of the rackings, of the ouillages, of the sulphiting with the wick or the pellet, of the fining with fresh egg white and of the numerous forgotten practices (oil, lime, plaster,…). But we must keep in mind that these traditions are not fixed; they are evolving towards new ways of working or new technologies which will constitute, in a more or less long time, the traditions of the future. This movement is important and allows us to understand why it is useless to oppose the techniques, of a more modernist connotation, to the traditions synonymous with seniority and immutability.

          During the maturation process, the wine must benefit from a certain number of interventions intended to facilitate its evolution and stability. Depending on the regions, the habits and the characteristics of the wines, the maturation can be a slow and spontaneous transformation associated with a minimum of operations. The wine matures on its own under a more or less close supervision. In this case, it is more the duration of the maturation than the work of the wine that intervenes. The wine is taken out of the barrel at the moment deemed most optimal. On the contrary, in other cases, the maturing process combines duration with a certain number of manipulations aiming at regularly eliminating the lees deposited at the bottom of the containers, ensuring a decarbonication of the wine and an oxygenation. The filling of the barrels is adjusted at an adequate frequency. In addition, treatments with tannins or yeast derivatives can be performed. This practice of maturation is more interventionist and dirigiste. It focuses mainly on the first four to six months, then the wine is rested for the summer period and the few months that follow until the decision to bottle. This is a traditional way of maturing for the Bordeaux region. In this second case, the maturation is divided into two phases: at the very beginning, an interventionist and active period during which the wine evolves rapidly under the influence of operations carried out at variable frequencies; then, later on, a calm period that allows the wine to evolve slowly under the effect of time, temperature variations and the porosity of the wood alone. Neither approach should be preferred; the best result prevails. Depending on the wines, the constraints and their real potential, we will choose the most suitable way.

          Racking is a very old operation, which does not necessarily require a pump or other motorized tools to be performed. For many decades, racking has been the only oenological practice in wine maturation. It is in fact a manipulation with multiple virtues that should not be reduced to its most obvious role, that of oxygenation. Racking consists in taking the clear wine from a container (tank or barrel) that is cleaned before putting wine back in. The first function of the racking is to eliminate the disorders and the various deposits which were sedimented slowly. It is a clarification role. The second is to allow, by a more or less important exposure to the air, at least at the beginning of the maturing process, a decarbonication of the wine which favors both clarification and oxygenation. The third one ensures the elimination, during the contact of the wine with the ambient air, of the compounds with reduced odor and certain fermentary notes, allowing the wine to reveal its true aromatic potential and in particular its fruity notes as well as its aromatic clearness. Finally, oxygenation, a direct consequence of racking, ensures the evolution of the composition and quality of the wine through oxidative mechanisms. This last function has a major importance in the future of the wine because the oxidation ensures both the stabilization of the color, the refining of tannins, a better resistance to future oxidations, the stabilization of the composition of the wine, its quality and the improvement of its colloidal structure. Racking can be compared to both breathing and cleaning. Racking is also an opportunity to adjust the free SO₂ content, to clean the barrels if necessary and to homogenize the different batches of wine.

 

          The management of racking throughout the maturing process is either motivated by a traditional rhythm – for example, one racking every three months in the first year – which is based on a customary habit and which gives satisfactory results, at least in the great rich, balanced and expressive wines, or the choice of the frequency and intensity of racking (strength of aeration, fineness of lees removal) is made each year and for each batch of wine according to the real need, motivated by the monitoring of the reduction of the cloudiness, of the odors of reduction or of the tactile sensations of the tannins. This approach is more flexible, less systematic, and requires regular monitoring of the wines and a significant amount of time spent tasting them several times a month (from two to four times). Finally, an additional alternative exists: that which is based on the principle of maturation itself (syn. education, training, formation). Thus, the younger the wine is worked on, the more favourable it is to the changes required of it. It is young that the wine needs the most oxygen and is the least sensitive to its harmful effects. To illustrate this, it is easy to compare in a cellar the effects of equivalent aeration on the same wine divided into two batches. The first one receives aeration during the three months following malolactic fermentation, the second one receives the same aeration nine months later. At the end of the maturation process, it is clear that the first batch is more balanced, expressive and fresh than the second, which shows slight signs of premature aging and a certain hardness in the mouth. We repeat that, from experience, it seems wise to us to concentrate 75 to 80% of the total oxygen intake during the maturation of the wines over a period of three to six months after the end of the fermentation processes. Often, during barrel-to-barrel or barrel-to-vat racking, the freshly emptied barrels are cleaned and then sulfited by various processes (wick, pellet, gas) or treated with ozone or other chemical or technological disinfection processes. When they are to remain empty for several days and even more so for several months, the practice is highly recommended. This is also the case before the summer to limit the risks of contamination or increase in volatile acidity due to ambient heating or summer vacations and less regular monitoring of the level of free SO₂ in the wines. On the other hand, between two rackings, at the beginning or in the middle of the maturation process, it seems that the operation is not of interest, at least for sensitive parameters such as volatile acidity and ethyl acetate.

          During barrel aging, and particularly in new barrels, the absorption of wine by the wood (five to six liters) and the evaporation of water and alcohol over the entire surface of the shell favor the reduction of the wine level inside the container. The vacuum thus formed can present risks of oxidation, development of acetic bacteria or “flower” yeast. The topping up allows, by the regular levelling of the liquid in the barrel, to compensate the inevitable loss of wine and to avoid the risks linked to the accumulation of air. In the cellars, topping up is frequent on new barrels and at the beginning of the maturing process; it can then represent one or two passages per week. Then they decrease to one or two per month. The topping up is also, in a secondary way, a mode of oxidation of low intensity. In the past, on small estates or for particular batches of wine, topping up was replaced by the deposit of non-calcareous stones at the bottom of the barrels, as the wine evaporated. More modern systems of inert balloons connected to gas bottles are used to maintain the level of wine in the barrels. In practice, topping up is not systematic. On a partially closed barrel, the evaporation of the liquid and the formation of hollows are compensated by the penetration of outside air through the bung hole. The oxidation on the surface of the wine is higher, but so is the decarbonication and the elimination of reduction odors. Regular filling of the formed vacuum can then be justified. The vacuum is comparable to an inerting mechanism fed by the barrel itself without any other intervention than the complete closing of the barrel. There is therefore no risk in keeping this volume of nitrogen above the wine. In previous experiments, we compared the oxygen content in the vacuum above the wines in the barrel; the results show that compared to the normal oxygen content in the ambient air (19% oxygen), the trapped air has 2 to 6% oxygen after 15 days of closing the bung. This phenomenon is relatively important since it favors a slow and continuous oxidation of the wine over long periods. Repeated topping up breaks the vacuum and cancels this mechanism. In order to avoid a large volume of hollows inside the barrels, it is advisable to regularly toast the first three to four months of the maturation, then when the loss of wine – also called consumption – slows down significantly, to stop toasting and to close the barrel tightly. This period corresponds to spring or early summer. To be on the safe side, it is a good idea to check the formation of this vacuum on a randomly selected batch of barrels by opening the bung and listening for the characteristic suction noise. Finally, it is advisable to carry out the topping up with healthy and correctly preserved wines. Vats with a floating cap or inerted with nitrogen are an interesting way to maintain these wines over time. It is important to regularly control the free SO₂ and to carefully eliminate the lees and the various disorders that can affect the clarity of the wines contained in the barrels.

 

          In the barrel, the seasons are important, firstly to respect the evolutionary phases of the wine, and secondly to avoid accidents due to the climatic conditions of the moment. The work sequences must be as much as possible in phase with the aptitudes of the wine. These will vary over time; respecting them will greatly depend on the success of the maturation and the improvements of the wine. It is illusory, at least if we are aiming at quality, to try to impose on the wine a working rhythm that does not correspond to these moments; thus, the technological time must be adjusted to the time of the wine. The most obvious example is the management of oxygen and oxidative periods. Therefore, the oxidative phases must be concentrated at the beginning of the work of the finished wine. The concern of clarification answers the same rules, for different risks. Obtaining the clarity of the wine and the careful elimination of the lees responds to a requirement of microbial risk in relation to the progressive increase in temperature during the spring and summer. Therefore, aging on lees or with yeast derivatives is recommended during the winter period. Even in the case of an air-conditioned cellar, summer accidents are not totally ruled out and their frequency also justifies cleaning the wine and the barrels. Temperature plays a major role in the kinetics of physicochemical mechanisms and biochemical and microbial phenomena. Thermal rhythms and their succession during the maturation period have a great importance on the wine evolution and its final quality. We have carried out several experiments to illustrate this phenomenon. Each time, we divided a batch into two series: the first one was aged at a constant temperature (e.g. 15, 13, 9 °C), the other one at a variable temperature according to the seasons or simulated by the air conditioning (e.g. 13-15 °C in winter, 15-17 °C in summer). The wines were tasted at the beginning of the experiment and after 12 and 18 months of aging. Below 10-11 °C, the evolution of the wines is almost interrupted and the tastings before and after aging remain very similar. As long as the wine is adapted to this duration of aging, the results are always edifying for temperatures above 12°C. Reasonable seasonal variations in temperature or simulation by air conditioning give better results. The impression of mellowness and the integration of the wood are underlined, as well as the tactile quality of the tannic structure of the wines. Thus, this natural choice ensures that the wine progresses more fully during the maturation process, towards its optimum quality in relation to its initial potential. Temperatures also largely influence microbial deviations, such as the volatile acidity of acetic bacteria, or the ethyl-phenols of Brettanomyces. Up to 10°C, the risks are very limited and accidents are rare; the temperature is too low to allow a sufficient metabolism capable of causing alterations. Between 10 and 15 °C, the risk is limited and largely depends on the pH. Below a pH of 3.7, things are comparable to what they are for values below 10 °C, while for pH above 3.7, the frequency of accidents becomes significantly higher. For temperatures between 15 and 20 °C, the risks increase exponentially, with priority for the highest pH. When the maturation is over, the questions of improving the limpidity, fining and setting a bottling date remain. Apart from any technological or scientific consideration on fining and filtration, the reflection must concern the necessity of the treatment(s) and not the mode and intensity of application of the treatment. We remind you that fining and filtration are curative interventions and not systematic oenological operations.

 

          There are many risk factors that must be monitored throughout the aging process. We will not go back over those that have already been discussed elsewhere, such as lees and temperature. On the other hand, others deserve to be studied in greater depth, because of their predominant and sometimes underestimated role. SO₂ remains an irreplaceable safeguard against the uncontrolled activity of the various residual micro-organisms that may accompany the wine during the entire maturation period. What matters when we talk about SO₂ is its activity and the regularity with which it is controlled and readjusted. For pH up to 3.7, free SO₂ is a sufficient value; above 3.7, the calculation of molecular SO₂ is essential, at least to be aware of the weakness of the real SO2 activity. A correct coverage in free SO₂ is of the order of 30-35 mg/L and 0.4-0.5 mg/L for molecular SO₂, which is not always allowed by excessively high pH values. In extreme cases, there are factors acting to overexpress the effectiveness of SO₂. This is the case, for example, of the alcoholic degree, since for values of 14 to 16% vol. it acts in synergy with SO₂. Finally, low temperatures (<11-12°C) clearly improve the efficiency of molecular SO₂ and contribute to the microbiological stability of the wine. But in addition to these key values, it is important to maintain a regularity of the SO₂ level in the wines; it is often when the SO₂ level drops that deviations occur. Often, two to three days with less than 15 to 20 mg/L of free SO₂ are enough to record sometimes serious accidents. This is why the regularity of the control and the adjustment of the doses is, as well as the level of SO₂, a critical point to watch. The summer period and the decrease in the number of employees in the cellars due to vacations are, with the rise in temperature, a frequent cause of uncontrolled evolution of the wines. The inter-seasons with changes in temperature and atmospheric pressure are also periods to watch. For example, at the arrival of spring, then during the transition from summer to autumn, the wines appear more closed, less expressive than they are before and after these two phases. Sometimes the wines become cloudy again, for a few days. It is also a time when the wine is more prone to accidents of all kinds.

 

           Not so long ago, coopers delivered good barrels… The grain of the wood, the geographical origin, the type of drying and even the toasting were not options offered to winemakers. It is quite recently, during the 1980s, that the barrel has experienced a diversification of its constitution and definition. From this period were born the notion of toasting level, low, medium, medium plus and high, as well as the geographical origin of the forests. Later, in the mid-1990s, the notion of grain allowed for more reproducibility in barrel deliveries. Today, certain series of barrels have special properties, such as enhancing the fruitiness of wines or modulating the tannin content of the wood, to name just two important examples on the market. In thirty years, the barrel has gone from being a standard product representing tradition to an oenological tool with different properties.

 

          Apart from economic considerations, the selection of barrels, like the percentage of new wood, can be based on different ways of thinking. First, the reasonable choice consists in taking into account the real aptitudes of the wines. This may seem obvious, but too often decisions are still disconnected from the different batches of wine available. It is necessary to be able to linger on the characters of the wine such as the minerality, the smokiness, the fruit and its freshness, the spicy nuances or the vanilla or even woody notes developed by certain wines. Secondly, it is the wine’s capacity to digest the new wood; on this point, it is more prudent to be below the wine’s potential than to risk over-oaking. The way in which the wine is made and the technical decisions taken before aging have a direct impact on the composition of the wines and in particular their polyphenolic structure, the nature and balance of which contribute to the response of the wine during aging in barrels, whether it is favorable or not. Thus, the vinification is a preparation for the aging in wood and must represent as such a parameter of reflection. If we were to use a shortcut, we would say that barrel aging is prepared on the grapes, in the vineyard as well as in the cellar.

     

          Several factors easily controllable in cooperage are likely to intervene on the quality and composition of wines. Thus, when ordering barrels, the choice of the origin of the wood, the nature of the drying, the toasting, the thickness of the staves can play a role in the success of the maturation of the wines. Oak has not always been the only type of wood used in cooperage; for more details on this aspect, we recommend you read our book on the subject. Even today, but in a more anecdotal way, we still find attempts to age wines in barrels made of various types of wood. The most serious one, or at least the one with a real oenological potential, concerns the acacia (Robinier pseudo acacia). This species gives excellent results, particularly on dry and sweet white wines from late harvested grapes, possibly botrytized. The wood gives the wine top notes, light, more floral and white fruits, very respectful of the wine and naturally discrete. We also find barrels made of eucalyptus dried for a long time, cherry wood and many other species and even larch wood in the Swiss Valais with a certain success. American oak or French oak – more broadly, the oaks of central and northern Europe – is still a choice that should not be. Indeed, these two groups of oaks are so different in their composition, their quality and their impact on the wines, that we should talk more about a choice of aging strategy. American oak has been used for a very long time in the maturation of whiskies and bourbons. In wines, its use is more recent. In the past, the quality of American white oak has been globalized, attributing to it a coarse grain, typical of rapid growth, wild aromatic notes, foxed and overly marked by the coconut character. Finally, the soluble polyphenolic fraction was often considered bitter and rustic. Under these restrictive conditions, these woods were almost exclusively destined for mid-range wines for which one was only looking to bring an olfactory supplement or for mass-produced technological wines to which American oak brings to some extent an additional level of complexity. It should not be forgotten that American oak barrels are less expensive than French oak barrels and therefore have a significant economic appeal. Strictly speaking, the name white oak (Lepidobalanus section), as opposed to red oak (Erythrobalanus section), includes the following species: Q. alba, Q. stellata, Q. lyrata, Q. marcrocarpa, Q. prinus, Q. muehlenbergii, Q. michauxi, Q. bicolor, Q. lobata, Q. garryana (North America), Q. robur and Q. petraea (Europe). Thus, white oak is not synonymous with Q. alba or even with North American white oak. This confusing name should be abandoned in favor of the species name. However, the species name is not always correctly used, since in cooperage, many species are used in association with Q. alba, which is always in the majority. We find: Q. prinus, Q. bicolor, Q. muehlenbergii, Q. stellata, Q. macrocarpa, Q. lyrata, Q. durandii and generally they are all assimilated to Q. alba. It is therefore necessary to check carefully the distinctive characters of these species. Q. alba has an ultrastructure comparable to that of Q. robur and Q. petraea. Nevertheless, it has a distinctive character that can give it particular properties. Its thylls are thicker than those of Q. robur and Q. petraea. The thickness of the thylls of Q. alba varies between 2 and 4 µm, whereas for Q. robur and Q. petraea it is at least ten times thinner. This particularity is important since it explains the possibility of producing watertight drums with sawn wood. On the other hand, this property allows to reduce the losses by evaporation and the gas exchanges with the outside. This characteristic is found in other species, including European species. The grain of the wood is also an important parameter since it conditions the intensity of the oxidative phenomena according to the porosity. It is true, and we have already mentioned it, that Q. alba was considered, until the last few years, as a species with a rapid growth rate, leading to coarse grains, often wider than our Limousin woods. If we compare Q. alba from different regions of the United States, we can see that in Missouri and Ohio, we are likely to find wood rich in vessels and therefore more porous. Density measurements on various oak species confirm that in southern Missouri Q. alba oaks have porosities close to Q. petraea as well as some other white oak species such as Q. stellata and Q. macrocarpa. Q. alba has a fairly well known composition. Compared to the two French species, this oak has a much lower dry extract, total phenol and ellagitannin content, but a much higher whisky-lactone content (Table 2). In our example, Q. alba has two to three times more than Q. petraea. It can be noted, for example, that Q. alba has a wide variety of norisoprenoid compounds. Gustatively, extracts of Q. alba are considered more bitter and aggressive than those of Q. robur and Q. petraea; in smell, it is often too fragrant, foxed and “wild”. In reality, little is known about the compounds responsible for these characteristics. Our work has identified in Q. alba varying amounts of gallotanins and depsides such as digallic acid, often considered bitter and harsh in hydroalcoholic solution. If we compare many species of oak from Asia, America and Europe, we note that common points can group together all the species potentially usable in cooperage. These are the presence of ellagitanins and typical aromatic compounds (B-methyl-g-octalactone, eugenol, vanillin).

 

 

          On a practical level, there are many recurrent questions that oenologists and cellar masters or technical managers of wineries are regularly confronted with concerning the management of the barrel stock. These are mainly questions about the most judicious way to put new barrels into service and to use used barrels. In my Cooperage Manual for barrel users, there are some answers. The questions are divided into two groups:

 

           a) New barrels. First, the quality and compliance with the specifications established at the time of the order must be judged; then, the main problems remain related to the conservation of the dimensions of empty barrels in a more or less dry atmosphere, cleaning practices of varying intensity to eliminate debris, and possibly disinfecting the barrels to get rid of hypothetical microbial contaminants.

           b) Used barrels. The problem is directly related to the conditions of preparation for a satisfactory conservation, then the methods of treatment of the barrels before their reuse. In addition, it is necessary to control the potential contamination of the barrels in relation to the altering micro-organisms and the atmosphere of the storage places.

 

          The most frequently asked questions are: How can new drums be stored in their original condition? How to treat them if they have dried out? What are the preparation practices before filling? For used barrels, we systematically find the concern of regeneration to still benefit from the contribution of the wood to the wine. Another important concern is how to prepare, preserve and reuse an old barrel and eventually, how to recover a barrel with moldy, stable smells, very advanced acetic and pungent notes and of course contaminated by chlorinated derivatives (chlorophenols and chloroanisols). To all these problems, there are answers, often simple and based on renewed experiments. However, when it comes to the use of barrels or any other cask, simplicity and common sense remain the golden rule.

         

          On new quality barrels, it is often useless to carry out numerous cleaning operations which only aim at eliminating a soluble fraction of the wood constituents. In a way, this means losing some of the wood’s contribution to the wine. In hot water rinses, traces of aromatic compounds are often found as well as appreciable quantities of ellagic tannins. If the wine is also left to stand for a few days in sulphite water, the rinsing water becomes significantly enriched with permanently lost wood compounds. With the exception of special problems such as the extensive drying of the barrels, we recommend a simple rinse with cold water in order to eliminate dust and small splinters of wood. In these conditions, we note by tasting that the first two to three months are marked by a significant woodiness which then diminishes; at the end of the aging process, the wines from barrels used raw (after a simple cold water rinse) are generally better rated than those from more heavily treated barrels. It is useful to remember that the tannins in the wood participate in the oxidative phenomena of the wines, so their presence is essential for long maturation. When we are looking for more rapid aromatization, it is more interesting to exhaust the wood partially in phenolic compounds. For short maturation periods, heavily leached and disgorged barrels cause slower woody tannins than simple rinses. However, the cost of using barrels would justify blending barrel-aged and tank-aged batches to control oakiness, which in this case is primarily an aromatic problem. There are cases where the use of disgorging and intense rinsing of the barrels is useful and even desirable. This is the case of questionable barrels marked by green and sap-like odors. To identify these characteristics, it is sufficient to collect a few shavings from the shell and to infuse them for two hours in a glass of wine; the tasting will quickly allow to judge the quality of the refining of the wood. If the defect is proven, it is advisable to rinse the barrel for 15 minutes with hot water (60-70°C), followed by filling the barrel with very hot, sulfited water, then emptying it after 48 hours, steaming it with moderate pressure (to limit the internal defibration of the shell) and finally rinsing it for 15 minutes (60-70°C). Under these extreme conditions, the woodiness remains minimal and the green and sappy characteristics are significantly reduced. SO₂ is often used to treat new barrels. However, after burning, very few viable micro-organisms remain in the wood mass, and they are not well adapted to the wine environment (inhibition by alcohol, which is very sensitive to SO₂, and liquid environment in micro-aerobiosis).

 

          During cooperage operations, especially during toasting, the wood is likely to undergo more or less strong tensions causing deformations or breaks of fibers. One of the problems that can concern barrel users is blisters and their abundance. During the toasting process, the interior of the shell gradually rises in temperature. Under these conditions, the steam from the residual water and the gases formed escapes at the same rate as they are produced; thus, the surface of the staves remains perfectly smooth. But in the case of a rapid rise in temperature or prolonged heating over an intense fire, the gases are formed in greater quantities; the result is a detachment of the tissues which then rise and form a blister. They are often considered as sources of microbiological contamination by promoting the stagnation of small quantities of wine. In reality, the first 3 to 5 millimeters of wood of the barrels are already impregnated with 4 to 6 liters of wine. In the often hollow central part of the blisters, we have never observed any free wine; even after decortication in the laboratory of blistered wood samples immersed in wine for several days, the result is unchanged. Furthermore, observations made by scanning electron microscopy show that the blisters do not contain more microorganisms than intact stave parts. Therefore, there is no higher microbial risk in the case of blistered barrels. To confirm the risk of microbial spoilage, we determined the total bacterial populations of the rinsing water of different barrels. It clearly appears that the presence of blisters does not modify the total bacterial population of the latter. On the other hand, the barrels with acetic odors have a very high bacterial population in their rinsing water.

          Let us now consider the particular case of drying out of the barrels during transport and storage. Up to a value corresponding to the saturation point of the fibers – a value close to 30% – the wood keeps its dimensions. When it dries out and its moisture content falls below 30%, the wood undergoes three-dimensional variations (wood shrinkage). Coarse-grained wood has greater volume shrinkage than tight to very tight-grained wood.
In concrete terms, when barrels have dried either during transport (this is especially true for export) or during storage before use in a too dry room, it is necessary to resort to rehydration of the wood. Depending on the intensity of drying, we recommend a number of effective and relatively simple protocols. For lightly dried barrels, there is no need to put them in water, the wine during the filling will be enough to give the pieces of wood their shape. For barrels whose joints are slightly apart, the barrel should be filled halfway with hot water (40°C) and left to soak each side for four hours. Then, it is possible to fill the barrel with water. When the drying is more serious, the operation is repeated over 2×4 hours. For very dry barrels, it is first necessary to proceed to the swelling of the bottom pieces by putting water on the top of the barrels placed on the ground and by turning over after four to eight hours according to the importance of the drying, on one side then on the other. Then, it is a question of putting in water the shell, one fills with water the barrel during one to two days. The two operations can be carried out simultaneously and the water is possibly sulphited at 0.05 or 0.1 %.

 

          In the case of used barrels, there is always a risk insofar as, most often, one does not know the conditions of maintenance of the barrels or even where they were stored. It is then preferable to have recourse to a careful examination of the barrel: clear and frank smell of SO₂, absence of acetic and mouldy character, good appearance of the barrels and the parts composing it. In addition to this basic examination, it is often recommended to visit the winery where possible to check the atmosphere of the barrels. For a more complete guarantee, the determination of chlorine derivatives on a sampling of chips from the outside and inside of the barrels is strongly recommended. 5-10% of randomly sampled barrels from each batch is satisfactory. An important aspect of barrel reuse is the sustainability of the wood contribution to the wines. By measuring the specific components of the wood, we can make various observations that are valid in the case of natural drying. The contribution of ellagitannins, whose fundamental roles in the maturation process have been mentioned, is practically ineffective after three years, mainly because their content has become insufficient. It is important to note that heating can modify the supply curves. In strong toasting, even if the contents in the burnt layers are low, the appearance of numerous microcracks on the surface of the staves allows a greater release of these tannins over time. The main explanation is a better accessibility of the wine in the wood mass. We have observed this phenomenon very often. The odorous compounds behave differently depending on whether they come from the wood or from the toasting. Wood aromas are released regularly and can significantly contribute to the aroma of wines for three to four years. On the other hand, toasting aromas are only localized in the first three millimeters of the staves exposed to the brûlot. Therefore, after two years, few toasting characteristics mark the nose of the wines. But one parameter remains to be considered: it is the wood’s refining method. It is easy to see that wood treated quickly by leaching leads to staves depleted in various positive or negative extractives, whereas a properly conducted natural drying process allows to maintain the wood’s contribution to the wines even after the first year of use. The results of repeated tastings in all the world’s major vineyards confirm this fact. It is certain that the woodiness is sometimes brutal the first year and then attenuates with time. But blending is often practiced and 100% new barrels is not common. This percentage is reserved for the richest wines, presenting satisfactory balance and a remarkable potential for amortization. Finally, the quality of the woodiness should never be judged before a minimum of six months of aging.

 

          During the aging period, few actions are carried out on the barrels. It is more a question of interventions on the wines through the management of the lees and oxygen. It is mainly during the racking that the inside of the barrels can be cleaned with hot water under pressure. Normally, acetic acid and sulfates from the progressive oxidation of ethanol and SO₂ accumulate in the wine-impregnated wood mass (about four to six liters). Sulfates contribute to the impression of hardness of the tannins, while acetic acid contributes to the increase of volatile acidity. During the reuse of the barrels, these different products accumulate in the wines. This mechanism can also be accentuated as the barrels age. On the other hand, if the barrels remain constantly full of wine without a more or less long period of emptying – from a few weeks to several months – this phenomenon is greatly reduced. The used barrels can then be used longer. For all these reasons, disgorging barrels that have remained empty for a certain period of time, before they are used, is in most cases a sufficient remedy. Another problem to be mentioned is the reduction odors. During maturation, the thiols hydrolyze and give dimethyl sulfide, which increases regularly during maturation. Generally, in new barrels, the contents of this compound remain systematically lower than in used barrels and vats. In practice, the preparation of used barrels can be summarized by the following procedures. For a drum to be kept empty, a simple rinse with water is recommended, followed by a careful draining (at least overnight). Indeed, if water remains on the walls, the efficiency of sulfiting decreases by producing sulfuric acid – at pH 7 and above, the SO₃ anion spontaneously gives SO₄ and sulfuric acid -; on the other hand, the penetration capacity of the SO₂ vapors is limited by the liquid water so that the sulfite yield is clearly decreased. Afterwards, 10 g per barrel are meched and hermetically sealed with a well-struck bung (wood or silicone). Before reuse, disgorging is essential to eliminate the sulfates and acetic acid produced. To do this, the barrel is filled with 0.05 or 0.1 % sulphite water and left for two to four days before being emptied, riddled and the wine is poured back in. The operation also allows to seriously rehydrate the more or less dried wood and to avoid leaks. If there is probably a lot of scale or lees residue adhering to the barrel, it is recommended to use hot water under pressure to completely clean the inner surface of the barrel. During the conservation of the empty barrel, it is useful to burn 5 g of sulfur every month to further protect the integrity of the wood.