Types of Ageing
Ageing is without a doubt the decisive stage in the Sherry production process: the most prolonged in terms of its duration and a stage which witnesses the appearance of organoleptic characteristics which give rise to a whole range of different types of Sherry wines.
Two types of crianza are carried out in the Jerez region: one that consists of storing and developing wine in wooden butts, where it undergoes slow, physico-chemical development influenced by surrounding conditions, known as envejecimiento (maturing) or oxidative ageing; and another known as biological ageing, a process that takes place under a film of yeasts known as velo de flor, during which the wine develops in a more dynamic way, driven by what goes on within the biological layer formed on its surface by specific indigenous ambient yeasts.
In the case of biological ageing the influence of the flor is decisive: not only does it protect the wine from oxidation by preventing direct contact between the liquid and the air contained within the butt, but also the interaction of the yeast and the liquid brings about significant changes in the same: to the already mentioned consumption of alcohol as a consequence of its being metabolised by part of the flor must be added the consumption and consequent reduction of another series of elements initially present in the wine, such as glycerine or volatile acidity. Biological ageing, on the other hand, brings about a substantial increase in the content of acetaldehydes which are responsible for the sharp sensation in the nose which wines aged by this process gradually acquire.
Oxidative ageing, however, facilitates the appearance of radically different characteristics in the wine: with a greater degree of alcoholic strength and in direct contact with oxygen in the air, the wine becomes gradually darker and is more clearly affected by the phenomenon of concentration produced as a consequence of the transpiration of specific elements in the wine through the walls of the butt.
Ageing is without a doubt the most decisive stage in the Sherry production process.
In accordance with the Pliego de Condiciones the ageing of sherry wines must be prolonged for a minimum period of two years. Frequently, the ageing time is much longer so that the wines may develop the distinctive characteristics of each type.
Although in the case of oxidative ageing it is in fact possible to carry out static ageing without blending wines of differing ages, the traditional system in the region (and the only viable method with which to successfully carry out biological ageing) is the dynamic ageing system known as "criaderas y soleras".
The nature and capacity of the containers used in sherry-making have evolved over the course of its long history. The earliest vessels were earthenware amphorae and jars, and these continued to be used for over two thousand years. From the Middle Ages on, when the significant advantages of using wooden casks for transporting sherry became apparent, these also came to be used as storage or ageing containers. Changing the nature of the receptacle was to prove a milestone in the career of the region's wines in that it was instrumental in effecting major changes in their composition and sensory properties, effectively creating the prototype of the sherry we know today.
The wooden casks used for ageing have varied widely in size, capacity and type depending on winery conditions and storage space. Toneles, toneletes, bocoyes, botas gordas, botas largas, botas cortas, medias botas, cuarterones and barriles, ranging in capacity from the 900 litre tonel to the 16.66 litre one-arroba barril, have all been used for ageing wine and their presence has configured winery spaces. Various woods - chestnut, local oak, American oak, and so on - have likewise been used.
Nowadays, although casks of various types are still in use in many bodegas, the preferred and most widely employed type is the American oak 600 litre (equivalent to 36 arroba) butt, also known as a bodega butt. This type of wood is preferred to any other because of the specific contribution it makes to sherry, and it is furthermore traditional: it has been used since the first trading exchanges with the Americas, from which Spain imported wood and to which it exported wine.
The butts are not usually filled to the top: in the case of butts used for ageing wine under flor they are filled up to 30 arrobas (500 litres), leaving a space equivalent to that of "two fists" of air. This permits the creation of a surface area upon which the flor may develop and provides a sufficient surface/volume ratio for the influence of this upon the wine to be ideal.
The wooden butt is neither a completely air-tight nor an inert receptacle since the wood is permeable to oxygen and also absorbs water from the wine it contains and then releases it into the winery's atmosphere. This transpiration causes the volume of wine in the butt to drop, the rate of loss increasing the drier the winery atmosphere becomes. This evaporation effect is known as merma and accounts for losses of 3 percent to 5 percent per year of the total wine stored. However, as the loss is essentially accounted for by water in the wine, this means that its other components are continually being concentrated.
After long years of crianza, this is discernible in the increased alcoholic strength of wines aged without the protection afforded by a film of flor. This concentrating effect is not the only modification that will take place in the wine: it will also be enriched by subtle, specific contributions from the wood of the butt, which will have been thoroughly wine-seasoned before being put into service as an ageing receptacle. Meanwhile, the wine will have been developing gradually in this special environment by virtue either of the gradual but continuous impetus provided by the dose of oxygen that the wood allows to penetrate into its interior, or, in a different - more dynamic and substantial - way, of biological ageing beneath a film of flor.
The Solera Criadera System
The traditional, genuine system used for ageing sherry wines is known as the Criaderas and Solera System. This is a dynamic system by which wines from different stages of the ageing process are blended together in order to perpetuate specific characteristics in the wine which is finally sold on the market, which is a result of combining all the different vintages.
The successful development of this ageing method requires a very precise arrangement of the sherry casks in the bodega according to the different levels of age, a process which takes place in what is known as the criaderas. Each solera is made up of various 'scales' or tiers, each in turn composed of a particular number of butts. The tier that contains the oldest wine is at floor level (the term 'solera' derives from the Spanish word for floor - suelo). The tiers placed on top of this, containing progressively younger wine the further away from the floor they are, are called criaderas (nurseries) and numbered according to their closeness in age to the solera tier (the closest being the 1st criadera; the next one, the 2nd criadera, and so on).
The solera, or tier pertaining to the oldest level of the ageing process, produces sherry ready for bottling. Periodically, a specific proportion of the wine in each of the butts making up the solera system is extracted, leaving them partially empty. This operation is known as saca (taking out). The space thus created in the solera (floor-level) casks is topped up with wine taken from the next oldest scale, namely the saca from the 1st criader which sits in the tier above. The space thus created in the 1st criadera is then in turn topped up with wine similarly removed from the 2nd criadera, and so on up to the youngest scale, which is then topped up with wine obtained from the añada system. The operation of topping up, or refreshing, the space created in a scale is known as rocío (sprinkling), and the whole process of effecting the sacas and rocíos in a solera is called correr escalas (running the scales).