Care of Oak Barrels

Excerpts from PROGRESSIVE WINEMAKING by Duncan & Acton Chapter 21 Cleaning and Conditioning Oak Casks.

It is extremely important to ensure that every cask or barrel in which wine will be stored or fermented is thoroughly cleaned and sterilised prior to use so that there is no danger of it contaminating the wine in any way. The wood of new casks which have never held wine contains quite large amounts of tannins and the like which would be leached out by a wine and would consequently coarsen its flavour were steps not taken to effect their removal beforehand.
Reconditioned casks are considerably better in this regard since the wood has already been in contact with the wine, but infection by spoilage organisms may have already occured and fresh wood surfaces may have been exposed during recoopering so that thorough cleaning is still imperative.  Secondhand casks must always be regarded with suspicion because superficial deposits, typically a thick black crust and/or spoilage organisms, may be present on, in or between the staves and must be eliminated before the cask is put back into service. A standard procedure for cleaning and conditioning newly acquired casks has therefore been devised, the details of which are described below. The winemaker will find it advisable to treat all such casks in this manner to minimise the risk of contaminating or even spoiling the first few wines for which they are employed. Fill the cask with cold water and continue to top it up regularly until any staves which may have parted due to the wood drying out have come together again and the cask is completely watertight. If leaks are still apparent after about 48 hours, further soaking is unlikely to effect any improvement and the cask should then be returned to the supplier on the grounds that it is faulty. Even when dealing with casks that are free from leaks from the very beginning, it is advisable to leave them filled with cold water for 12 to 24 hours to allow the wood to absorb water and become saturated with moisture. After this initial soaking period, pour off the water and refill the cask with a hot solution of washing soda, using about 900 grams of the later per 22.5 litres of water. If the cask is in very poor condition, it should be treated with a boiling hot solution of washing soda, but normally hot water drawn directly from the tap will suffice. Leave this solution in the cask overnight, then pour it off. In most cases, its colour will be very dark brown and reconditioned or secondhand casks may give a muddy solution if any black scale has been dislodged from the interior. This treatment should be repeated with casks whose condition is poor, but normally this will not be necessary. Once the final washing soda solution has been poured off, rinse the cask briefly with one or two changes of cold water to remove any residual soda from the surface of the staves. Next fill the cask with very hot or boiling water and allow to stand overnight. This treatment will extract the washing soda solution absorbed by the staves and will complete the removal of tannins and so forth from the wood. It should be repeated until the water emerging from the casks is either colourless or has a pale brownish tint. Two or possibly three such hot water extractions will normally be ample, after which the cask should be rinsed out with cold water. At this stage, the interior should look clean and smell
sweet. All that now remains is to sterilize and condition the cask. First of all, partially fill the cask, say to about 10% to 20% of its capacity, with dilute sulphite solution (your stock solution diluted with nine volumes of water). Bung down the cask tightly and roll it around vigorously for half to one hour (with occasional rests, of course!) so that the sulphite penetrates into every corner. This procedure not only ensures complete sterility but will also neutralise any residual washing soda which may some how have escaped complete removal previously. Finally, pour out the sulphite and rinse the cask with several changes of cold water to wash out any excess of sulphite. The cask must now be “conditioned”. Some nondescript but sound wine to which a little citric acid and tannin have been added is introduced into the cask, generally one bottle of wine containing a teaspoonful of citric acid and a large pinch of tannin being used per 22.5 litres of capacity. Add this wine to the cask, bung down tightly and roll around vigorously for half to one hour. During this period, the wood will absorb body, acid, tannin and other substances from the wine which will emerge completely undrinkable.  Pour out the spent wine and fill the cask with fermenting must within the next hour. It is preferable to conduct at least one fermentation in the cask before it is used for storage purposes because conditioning is then completed with the minimum risk of the character of the wine therein being modified by the wood absorbing certain constituents. Small casks, (less than 9 gallons in capacity) should always serve for an initial period as fermentation vessels for this very reason. The treatment outlined above is undoubtedly quite lengthy and arduous, but it is extremely reliable and well worth the trouble. Moreover, there is no need to repeat it again providing the cask remains sound and is never allowed to stand empty for more than an hour or two after a wine has been removed. The golden rule here is to never empty a cask without having another wine available for refilling it. In this way, the cask is kept in continuous use and need only to be rinsed out with cold water to remove any deposits left by the previous wine before introducing another.