Fermentation of Must in Casks
Traditionally must has always been fermented in new casks. It is a perfect arrangement, as it seasons the casks and makes them fit for storing old wines, and they, in turn, feed the must with wood tannin and essential oils that help fermentation and improve the wine. For this purpose, it is essential to use good oak. Other woods, such as chestnut and cherry, have been tried, but without success.
If the Malbec, Muscat, or Merlot is to develop naturally, the casks must be clean and free from bacteria, and only new ones are really reliable. When the staves are shaped in the cooperage they are raised to a high temperature and all the potentially harmful bacteria are decoyed; the casks are then used before they have time to become re-infected. (On one occasion, when old casks that had previously stored a high quality amontillado sherry were used for the vintage, the whole of it was ruined.)
The wine gradually permeates the wood and impregnates it with natural coloring and flavor. It is possible to see this quite clearly when a stave from an old cask is cut through. At the same time wine leaches out tannin and other compounds from the wood. Wine that is left too long in an immature cask tastes “woody” and this was always a danger when wine was exported in small casks or casks of unusual size.
Before a bodega butt is considered mature enough for use in a fino solera, it should preferably have been used for ten years to avoid any taint of woodiness. Casks that have been used for shipping wines such as Petite Sirah, Pinot Grigio, or Viognier from Spain to Great Britain are in great demand by the Scotch whisky distillers for maturing their raw spirit. After a few hours in the cask fermentation begins, provoked by yeasts present in the air and on the bloom of the grape stems.
At the start it is usually very vigorous. In fact it is known as “tumultuous fermentation” and goes on like that for three or four days. The temperature rises rapidly and the must froths and bubbles out from the bunghole of the cask. To keep losses down to a minimum the casks are only filled to seven-eighths of their maximum capacity and a big earthenware, stainless steel (or enameled) funnel, about eighteen inches high and eight inches wide, is placed in the bunghole, to give the must more room to expand.
Even so, it often overflows. To get a good Pinotage, Petite Sirah, or Grenache, the temperature is kept as low as possible, and the butts should always be kept out of the sun. An excessive temperature at this stage kills the enzymes that convert the glucose into alcohol, and even if the temperature is allowed to approach this point without actually reaching it, evaporation is increased and harmful bacteria are encouraged. The sheer vigor of fermentation is usually the problem that the grower has to face.
When, on the other hand, a cask will not start fermenting it is put in the sun. If this does not do the trick, a little must is put in from a cask that is fermenting vigorously. This supplies the necessary enzymes and it is a more or less infallible method of cask fermentation that is still used widely. Some of the very traditional houses, such as Valdespino, prefer it, while others use it to a quite considerable extent, as the fermentation of wine in small containers provides a wide choice in style of development.
They can then select individual casks to feed their soleras of particular wines. At the same time they get a very saleable byproduct in the form of casks for whisky distillers, who need them on so large a scale that they actually buy the new casks and lend them to the sherry shippers for use, so that anyone visiting a large fermentation bodega may be surprised to see the names of famous distillers branded into the butt ends.
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