Barrels, oaks, and oenological miscellania.....
Tannin is the reason that white oak is the preferred wood for whiskey barrels.
Actually, this is not quite correct. The reason why WHITE oaks (
Quercus, Subgenus
Quercus) are used for cooperage is that the wood's cell structure contains "plugs" of material in the vessels and tracheids of the wood (called tyloses) that create a water-impermeable barrier so the contents of the barrel are contained. Tannin could be easily obtained from many species, including red oaks; e.g. if barrels were made of red oak (Subgenus
Erythrobalanus) there would be plenty of tannins available for subsequent wine flavor profile refinement, but the barrel contents would slowly leak out, and the product would be lost. White oaks maintain the barrel contents because of the tyloses in their wood, although there still is some evaporation and liquid volume loss with most wines/spirits stored "on the wood" while they condition, necessitating periodic 'topping-off' in many cases. It isn't really the presence of tannins as the primary reason why white oaks are used.
I suspect that they char the interior to somewhat reduce the amount of tannin that goes into the whiskey.
The charring of the barrel's interior provides a number of benefits for both wines and spirits stored in them, including reduction (but not elimination) of tannins, as well as adsorbing a range of organic molecules produced by fermentation, or carried through the distillation process, due to the 'charcoalization' zones in contact with the conditioning liquor.
Wine barrels are also made from white oak, but instead of only being used once they are used many times and the first time that they are used for secondary fermentation, the wine is not considered table quality because of the high level of tannins. The first batch is to "season" the barrels. Some amateur vintners just toss some white oak shavings into their glass carboys to add a bit of tannin to the wine.
The 'secondary fermentation' in wines is typically a malolactic fermentation (with bacteria) during barrel conditioning that is common in many different wines, and does not directly relate to the aging of the barrel, since it often accompanies most wine fermentation cycles, especially with red wines. It's true that the amount of tannin contribution from the barrel diminishes with age and 'cycles' of wine conditioned in it, but it is not related to the secondary fermentation process
per se. Most commercial wines produced today that are stored in oak cooperage undergo secondary fermentation, whether it is their first cycle in the barrel or 6th cycle. Most primary fermentation of wine today (with the exception of generally small "old school" wineries) is done in stainless steel fermenters. If the primary fermentation is racked to a stainless secondary fermentation vessel, Bill is correct that oak chips are added to allow the chemical processes to continue that would have been enabled by storage in wood barrels (storage on wood chips is not just by amateurs, but also by some commercial vintners). Usually the wood chips are toasted prior to adding to the secondary fermenter to bring out different chemical characteristics, (e.g. development of vanillins and other terpenoid compounds). There's a lot of complex chemistry the wine undergoes after the yeast have had their way with the must....this emphasizes the importance of conditioning the wine following primary fermentation, as well as maintaining good racking and clarification processes as the wine ages.
"Cheers"....
Rob
