How is your Scotch “finished"?
Aging whisky is an art that endeavors to produce optimal taste and smoothness with a precise blend of ingredients and method. Many producers (specifically of Scotch whiskies) have reintroduced the practice of storing the spirit in casks that were previously used for wine, called “sherry finishing.” Contemporary poskim question whether this process may place the resulting spirits under the issur d’rabbanan (Rabbinic restriction) of stam yainam (wine produced by non-Jews). The wine product that remains absorbed in the walls of the empty casks would seemingly be halachically insignificant according to the principle of bittul b’shishim (nullified by a volume of sixty times to one) —or even, as in some cases of stam yainam, bittul b’shishah (six parts to one). However, there are caveats that override the dispensation of bittul: If the sherry imparts a distinct flavor to the whisky—milsa d’avida l’taama (something added to provide taste)—it cannot be batel; in addition, whisky matured in sherry-infused barrels may be considered derech asiyaso b’kach (the recipe usually includes this ingredient) and likewise not subject to the rule of bittul.*
One aspect of matured whisky which is controversial—among the whisky-makers, as well—is whether the wine actually adds flavor to the end-product. Some maintain that it is simply the quality of the wood that helps the whisky process or, as some experts posit, the earlier production of the sherry neutralizes the negative effects new wood casks would have on the whisky. If either is the case, the sherry would indeed be batel.
In practice, if people drinking the whisky do not detect the influence of sherry (according to some whisky connoisseurs a short “sherry finishing” once the whisky is already quite aged does not necessarily improve the product) there is room for leniency, since the improved taste may be attributed to any one of many factors. If indeed the improved taste is not attributable to the wine itself, it may be a case of applying the halachic principle of zeh v’zeh gorem (caused both by this [kosher] and by this [non-kosher]): the smooth, fruity flavor is a result of a kosher component (the wood) which is ostensibly at least as important to the product as the non-kosher ingredient (the sherry) and can therefore override it.
In addition, the fact is that sherry finishing is an expensive prospect for whisky producers; bottles not labeled as such are possibly not of concern. However, the kosher consumer should be stringent not to use whisky brands that clearly label their product as having been aged in sherry or other wine casks. Kashrus agencies have listings of the kosher status of many types of spirits—including which whiskies contain sherry, which possibly do, and which are not a problem at all.
*See Halacha #536 for a list of bittul b’shishim exclusions in commercial food production.