Ordinary Scotch, English and Welsh whiskey, whether Single Malt or Blended, without any mention of the use of sherry, port or other wine casks on the label or marketing literature, can be consumed confidently without any Halachic concerns relating to potential contact with ex-wine casks. No additives to these whiskeys are permitted, with the exception of Caramel Colour which is kosher.
As far as Irish Whiskey is concerned, as a direct result of the work of the KLBD with the distillers, Irish legislation has been changed and it is no longer permitted to add flavour enhancers (including those which are wine based) to Irish whiskey. Therefore all whiskey bottled in Ireland since mid 2009, without any mention of the use of sherry, port or other wine casks on the label or marketing literature, can also be consumed confidently without any halachic concerns.
Whiskey which has been matured in wine casks has been subject to detailed Halachic consideration by major poskim (including Minchas Yitzchok Vol 2, 28 and Igros Moshe Yore Deah Vol 1, 62 and 63) who did not forbid its consumption. There is a new process however, known as Wine Cask Finishes (also referred to as double or second maturation) which is a secondary process in which fully aged and mature whisky is returned to specially commissioned and primed wine casks for a further period of maturation, a process some claim imparts a recognisable taste of the wine. This new process may not be covered by all the aforementioned heterim and accordingly some may wish to avoid products so labelled.
All KLBD certified whiskies are matured exclusively in non-wine or kosher wine casks. For a list of KLBD certified Scotch Whisky click here and for Welsh Whisky click here.
Other whiskeys produced overseas (American, Japanese, Norwegian etc) are not necessarily subject to the above mentioned controls and only those kosher certified or approved can be recommended.