In my tasting notes I have frequently complained about a certain nutty bitterness that seem to be present in some way or another in a lot of whiskies, most of them (though not all) having matured for some considerable time in used sherry - and or wine casks. I also found it in some Bourbons by the way. In all cases I did not like it at all and found that it could really spoil the nosing and tasting experience. I certainly would not buy these whiskies again if I could help it. Problem is that I don't really know what causes this bitterness.
One of the possible villains is sulphur dioxide. This product is used in vineyards to prevent the growing grapes from oxidising. It is also added to Wine and Sherry Casks to kill bacterias. Or maybe it is added to all casks these days with food safety and all.
Now the funny thing is that apparently not everybody is able to pick up the smell of sulphur unless it has reached the rotten egg stage where it is hard not to detect it. This seems to be a genetic issue although experiments show that smokers and heavy coffee - and/or black tea drinkers have more trouble identifying it. Most people who do detect it compare the smell with burnt matches and/or flints. Some people even like a bit of it in their whisky as it adds another component just like peat or smoke or iodine, also stuff that not everybody cares for. Some people believe we should be advised via the label if and when a whisky contains sulphur.
As I said, I'm not really sure if it's the sulphur that causes the bitter taste and finish in many whiskies I've tasted so far. But if the answer is yes, I believe the industry should start looking for odourless - and tasteless alternatives as the sulphur may drive many new consumers away from our fine spirit.