When tasting the National Distillers Old Taylors I mentioned yesterday in the daily tasting thread, I was struck by the deep butterscotch-like taste of the whiskeys.
Not all whiskeys from 30-40 years ago and more have that taste, but many do.
In 2006, a special event was organised by Brown-Forman at which a number of historical bottles of Old Forester were tasted, dating back to whiskey that would have been distilled in the 1940's. John Lipman reported on the tasting on another website and said that the older ones all had a butterscotch-like taste.
I have speculated that the use of older trees for barrel wood might have imparted this complexity but I now am wondering if fermentations then might have contained larger amounts of diacetyl than today.
Interested readers may refer to the discussion of diacetyl in Wikipedia. Its alternate chemical names are given and other properties discussed.
Wikipedia states that in brewing (and many beer fans are familiar with the buttery dicetyl taste in certain ales), a "diacetyl rest" is often allowed to occur after fermentation. This allows the yeast to re-absorb the diacetyl produced during active fermentation. The entry explains exactly how (in chemical terms) diacetyl is created, it is a substance that exits the yeast cells but can somehow re-enter them with a consequent neutralization of the taste associated with it.
I wonder if Old Taylor fermentations in the 1980's and earlier might have been done in a way to favor production of large amounts of dicetyl, or at least, that is how the ferments occurred at the time for this brand and some others in the ND stable (and possibly again in some other distilleries for some other bourbons). As discussed earlier in a different context, Old Taylor may not have had this keynote taste in the 70's and earlier, so it is possible that either intentionally or otherwise, the fermentations were conducted in the 70's and 80's in a way to impart this character but not earlier.
The fact that other bourbons had the taste suggests to me it was a traditional bourbon flavour - not the only one to be sure, but one type of flavor associated with some distilleries in Kentucky.
In saying all the above, I am assuming of course that diacetyl can equal atmosphere pressure under sufficient heat (i.e., vaporize) but this seems clear from the Wikipedia entry which states a vaporisation temperature for diacetyl of 88 degrees C. This would be something over 200 F. and clearly since water and other compounds boiling at around that point enter a whiskey distillate so might diacetyl.
Also, I can foresee that old-time distillers might not have wanted to rest the ferment for two to three days after its completion. Doing this might have been excessively costly and interrupted unduly the efficiency of the mashing/fermentation/distilling cycle.
I'd be interested in anyone's comments especially brewers familiar with diacetyl and its properties or SB-ers with laboratory or chemistry experience. (Ed, any thoughts?).