hist-brewing: Brandy and fortified wines

Jeff Renner nerenner at umich.edu
Fri Sep 22 06:30:36 PDT 2000

At 4:28 AM -0400 9/22/00, PBLoomis at aol.com (who must suffer from 
insomnia) wrote:
>  > On Thu, 21 Sep 2000, Jeff Renner, quoting Lechine, wrote:
>>   > "It [Amantillado] averages 18% of alcohol by volume, though with age
>>   > it may reach 24% and 25%."
>In a message dated 9/21/00 7:20:07 PM Central Daylight Time,
>dbeistle at arches.uga.edu writes:
>>   This phenomenon continues to puzzle me; I would expect the alcohol in a
>>   wine to evaporate through the walls of a cask much more rapidly than the
>>   rest of its component liquids. Boiling point lower than water = more
>>   evaporation than water; more evaporation = reduced concentration.
>>   Where does my reasoning go amiss here? Thanks in advance for the
>>   clarification.
>     I'm glad you asked that, Don.  I was about to ask the same thing,
>following the same reasoning.
>     Scotti
>     "There are no stupid questions, except the ones we don't ask."

"David Murray" <davidmurray56 at hotmail.com> wrote

>The barrels, usually oak, allow the water molecule to get through but not
>the larger alcohol molecule.
>This is how the wine gets stronger.

It apparently is not as straight forward as this.  Climate seems to 
have something to do with it, as in Scotland, whisky decreases in 
alcoholic content as it ages, but in Kentucky, it increases!  No 
doubt  humidity and temperature, and perhaps temperature swings, play 
major roles.  Or perhaps it is that US whiskey is aged in new 
barrels, which are then often used in Jerez for sherry, and then in 
Scotland for Scotch.  But some Scotch is aged in bourbon barrels 
without the intermediate stop for sherry.

"The Book of Classic American Whiskeys"  (1995) (a book with some 
errors, unfortunately, but still a worthwhile reference) by Mark 
Waymark and James Harris, says this (p. 69-70 - I have broken this 
into smaller paragraphs for easier reading.  I really have to set up 
my scanner for character recognition so I can stop keying this all in 

"Not all that happens in the barrel is easily understandable.  For 
example, the alcoholic strength is apt to change during the aging 
process.  In the Scotch industry, the whisky, aged in Scotland, of 
course, tends to lose alcoholic strength.  but in the American 
whiskey industry, it tends to go the other way.  Especially in the 
barrels located in the upper ricks of the open warehouses, the 
alcoholic proof rises year by year.

"Now, why should this happen?  There seemed to be as many answers to 
this question as people we asked it of.  One master distiller, 
sucking air through his teeth when  we posed the question, suggested 
what is known as the 'osmosis theory' - that the water molecules, 
being smaller than alcohol, could more easily gain passage through 
the oak, thus leaving the barrel at a greater rate than  the alcohol.

"A second distiller we asked tucked his hands under his best, gazed 
off into the distance for a moment, and spun a delightful story about 
how, as the barrels heated up in the hot summertime, the alcohol 
would turn to vapor within the barrel and by the increasing pressure 
exerted by the gases trapped within the barrel the water would be 
squeezed out proportionately more rapidly than the alcohol.  (Maybe 
it was the taste testing we had already done that day, but we had a 
hard time following the rationale here.)

"Yet another hypothesis was that because it is hotter in Kentucky and 
Tennessee than in Scotland, the heat favored the evaporation of water 
over alcohol.  (That one really had us stumped.  Since the 
evaporation point of alcohol is lower than that of water, wouldn't 
you expect the alcohol to evaporate proportionally faster, thus 
lowering the proof rather than raising it?)

"Yet another noted distiller suggested that the year-round humidity 
of Scotland favored the proportionately greater evaporation rate of 
alcohol there, whereas the relatively less humid climate of Kentucky 
(especially in the upper floors of the warehouses in the hot summer 
time) favors a relatively greater rate of evaporation for water.

"Finally, we asked this of a chemist involved in the industry, 
expecting to be told the authoritative scientific opinion.  He pauses 
a moment, bit his lower lip, and finally responded, "You know, I have 
no idea.  I really can't explain it."

"Whatever the reason, that's how it works.

In an interesting experiment, [bourbon distillery] Maker's Mark, 
which sells most of its used barrels to the Glenmorangie distillery 
in Scotland, has been aging a barrel of Glenmorangie single-malt 
Scotch in one of its warehouses in Kentucky.  A comparative taste 
test of the outcome will be both amusing and instructive."

Does this make it any clearer?  I thought not.  I am sure there is an 
answer, even if the industry chemist didn't know it.

Jeff Renner in Ann Arbor, Michigan USA, c/o nerenner at umich.edu
"One never knows, do one?"  Fats Waller, American Musician, 1904-1943

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