According to Wikipedia:
http://en.wikipedia.org/wiki/Irish_coffee

"Although this beverage may be served without sugar,
doing so would prevent the cream from floating on top
of the coffee in the traditional manner."

The recipe given has only 1 teaspoon of sugar
in 150 ml of Irish coffee, and this seems hardly
enough to modify the properties of the liquid
at all. Certainly not enough to substantially
change it's specific gravity, osmotic pressure,
ionic strength, etc. So how does it affect
floating the cream on top? Is this just a
bartender's myth?

It seems to me this is a ripe field for
experimental chemistry, if it hasn't been
done already. Why do chemists study stuff
that we can't use -- like the solubility of
noble gases in organic solvents -- when there
are practical problems of everyday life for
which everybody can benefit from the research?

On Sep 12, 2:37 pm, Mark Thorson > wrote:
> According to Wikipedia:http://en.wikipedia.org/wiki/Irish_coffee
>
> "Although this beverage may be served without sugar,
> doing so would prevent the cream from floating on top
> of the coffee in the traditional manner."

It changes the fluid's average density? Similar to adding salt...

David A. Smith

So, you can't believe everything you read on the Internet? Go figure.

As you may know, Irish Coffee is supposed to be layered, coffee on
bottom and cream on top. The coffee is supposed to be dark. The
cream is not supposed to dissolve in the coffee, if it is done right.
And if the cream does not float on the coffee then it will dissolve in
the coffee. So, what makes it float?

My understanding is that it took a lot of experimentation to create
Irish Coffee, the properties of solutions can be complicated and water
is the exception to many chemical/physical rules.

So, what makes it float?

My understanding is that the factors at work are liquid density,
surface tension, and a spoon.

Plus if you whip or partially whip the cream, not that you would
actually do that, just hypothetically, then you would put air in the
cream making it less dense. And if you completely whip the cream then
the whipping will help cause the cream to stick together, like a
solid, which will help keep it from dissolving in the coffee.

Some of the ingredients in Irish Coffee make it harder to make the
cream float, so some tricks, some compensating factors, are needed.

But basically the sugar makes the coffee denser and increases surface
tension.


Density

The cream is less dense that the coffee so it has a tendency to float
because of that, just like cream floats on milk.

The sugar makes the coffee denser. There is some proof at
http://biology.clc.uc.edu/Courses/bio111/pop.htm.

The whiskey will make the coffee a little less dense, since whiskey is
less dense than water. Please see http://www.henriettesherbal.com/ecle...1/spirits.html.

(If specific gravity that is less than one then it is less dense than
water. If it is greater than one then it is denser than water.)

My understanding is that coffee has about the same density as water,
but the stronger the coffee, the denser it is.

Hot coffee (hot liquid in general) is less dense and cold coffee. In
convection, hot liquid particles rise and cool liquid particles sink.


Surface Tension

I think the whiskey decreases the surface tension of the coffee.
Liquor decreases the surface tension of water.

I think the sugar increases the surface tension of the coffee. I
realize that this might contradict the Wiki article on surface tension
http://en.wikipedia.org/wiki/Surface_tension. But please see:
http://acs.confex.com/acs/csss06/techprogram/P30854.HTM
http://www.bio.net/bionet/mm/neur-sc...ry/027102.html


Spoon

Pouring the cream onto the spoon helps keep the cream from plunging
into the coffee. It redirects the force of the poured cream sidewise
instead of down, so it is less inclined to break the surface of the
coffee.


Thank you.

While researching my answer I found the gems below to be amusing.
Hope you do too.

"I used work in a hotel bar in Ireland, while there I made 100's of
Irish Coffees. The key is not to have the cream fully liquid or fully
whipped, somewhere is the middle is best. Pour the cream on to the
back of a hot spoon at the side of the glass to insure the cream
floats!"

"NO! NO! NO! You don't whip or beat the cream at all! The trick is to
get the fresh cream to slip over the spoon onto the coffee. The sugar
seems to be the secret - it increases the density of the coffee (or
something) so that if you do it right, the cream just floats just
right.
I don't know how many so-called "Irish Coffees" I have sent back in
the US 'cos they used WHIPPED cream!
And yes ... I am Irish ..."

http://www.barkeeper.ie/page.asp?menu=104&page=492
http://www.tablehopper.com/archive/2..._01_index.html
http://mylifeasareluctanthousewife.b...coffee_07.html
http://www.thebuenavista.com/irishcoffee.html
http://www.fantasy-ireland.com/Irish...e-recipes.html
http://www.sciencebuddies.org/mentor...hem_p021.shtml

And I forgot to think about surface tension and temperature. Surface
tension decreases as temperature increases, so it is harder to float
cream on hot coffee than it is to float cream on cold coffee.
http://www.newton.dep.anl.gov/askasc...0/phy00547.htm

John,

Tried to email you, but your email address bounced. Looking for an author
for our site. If you are interested, modify my email address as noted and
drop me a line.

Cheers,

Dan

--


Bar None Drink Recipes

Servin' the Web Since 1995
http://www.barnonedrinks.com


On Tue, 16 Oct 2007 15:56:27 -0700, wrote:

>And I forgot to think about surface tension and temperature. Surface
>tension decreases as temperature increases, so it is harder to float
>cream on hot coffee than it is to float cream on cold coffee.
>http://www.newton.dep.anl.gov/askasc...0/phy00547.htm