From Drinks
Editor's Note: Please welcome Kevin Liu of Science Fare to SE:
Drinks. We're pumped to have him here to share a bit of cocktail
science.
[Photographs: Kevin Liu]
The difference between a perfectly balanced cocktail and a so-so
one often comes down to ice. How does ice affect temperature?
Dilution? Since as much as half the volume of a cocktail can be
melted ice, why not pay a little more attention to what you put in
your glass?
If you spend time at fancy cocktail bars, it's quite possible
that you've heard a few things about ice that that aren't quite
true when you put them to the scientific test. Today, we're
debunking those myths and clearing up a little of the science
behind the chilly stuff.
Myth #1: Impurities in water lead to cloudy ice.
False. Impurities in water, such as dissolved
minerals or gases, are part of the what makes ice cloudy, but there
are ways to freeze perfectly clear ice without using boiled or
distilled water.
4 factors can make ice cloudy and any technique for making clear
ice has to control for each of them. Here are the culprits, in
order of importance.
- Ice crystal structures. An ice cube is made of
crystallized water molecules. When you freeze ice fast, crystals
start forming in many different locations simultaneously. When
water molecules join these crystals, they automatically align
themselves into formation. The problem is that if you have a
crystal that starts to form in one location and another crystal
that starts to form in another and they aren't perfectly aligned,
when they meet, they won't be able to join up cleanly, which causes
cracks and imperfections, resulting in cloudy ice.
Think of it like building a large brick wall. If I start building
from one side and my friend starts building from the other, chances
are that when we meet in the middle, our two halves won't be
perfectly in sync with each other, leaving holes and cracks. But if
we work slowly, building it up a layer at a time starting from a
single point, we end up with a much tighter, more regular
pattern—this is exactly what happens when you freeze ice slowly and
directionally.
- Supercooling. While a slow freeze helps to
create the perfect crystal structure, temperature of freezing is
the biggest determinant of whether large crystals will form.
Chocolatiers know that the best chocolate is chocolate that has
been "tempered," or manipulated to solidify at a temperature right
around 32°C. Only at this temperature will ideal crystals
form in the chocolate. Similarly, large, transparent ice crystals
only form when ice freezes near water's normal freezing point,
0°C. When liquid water goes below 0°C without
freezing, it's called "supercooling" and the crystal structures
formed are smaller and less transparent. Due to a variety of
factors, supercooling is actually the norm in home freezers, not
the exception.
- Expansion. Ice is less dense than liquid
water, which means that for the same mass, ice occupies more space.
Water has to expand as it freezes. When freezing happens too fast,
this expansion can leave behind stress lines and cracks.* This also
means that if you add a perfectly clear ice cube to a
room-temperature spirit, it will crack. If keeping the ice clear
for presentation is important, make sure to chill the drink first,
then add the clear ice.
- Impurities. Yes, impurities can cause
cloudiness, but your water would have to be pretty minerally for it
to be an issue. To see the effects of impurities in the extreme,
try freezing a cube of salt water. The freezing process will force
the salt to the very outside and very center of the cube, leaving
salt-free but extremely "crunchy" ice in between. The ice is
crunchy because air now occupies the space that has been vacated by
salt. As you would imagine, the cube would be very opaque. But
since good filtered water should be below 30 parts per million
total dissolved solids, the effect of those impurities getting
squeezed out are minimal. The real concern is dissolved oxygen.
When ice freezes quickly and randomly, air bubbles get trapped and
contribute to a cloudy appearance. Freeze slowly or directionally
and the air bubbles get pushed out.
If distilled water doesn't work, what does? For all the reasons
listed above, the clearest ice is ice that freezes slowly and
without supercooling—that is, ice that forms right at 0°C.
So how do you do that?
Method 1: Use a cooler. The best known method
is Camper English's directional freezing method. Camper freezes ice
in an open igloo cooler in his freezer so that the ice freezes from
top down, layer by layer. The top of the ice stays clear while only
a bit at the bottom ends up cloudy.
Method 2: Use a temperature controller. I've
written about my personal favorite method. I hook up a sous-vide temperature controller to a mini
fridge so I can guarantee my ice freezes at just below 0°C.
The temperature controller turns the fridge on and off based on an
algorithm that takes into account factors like insulation and air
flow to maintain a more constant temperature than the fridge would
be able to maintain on its own. I've found that by tweaking the
right parameters, I can keep the temperature within a + or -
1°C window. This technique works best if you don't have to
open the door to the fridge much throughout the day.
Method 3: Start with hot water. While I was
working on my cocktail science book, I spoke with former NASA
cryogenic engineer Doug Shuntich, who pointed out that depending on
your freezer conditions, simply starting with hot water can help.
When hot water freezes, it moves around more due to convection,
which can actually help to prevent supercooling and "encourage" the
water to freeze closer to 0°C.
Any technique you can use to get your ice to freeze at
0°C should work. For example, since impurities in ice
actually help prevent supercooling through a process called
nucleation, it's possible that an intentional impurity, like a mint leaf, could actually make your ice
more clear by forcing it to start forming crystals in a
localized spot; the area right around the leaf will be imperfect,
but the rest of the cube should form more clearly.
*Watch this video to see ice grow after it's already
frozen.
Myth #2: You should never add ice to Scotch
More false than I thought. The basic argument
for not adding ice to Scotch is this: ice waters down the Scotch
and chills it. When you chill Scotch, fewer aromatic compounds from
the spirit get released into the air, which means you experience
much less of the Scotch's potential. All of this is true.
So why might adding ice to Scotch be ok?
First off, even the most prestigious Scotch makers acknowledge
that some Scotches benefit from a little water.
Water changes the solubility of some aromatic
molecules, which means a few drops can help highlight
particular flavors or mask others.
Scotch is pretty strong stuff (in the academic literature,
Scotch has been used as a particularly intense spirit in alcohol taste
tests.) Cooling down and diluting the Scotch reduces the burn that
some people, especially supertasters, might experience. And
that might make the Scotch more palatable for them. If anything,
the popularity of whisky stones proves that there is a market for
chilled Scotch.
What about all that lost aroma due to
chilling?
The concerns over lost aroma deal primarily with orthonasal
olfaction, or the sensations derived from aromatic compounds
that enter the nose through the nostrils. But the tastes we derive
from food (or Scotch) also depend on aromatic compounds that enter
the nose through the back of the mouth. See pretty picture,
below.
This image was first published in
my book.
So the point I'm making is this: although chilled Scotch won't
be shooting aromatic molecules all over the place while it's still
in the glass, as soon as it gets warmed by body heat in the mouth,
those molecules will become volatile and travel up the
back of the mouth into the nose via retronasal
olfaction.
Myth #3: Larger ice cubes melt more slowly
Depends. You've probably heard that large
blocks of ice are better for drinks because larger ice melts more
slowly. The argument usually goes something like "more surface area
= faster melting = more dilution." It turns out that surface area
does matter, but perhaps not the way you think it would.
But, let me come back to that in moment.
First things first.
Whenever we talk about ice and chilling, it's important to
remember that there is no chilling without
dilution. The vast majority of the chilling power of ice
comes from the heat of fusion—that is, the heat ice sucks up from
its surroundings when it turns into water. And since it takes 80
times as much energy to melt a gram of ice as it does to raise a
gram of solid ice one degree in temperature, any significant change
in the temperature of a drink correlates directly with the amount
of ice melted.
What happens when you add equal masses of small
rectangular vs. big spherical ice to a room-temperature glass of
Scotch?
In the glass with small ice, the extra surface area of the ice
would lead to very fast chilling and dilution. The drink would
quickly drop down to around 0°C or just below** and stay in
that rough temperature range until you finished your drink.
In the glass with a big sphere of ice, chilling and dilution
would occur more slowly because spheres have the smallest ratio of
surface area to mass. The Scotch surrounding the sphere would
eventually chill to 0°C, but the ice would also melt a bit
and probably float, which means the bottom of the drink would
probably be closer to 4°C* because water is densest at that
temperature and the sphere would not be able to chill fast enough
to generate the convection necessary to circulate the Scotch. Of
course, simply stirring the drink a little would chill it more.
Now that we know the conditions under which big ice does melt
more slowly, let's look at a situation where the opposite is
true.
What happens when you add equal masses of small
rectangular vs. big spherical ice to an
Old-Fashioned that has been chilled down to
0°C?
In both cases, when you add the ice, the temperature gradient
between ice and surrounding pre-chilled cocktail would essentially
be zero, so relatively little initial melting would take place. As
you drank the two cocktails, the ice in each would melt as heat
would be lost to the surrounding environment. Whether or not the
large ice melted more slowly would depend on insulation, air
temperature, and volume of cocktail to ice, but in most situations,
the sphere would likely be able to keep up with heat loss, so the
two cocktails would chill and dilute at almost the same rate.
Why might smaller ice be preferable to large in some
cases?
If, as you drink your cocktail, the large ice gets exposed to
the air. Then what happens is that the big ice starts cooling the
atmosphere instead of your drink and you get additional dilution
with no added chilling. It can be easier for small ice to rearrange
and stay submerged in a drink as you sip it. So in the case of a
chilled Old-Fashioned, all that really matters is you use ice that
stays submerged for as long as you intend to drink the
cocktail.
Does that mean we should use crushed ice for every
drink?
No—you also have to consider water that is on the surface of the
ice before you add it to your drink. Small ice has tons of
surface area. As a result, it accumulates surface
water—liquid water that builds up on the outside of the
ice through melting and through condensation. When you add small
ice to a drink, that surface water immediately dilutes the drink
without adding any chilling benefit.
Of course, this is really much more of an issue if you are in a
bar situation where ice is stored at room temperature. If you use
lots of small ice directly from the freezer, surface liquid should
be insignificant.
So, what ice do I use? When I'm drinking
cocktails home, I'm perfectly happy using lots of small cold ice
cubes straight from the freezer. But that doesn't mean I don't like
big cubes—they may not make a difference in chilling, but they're
still pretty [ahem] cool.
*Although water is densest at 4°C, the temperature at which
mixtures of alcohol and water will be densest will vary based on
ABV.
**A mixture of water and ethanol has a lower freezing point than
water by itself, so the incredible cooling power of melting ice
*can* take a drink below 0°C. But, in the case of a big ice ball
and room-temperature Scotch, the effect probably won't be
significant. See myth #5, below, for more.
Myth #4: Egg-based drinks always benefit from a "dry
shake"
False. It turns out that drinks that contain
only egg whites do benefit from a dry shake (that is, shaking
without ice), but drinks that contain whole eggs do not.
What does this "myth" have to do with ice? Dry shaking isn't so
much about dry vs. wet as it is about temperature. As any baker
knows, an egg white foams form much more easily at room temperature
than when chilled, which is why a dry shake will create a foamier
egg-white-based drink.
Whole-egg foams are different because they contain fat from the
egg yolk and so are not as much affected by temperature.
But all that doesn't change the fact that two separate shaking
processes is a huge pain in the butt for bartenders, so here are
some tips for making great egg drinks without worrying about a dry
shake.
For egg-white-only drinks
- Stabilize with acid. Acids help to stabilize
egg-white foams. Bartender Andrew
Cameron says that for large batches of Pisco Sours, you can
pre-shake egg whites with cream of tartar and then use scoops of
that foam instead of fresh-cracked egg whites when shaking each
cocktail.
- Use a cream whipper. Egg foams are emulsions.
But they don't just contain water and oil. Air also plays an
important role in texture. To guarantee an amazingly smooth foam
every time, use an ISI whipper charged with N2O to create the perfect foam for a Ramos Gin Fizz
For whole-egg drinks
- Extra fat. Dry shaking a whole egg doesn't
help a foam form, but it does let you get your egg emulsified with
less ice melt. If your drink ends up a little watery for your
taste, don't be afraid to add a barspoon or two of neutral
vegetable oil. Egg-based drinks are emulsions and it is the proper
balance between fat and water that creates the ideal texture.
- Magic Powders. To beef up the creaminess of
your flips and nogs, you could consider using gomme syrup if you have it, or adding a tiny
bit of the hydrocolloid Xanthan gum, a bacterium-derived thickener
often used in gluten-free baking and the secret behind creamy
blended coffee drinks.
You can read more about using eggs in cocktails
here and
here.
Myth #5: Shaking a Martini "Bruises" the Gin
TRUE! Well, sorta.
The fact of the matter is that a shaken drink rapidly reaches an
equilibrium temperature well below the freezing point of water. For
a stirred drink to reach the same temperature, a bartender would
have to stir for nearly 2 minutes.
Because of physics and stuff, a colder drink translates into a
more diluted drink, as ice does not chill unless it also melts. In
a test done by Gizmodo, a shaken cocktail ended
up at 48 proof while its stirred twin finished at a much higher 65
proof.
What if you stir long enough that you do manage to get a
stirred drink as cold as a shaken one?
The two drinks would probably still taste different because the
violent action of shaking a drink aerates it. Although the tiny
bubbles that affect texture dissipate relatively quickly, at the
molecular level atmospheric oxygen and carbon dioxide will stay
dissolved. Whether this makes a noticeable effect on taste is
questionable, but it's certainly possible.
A short public service announcement
These little tidbits about ice are really just the tip of the
proverbial... well, you know. I think it's always worth knowing the
science behind your drink; but, as with all science, you shouldn't
just take my word for it. Keep in mind that real-world factors like
glassware, room temperature, even humidity, will affect your
results.
Have you played around with cocktail science at all? Any
questions out there about ice in drinks? Feel free to share your
experiences or tell me how totally wrong you think I am in the
comments.
About the author: Kevin Liu likes to
drink science and study cocktails. Wait, that's backward. Ask him
geeky food and booze questions on twitter @kevinkliu. While you're at it,
check out his
book about cocktail science and his blog about food and
science
Special thanks to Mike, Angus,
Andrew, Doug, and Jimmy for helping with this
article.
An award winner may times over, 
