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.
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.
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.