Cocktail Science

Studying the science behind your drinks.

Cocktail Science: All About Foams

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[Photo: Quinn.Anya on Flickr]

The head on a pour of Guinness, the crema of a perfectly pulled espresso shot, the froth from a malted milkshake. Creamy, bubbly, and aromatic, each of these naturally-occurring foams adds an extra dimension of texture to drinks.

Today, we'll look at some traditional and not-so-traditional techniques for making and perfecting foams for cocktails.

What Is A Foam?

All foams are a subcategory of dispersions. In chemistry, a dispersion is the evenly-spread mix of one material into another. When a solid is dispersed into a liquid (like coffee grounds in hot water), it's called a suspension. When a liquid is dispersed into a liquid (like oil in a vinaigrette) it's called an emulsion. And when a gas is dispersed into a liquid, it's called a foam.

If you've ever made mayonnaise or vinaigrette, you may know that naturally-occurring emulsifiers in egg yolks and mustard help to hold the oil and water portions of these recipes together.

Today, we're looking specifically at foams. In foams, surfactants reduce the surface tension of a liquid, which in turn affects the amount of pressure that can build up in bubbles before they pop. And when bubbles don't pop, a foam forms.

Tweaking foam texture

A foam on top of cocktail is like any other garnish. Not all drinks need the same garnish, and some require none at all. Likewise, some drinks need a thick, heavy foam while others benefit most from no more than a slight froth.

All foams are composed of two distinct parts: a dispersed phase (the bubbles) and a continuous phase (the liquid.) Tweaking the characteristics of either of these two phases changes mouthfeel.

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[Photo: Kevin Liu]

The mouthfeel of a foam is most directly affected by two variables: viscosity and creaminess.

  • Creaminess is defined by the particle size of the dispersed phase of a foam. The human tongue can distinguish particles larger than about 30 microns in size.* That means that if all the bubbles in a foam are smaller than 30 microns, the tongue will perceive a perfectly creamy foam, while anything larger than 30 microns can be perceived as more grainy or bubbly.
  • Viscosity is a property of liquids and is defined by the force it takes to move a solid through the liquid. Think of molasses or maple tree sap as examples of viscous liquids. The overall viscosity of a foam is affected both by particle size and the viscosity of the continuous phase (that is, the liquid). Thicker liquids make thicker foams.

Of course, real foams made by cooks and bartenders are rarely ideal. For example, fruit juices and purees are suspensions, and egg yolks are emulsions, which means that the texture of foams made with these ingredients will depend not only on bubble size and viscosity, but also on the particle size of suspended solids and the interactions between oil and water.

Suffice it to say, there are a fair few ways to make foams. Here, I'll cover the easiest and most useful techniques I know of.

*1 micron = 1/1000th of a millimeter. The diameter of a human hair is between 17 and 180 microns.

Egg Foams: Simple, easy, and good for most situations.

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Fizz a la Violette from Barmini in Washington, DC [Photo: Kevin Liu]

For a simple cocktail that really benefits from the addition of an egg white, look no further than the classic Pisco Sour. Without the egg white, the Pisco Sour is exactly what it sounds like—a tart, lemony drink reminiscent of a daiquiri. With egg white, though, the drink transforms into a creamy decadence. Plus, the thick foam keeps the Angostura garnish from bittering the rest of the drink, and enhances the drink's aroma.

  • To use egg whites in a pisco sour, first shake all ingredients without ice in a cocktail shaker. Shaking the egg white while warm helps build the foam. Then add ice and other ingredients and shake to chill the drink.

Many sours, particularly those based around lemon, benefit from some frothiness. In fact, many bottled sour mixes contain a small amount of dried egg white to lend drinks a slightly frothy texture.

Another classic egg white preparation can be found in the Ramos Gin Fizz. This cocktail calls for both egg whites and heavy cream. The drink is famous both for its luxurious texture and for the fact that it historically took over 12 minutes per drink to make.

The long preparation time probably has to do with the fact that while the fat in the heavy cream makes the foam more stable, it also makes it more difficult to form a foam.

  • To make a big, poofy, fizz-style foam, dry-shake all ingredients for two to three minutes, then shake with ice, strain, and add soda water. For a few easier techniques, see below.

Dry Shaking Optional?

Dry shaking works because egg white foams form more easily when warm, but any technique for introducing air works. Here are some options:

  • Pre-make the egg white foam by itself with an immersion blender, then add a few spoonfuls to each Pisco Sour order. For batches, a teaspoon of cream of tartar per 8 egg whites helps stabilize the foam.*

  • Add all the ingredients of a Ramos Gin Fizz into a blender and turn a 10-minute shake into a 1 minute blur

  • Use a whipping siphon with egg white alone or with your choice of flavorings to dispense foams a la minute.

A few factors to consider.

  • The fresher the egg, the more stable the foam that forms. This has to do with the strength of the proteins in the albumin.
  • Acid helps to stabilize an egg foam.* When dry shaking, shake the egg white with the other cocktails ingredients, as they will usually include an acid such as citrus juice.
  • Due to the high pressures involved, a foam made with a whipping siphon will always be creamier than a foam made by dry-shaking or blending—this is not always desirable (see the light and frothy section, below).

*Cream of tartar, like lemon juice, is an acidifier. Acids help egg white foams form by interfering with sulfur bonds that would otherwise make the foam collapse.

Gomme syrup: The classic forgotten mouthfeel modifier

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3 Whiskey Sours: Made from left to right with gomme syrup, egg white powder, and methycellulose (the methylcellulose foam is spooned on top) [Photo: Kevin Liu]

Page through any classic cocktail book and you're likely to find recipes calling for "gomme syrup," "gum syrup," or simply, "gomme."

In the past, gomme syrup was made by combining sugar, water, and gum arabic in a rough 1.5:1:1 ratio.

Gum Arabic is a hydrocolloid* made from the sap of the acacia tree. It's unique in that it acts as both an emulsifier and a thickener. Gomme syrup in place of simple syrup gives cocktails a slight rich, creamy mouthfeel and light froth.

Although gomme syrup remains commercially available today and can also be made at home, political instability in the regions of the world where gum arabic has traditionally been harvested have driven up prices for it.

Making Modern Gomme

I'll skip the research and science here, but my version of gomme syrup calls for much less gum arabic and substitutes in egg white powder for emulsifying power and xanthan gum for thickening.

Get the Recipe »

A note on "scaling"
In the recipe for gomme syrup and in a few instances below, I call for a "1% scaling" of this or a "0.5%" scaling of that. Here's what I'm talking about.

When working with powerful hydrocolloids, especially in small batches, a little goes a long way. A "1% scaling" means that a recipe calling for 100 grams of water (which, at sea level, is the same as 100 mL) would need 1% as much of the hydrocolloid, or 1 gram of powder for every 100 grams of water. A cup of water comes out to about 240 mL, so a 1% scaling would be 2.4 grams.

For these small measurements, I use a cheap jewelry scale and a paper muffin cup. If you don't want to go to this trouble, you can ballpark 1/8 teaspoons as 0.5 grams, try the recipe, and go up from there. Xanthan is the most powerful of the hydrocolloids—use less of it than you think you need and add a little bit at a time until you get the texture you want.

*Hydrocolloids describe any polymer that can be dispersed in water. Different types can thicken, emulsify, or form gels. They're not as scary as they sound: the "polymer" portion of hydrocolloids are no more than starch or proteins. Flour and gelatin, for example, are both hydrocolloids.

Beer Foam

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Nothin' but beer. [Photo: Kevin Liu]

If you're a fan of Guinness stout or Boddington's pub ale, then you're familiar with the rich and creamy foam that perches on each. But what if I told you you could create that same rich foam with any beer?

The secret behind Guinness' signature head lies in the use of nitrogen gas in addition to carbon dioxide to create smaller bubbles. The science behind why this works is still the subject of some debate, but I know that nitrogen is important because, well, I tried it.

The above image is of me squirting a can of Pabst Blue Ribbon that had been charged with a CO2 canister and one or two N2O canisters in a whipping siphon. As you can see, the result is a rich, velvety foam reminiscent of Guinness, but tasting of light beer.

Almost all beers contain natural starches and proteins that act as foam stabilizers and emulsifiers. Many beer producers will also add foaming agents to their beer formulations to achieve the perfect amount of foam on top.

When I experimented, a straight PBR foam lasted for about 3 minutes before the tiny bubbles formed larger ones and the foam started to melt away. I found that dissolving a pinch (scant 1/8 tsp) of xanthan gum per 12 oounces of beer helps stabilize the foam. Just keep in mind that if you decarbonate the beer to dissolve the xanthan, you'll want to charge the whipping siphon with a charger of CO2 in addition to the N2O canisters.

Light and Airy Foams and "Bubbles"

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[Photo: foam on Flickr]

You've probably heard the term "air" used to describe particularly light foams, often used in modernist cuisine. Depending on what ingredients you use, these foams can range from big bubbles stacked on top of a drink to a substantial yet effervescent froth reminiscent of Champagne.

Gelatin
Simple store-bought powdered gelatin produces foams that are just slightly lighter than egg white foams. Use a scaling of 1%. The foam will become more dense and stable the closer it gets to refrigerator temperature (about 34°F). Gelatin foams can be dispensed with a whipping siphon or blended with an immersion blender.

Xanthan
Remember above how my recipe for modern gomme syrup calls for xanthan gum as a thickener? I like to use xanthan in everything from smoothies to stews because it dissolves without lumps in any temperature and a tiny amount can thicken a lot of liquid.

To create frothy xanthan-only foam, try 0.5% to 0.8% scaling with a strongly-flavored fruit juice, like pineapple or watermelon. After combining the xanthan with the juice, allow to rest for 5 minutes (the mixture will thicken slightly with time) then dispense with a whipping siphon.

To make big, pretty cocktails bubbles, you'll need both xanthan gum and egg whites as well as a cheap fish tank bubbler. Luckily, the doodad can be multipurposed: I use mine to circulate my DIY immersion circulator when it's not being used for bubbles :-).

Lecithin
Lecithin is an emulsifier found in egg yolks and can be used to make delicate, bubbly foams. Use powdered soy lecithin, not the liquid type that's intended for oil-based applications. The technique is pretty simple: use an immersion blender to froth up your liquid with 0.5% scaling lecithin and scoop the foam off the top.

The biggest pain about using lecithin is that the stuff I've bought bunches up hard like brown sugar over time and can be a pain to use. It's also not as widely available as xanthan or gelatin. More info on lecithin here.

Unique applications, batching, and dietary restrictions

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[Photo: Kevin Liu]

The techniques for making cocktail foam we've covered thus far should let you create any texture you want without much fuss. But, there are a few other ingredients you can try for special applications.

If you are part of an at-risk population and need to avoid the consumption of raw eggs, feel free to use pasteurized or powdered egg white instead of raw. I prefer using powdered because you can add more powder to bulk up your foam without diluting the drink.

The standard is 2 teaspoons of powder and 2 tablespoons water per egg white. Combine the powder and water in warm (not hot) water. A fork or immersion blender helps. Egg white powder is also sold as albumin powder.

If you're allergic to eggs, you can use egg-free hydrocolloids designed specifically to mimic the texture of an egg foam. These hydrocolloids can be useful in situations when you need large batches of foam: they stay stable for hours at room temperature, there's less risk of microbial growth than with egg whites, and it's a lot easier to scoop a few spoonfuls of powder than it is to crack and separate dozens of eggs.

The image at the beginning of this section is of a foam made with Methycellulose and xanthan gum, and you can get the recipe here.

  • Versawhip is made from soy protein and can be used to create both hot and cold foams. It's easy to use: use a scaling of 1% and beat with a blender until a foam forms. Add xanthan gum (0.5% scaling) to create a thicker foam. Unfortunately, the problem with Versawhip is that it tastes bitter and metallic, so the foam has to be sweetened to mask those flavors. For that reason, I rarely use the stuff.
  • Methylcellulose describes a wide variety of hydrocolloids that do different things. I use MethylCel F50, a formulation best known for its ability to form gels at high temperatures. It works just as easily as Versawhip, but it has less of an undesirable aftertaste. Once again, a 1% scaling with 0.5% xanthan works well for a thick cocktail foam.

For vegans, both Versawhip and methycellulose contain no animal products. Another option is agar (or agar agar), a seaweed-based replacement for gelatin. Use agar as you would gelatin, but keep in mind that agar will gel at a higher temperature, so you may need to use less to achieve the same texture as an equivalent gelatin foam. Agar is available online and at many Asian groceries.

What have you tried?

In this article, I've tried to cover the techniques and tricks most useful and easy to implement at home. I'm sure there are other methods out there—what bubbly ideas do you have?

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

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