Drinks are generally one boring, homogeneous colour. Usually it doesn’t matter because you’re in a dark bar and we’re drinking for the flavour (or more likely, to forget). But sometimes it’s fun to shake things up. Add a little colour, add a new dimension to your drink. And wouldn’t you know it, the key ingredient is science.
It’s Sunday, you’ve made it through the long week, and it’s time for Happy Hour, Gizmodo’s weekly booze column. A cocktail shaker full of innovation, science, and alcohol. Who are you calling a pousse, buddy?
Layer Upon Layer
Meet the pousse-café, also known as a layered drink, or a stacked drink. It might look like an expertly made latte, or a Rocket Pop, and that’s all fine and dandy. What it actually is, though? A drink with several different types of liquid (typically between three and seven) carefully layered on top of each other. It’s an eye-catching effect because it looks like it defies the laws of physics.
On the contrary, it’s actually the laws of physics that makes it possible. For the lowdown, we once again tapped our friend Timothy Zohn at AQ Restaurant and Bar in San Francisco.
Specific Gravity
The secret sauce that makes these layered drinks work is specific gravity, or what’s more commonly known as “relative density.” It refers to the mass of a unit of volume of one substance, compared to the mass of the same unit of volume of another substance. So, for example, say you have 30mL of water and 30mL of gold. They both occupy the same amount of space, right, since a millilitre is a measurement of volume? But the gold is much heavier, therefore it has a greater relative density (specific gravity). If you put the two substances together in a bottle, the gold would sink and the water would float.
The same principles are at work in our pousse-café. Say water is our reference substance (which it usually is in science). That would give it a specific gravity of 1.0. Pure ethanol alcohol generally has a specific gravity of around 0.8, which means its relative density is less than water, and so it floats on top. Sugar, on the other hand, is heavier than water and so when it is dissolved in a fluid it increases the relative density of it. Since sugar and alcohol are both common in almost all of the ingredients of a pousse-café this creates an interesting balancing act.
The sweeter something is, the denser it will be and thus more likely to sink. But, the more alcoholic it is, the less dense it will become, and will be more likely to float. Grenadine, which has tons of sugar and no alcohol, sinks like a boxer throwing a fight. Bacardi 151, which is low in sugar but high in alcohol, floats like a 5th grader on Robitussin. It’s all the liquids in between, however — those multi-coloured liqueurs which are necessary for the drink’s distinctive look — that are a little harder to pin down, since they all have different ratios of water, sugar and alcohol (plus whatever else is in them).
It’s worth mentioning that specific gravity is affected by two other factors. There’s pressure, which generally speaking is not something you need to worry about in this case (the subtle changes in environmental pressure aren’t generally strong enough to affect this drink). But then there’s heat, which can play a critical role. Heating a liquid causes it to expand, which decreases its density, making it more likely to float. If you’re heating all of the ingredients equally, then it doesn’t matter because remember, we’re talking about relative density. However, if one ingredient is coming out of the freezer and another is coming out of the kettle, you might accidentally make your sinker float and your floater sink. In other words, this is a lot easier when everything is at room temperature, though if you want to try using heat to your advantage (like how a barista gets hot espresso to sit on top of warm milk, but under the less-dense foam), have at it.
Making The Drink
There are a few simple rules to follow:
- You want to use a narrow glass to maximise the thickness of each layer without using a ton of booze. There actually is a specific pousse-café glass, but only your crazy aunt has those, and do you really want to go over there and borrow one? No. Just use a champagne flute or a skinny shot glass.
- The order in which you pour is the most important aspect. You want to start with the liquid that has the greatest specific gravity, and decrease as you ascend. That way they liquids won’t try to pass through each other, which will mostly likely just end in blending. Terrible stupid blending! Start with the sweetest, least-alcoholic liquid, and go up from there until you have the least-sweet, most-alcoholic liquid at the top.
- Pouring technique is critically important. This definitely takes a steady hand. If you pour too fast, the top layer will plunge down into the one under it, which will cause mixing. That ain’t pretty, and prettiness is what this drink is all about (prettiness and science). The top of a bar spoon (as Tim demonstrates in the video) is pretty much the ideal tool, but you can pour over the back of a regular spoon, too. Just position the spoon right above the last layer, and pour as slowly as humanly possible. This is why bartenders hate you for ordering these.
For the pousse-café in the video, Tim used Creme de Menthe, Campari, Averna, and Absinthe, in that order. You can see that the difference in gravity between the Creme de Menthe and the Campari is quite large by the way they practically bounce off each other. In contrast, the Campari and Averna are extremely close in gravity, and you have to go very slowly to keep them from blending. Tim added a a few drops of water, to break out an additional layer of colour (and flavour) in the absinthe, and finished it off with a pinch of sea salt. That might sound weird, but those bottom layers are so sweet — the salt cut really through that in a pleasing way and made it much tastier.
Remember, not all of the layers have to have alcohol in them. Lime juice is very high in sugar and has no alcohol, which, combined with its sourness, makes it a great bottom layer — think of it as a chaser. There are dozens of other pousse-café recipes out there and a quick search will find you some tasty ones. Dallas Bartenders, for example, not only gives you a ton of recipes, but they have a chart of the specific gravity of a bunch of different spirits and liqueurs, so you can use that knowledge to do your own experimentation. Very handy.
As a quick shortcut, here’s how some popular liquors rate:
Light: Southern Comfort, Sloe Gin, Brandy
Medium: Triple Sec, Amaretto, Blue Curacao
Heavy: Kahlua, Grenadine
Drinking It
There are two schools of thought here. One says you just knock it back like a shot, and let all those flavours hit you in rapid succession. The other says that you should sip a pousse-café through a straw, one layer at a time, so you can enjoy each as you go. Which is best ultimately depends on what recipe you went with and your personal preference.
It should be noted that these are, almost without exception, incredibly sweet. The pinch of salt certainly helps a lot, but this is almost definitely a hangover waiting to happen. If anything, they’re dessert shots, and they’re certainly fun to look at. Frankly, they’re not really our cup of tea, but we just couldn’t pass up the chance to demonstrate some fun scientific principles through drinking. And who doesn’t have a sweet-tooth every once in a while?
Timothy Zohn is the Bar Manager at AQ Bar and Restaurant in San Francisco, He has spent the last eight years working at some of the best bars in the city with some of the best bartenders in the country. He is proud to call San Francisco home. You can find him on Twitter @TimothyZohn
Thank you to Tim and the AQ Bar and Restaurant for their time, and to Trevor Easter for the love connection.