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Why absinthe turns opaque when you add water, and why it stays that way. [Photos: Kevin Liu]

I still remember my first encounter with absinthe. I was at a friend's rooftop party on a beautiful day when he pulled a neon-fluorescent bottle out of nowhere and proudly challenged only the most daring of our group to try a shot of the Green Fairy.

Fast forward a few years, and I've acquired quite a taste for the herbaceous, complex, and deceptively high-proof spirit. The worst versions smell like licorice and taste like cough syrup, but many fine brands worthy of a sample now populate store shelves.

If you've tried absinthe, you know that while it's perfectly acceptable (albeit a little intense) to sip the spirit neat, most fans of the stuff prefer to add a little water before drinking. Just as with whiskey, water can mellow out harsh edges in absinthe and even cause aromatic compounds to become more volatile (and therefore more tasty).

If you add water to whiskey, the liquids blend and swirl in the glass, but they eventually settle into a uniform mixture pretty visually indistinguishable from the original high-proof pour. Add water to absinthe (or ouzo, or several other anise-flavored spirits), and something strange happens: the drink suddenly turns milky.

The Louche: What Gives?

I had never really thought that this cloudy effect—called the louche—was anything particularly noteworthy. In fact, if you ever went through high-school chemistry, you probably saw a similar effect while practicing titration—adding one liquid to another drop by drop.

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When you titrated in class or when you've added water to absinthe, maybe you've seen the sequence I've illustrated above. The target liquid starts out clear because everything is in solution. As you add a second liquid, a cloudiness starts to form. That's because you've changed the balance in the system by adding a new player and suddenly other chemicals don't want to stay in solution anymore.

As I've written about in the context of fat-washing spirits, alcohol can dissolve both polar (water-loving) and non-polar (oil-loving) molecules. Since aromatic essential oils are nonpolar, they are ok with hanging dissolved in alcohol. Add some really-polar water, though, and the essential oils aren't so happy anymore and start coming out of solution.

Straightforward enough, right? Here's where it gets interesting.

Notice the third frame in the illustration above, labeled "precipitation." When a solution goes cloudy, it's usually due to chemicals coming out of solution. Eventually, those chemicals will completely separate from the original solvent. Think of it this way: shake up oil and water really hard and it goes cloudy. Let it rest for a few minutes, though, and the oil and water eventually part. Likewise, mix up coffee grounds with water and the coffee will stay murky in your French press for quite a while. Let it sit, though, and eventually those grounds settle to the bottom.

That's the way the world of solubility and precipitation works.

Except, my friend, in the case of the absinthe louche.

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As it turns out, when you add water to absinthe, the cloudy mixture that results will stay cloudy and will not further separate for months. It's about as weird as if you were to toss a handful of sand into a swimming pool and the pool stayed cloudy for the entire summer.

I'm not just making this stuff up and it wasn't even cocktail nerds who got all excited about it in the first place. Scientists call the louche phenomenon the "ouzo effect" after the popular anise-flavored Greek spirit. Here are a few choice quotes from a recent scientific summary of the phenomenon:

  • ...the common belief is that the ouzo limit cannot be explained by classical thermodynamics at equilibrium...
  • ...In this context, the very definition of 'phase diagram' becomes ambiguous...
  • ...the ouzo effect is a versatile way of forming nanometric droplets of a liquid encapsulated in a protective shell...

What I think they're trying to say is "OMG you broke the physics" and also, "your cocktail is made of nanotechnology."

I looked through some of the other literature on the ouzo effect, and as far as I can tell, the phenomenon results from the unique characteristics of anethole (the essential oil responsible for anise flavor), high-proof ethanol, and water.

In normal situations, it takes an emulsifier or added energy (like blending in a blender) to keep two normally-separate liquids emulsified. For example, proteins in cow's milk act as emulsifiers while many nut milks have to be blended or shaken for the best texture. Researchers still don't completely agree as to why the combination of ethanol, water, and some essential oils behaves differently—why an emulsion spontaneously forms with no added energy or emulsifier.

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From what I can understand, the structure of the particular molecules in absinthe and other anise-flavored spirits allows energy to be transferred in such a way that a milky emulsion ends up being all parties' favorite state of being. There a few theories as to exactly how this works, with debate that involves topics like the "Marangoni effect," "Ostwald ripening," and Brownian motion, all of which are over my head. Regardless of the particulars, the effect is inarguably observable and repeatable. In fact, the paper I cited above as well as this paper go so far as to coin a new zone in chemical solubility charts called the "ouzo zone" that might have novel applications in food science, nanotechnology, and drug delivery.

So, the next time you're sipping absinthe (or ouzo, or pastis, or sambuca) make sure to tell all your friends about the crazy science contained in each bottle. Then again, maybe not.

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