Tuesday, April 23, 2013

Cafe au Lait at Home

While I love espresso machines with all my heart, I doubt I will ever have one in my home. Money, of course, is one obstacle. A good commercial machine like a La Marzocco Linea runs about $5k used. While there are a multitude of models of home espresso machines, most all of them have quirks of one sort or another. Even if they can reach nine bars of pressure, most home machines have issues with temperature stability—issues that commercial machines largely avoid due to their greater mass. And that's not even mentioning the grinder, which in many respects is more important than the espresso machine itself.

Cost aside, making espresso inherently involves a certain amount of waste. It takes at least a couple of shots to dial in the grind on a coffee, and dialing-in has to be performed daily (if not more often) as well as whenever switching coffees. In a commercial context, a few wasted shots a day is not a big deal, but for a home enthusiast who's only drinking a couple of shots a day, dialing-in could mean that 50% of the shots pulled are test shots.

For these reasons, I mostly drink brewed coffee at home. I've used the Hario V60 cone more than any other device, although I also have a Kalita Wave and an Aeropress. However, a lot of people really like steamed milk, and it's nice to be able to offer guests that option. So I recently acquired a Bellman stovetop steamer, with the intention of using it exclusively for hot chocolates and chai lattes. But, curiosity getting the better of me, I've started using it for coffee drinks as well. Since I don't have an espresso machine, I can't make lattes or cappuccinos, but I can make cafe au laits with very strong coffee from an Aeropress. There are, however, limits to this technique.

The biggest difference between an Aeropress and an espresso machine (there are, of course, many differences) is that the Aeropress is an immersion brewing device, while espresso is a percolation brewing method. What that means is, the Aeropress incorporates a steep time during which most of the extraction occurs, while an espresso machine is continually pumping in fresh water for the entire extraction.

As Vince Fedele recently confirmed (see the fourth-to-last paragraph of this post for more details), immersion methods of brewing coffee require a greater amount of coffee to reach the same level of extraction. This effect is exaggerated at very high doses. Therefore, in order to make coffee by Aeropress at a similar strength and level of extraction as espresso, you have to use a lot more coffee.

Option One: Whole Hog

So what happens if we try to mimic a good shot of espresso with an Aeropress? For this trial, we'll choose to model a moderate ristretto shot. We'll aim for a liquid yield of 40 grams and an extraction of 19%. Let's figure out how much coffee and water I should use.

For immersion brewing, the equation to calculate extraction is:

Extraction[%] = Water[g] * Strength[%] / Coffee[g]

Since the Aeropress retains 1.5 grams of liquid per gram of coffee, we can calculate yield as:

Yield[g] = Water[g] - (1.5 * Coffee[g])

Espresso can range in strength from 5% to 15%, depending on how the shot is pulled. Let's shoot for 10%, which is about average for many specialty coffee shops. (That's about what a 20g dose for a 40g yield comes out to at 19% extraction.) If you work through the equations, what quickly becomes apparent is that achieving coffee at 10% strength by immersion is hugely wasteful. Sure, you can keep adding more coffee, but your yield will keep decreasing, and if you add more water to increase yield you also have to add more coffee to maintain 10% strength.

What that means is, at 10% strength, for every 100g of water you add, you'll only get 20g of liquid, because the amount of coffee you have to add (53 grams, in this case) retains so much. And with infusion methods, all of that retained liquid is extracted coffee. So if you wanted to produce 40 grams of liquid at 10% strength in an Aeropress, you would have to use 106 grams of coffee! (If anyone wants to check my math, shoot me an email.) And I don't think that you can fit that much coffee and water in an Aeropress. So, if we're defining espresso as coffee at a similar strength and volume as what is typical for specialty coffee, then it is impossible to make espresso in an Aeropress.

Option Two: Half Hog

So let's lower our sights a little. How about if we aim for 5% strength? Now we're only losing 39% of our liquid in the grounds. So we only have to add 65 grams of water and 17 grams of coffee to get our 40 grams of liquid. But we'll also need to increase our liquid yield in order to (partially) compensate for our loss of strength. So let's scale the recipe up to produce 60 grams of liquid, using 25.5 grams of coffee.

Now that we have our brew ratio sorted out, we can move on to the actual brewing process. In order to get a proper extraction, a very fine grind should be used. I generally stick with 200F water for everything, so that's what I used for this method. I brew the Aeropress inverted, aiming for a total brew time of 2 minutes, including pouring and pressing. Pressing should be done slowly to avoid channeling.

To be clear, this is not true espresso. It is very strong brewed coffee. It lacks the sweetness and body of a good shot of espresso, but is superior to anything I've had out of a superautomatic machine.

Milk Steaming

The Bellman steamer is not an ideal steaming device, mainly because it has a single-hole tip and limited steam pressure. It's also more difficult to clean than commercial steamers because the tip isn't removable. However, with some practice I've been able to make latte art quality microfoam fairly consistently. Some general tips for using this steamer:

-Preboil the water in an electric kettle. It's much faster than waiting for the water to heat up on the stove.
-Only fill the steamer halfway.
-Wait for the safety release valve on the steamer to start whistling before you try to steam anything.
-Keep the steamer on medium-high heat while steaming.
-Use a 12 oz pitcher.
-The Bellman takes much longer to heat up milk than a commercial steam wand, though foaming may occur rapidly. For this reason, it's easy to overaerate your milk. If you do overaerate, scoop off excess foam.

This method makes a nice small (4-5 oz) milk drink.

Thanks to NetPhilosopher on coffeegeek.com, on whose work I based this post.


  1. Good article, are you on twitter? I don't know how to follow you or your blog!!

  2. I'm not on twitter, but I've added an option on the righthand popout bar to subscribe via email or RSS