Stop the press

Getting the best from your AeroPress is not as easy as it may seem, Jem Challender explains the theory

Picture this: You’re in the pub, and the cool change the forecast promised hasn’t blown in yet. You’ve spent a half hour getting to know a nice pale ale, and you’ve already made the decision to keep the relationship monogamous. The bartender comes over and asks, ‘The same again?’ You nod, and then halfway through the pour, the keg runs out. Not only that, but what did make it into your glass is cloudy, like milky Assam tea — nothing like the golden glory you were expecting.

A range of things could happen next. But the worst-case scenario is when the bartender trots off, saying, ‘I’ll just change the keg’, returns minutes later, tops up the half-filled pint glass, and hands it to you. If you’re the congenial type and accept the pint, you’ll notice the beer tastes completely grainy and considerably less bright and fruity than it did before the bartender poured you the dregs. The cause of this drop in flavour is ‘turbidity’.

To find out what the current best practice is when it comes to serving beer that contains sediment in the keg or the bottle, we contacted the head brewer and founder of Bentspoke brewery in Canberra, Richard Watkins. Richard explained, ‘I’m not into the roll or shake [of bottleconditioned beers like Coopers], the yeasties have done their job so leave them alone. Best to pour the beer slowly into a glass and leave the yeasties in the bottle. If you do decide to pour the yeasties into the glass, then allow to settle before sampling’.

‘Turbidity’ is not necessarily a bad word in the realm of coffee; you’d be hard-pressed to find a more turbid beverage than espresso. The problems begin when the turbidity levels are high but the strength (measured as total dissolved solids) is low. When coffee turbidity is unduly high, we at Barista Hustle look for ways to dial it back by trying different approaches to brewing.

Unquestionably, the brew method that most dramatically demonstrates swings in turbidity is the AeroPress. We at Barista Hustle spent a long time trying to find out how to make AeroPress brews more transparent, with better flavour clarity. We found scientific proof that AeroPress turbidity is adjustable.

Accurately measuring turbidity isn’t simple. You could use a $400 turbidity meter, such as one the EPA might use to measure clay siltation in a waterway — but that meter would be useless to measure turbidity in a complex, volatile substance such as brewed coffee. In coffee, turbidity changes constantly as a coffee brew cools. Some chemicals will rapidly precipitate out, forming solids. Therefore, to measure coffee turbidity, a fancier piece of equipment is required. We asked Bulgarian scientist Nasko Panov to run some tests for us using a UV spectrophotometer that measures the light absorption of particles in suspension. Panov used Kevin L. Goodner, PhD’s method for measuring turbidity in tea.

Panov’s experiments answered four very specific questions:

1. Does layering two filter papers reduce turbidity?

Yes, using two papers helps, albeit by a very small amount. Many baristas, including two-time World AeroPress Champion Jeff Verellen, have been doubling up their filter papers for a while now. Sometimes, at Barista Hustle even use three.

2. Is turbidity lower if you allow a coffee bed to form on the filter before you plunge?

The turbidity of brews produced when the coffee bed was allowed to settle onto the filter paper before plunging was virtually identical to the turbidity of brews plunged immediately after the AeroPress was inverted.

3. Does squeezing every last bit of out of the AeroPress affect turbidity?

Nasko’s results absolutely busted any notions that you mustn’t squeeze out the final few drips: he observed virtually no difference in turbidity levels between AeroPresses plunged to the point where you can hear the air come out and brews in which the plunging was halted before that point.

4. What's the effect on turbidity of plunging very slowly and gently compared with plunging fast and hard?

Here’s the game changer: Should you plunge as hard as you can? The answer is... absolutely not! Have you noticed the small gaps positioned every few millimeters around the base of the AeroPress filter cap? Applying very hard pressure as you plunge can turn these little gaps into escape routes for undissolved coffee solids. If you want to produce clean, transparent brews with a crisp clean mouthfeel, screw the filter cap on tight. Nasko found that an extremely hard press led to dramatically higher turbidity levels — 71% higher in some cases.

Nasko explains: ‘I intentionally hard tightened the AeroPress filter cap. Afterwards, I made some measurements with a loose cap, but still reasonably tight. If I was correct with my assumption, there should be a significant difference between the two methods when the plunging pressure is high.’ The results from retesting confirmed Nasko’s expectation: tightening the filter cap reduced the turbidity of brews made with a hard plunge — by 14% on average.

If you’re the type who rolls your Coopers Red ale along the bar top before opening the bottle, you might prefer your AeroPress brew produced with an ultra-hard plunge. We’re not saying that’s wrong. After all, a drink like hot chocolate is as turbid as it gets — and who doesn’t love hot chocolate? But if you prefer your AeroPress brew to have a silky-smooth mouthfeel and flavour clarity like what you’d expect from the best pour-over, take our advice: double up the filter papers, tighten up that AeroPress filter cap, and dedicate at least 45 seconds to the plunge. It sure is nice when science gives you a reason to slow down and take it easy.

Jem Challender is the Dean of Studies at Barista Hustle (BH) in Melbourne. He’s a former UK Brewers Cup Champion and the author of 10 online courses about coffee production. Caffeine readers can access Nasko’s paper on turbidity for free at baristahustle.com