Coffee Science: How to Make the Best Pourover Coffee at Home

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There seem to be new coffee brewing gadgets showing up all the time, but sometimes it's hard to separate what's worthwhile from what's a waste of money. It's helpful, then, to back up and try to understand how coffee brewing works and how brewing equipment differs. Then, you can make an informed decision about which devices and methods might work best for the type of coffee you like to drink.

Today we'll focus on the physical and chemical processes that make up coffee brewing, starting with one of the most simple (and increasingly popular) methods: pourover.

Pourover coffee starts with (freshly) ground coffee, a filter, and a filter holder, often called a 'pourover dripper.' At the most basic level, pourover brewing involves pouring water over and through the grounds to extract the coffee flavors into your cup or serving vessel. Seems simple, right? But let's get a few levels deeper!

All coffee brewing methods involve the same three general phases: wetting, dissolution, and diffusion. Each phase is linked to the others, and they affect what comes next in some important ways.

Why Pouring is Different

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Pourover coffee (unlike some other methods) continuously replenishes the liquid surrounding the coffee grounds with new, fresher water. This promotes a faster, more efficient brew. On the other hand, that fresh water also has a tendency to extract more from the surface layers of the grounds. It's sort of like frying cubed potatoes in a seriously hot pan. Compared to a cooler pan, your potatoes will cook faster, but there's also the risk that you'll overdo it, especially on the outsides.

Pouring one stream of water, rather than a dozen or more little streams from a coffee-maker's shower head, results in a brewing environment that's a few degrees higher, just from reducing the surface temperature loss from those narrow water streams. Temperature and water quality affect the overall reaction rate of our little coffee chemistry set (hotter, cleaner water generally means faster).

Wetting

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Wetting is just what it sounds like: the coffee is dry and you make it wet. The reason you need to think about it as an entire phase is because it's not as simple as it sounds. One of the major byproducts of roasting coffee (you didn't think coffee beans are born brown, did you?) is carbon dioxide gas. For lighter roasted coffees, that carbon dioxide is literally trapped in the cell structure of the coffee bean, and leaches out slowly over weeks. With dark roasted coffees, the roasting process has physically blown a hole in every cell, and most of the CO2 is out within just a few days. This attribute of dark roasts (coffee roasted past the "second crack" phase) is also why I'm giving you different recommended specs for brewing below—brewing darker roasts is more efficient.

When you hit the coffee grounds with hot water, CO2 is able to escape and it bubbles out. The problem is that if carbon dioxide gas is going out, water isn't able to get in. I like to picture shoppers on Black Friday. If you opened the store for business at the same moment there was a panicky fire drill, you could have a mess on your hands. That is, unless everyone wanting in waited until everyone wanting out got out.

As you start your pourover brew, you'll want to add just enough brewing water to wet all of the grounds, then it's good to stop and let the gas escape for about 30 seconds or so. You'll see the bed of grounds swell and expand, resulting in what coffee professionals call a "bloom."

Dissolution

The word "dissolution" looks a lot like the word "dissolve," and that's just what it's about. Once the coffee grinds are fully wetted, the hot water will dissolve the solubles (also called solutes) in the beans' cells.

Part of what makes great coffee brewing difficult is that the complex cocktail of organic substances in coffee includes both pleasant and unpalatable types. Lucky for us, it's one of the convenient facts of coffee chemistry that the desirable and tasty solubles dissolve in water more readily than the unpleasant-tasting substances, so a getting a tasty brew is all about stopping the brewing at the perfect moment—after you've dissolved the good flavors but before those nasty flavors start to dominate.

Diffusion

Diffusion is about taking that dissolved stuff and transporting it out of the coffee grounds via that term you might not have heard since school: osmosis. The cell wall structures of our coffee grounds are semi-permeable membranes, so the osmosis pressure drives the brew out of the highly concentrated chambers of the coffee grounds out to the more watery surrounding environment.

Timing and Adjustments

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Most of the roasted coffee bean, about two thirds of the bean's mass, is insoluble cellulose. The other third is dissolvable in water. Of that soluble third, most of it is the good stuff, particularly various organic acids and sugars. The rest are longer-chain molecules that we associate with astringent and bitter tastes. Where we find the happy balance is at the 19-20% point, that is, if you extract the first 19-20% of the mass of the coffee, we tend to find the best flavor balance. More than that and you'll find those astringent and bitter flavors start to dominate. Less than that and you'll find the resulting flavors thin and unbalanced, and with lighter roasted coffees, unusually sour. Timing really is what makes or breaks your coffee brew.

One problem is that we're not dealing with coffee grounds that are all exactly the same size and shape. Since the grind sizes aren't uniform, some bits (the finest grounds, we call "fines") will get to the nasty-flavor level before the larger sized grounds. How much you'll have to deal with this problem depends on the quality of the grinder you're using.

Tuning your pourover brewing means finding the right combination of grind size (coarser or finer), recipe (ratio of coffee to water), and brew time. But how quickly the water will drip through your coffee bed depends on how much the coffee bed itself slows down that flow. More coffee or finer-ground coffee will result in a slower flow, and the opposite is true as well. One of the downsides of pourover brewing is that the flow of liquid is so inextricably linked with both the grind size and bed depth. Getting the best brew can involve a lot of trial and error!

While pourover brewing doesn't require a special pouring kettle, a narrow spout does make it easier to control what you're doing. One of the big differences between pourover coffee and brewing coffee in a drip machine is that with pourover, you're able to clearly observe everything that's happening as you brew. Having a narrow-spout kettle helps maximize control, and direct water right where you want it to go.

You can eke a few more degrees out of the brew temperature by maintaining a fully wet brew bed. Letting your coffee bed dry out can drop your effective brew temperature by 5°F or more, because that much water evaporating from so much surface area is enough to cool things down significantly. A higher temperature keeps the chemical reactions speedy, and while it's technically possible to have your brewing water too hot, you're going to be fine in most situations.

Try it at Home!

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Here's my basic technique for making pourover coffee at home. You'll need to experiment with the different variables and taste your results to pin down the method that works for you.

Have a watch or stopwatch handy to time your brew. Your phone might have one hidden in the 'Clock' section somewhere.

1. Start with a grind size around that of coarse sugar. (Think Sugar in the Raw.)

How much: Most pourover drippers work best when they're between one half to two-thirds full of coffee grounds. Any less than that, and there won't be enough coffee to restrict the flow. Any more, and your dripper may overflow. You'll also want to make sure you're dripping into a large enough vessel. If you're the more precise-measurement type, a good coffee-to-water ratio is between 60-70 grams of coffee per liter of water (a mass ratio between 1:16 and 1:14.)

2. Get your clean (filtered if you need it) brew water ready. You'll be using water that's about 30 seconds off boil if you're pouring straight out of your boiling kettle, or immediately off boil if you're pouring into a second pouring kettle. I like about 207°F for medium to light roasts, and about 10° lower for dark roasts.

3. Start your clock and add enough water to soak all of the coffee (a little premature dripping is okay). Wait for the coffee bed to stop the initial swelling (about 30 seconds) before adding more water.

4. Continue your brew. Try to pour quickly, gently, and evenly across the surface of the coffee, pausing between pours to pace your brew to your target brew time (see below). The distance that your brew water drops can affect brew temperatures, as well as increase or decrease the amount of agitation that the falling water creates wherever it falls in the coffee bed. In general, the lower you pour from, the better, if for no other reason than it's the easiest to create and maintain consistency.

When you stop adding water, your dripper will continue to drip for between 20 and 60 seconds.

Your target total brew time is about 2.5 to 3 minutes for dark roasted coffee, and 3 to 4 minutes for medium to light roasted coffees. This includes the dripping time after you stop adding water.

Make adjustments! If your coffee tastes weak, you're probably grinding too coarse, so try a finer grind the next time around. If your coffee tastes too strong, next time use a little less coffee, or just add a bit of hot water to the finished brew to taste.

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