Coffee | The Science Behind Brewing

There are hundreds of ways of making a coffee, and everyone thinks theirs is the best. But what's the science behind coffee's flavour?

Modern baristas have turned making coffee into an art, and many of us are addicted to their work. These days, there are hundreds of different ways of making coffee, and which cup is ‘the best’ depends on your own taste. But no matter how you brew, every cup of coffee is underpinned by the same seven key steps.

Join me on a journey from bean to cup, covering everything you’ll need to know to brew the finest coffee the world has ever seen and understand why it tastes so damn good.

Step 1: Buy hand-picked 100% Arabica beans

First things first – you can’t make a great coffee with the wrong ingredients.

There are two major species of commercial coffee plants in the world: Coffea arabica and Coffea robusta. Robusta beans are easier to grow and cheaper to buy, but Arabica beans are widely considered to make better-tasting coffee (hence why most high-street coffee chains proudly boast that their coffee is 100% Arabica).¹

One reason for this is that Robusta beans contain more caffeine. Whilst that might sound like a good thing, lots of caffeine actually makes coffee taste harsh and bitter. Arabica beans contain less caffeine and more fat-based compounds, which make coffee taste smooth and rich.

It’s not just the species of coffee bean that matters, though: how a bean is grown and harvested matters too. Even Arabica beans contain loads of those harsh chemicals before they ripen, so even if there are only a few under-ripe beans in a batch (which inevitably happens when you harvest an entire crop at once with a machine), it can really ruin the taste of your final brew.

Step 2: Roast Your beans at 200°C

Before roasting, coffee beans are green, and they contain a high concentration of molecules which both smell and taste unpleasant (like trigonelline, chlorogenic acids and aldehydes).

Roasting coffee beans at high temperatures (around 200°C) breaks down most of these molecules. The high heat drives other reactions that replace the unpleasant molecules with ‘aromatic’ molecules.²

Roasting coffee beans for longer and at a higher temperature (a darker roast) means the amino acids and sugars inside the beans will have more time to react with one another, creating more of these delicious aromatic compounds and breaking down more unpleasant compounds will be broken down, which gives a deep rich taste to the final brew.

But don’t roast your beans for too long. When roasting high-quality coffee beans, a dark roast can overpower the delicate flavours some connoisseurs are searching for, so a shorter, lighter roast can be the way forward. Even worse, if your beans stay in the oven long enough to reach a toasty 250°C, they’ll crack in half, and all the flavour you’ve created will evaporate.

Tip: Store your roasted beans in an airtight container. Remember that roasted coffee can go bad: those delicious aromatic compounds are fragile, and they’ll break down if exposed to air, light or water.

Step 3: Grind it fine and fresh

Grinding beans into a powder increases the surface area of your coffee beans. This helps the flavour locked inside your beans to escape.

How you grind your coffee determines how long you should brew it for: finely ground coffee has a higher surface area, so you’ll pull out the flavour more quickly. But, it’s also easier to burn or over-brew a finely ground coffee, giving you many bitter compounds that can ruin the richer flavours.

It’s also crucial to use the right grind for your chosen brewing method. For example, using an espresso machine that makes coffee in under a minute means we need to use a fine grind or else the coffee won’t have enough time to brew and will come out watery and tasteless.

Tip: Ground coffee goes stale more quickly than roasted beans, so always use freshly ground coffee

Step 4: The tech – use an espresso machine

Now we can finally get down to actually making our coffee. But how?

There’s never been one way of doing it: you could use an espresso machine, a French press, or a complex contraption that looks like it belongs in a lab rather than your kitchen.

But no matter what method you use, brewing a coffee can be broken down into a few simple variables:

How coarse or fine you grind your coffee beans,
The water you use, and
What temperature and for how long you brew it.

But remember, avoid paper filters! Most of the tastiest compounds in coffee are fat-based, which means they’ll end up stuck to the filter paper rather than in your cup.

If you're brewing coffee at home, your best bet is to ignore all the complex new technologies and just stick with a traditional espresso machine, brew your coffee under pressure rather than using a filter, and focus on getting everything else just right.³

Step 5: Use hot water (92-96°C)

Water makes up 99% of a cup of coffee, so if you want the perfect warm cup of coffee, treat the water you use with the respect it deserves.

First: Use hot water between 92-96°C. Any hotter and it will burn your coffee and ruin the taste. Any colder, and you won’t extract enough of those aromatic compounds, leaving you with a cup of brown water.⁴

Second: Use ‘hard’ tap water. Hard water contains lots of calcium and magnesium ions, which stick to the molecules we want, bringing out more flavour from your coffee.

Lastly, use the right amount of water. Too much will dilute the flavour you’ve worked so hard to get hold of. Too little, and you won’t have any coffee to drink. For an espresso, weigh your coffee and measure out 15ml of water for every 1g of coffee you’re using. It might sound pedantic but just think of it as a science experiment.

Step 6: Drop the pressure & take your time

So our brewing has begun – but when does it end? Striking a balance between flavour and bitterness is far from easy.

The very first compounds extracted from coffee are acidic and horrible, but leaving coffee brewing for too long increases the risk of burning it and leaves you with more bitter compounds, which overpower any other flavours.

The brewing time all depends on what pressure we set our machine to. The higher the pressure, the faster water flows through the coffee, so the less time we need to leave it. Traditionally, coffee shops (and most consumer espresso machines) make an espresso under ~9 bars of pressure (which takes around 20-30 seconds), but some experts argue that brewing at a lower pressure (7-8 bars) for a little longer extracts a little bit more of that sweet spot of rich espresso goodness.⁵^,⁶

Step 7: Drink it while it’s hot

We’ve made it: a handcrafted cup of precision and perfection (unless you like your coffee with milk, in which case jump to the bonus step below). All that’s left now is to enjoy the taste of your perfect cup, just remember one more thing, though: drink it while it’s hot (unless you prefer cold brew).

What we call ‘flavour’ is much more than just how things taste: the texture, temperature and smell of food also contribute to its flavour, which is why a blocked nose stops you from tasting much at all. In a hot coffee, all those rich, creamy flavours will evaporate off your tongue and reach your nose, creating the authentic full-bodied flavour we aim for.

Last but not least, experiment. Master barista Frederick Gonzalez says,

Coffee changes as you move these pieces around, and the perfect coffee is different for everyone. If you want to delve deeper, there are plenty of people out there who'd love to get you hooked; check out Standart Magazine, Barista Magazine, The Daily Grind and Coffee Talk for starters!

Created by Kirstyn Byrne

Bonus Step: Add Steamed Full-Fat Milk

‘Latte art’ takes practice and dedication to get right, but there's also a science behind why we froth milk before adding it to our coffee.

Adding warm milk helps keep your coffee warm, but heating up milk also breaks down some of its lactose into other sugars (like glucose) which makes it taste much sweeter. To get the taste just right, steam your milk to around 60°C, but don’t overdo it – any hotter and it’ll curdle.

Steaming milk also creates a ‘microfoam’: a foam so fine and creamy that you can’t even see the individual bubbles.⁷ As you steam milk, its proteins stick to air bubbles and stop them from bursting, while milk fats try to pop all these bubbles. This means it's way easier to create a strong and stable milk foam with low-fat milk, but it’ll also have lots of huge bubbles in it.

If you want a true microfoam, you need more fat around to pop everything but the smallest bubbles, so it’s best to struggle with full-fat milk. Traditionally, that meant cow’s milk was your only option (as alternative milks are very low in fat), but vegans, you’re in luck: ‘barista’ edition milk alternatives have lots of added unsaturated fat, so it foams up almost as well as the real deal.