A Perfect Cup of Coffee | 7 Science-Backed Steps
May 28, 2019 Aran Shaunak By Aran Shaunak

A Perfect Cup of Coffee | 7 Science-Backed Steps

There are hundreds of ways of making a coffee, and everyone thinks theirs is the best. But is there a scientifically ‘perfect’ coffee?

Modern baristas have turned making coffee into an art form, and we're all addicted to their work - even paying exuberant prices for a pretty average coffee on a daily basis. But what if we knew how to do better?

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 Bean

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’s 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: Dark Roast Your Beans at 200-240°C

Before roasting, coffee beans are actually 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 (200-240°C) breaks down most of these molecules. The high heat drives other reactions that replace the unpleasant molecules with so-called ‘aromatic’ molecules, which taste and smell far better.²

If you roast coffee beans for longer (a darker roast), 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. So, you’ll end up with a better tasting coffee.

But, don’t roast your beans for too long or else they’ll get too hot and some of those aromatic compounds will start breaking down into less tasty things. Even worse, if your beans reach a toasty 250°C they’ll crack in half, and all the flavour you’ve created will evaporate away.

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 flavour more quickly. But, it’s also easier to burn or over-brew a finely ground coffee, giving you lots of bitter compounds that can ruin the richer flavours.

It’s also key to use the right grind for your chosen method of brewing. For example, using an espresso machine that makes coffee in <1 min 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 even more quickly than roasted beans, so always use freshly-ground coffee – no excuses.

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 just one way of doing it: you could use an espresso machine, a French press, or a complex contraption, which 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:

  1. How coarse or fine you grind your coffee beans,
  2. The water you use, and
  3. 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.

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 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.

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 actually have any coffee to drink. For best results weigh your ground 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, so really short brewing times don’t work very well. 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.

Baristas in coffee shops generally make an espresso under ~9 bars of pressure (which takes around 20-30 seconds), but they must be in a rush. Using less pressure (~4 bars) but allowing the coffee to brew for longer (almost a minute) will make it easier to hit that sweet spot of rich espresso goodness.

So, drop the pressure and take your time to make your perfect cup of coffee.

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.

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

Don’t even think about icing a coffee this good.


Created by Kirstyn Byrne

Bonus Step: Add Steamed Full-Fat Milk

Forget ‘latte art’ – it’s an art that will take tons of practice and dedication. But getting the perfect texture and flavour in your froth is something science can help with.

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 make 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. As you steam milk, its proteins stick to air bubbles and stop them from bursting, while milk fats try and pop all these bubbles. This means it’s way easier to create a strong and stable milk foam with low-fat milk, but it also means it’ll have lots of really large 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 on 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: Oatly’s new ‘barista’ edition milk has lots of added unsaturated fat, so it foams up almost as well as the real deal.

References

  1. Buffo, RA (2004) "Coffee flavour: an overview." Accessed 26/04/2019.
  2. Petracco, M (2005) "Our Everyday Cup of Coffee: The Chemistry behind Its Magic." Accessed 26/04/2019.
  3. Parenti, A (2014) "Comparison of espresso coffee brewing techniques." Accessed 27/04/2019.
  4. "Espresso Recipes: Time." Barista Hustle. Accessed 27/04/2019.
  5. Sebba, F (1971) "Microfoams — an unexploited colloid system." Accessed 29/04/2019.
  6. "Brew Temperature and its Effects on Espresso." Five Senses Coffee. Accessed 27/04/2019.