For most of us, the way in which we brew our coffee is a matter of habit and tradition. We do the same things every time because we know they work.
Occasionally, we might get wild and add some vanilla or cinnamon, but few people spend much time pondering the finer points of their technique or even the science of coffee.
What actually takes place between bean and brew?
Is coffee healthy?
Does the process you use affect the taste of the final product, or is this all in our heads?
Most importantly, what can science tell us about preparing barista-quality coffee at home?
The Chemistry of Coffee
Just what is in the typical cup of Joe, anyway?
To state the completely obvious, coffee is water with stuff mixed into it. This “stuff” consists of more individual compounds than you can shake a stick at.
All of them together make up the flavor we call “coffee”, but none of them taste like coffee by themselves. Some are fragrant, some are bitter, some are sour; getting the best possible brew means extracting all of them in a balanced way.
Altogether, the material pried out of the beans makes up about 1.2 to 1.5% of coffee brewed using filter, French press, and similar methods, rising to a whopping 8 to 10% for espresso.
It comes as no surprise that espresso machines produce a more potent drink, but the science of coffee shows us how large this difference really is.
Ground Size Matters
If you buy branded, pre-ground coffee at the supermarket, you’re almost certainly not getting the best possible flavor every morning. Using a grinder takes two minutes extra, but you’ll be more than pleased with the results.
According to the science of coffee, there are three main reasons for this:
Ground Coffee Loses Its Subtle Flavors Quickly
Coffee beans, once roasted, stay fresh for about a month if properly stored.
Should they be broken up into grounds, however, you’ll notice a much flatter flavor after less than half this time. In fact, it takes only minutes for freshly-ground coffee to become significantly less potent.
Smaller Particles Lead to More Surface Area
This is basically a no-brainer: the more contact hot water makes with your ground coffee, the quicker the flavor can move from bean to brew.
If your coffee tastes sour and anemic, the flavor hasn’t been properly extracted. This isn’t just a question of how much time it takes to make a cup, either: steeping the grounds for too long in order to compensate for a too-coarse grind results in a very bitter, highly caffeinated drink. The least appetizing parts of coffee are typically the last to dissolve, so balancing timing and ground size is critical.
Ground Size Actually Changes the Flavor Profile
Closely related to the previous point is the idea that flavor molecules need some time to travel from the interior of each particle to the surface, where the hot water can scoop it up.
To give a highly inaccurate analogy, sunlight needs only 8 minutes to travel the 93 million miles to Earth – but it may take thousands of years for a light particle to get from the core of the sun to its surface.
In a French press, a courser grind is preferred. Not only does this stop particles from passing through the sieve, but the several minutes during which the coffee gets to steep also allows efficient extraction for a more full-bodied flavor.
In an espresso machine, by contrast, water is only in contact with the grounds for seconds. A much finer grind is therefore used, and even so fewer of the more robust (and potentially offensive) aromatics end up in the final product.
If you own a grinder (or have an espresso machine with an integrated, adjustable grinder), you can experiment with different ground sizes as well as roasts and beans. The difference in flavor isn’t dramatic, but it’s definitely noticeable.
How Roasting Affects Flavor
There’s a persistent myth that dark roasts – coffee beans that have been baked for longer – are “stronger” than light roasts.
The truth is somewhat more complex.
Coffee made from green beans isn’t great; they have a pronounced herby flavor with little depth that few people will enjoy. The magic only really starts to happen when they’re kept at around 300 °F (150 °C) for a few minutes. In terms of the science of coffee, what happens is called the Maillard reaction.
As the roast gets darker, some of these nuances start to disappear, making different blends of coffee taste more alike.
At the end of the day, your personal preference is really the only deciding factor. If you like a bit of variety, though, you’ll probably want to get cheap beans roasted to a deeper shade, allowing the more expensive stuff to express its individual quirks with a light roast.
The Science of Coffee and You
Surprisingly, there exist several controversies on what exactly constitutes a good cup of coffee, even among experts. You can get multiple, dearly held views, for instance, on whether crema is important to espresso’s flavor or basically just cosmetic.
At the end of the day, your own tastes should be the deciding factor, but certain basics can’t be ignored. Apart from those mentioned above, you should certainly use pure water that’s neither too hard nor too soft, measure the amount of coffee and water like a chemist, and remember that no amount of science can compensate for poor ingredients.