blog post

How to Avoid Bitter Coffee and Why it Exists in the First Place – a scientific look at brewing

Coffee. Not only can it be used in scientific experiments, but every day the delectable flavors derived from the humble coffee bean wake up and fuel a substantial proportion of the population.

Image adapted from source1 and source2.

Image adapted from source1 and source2.

But just how do those delicious flavors get from the bean into your cup? It all comes down to a process called extraction. In this case, extraction is the ability to “pull” a particular chemical compound of interest out of the myriad of other chemical compounds that comprise the bean we know and love.

Extraction is the key to a great cup of coffee. Many are led to believe that good coffee only comes from a special, expensive source—for example the coveted single-origin, shade-grown variety. Much like mining for gold, however, without the correct techniques, you end up with a lot of muddy garbage that ends up covering up the good stuff you wanted in the first place!

By understanding the main extraction processes that occur while brewing, one can reliably get a great cup of coffee.

The trick is to know which coffee bean compounds you want and which you don’t want. Inside each bean are a variety of compounds that give coffee a distinct flavor. When brewing, water essentially “beats” at the coffee grinds to dissolve these compounds. Different flavor compounds come out of the bean and into the water at different rates.

I will broadly divide these compounds into three categories (though there are numerous ways to do this). As water contacts the coffee, the first flavors to come out are bright, acidic, and fruity flavors. With time, one reaches the more earthy and caramelized flavors, consisting of more complex carbohydrates and oils. These flavor compounds take more time to dissolve, somewhat due to their complexity. Finally, after enough time, bitter, astringent compounds leave the bean. These are the ones that give coffee a bad name.

As more and more flavors come out over time, the yield of the coffee increases.  A low yield coffee may taste a little more sweet and bright, but also won’t be developed—as only the first category of compounds has had a chance to escape the bean. A high yield coffee will taste bitter—as all the flavor compounds, even the bitter ones, have had time to dissolve in the water. The key to coffee brewing is to get the yield in a sweet spot where the right mix of flavors comes out.

But time is not the only factor! This is where our friend the coffee grinder comes in. As it breaks the coffee bean into smaller and smaller particles, it changes how quickly the flavor compounds can be extracted from the coffee bean. Smaller particles enable all compounds to dive into the water more quickly than if they were trapped in larger particles, as more of the bean is physically exposed. The “quick-releasing” fruity category still comes out the quickest and the “slow-releasing” bitter category still comes out slowest.

Consider a the large, unground bean: as water is allowed to contact the large coffee bean “particle,” the outsides of the bean can become over-extracted while none of the insides are even touched! Consequently, brewing with whole beans would produce extremely weak coffee, which is why nobody does it.

This is where grind size comes into play. The goal is to have enough flavor compounds dissolved into the water so that it there is something to taste, but you also want to make sure that you are tasting the most enjoyable compounds of the coffee.  You don’t want to use whole beans, as described above.  However, if you grind your coffee into super tiny particles, before you know it all the flavor compounds, even the bitter ones, will have jumped into your cup.

So, good coffee requires optimizing extraction via the precise triangulation of coffee grind size with exposure of those coffee particles to water, as shown in the graph below.

Coffee size and extraction

Why is this on a nano blog? Excellent question! To be honest, it’s here mainly because I’m a total coffee addict. However, the fact that properties change with size is super important to understanding why nanomaterials have such unique properties. Just as the properties of the coffee particles change as they get smaller (releasing their flavor compounds more quickly), the properties of all materials change in dramatic and beautiful ways when you get into the nano-size range. This is why scientists are so interested in nanomaterials and is why they have such potential for revolutionizing existing technologies.

So, want to get your hands dirty? Be a chemist and experiment! A great thing to do with a new coffee is to brew it in a drip coffee maker and collect collect the drops in 3-4 second intervals to isolate the different flavors. Taste them yourself to see how every coffee type has a slightly different profile.

Additional coffee complexities

Blade grinders haphazardly chop the coffee and produce particles of different sizes. This means that, even at theoretically optimal conditions, there will always be some smaller coffee particles being over extracted (bitter coffee) and some larger particles being under extracted (waste of coffee). Burr grinders are an effective way to produce coffee particles of all nearly the same size. Tip: If you have a blade grinder like me, grinding smaller batches, while shaking the grinder, allows for burr-grinder-like results!

Happy brewing!

References and Further Reading
Brewing Coffee – a Framework (all based on the fundamentals of extraction!)
Coffee: Recent Developments
The Complexity of Coffee (subscription required)
Comparison of nine common coffee extraction methods: instrumental and sensory analysis (subscription required)

3 thoughts on “How to Avoid Bitter Coffee and Why it Exists in the First Place – a scientific look at brewing

    • Thanks! The fundamental processes are identical: it’s which flavors one desires are quite different. So while you won’t usually vary, say, the tea leaf size, it’s important to pay close attention to the water temperature (aka the speed at which you pull compounds out) and time (which compounds you actually select to come out).

      The windows of optimal temperatures and times are much more variable tea by tea (on the order of 30 degrees C for some teas!), and it all comes back to what flavors/compounds one wants.

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