Adding tonic to cold brew or espresso is nothing new. For the past few years, specialty coffee enthusiasts have been enjoying the refreshingly fizzy and caffeinated beverage to quench their thirst and get a pick-me-up during the warmer seasons.

Philadelphia, in particular, has seen a rise in the espresso and tonic combination. While some proclaim that the bubbly combination results in a bright and citrusy flavor, the addition of tonic to espresso actually creates a less than pleasant experience for the taste buds. If you didn’t know, tonic is both sweet and bitter due to its primary flavoring agents, cane sugar (sweet) and quinine (bitter). If the drink is simply espresso and tonic, it doesn’t matter what kind of tonic water or espresso you’re using; the concoction will always taste bitter even if you’re using light roast espresso, which is less bitter than darker espresso.

So why is the popular espresso and tonic so bitter? Let’s talk about grind size and flavor extraction.

When you grind coffee beans, you get some grounds that are the intended size, some that are smaller and others that are bigger. Generally speaking, the finer you grind coffee/espresso, the more uneven the grind size particle distribution is. The grind size for espresso tends to be on the finer side of the spectrum. When water flows past/through coffee/espresso grounds, the amount of flavor extracted depends on the surface area of the grounds (i.e. the size of the grounds). The amount of flavor extracted also depends on many other factors, but we aren’t worried about those here. The bigger the surface area of the grounds (the smaller the grounds), the more flavor is extracted. The smaller the surface area of the grounds, the less flavor is extracted.

Hold on. Read the last two sentences of the previous paragraph again. We hope you’re saying, “Don’t bigger grounds have more surface area than smaller grounds?” and of course, you are right. But, many many more smaller grounds can fit in the same volume as bigger grounds, and the cumulative surface area of the large number of smaller grounds is much bigger than the cumulative surface area of the smaller number of larger grounds. Think about a marble and a basketball. The basketball has more surface area than the marble. But if you fill the entire volume of the basketball with marbles (there will still be little spaces between the marbles), the surface area of all the marbles combined is much bigger than the surface area of the basketball. If you’re still confused, another example would be sheets of paper stacked on top of each other (there are tiny spaces between each sheet) vs. a single solid box. So yes, that wasn’t a typo. There are always way way more small grounds than large ones. The smaller grounds have more surface area, so more flavor is extracted from them. The larger grounds have less surface area, so less flavor is extracted from them.

Back to espresso. As we just stated, the bigger particles have less flavor extracted while the smaller ones have more flavor extracted. When you’re drinking it as straight espresso, the unevenness in flavor is more difficult to notice because the drink is so strong. When you dilute it (make it weaker), the unevenness becomes noticeable. You’ll taste both underextracted (sour) and overextracted (bitter) flavors. This is why Americanos don’t have the same clarity of flavor and sweetness that batch-brewed or pourover coffee have as the grind size for batch-brew and pourover is much coarser than for espresso, so the flavor extraction is more even.

Diluting unevenly extracted espresso (even the world barista champions extract espresso unevenly) with tonic makes that flavor unevenness more apparent because the strength is lower. As we already mentioned, the unevenly extracted espresso is both sour and bitter while the tonic is both sweet and bitter. Sour and bitter plus sweet and bitter results in the espresso tonic having a little bit of sourness, a little bit of sweetness and a whole lot of bitterness. Bitterness isn’t really our thing (balance is), but we get that some people are into it. If that’s the case, you’ll love the espresso tonic.

If you’re looking for something a bit more balanced, then look no further than the coffee tonic, a blend of iced coffee and tonic. Note that here we are talking about Japanese flash-cooled iced coffee not cold brew or traditional iced coffee.

As with brewing hot batches or hot pourovers, when brewing iced coffee, the grind size is quite coarse, so the flavor extraction is much more even. As a result (if the coffee itself is good quality and has been roasted well), the iced coffee tastes sweet, with some acidity (sourness, but in a pleasant way), and of course a little bit of bitterness (coffee is inherently bitter, but we try to minimize it). Dilution here isn’t nearly as much of an issue as it is with espresso. With an espresso tonic, the majority (at least 75%) of the drink is tonic, and just a small part is espresso, so the espresso is diluted a lot. A well-balanced coffee tonic will be about equal parts coffee and tonic, so it isn’t diluted as much, and the flavor is much more even to begin with. As a result, the drink is sweet, with some acidity and some bitterness. In other words, it’s balanced.

The coffee tonic is particularly excellent when made with a light-roasted washed Ethiopian coffee that is sweet, fruity, and floral.