The Science Behind a Great Cup of Coffee

My favorite coffee drink is a cortado — and it definitely takes a professional to make it correctly. . (Photo: Carrie Anne White/Shutterstock)

I didn't drink coffee in any serious way until I was 29 and began graduate school. So my coffee-quaffing that began a decade ago coincided with the rise of the artisanal java cafes and frankly, it's been a love affair ever since. You might assume I have expensive beans in my freezer, and a complicated coffee-brewing setup at home, but I don't even have coffee in my house. That's because, though I've tried, I've never come close to the delicious cortado (my coffee drink of choice, which is 2 ounces of espresso and 2 ounces of milk) that a good barista can make for me.

There's a reason for that, and it comes down to the complex chemistry of coffee, water and milk, the quality of the beans and the talent and training of the person bringing all that together.

As materials scientist Christopher Hendon points out, coffee is one of the few beverages that isn't served to you as-is, with flavors and quality pre-determined by the producer (like, say, a glass of beer or wine or even juice). That means a handmade coffee drink is more like a cocktail than a soda. (Which is why I always tip my barista $1 per drink, as I would a bartender.)

Coffee is more than the beans

First, there's water chemistry, which can change the flavor of the final product. Coffee is acidic, and that can be affected by the acidity of the water used to make it, leading to a more- or less-acidic cup. "Brew water containing low levels of both calcium ions and bicarbonate (HCO3−) – that is, soft water – will result in a highly acidic cup, sometimes described as sour. Brew water containing high levels of HCO3− – typically, hard water – will produce a chalky cup, as the bicarbonate has neutralized most of the flavorsome acids in the coffee," writes Hendon.

Most good cafes will be fussy about the water that goes into the espresso shot that is the basis of your drink for that reason. At home, getting the water right can be enough of a hurdle to keep you from getting your flavor right. I personally like the taste of mineral-rich "hard" water from my well, but it doesn't make for great coffee.

According to the Arrhenius equation, which states that chemical reactions happen more quickly at higher temperatures, the temperature of the water is another important part of the chemistry of coffee. Higher temps mean more coffee flavors are extracted from the beans, but also means that more of the flavors you don't like will come through, too. How long you brew the coffee, what method you use, and how long the ground beans are exposed to the hot (or cold) water can all change the flavor.

Temperature also affects, and is affected by, the surface area of the coffee beans that are exposed by grinding. Even if you grind your own beans before brewing, it gets complicated fast: "Simply grinding more fine to increase extraction invariably changes the brew time, as the water seeps more slowly through finer grounds. One can increase the water-to-coffee ratio by using less coffee, but as the mass of coffee is reduced, the brew time also decreases," explained Hendon.

The 'how' counts a lot, too

Various methods, from French press to pour-over to espresso machine (and don't forget cold brew!), all play with the variables of water temperature and brew time. Baristas fuss over brew times, grinding beans fresh for each cup, and tamping just-so — and all are adjusted depending on the coffee being served. So a lightly roasted Ethiopian bean will get a different treatment from a darker-roasted Costa Rican.

I haven't even touched on the types of beans, where they're grown and how they're roasted. A great cafe will have baristas and/or a manager who tests each variety of bean and makes changes to the brewing process accordingly.

Taking all that into consideration, I'll happily spend $4-$5 on a small cup of perfectly made espresso and milk a couple times a week. I'd rather drink less coffee and enjoy it more. Plus, I'm just not dedicated enough to learn about all those variables. But if you want to DIY your coffee-drinking experience, I'd imagine it can be a fun and challenging project.

Coffee, as in life, is all about what works for you. For all of you who just love a fresh pot of home-brewed coffee, more power to you. The rest of us will sit back and appreciate the coffee and espresso drinks that are made for us by people who have a passion for learning about, testing and making them.