I know you’re trying to relax, but I’m still going to make you do some math. Ready? Here’s my favorite equation: beer = malt, hops, yeast, and water. Understanding that equation helps you order the right beer in the right circumstance. You in the mood for the flavors of toast or chocolate? Then you want a malt-forward beer like Chuckanut’s award-winning Dunkel. Or perhaps you want to annoy your evolutionary ancestors by opting for the taste of bitter fruit. Then you want a hoppy beer like Aslan Batch 15. Or maybe you want to tantalize your taste buds with the spiciness of pepper or clove. Then you want a yeast-forward beer, like Wander Wild Warehouse. But what about that last ingredient – what about water? Well, you want your beer to be in liquid form, right? Then water is the key. But water plays a more interesting role in beer than simply making it a liquid. And today of all days, it’s appropriate to pay homage to the role of water in brewing, because water is the reason why we drink stout on St. Patrick’s Day.

Emblazoned on a beam a the Guinness brewery in Dublin.

The story is not that complicated, actually. You know how you’re not supposed to go swimming in a lake in the middle of a thunderstorm? Right, everyone knows that. But do you know why you’re not supposed to go swimming in a lake in the middle of a thunderstorm? It doesn’t have anything to do with the hydrogen and oxygen molecules that combine to make H20. Rather, it has to do with the other stuff that is found in water, the minerals that differentiate tap water from distilled water. Well it’s that other stuff – the minerals found in tap water – that plays a crucial role in brewing, too, and which explains why Dublin has a historical association with stout.

Brewing is an acidic enterprise: in order to get fermentable sugars out of malted barley, you have to let it bathe in a slightly acidic bath of hot water. And the way to get that bath to be at the Goldilocks level of acidity is to make sure that it includes the right concentration of minerals. Now it turns out that when you combine water (which is neither acid nor base) with heavily roasted malts – like the dark barley used to brew a stout – you end up with a bath that’s a lot more acidic than the water you started with. This is because when you roast barley you initiate the infamous chemical reaction called the Maillard reaction, which is the same thing that happens when you put a piece of bread in the toaster. One of the products of the Maillard reaction is acid, so darker malts tend to make brewing water more acidic than lighter malts, just as toast is more acidic than bread (and tastier because of the contrast that the acidity provides).

The reason this matters for us today is that the naturally occurring water in Dublin contains so many minerals that fight acidity – i.e., it is high in alkalinity – that drastic measures were required in order to balance out that alkalinity and get the brewing water to be at that Goldilocks level of acidity. How to do it? Well, one option, as we’ve seen, is simply to throw in some heavily roasted barley. And voila: the stout is born.

(By the way, in the Czech Republic they have the opposite problem: the naturally occurring water is so easy to acidify that dark malts would make the brewing water chemically inhospitable: hence the light-colored beer we call pilsner, which hails from Plzeň.)

Of course, modern brewers don’t have to use naturally occurring water: they can add and subtract minerals at will (using reverse osmosis filtering, for example, or adding brewing salts) to allow them to get just the right water chemistry. But the history is still worth knowing: the reason you’ll be drinking a stout today isn’t just because it’s what the Irish happened to find appealing; instead, it’s because they had to brew something dark in order to have beer to drink at all. Necessity is the mother of invention, after all, and it’s hard to think of any more pressing need than an alcoholic beverage with which to say sláinte!