Attenuation is not all you need: The Space Guide to Beer

Abstract

The BJCP 2021 Style Guidelines describe 107 beer styles with numbers for gravity, bitterness, color and alcohol, plus pages of prose about how each should look, feel and taste. Read as a map, the guide is a set of boxes in a space of many dimensions, and it can be asked plain questions: how much of the space a beer could occupy does no style claim, where is that space, and does anyone brew there? In the four numbers every brewer uses — original gravity, final gravity, bitterness, color — about 80% of the space inside the guide's own limits is claimed by no style, and the largest empty regions are proven empty by a solver rather than merely sampled. That figure barely moves under any reasonable change to how the space is drawn (never below 65%): about four fifths of the plainest description of beer lies outside every style, so brewing strictly to the guide confines a brewer to about a fifth of the space between the guide's own extremes. Adding seven qualities the guide describes in words — carbonation, body, finish, haze, hue, head retention, sourness — the claimed share falls to about 1% or less; adding fifteen yes/no traits such as hoppy, roasty or funky, to about one combination in 24,000. The emptiness is not a sign of neglect. Random guides built from equally narrow, sensible boxes leave about as much space unclaimed — the guide slightly more, because its styles cluster. The guide's typical style is close to Belgian Pale Ale; the typical homebrew is a little paler and more bitter. In 61,530 homebrew recipes, the empty regions are rarely brewed, and most of that shortfall is matched by the ordinary link between strength, bitterness and color; the rest — a tenth to a fifth — is consistent with brewers following the guide, which raises the question of whether a guide written to describe beer has come to prescribe it. The report describes where the gaps lie rather than ranking them, and keeps every assumption and every number traceable.