Every article on Hop Creep treats it like a problem. Something nasty to be avoided at all costs. Personally however, I think they’re wrong.
Simply put, Hop Creep is when the enzymes on the hop used in dry hopping start to convert the remaining starches in the beer to sugars, which the residual yeast will then convert to alcohol – and carbon dioxide. It’s referred to as over-attenuation and can make bottles and cans explode, and kegs gush foam. Modern brewers do not like this side effect of the high rates of dry hopping.
But like so many things in brewing that are seen as bad, there’s actually a place for this, and it explains a few things about heritage brewing that have seemed a little strange.
Historically some beers (mostly IPAs destined for India) were fermented right out, no residual sugars left, as flat as possible before being put into barrels with a handful of hops, then stored out in the brewery yard packed with straw for a year. It seems a bit daft to do this, until we take into account Brettanomyces yeast and hop creep. Brettanomyces is a much slower acting yeast than the far more common Saccharomyces that is used in beer. Whereas Saccharomyces yeasts will ferment in about a week, Brettanomyces yeasts will take 6 months to a year. Whereas Saccharomyces yeasts will clean up after themselves in about a week, Brettanomyces yeasts will take about 6 months.
Keeping these barrels in the yard in all weathers though for that year doesn’t seem to make sense, until we remember that they were packed around with straw, which is a very good insulator. The beer in these barrels would have warmed up, which for the amylolytic enzymes on the hops was a good thing.
When we mash in the grains to make beer, the temperature of the mash is very important to the activity of the amylolytic enzymes on the grains. Too cold and there’s no activity at all. Too hot and the enzymes die off. But a long, stable warm temperature will see those enzymes happily converting all the remaining starches to sugar to feed the yeast. And that yeast converts it to alcohol, and carbon dioxide. And that carbon dioxide will keep the pressure in the barrels up, gently seeping out through the wood, and preventing oxygen from getting to the beer. Also at that temperature the essential oils in the hops would more easily seep out into the beer.
Why make sure the beer is totally flat beforehand though? Because when the beer is totally flat, the faster acting Saccharomyces yeasts have fully finished fermenting everything they can, and will drop out of the beer. If that doesn’t happen, they’ll start working on the sugars produced by the hop creep and you get exploding barrels, or these days cans and bottles. And that is not a good thing.
With all this in mind then, we can see that hop creep, the now frowned upon bad thing in brewing was actually a very vital part of brewing and beer stability in the past. It meant that beer lasted longer, much longer, and was able to be shipped halfway around the world arriving in perfect condition.