One of the more interesting chapters in brewing history is the ver-r-r-y slow shift from sweet ales spiced with gruit to hopped beers. The first time anyone mentioned using hops in beer was 822, when the abbot of a French monastery wrote about it. It would be something on the order of three hundred years before hopped beer enjoyed any kind of popularity, though.
It's not hard to see why it would have taken so long. For one, hopped beer is hard to make. In order to get the full benefit, brewers have to boil the hopped wort for an hour or more. Gruit beer may not even have been boiled; why would brewers have gone to the trouble to collect and burn wood when they could extract the flavor from spices without a long boil? (Brewing, a domestic chore until the monks took it up in the seventh century, was little-documented.) When brewers started working with hops, they wouldn't have understood this feature, and it would have posed technical challenges. Perhaps more importantly, the flavor of gruit beer and hopped beer is quite different. It's clear this was the main barrier. An acquired taste, people initially rejected the new product wherever it was introduced. In Britain, the last frontier, locals managed to stave off the stuff until the 1500s.
The reason hops won out was not aesthetic, it was biological. The Hanseatic towns of Bremen and Hamburg were the first to perfect hopped beer, and they had a huge market advantage: their beer kept. They could brew it, ship it to Amsterdam, and even then it would outlast beer brewed locally. Until the introduction of hops, beer had a shelf life of days before it would curdle into vinegar. Hopped beer was still saturated with wild yeasts, but fewer, and the hops delayed the souring process. Hops made it possible to ship beer and turned it into a regional commodity--and therefore a commercial one.
I recount all of this history because a thought occurred to me that I wanted to bounce off you--for you are collectively smarter than I. Breweries long avoided brewing in summer because the wild yeasts were too virulent to make decent beer. Until as late as the 19th century, lots of breweries quit making beer in the summer because of this. Of course, beer was hopped, so it would keep long enough that people could drink beer throughout the summer. Here's the question, though: before hops, there's no way beer would have kept throughout the summer. Wouldn't brewing have been a strictly seasonal pursuit? If so, it's no wonder it remained a domestic chore, not a viable commercial activity. Am I missing something?