In the standard-issue American brewery, the brew kit is a technical marvel designed to make chemical reactions happen within precise specifications—this is as true of the plant in St. Louis as it is at the brewpub around the corner. When building an American brewery, people think of chemistry, not history. But elsewhere in the world, in places where beer has been brewed for centuries, many breweries see their gleaming copper tanks as tools of preservation, not just enzymatic conversion. You find weird practices, like decoction mashing (Czech Republic), turbid mashing (Brussels), carbonation with recaptured CO2 to Reinheitsgebot-compliant (Germany), and in London, a thing called parti-gyle brewing.

Back in the olden days, brewers didn’t know that much about chemistry or microbiology. They understood the basics of mashing, but didn’t know which enzymatic conversions were happening at certain temperatures. Even if they had known, they wouldn’t have been able to control temperature, anyway (the thermometer wasn’t adopted until the 1760s). That’s where various oddball mashing schemes were invented. In parti-gyle brewing, a brewery would pack the mash tun full of malt and then draw off multiple worts (“gyles”). In popular understanding of the technique, the first wort would make a strong beer and the second would make a “small” beer. That kind of linear wort-collection was practiced—Belgians regularly made multiple worts, too—but in parti-gyle brewing, the worts are later blended, producing beers of differing strengths, but not strengths identical to the worts.

It’s possible that a brewery here or there still dabbles in parti-gyle brewing, but no one makes it such an integral part of its process as London’s Fuller’s. It’s the process Fuller’s uses to make its three flagship beers—Chiswick Bitter, London Pride, and ESB (along with a rarer specialty beer, Golden Pride). All of them come from the same mash and go through a boil with the same hops and hop schedule; their distinctiveness comes from the blending of wort.

Fuller’s uses parti-gyle brewing to make its three flagship beers. Photo by Jeff Alworth

The way it works is conceptually simple, though there’s quite a bit of math involved. Fuller’s uses two mash tuns, staggered so that one is resting while one is being collected into a copper. Georgina Young, Fuller’s current Brewing Manager, described it for me. “The mash tun stand is simple infusion at 65° C [149° F] of 50 minutes. We then recirculate the bed for 10 minutes until the wort is clear and start collecting it in the copper.” They collect the first worts from both mash tuns—the high-gravity worts—into one kettle (known in Britain as a copper). Young continues: “When [the first kettle] is full (520HL) we continue the sparge on both mash tuns but collect the [small] worts into the second copper. The last runnings are typically 1005 SG for both mash tuns.”

They boil the strong and weak worts separately, adding hops at the start and end of the 60-minute boil. “So we now start proportioning the wort to make the beers,” Young explains. “We know the target extract, so all we have to do is calculate how much first wort and then second wort is needed to achieve 260HL in each fermentation vessel at the correct original gravity.” And from these two worts come the four beers, in strengths of 8.5% (Golden Pride), 5.9% (ESB), 4.7% (London Pride), and 3.5% (Chiswick).

I know that among the avid beer fans of London, Fuller’s is not considered a happening brewery (although I think it is broadly admired for taking risks and reaching out to the proliferating new breweries). For my money, London Pride is one of the finest beers brewed in that or any country, and if I could teleport myself to, say, the Jack Horner for a pint and pie, I would do so at least once a week. Cask ales are made spectacular by subtle degrees, and London Pride has a balance of flavors—soft, nutty malt, stiffening minerality, apple esters, and citrus-to-marmalade hops—that make it a perfect bitter.

I figured that a brewery like Fuller’s wouldn’t possibly continue such a strange practice unless it had an effect on the beer, but flinty, pragmatic Head Brewer John Keeling shot this idea down. “Some effect on the way the maillard reactions happen in the two kettles?” I suggested. “A difference in hop utilization?” Nothing of the kind, he said. “It is quite simple really. Parti-gyles are the most efficient way of using a mash tun both in terms of speed and in terms of extract.” (In a parti-gyle, no sugars are left behind.) “It is not complicated and is rather simple and elegant.”

Derek Prentice and John Keeling of Fuller’s. Photo by Jeff Alworth

Well, maybe. Following my visit to the brewery back in 2011, then Brewing Manager Derek Prentice pointed out at least one advantage it had on the beer. “As you appreciate from your visit here, it is considerably different from producing different gravity beers from a single wort or fermentation.” He offered one example. “Dark malts typically will be readily solubilised and hence more prevalent in first worts and the copper loading of the hops into the different copper lengths are adjusted for the specific parti-gyle. Once the wort streams are blended into the fermenter they will impart unique characteristics into the final beers.”

As Keeling says, the parti-gyle system has the benefit of being especially efficient—but I still have a hard time believing that’s the reason Fuller’s still uses it. These old practices have a way of sticking around, and the more important reason they do is because no one can argue with the beer they produce. In the late 1970s, owners of Fuller’s recommitted themselves to making exceptional beer, and began spending on modern equipment and focused on careful, precise brewing practices. Between 1978 and 1989, the three parti-gyled bitters each won a medal as Champion Beer of Britain, the country’s most prestigious cask ale award—and ESB won it three times. They have gone on to win boatloads of awards.

Maybe it is just a more practical, efficient way of making beer. (Though if so, it’s curious that no other breweries employ it.) Or maybe Keeling is just happy to keep its benefits to himself.

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Jeff Alworth is the author of the forthcoming book, The Beer Bible (Workman, 2015). Follow him on Twitter or find him at his blog, Beervana.