Rhubarb contains oxalic acid, the same compound in sorrel, which is added to BlueJacket’s Sidewalk Saison. Oxalic acid can cause health problems in large doses, but it is concentrated in the rhubarb leaves, with malic acid (also found in apples and grapes) the predominant acid in the stalks. Malic acid is often converted to lactic acid by both malolactic bacteria, which are sometimes used in wine fermentations, and by the lactic acid bacteria commonly found in mixed-fermentation sour beers. Lactic acid tastes "softer" than malic, whose sharp flavor I associate with SweeTarts candy.
When I bought a few pounds of rhubarb this past spring, I was planning to add it to last years' Dark (Red) Saison V, but after tasting how acidic it was already, I added membrillo (quince paste) instead. This Berliner weisse hadn't soured as much as I wanted after a year, so I decided it was a good opportunity to use the rhubarb I had cleaned, vacuum-packed, and stored in the freezer. Like most things beer, I wasn't the first to brew a sour beer with rhubarb. The two most popular are probably Cantillon’s annual-experimental Zwanze (both 2008 and 2012), and New Glarus Strawberry Rhubarb, the former dry and sour, the later sweet and tart. I wasn't even the first homebrewer, what with Ryan Brew's Rhubarb Berliner Weisse.
Appearance – “Wine-cooler" pink as a friend described it recently (the photo doesn't do it justice). Not quite clear, but pretty close. The bright-white head unsurprisingly fizzes away within a minute of pouring. The second pour, containing the dregs, actually appears pinker than the first.
Smell – The aroma is clean, lightly doughy, and fruity. Rhubarb is tricky to describe, it isn’t distinct/powerful like cherries, raspberries, or peaches. It certainly shares some subtle similarities to its frequent companion, strawberries, but considerably milder. Luckily despite the similarities in appearance to celery, it has none of its green-vegetal character.
Taste – I added about 1.5 lbs/gal of rhubarb, and while the oxalic/malic acid enhanced the acidity it didn’t make the beer excessively sour. The fruity flavors meld well with the mellow base beer, not obscuring its mild wheaty flavor. The Brett (mostly Trois/Drie) adds a softy farmyard-hay flavor, nothing aggressively funky. The soft red-fruitiness lingers into the finish. Dry, but the fruitiness prevents it from coming across overly so.
Mouthfeel – Light, crisp, and spritzy, as it should be. About the maximum amount of CO2 (3.4 volumes) I’d put in a bottled beer, on opening foam starts to slowly build in the neck.
Drinkability & Notes – Probably not the ideal time of year to drink this light and refreshing, but that isn't stopping me from enjoying it! With the added flavor of the rhubarb I don’t miss the decotion mash I usually perform for Berliners (this was my first batch with a single-infusion mash). I still need to write up notes on the plain half, we'll see if that holds true.