While I love well-conceived and expertly crafted traditional sour beers (with their mouth-puckering lactic acid-produced sourness), I wanted to produce a different and what I consider uniquely Japanese sourness in our Japanese Koji Sour beer. I think we’ve hit the mark.

Japanese koji is a fungus which is used in the crafting of many traditional Japanese fermented foods and beverages. There are two types of Japanese koji (yellow and white). In sake production, yellow koji is the fungus of choice. In shouchu production, white koji gets the call. Koji is used for the purpose of converting the starch in grains into sugars fermentable by yeast. In the conversion process involving white koji, ample amounts of citric acid are produced which lowers pH and yields a delicately sour citrusy flavor.

It is this delicate and delightfully crisp sour-citrus character that we are pursuing when incorporating Japanese white koji into our beer brewing process. The white koji is added directly to the malt mash where it works its acidifying magic. Unlike in traditional sour beers, which derive their mouth-puckering sourness from the bacterial (lactobacillus) production of lactic acid, the citric acid production by Japanese white koji lends a softly tart and pleasingly refreshing character to our Japanese Koji Sour beer.

In this batch #006 (Daidai Session Sour) version of Japanese Koji Sour, we add both juice and zest from locally grown daidai fruit in two separate additions: first on the hot-side after the boil and then on the cold side in the conditioning tank after primary fermentation. This contributes further notes of fruity-citric tartness and spicy aroma to the beer.