Next month, The Rare Barrel Beer Co. will be releasing Keen Observation, a golden sour beer aged in oak barrels with apricots.

In particular, the newest beer from the Berkeley, Calif.-based brewery is aged in unrinsed oak barrels used to previously age Map of the Sun, which was also a golden sour aged with apricots. This makes keen Observation a member of The Rare Barrel’s Echo Series.

The Echo Series is an experiment with unrinsed barrels, in which the yeast, bacteria, beer, and fruit from the previous beer aged in a barrel is allowed to remain and blend with and influence the next beer. The process by which this is done is fairly straight-forward. The majority of the beer in a barrel is removed and placed in the blending tank. The remaining beer is removed with a sight glass until yeast and bacteria are seen in it. This usually results in a few inches of beer being left in the barrel, which would normally be rinsed. In this instance, instead of rinsing, the next beer is added directly to the remnants.

Keen Observation has an ABV of 5.7 percent and will be available in 750ml bottles when it goes on sale at the brewery on May 5.

Update (April 26, 2016) — This post was updated to include details on the release date. It was originally published on April 11, 2016.