Left Hand Brewing Co. has issued a recall for bottles on one of its most popular releases that “do not meet brand specs due to a foreign brewing yeast.”

According to post on the Longmont, Colo.-based brewery’s website, the beer in question is Milk Stout Nitro, a six percent ABV milk stout brewed with lactose that was first released in bottles in 2011. The recalled beers include Milk Stout Nitro bottles with best by dates of 10/16/16 – 2/28/17 as well as any bottles that are included in the brewery’s Mountain Mixers variety package with codes 130B – 242A.

Like other breweries, Left Hand uses specific yeast strains for our ales and lagers. Each yeast strain gives specific desirable characteristics to the beers and are a key ingredient in our recipes. Unfortunately, a foreign brewing yeast, which looks just like our own yeast, came into contact with our house ale yeast and impacted the gas in our bottles for specific periods of time and under certain conditions.

Because the yeast looks similar to our own ale strain, it was not identified until the Milk Stout Nitro bottles arrived in the market, with the gas mixture further impacted under specific conditions. We have discovered that the foreign yeast strain is creating a secondary fermentation inside of the bottles and is causing an increase in CO2. The resulting effect is changing the gas qualities of the beer and causing the beer to overflow and disrupt the customary nitro cascade.

Consumers who believe they have affected bottles should return the product to the location where it was purchased, and will be issued replacement bottles that will be available in approximately six weeks.