In-bottle secondary fermentation of self-brewed beer Yeast is live, so the second fermentation should be done in the bottle as soon as possible after the first fermentation is complete. The characteristic of yeast is to breathe out after eating, and to turn alcohol into alcohol after eating sugar. By the time the first fermentation is complete, the beer is almost free of sugar, so no more alcohol is added (note that after the first fermentation the alcohol content will be very low, around 3%).