Bottle conditioning refers to the process by which the beer is naturally carbonated in the bottle as a result of fermentation as opposed to being carbonated prior to filling. Oftentimes additional sugar or krausen is added to the beer prior to bottling or directly to the bottle so that the yeast will have enough sugar available to properly carbonate the beer.
A suitable fermentation temperature must be maintained for the conditioning beer to allow the yeast to adequately carbonate the beer. Since viable yeast is present in a bottle-conditioned beer, this provides an additional component of flavor that develops further as the beer ages. A slight layer of yeast on the bottom of a bottle of beer may be a sign that the beer had been bottled conditioned, but may also be due to poor filling or residual clarification of a non-filtered beer. The bottle and cap should always be sanitized before bottling occurs.