What is "refermentation"

n. The act of fermenting again, the fermentation of something that has already been fermented.

Refermentation, also known as bottle conditioning or bottle refermentation, can be an adverse side effect for opened wine, but many brewmasters purposefully use refermentation to create greater complexity and aging ability in their brews. There are two types of refermentation: 100% refermentation and partial refermentation. 100% re-fermentation or bottle conditioning is when flat beer is bottled and all the carbonation comes from fermentation in the bottle, see Brooklyn Brewery Local 1. Partial re-fermentation, commonly referred to as bottle-conditioned, involves adding artificial carbonation into a beer, then adding yeast and sugar into the bottle to add further carbonation during the second fermentation. Bottle-conditioned beers are only partially bottle-refermented. It is very rare to find 100% bottle-conditioning.

Typically refermentation leaves sediment, or lees, in the bottle.

One benefit of refermentation is that it removes any oxygen in the bottle thus preserving its life and prevents spoiling. The yeast then naturally creates carbonation instead of artificially carbonating. Many professional beer tasters can tell the difference artificial and natural carbonation: Natural carbonation is more delicate and fine, rather than an aggressive and sharp one.