When the sugars in the fermenting beer have been almost completely digested, the fermentation process slows and the yeast cells begin to die and settle at the bottom of the tank. At this stage, especially if the beer is cooled to around freezing, most of the remaining live yeast cells will quickly become dormant and settle, along with the heavier protein chains, due simply to gravity and molecular dehydration. Conditioning can occur in fermentation tanks with cooling jackets. If the whole fermentation cellar is cooled, conditioning must be done in separate tanks in a separate cellar. Some beers are conditioned only lightly, or not at all. An active yeast culture from an ongoing batch may be added to the next boil after a slight chilling in order to produce fresh and highly palatable beer in mass quantity.