When it comes to bottle conditioning, the need to add additional yeast depends on the age of the beer and the fermentation conditions it has undergone. In general, if a beer is young enough or the fermentation conditions were not particularly hostile, there should be enough yeast cells in solution to adequately produce carbon dioxide during bottle conditioning. This means that adding additional yeast may not be necessary.
One factor to consider is the age of the beer. If the beer is still relatively young, the yeast cells that were present during fermentation should still be viable and active. These yeast cells will continue to ferment any residual sugars in the beer, producing carbon dioxide and creating the desired carbonation in the bottle. In this case, there should be no need to add additional yeast.
However, if the beer has been aged for an extended period of time, the viability and activity of the yeast cells may have decreased. Over time, yeast cells can become stressed or die off, which can result in a slower or incomplete fermentation during bottle conditioning. In such cases, adding fresh yeast can help ensure a successful secondary fermentation and proper carbonation in the bottles.
Another consideration is the fermentation conditions the beer has undergone. If the fermentation process was carefully controlled and the yeast was given optimal conditions to thrive, there should be a sufficient amount of viable yeast cells in the beer. However, if the fermentation conditions were less than ideal, such as if the yeast was exposed to high temperatures or other stressors, the yeast cells may have been damaged or killed off. In these situations, adding additional yeast can help ensure that enough active yeast cells are present to ferment the sugars and produce carbon dioxide.
It's worth noting that crashing and most fining agents can cause some yeast cells to drop out of suspension, potentially reducing the number of yeast cells available for bottle conditioning. However, even with these processes, there should still be enough yeast cells in solution to adequately carbonate the beer. It's also important to keep in mind that some fining agents, such as gelatin, may remove proteins and other compounds that can help yeast cells flocculate and settle out, leading to a clearer beer but potentially reducing the number of yeast cells available for bottle conditioning.
In my personal experience as a brewer, I have found that for most beers, especially those that are not heavily aged or exposed to harsh fermentation conditions, adding additional yeast for bottle conditioning is not necessary. The yeast cells that remain in solution after crashing and fining are typically sufficient to carbonate the beer adequately. However, in situations where the beer has been aged for an extended period of time or the fermentation conditions were less than optimal, adding fresh yeast can be a good insurance policy to ensure proper carbonation in the bottles.
Whether or not to add additional yeast for bottle conditioning depends on the age of the beer and the fermentation conditions it has undergone. If the beer is relatively young and the fermentation conditions were not overly stressful, there should be enough yeast cells in solution to adequately produce carbon dioxide. However, if the beer has been aged for a long time or the fermentation conditions were less than ideal, adding fresh yeast may be beneficial to ensure a successful secondary fermentation and proper carbonation in the bottles.