Final answer:
You shouldn't retain yeast in the bottle during fermentation when the alcohol content reaches above 12 percent as it becomes toxic to yeast, halting fermentation.
Step-by-step explanation:
You should not retain yeast in the bottle when making beer or wine if the level of ethanol exceeds a certain percentage. Typically, ethanol above 12 percent is toxic to yeast, which can kill them or inhibit further fermentation. In brewing and winemaking, fermentation is a critical process where yeast converts sugars into carbon dioxide and alcohol under anaerobic conditions. However, if the alcohol content rises too high, it can be detrimental to the yeast, stopping fermentation, and potentially ruining the beverage.