how to stop fermentation?
Can you post a recipe? I would not suggest trying to stop the yeast before they are done. You are more likely to leave off flavors, like green apple from unfinished fermentation. Off the top of my head the three things that might need to be looked at are: 1. Original Gravity – are you starting high enough? If you have less fermentables to start with, the resulting gravity will be lower. 2. Recipe formulation / Process tweak – there are several ingredients and processes that can affect final gravity. Simple sugars are highly fermentable. Crystal malts and dextrin malts are not. If you are all grain, mash temp can also affect it. 3. Contamination – Wild yeast and bacteria can eat the things regular yeast leave behind, dropping the gravity. Check your sanitation techniques.
