Yeast nutrient is used to nourish yeast and allow it to complete the fermentation. For the best results, add the yeast nutrient during the last 10 minutes of the boil.
Yeast nutrient does not speed up fermentation. However, it will make a great catalyst and will allow your yeast to eat more efficiently. It will reduce unwanted byproducts of fermentation and benefit the final taste and smell of your beer.
For average-gravity level beers, it’s likely that you won’t even need nutrients. Although, a little pinch or two won’t hurt. For higher gravity beers, you may need some nutrients to help your yeast eat up all of the sugars present.
Trial and error can be useful here; the addition of nutrients is contingent on many factors and variables. To be safe, follow the instructions of the product you’re using.
Put warm water in a large enough bowl or container. Then, add the nutrients and allow them to dissolve into the water. Finally, add the solution into the wort with about 10-15 minutes remaining.
You can boil the nutrient to sanitize it, but the yeast will eat up harmful bacteria, and the need to boil the nutrient becomes nonexistent. If you’re still skeptical, there is no harm in adding it with 10-15 minutes left in the boil.
If you use too much nutrient, your yeast might not be able to eliminate all of it, making for an off-tasting final product. If you add too little, your yeast might not get enough nutrients to be able to finish fermentation.
Don’t add yeast nutrient to secondary fermentation. By then, your beer will be mostly done fermenting, and adding more nutrients will either leave your beer tasting funny from the leftover, or it might continue fermenting even after bottling, causing a chaotic eruption of bottles and cans.