Yeast nutrients do several things depending on what the nutrient is.
they
1 provide food for the yeast
2 help make the yeast cells stronger by thickening the cell walls, this in turn can give you cleaner less funky beers.
3 help keep the yeast cells healthy so they carry on fermenting until all the usable sugars are used up
4 help the yeast deal with stressful environments
there are many other benefits therefore its advisable to add nutrients, you will never get all the required nutrients out of the grain, you get most but not all.
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