I've seen several beer, mead, and wine recipes that call for yeast nutrients. I'm even now following a mead recipe that calls for staggered nutrients. But my question is, what exactly are yeast nutrients? What is in these nutrients that isn't in wort/must that the yeast needs or wants?
You see this mostly in wine, cider and mead making because those sources of fermentable sugars lack some of the nutrients needed by yeast for proper fermentation, which are usually present in malt-based worts. They are (aside from fermentable sugars):
- Amino acids/nitrogen - All-malt worts supply plenty of readily assimilable nitrogen. This is a big need though for wine, cider and mead fermentations as there isn't usually sufficient present in the must/juice.
- Lipids - Sufficiently present in wort. Added dissolved oxygen will be taken up by yeast and allows it to synthesize its own lipids, though I don't know if this is common practice outside beer-making.
- B vitamins - not sure how little there is in non-malt fermentables, but will be in good supply from regular wort.
- Other trace elements, mainly ions of calcium, magnesium, zinc, phosphate and sulfate. Zinc is the only one of these not present in (usually) sufficient quantities in all-malt beer wort (supplied by both malt and water), making it the only necessary addition for such beers. In wine/cider/mead most of these will be deficient.
What's in some yeast nutrients?
- Diammonium Phosphate is commonly used as a source of nitrogen and phosphate.
- Zinc sulfate - for zinc, and sulfate.
- Dead yeast/yeast hulls - has pretty much everything live yeast need, nutrient-wise. Supplies lipids, B vitamins and trace elements. Yeast hulls can also decrease the inhibitory effect of certain fermentation byproducts.
Yeast nutrients are often proprietary (as in they won't say exactly what's in there), and each will supply a different mix of nutrients, usually specifically targeted towards a certain kind of fermentation (i.e. wine, mead, for enhancing beer, etc.) so make sure you're getting the right thing for your particular needs.