What are yeast nutrients and how are they used?

Most yeast nutrient blends contain amino acids, inorganic nitrogen
(ammonia), B-vitamins, sterols, unsaturated fatty acids and oftentimes
autolyzed yeast which gives a mixture of all of these components. These
blends are typically used when making wine, cider or high adjunct beers
to provide critical growth factors required by yeast. Fermentations
lacking yeast nutrients are usually sluggish with a tendency to become
stuck.

Brewer’s wort is a very rich medium and has most
everything that yeast require for a good fermentation. In fact, the
practice of re-pitching yeast from one batch to another usually carries
some autolyzed yeast with it and yeast extract is a good source of
vitamins, amino acids and fatty acids. When yeast grows, amino acids
and nitrogen are required for protein synthesis, sterols and fatty
acids are used to build cell walls (yeast can synthesize these
compounds as well as use external sources) and B-vitamins are used as
co-factors in yeast metabolism.

Brewers typically do not add these sorts of nutrient blends unless
brewing high adjunct or very high gravity brews. Zinc is one nutrient
often added to wort, as yeast requires some zinc for growth. Wort zinc
levels should be between 0.10–0.15 ppm. Zinc can come from copper when
using copper brewing vessels, but most equipment is stainless steel, so
zinc additions are helpful.

One problem with adding zinc salts, such as zinc sulfate, is that much
of the zinc is lost in trub. Biologically available forms of zinc can
be enhanced by growing yeast in a zinc-enriched media and then drying
the yeast for use as a yeast nutrient. This form of zinc has lower trub
losses when added to the brew kettle.

I use such a product for every batch of beer we brew at our brewery and
have used this nutrient, called Servomyces, for the last eight years.
If wort is low in zinc, lagging fermentations are seen as well as
poorly flocculating yeast. A friend of mine working for a very large
brewery told me that they have a target yeast density following primary
before transferring to the lagering tanks and that this brewery adjusts
wort zinc levels to influence cell density after primary. If the cell
density is too low, they back off on the zinc to reduce yeast
flocculation. If the cell density is too high, the zinc dose is
slightly increased.

In my experience brewing all-malt beers and some beers with about 25%
adjunct, I have never felt the need to add any nutrients to wort other
than zinc since wort is really the ideal nutrient source for hungry
yeast cells!

Response by Ashton Lewis.