What are yeast nutrients and how are they used?

Q: What are yeast nutrients and how are they used?
— James Warren • Albany, New York

A: 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!