Wine yeast have a high demand for nutrients, not only during rehydration, but also throughout fermentation. Nitrogen deficiency is the most common cause of fermentation problems, but also the most easily remedied (nitrogen deficiency will be discussed in a later post).
Micro-nutrients (sterols, lipids, vitamins, and minerals) are naturally
occurring in grapes, but are also commonly deficient in juice/must.
Unfortunately, such deficiencies are not typically identifiable prior to
fermentation, meaning winemakers have a difficult time forecasting their corresponding negative effects on wine
quality; that being said, one can usually guarantee problems with such micro-nutrients if grapes have been subject to poor growing conditions (mold/rot) or have a history of deficiency. Fermentation nutrients are a class of products that help remedy these deficiencies and can be classified into two different categories: complex yeast nutrients and vitamin supplements.
Complex yeast nutrients
such as Fermaid K/O, Bioactiv, and Superfood, are proprietary blends
of nutrients; many provide some level of nitrogen (whether inorganic, organic, or both), but all provide sterols, fatty acids
(lipids), and vitamins (thiamine, biotin, etc.) at varying levels. Inactivated yeast cell walls present in these products absorb
medium-chain fatty acids that are toxic to yeast, helping prevent alcohol toxicity and yeast stress that would lead to undesirable aroma and flavor development. They also provide
nucleation sites that help keep the yeast in suspension throughout
fermentation. Winemakers have varying views on when, why, and how to use complex yeast nutrients (% additions at different stages throughout ferment, combination of more than one product, only for stuck ferments, etc.). I have used them in most of my winemaking experiences and believe they are very beneficial, if not essential in many cases, to successful fermentation.
such as Cerevit and Vitamix, provide an array of key nutrients such as
thiamine, biotin, nicotinamide, magnesium sulphate, calcium
panthothenate, and folic acid. Though many of these micro-nutrients are contained in complex yeast nutrients, they can still be at deficient levels. Vitamin supplements help ensure that undesirable sulfur compounds are not created due to yeast stress. Again, winemakers all have varying views on when, why, and how to use vitamin supplements (at certain brix levels, only with problem ferments, etc.); the main thing is to make sure that legal levels of certain micro-nutrients are not
exceeded, particularly when using vitamin supplements in conjunction
with out yeast nutrients (for example, Cerevit added at its recommended
rate contains the legal limit of calcium panthothenate). With any hint of a problem or for higher price-point wines, I recommend using vitamin supplements as a safety precaution for several reasons: they don't add a whole lot of money to your production expense, don't
effect aroma/flavor, and give you peace of mind.
Check out these related links: Yeast Rehydration Nutrients, Successful Wine Fermentation - Introduction, Selecting the Right Yeast (Part I, Part II), Preparing Juice/Must for Fermentation, Indigenous Yeast, Inoculation, Yeast Propagation (Part I, Part II, Part III).