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[nbbeerguy]

Types of Yeast NutrientsYeast nutrients come in several different forms, including inorganic nitrogen (ammonium salts), organic nitrogen (amino acids), yeast extracts and autolyzed yeast, micronutrient blends of vitamins and minerals, and complex yeast nutrients that are a combination of some or all of these. Complex yeast nutrients can also include non-nutritive ingredients like yeast hulls, cellulose, fining agents, like bentonite, and sodium caseinate and tannin.Inorganic NitrogenYeast can readily assimilate inorganic nitrogen in the form of dissolved ammonium salts. The fact that fermentations frequently need additional phosphate as well as nitrogen has made di-ammonium phosphate (DAP) the inorganic nitrogen source of choice for the U.S. wine industry. Outside of the U.S., much cheaper di-ammonium sulfate (DAS) salts, a common fertilizer, often are used as a nitrogen source. Food-grade and FDA-approved, sources of DAS exist, but U.S. winemakers should be aware that DAS is not currently listed on the TTB list of materials authorized for the treatment of wine and juice (Title 27 CFR 24.246).A diet of sugar and DAP isn't really a balanced diet, even for yeast. It's kind of the yeast equivalent of gorging on candy. The yeast live so fast that their cell membranes get flabby, and they die an early death pickled by acid (H+ ion accumulation) and ethanol.Organic Nitrogen"Organic nitrogen" really means amino acids. Most commonly amino acids used in yeast nutrients are from yeast in the form of yeast extracts or autolyzed yeast. Ideally, one would really like to know the exact composition since some amino acids like proline cannot be used by yeast under anaerobic conditions.Nitrogen BlendsNitrogen blends are simply a combination of DAP and amino acids. Since DAP is much less expensive, this usually means that any such blend is predominantly DAP. Ideally, one would really like to know the exact ratio between the two as well as the composition of the amino acids.Yeast ExtractsTrue yeast extracts, as opposed to autolyzed yeast, are relatively uncommon as yeast nutrients. This has more to do with the idiosyncrasies of European regulators than anything else. The concern seems to be that if yeast extract was allowed as a nutrient, unscrupulous producers might add yeast extracts to finished wine to improve the mouthfeel. This could allow producers from "lesser" wine regions to produce better tasting wine, and one mustn't allow that.Sarcasm aside, this is probably a genuine concern, for European bureaucrats, that is. Unfortunately, since most yeast nutrients are designed in Europe, this means that most yeast nutrients contain autolyzed yeast (see below) rather than yeast extracts. The American-designed Superfood is a notable exception.Autolyzed YeastAs noted above, autolyzed yeast are simply dead yeast cells that have been treated so that their cell walls have ruptured. Although yeast hulls do have fermentation benefits of their own (see below), it's the yeast extracts inside that matter nutritionally.Not all yeast are autolyzed equally.Partially autolyzing yeast is relatively easy: heat them at a particular pH and at least some of the yeast cells will burst and spill their guts. The problem is determining how many have done so. It is not recommended that winemakers rely upon partially autolyzed yeast to provide much nutrition. As any winemaker that has produced a sur lie Chardonnay should know, it takes many months for yeast to undergo autolysis in wine. That's why we leave Chardonnay on their lees for five to nine months.Getting all the cells to autolyze takes a bit more effort. Fully autolyzed yeast are widely available as food additives, but these have been intentionally designed to maximize their meaty, umami flavors and therefore probably would have negative sensory impacts on most wines.Micronutrients: Vitamins and MineralsMost of the micronutrients used in yeast nutrients come from the dead yeast used to make the nutrients. As winemaking consultant and scientific director of The Vinotec Group, Lisa Van de Water, noted, "If you are making a yeast nutrient, you grow up a tank full on nice healthy yeasts. You put the yeast cells through heating procedures, some short and not very effective, some longer and much more effective but also more expensive (which explains at least some of the wide differences in ingredient prices). You use the yeast themselves to feed other yeasts. So we don't really know every compound that is in it. We just know that yeasts want these ingredients, or they wouldn't have taken them up into their cells."The key vitamins for fermentation, Van de Water noted, "Supplement those naturally occurring in the yeast cell, include thiamine, calcium pantothenate, niacin, myo-inositol, pyridoxine (B6) and biotin. The percentages of these are fiercely proprietary, but new vitamin mixtures include a lot of pantothenate because of Charles Edwards' work on the key role of pantothenate in sulfide prevention."Complex Yeast NutrientsAlmost all complex yeast nutrients are a blend of autolyzed yeast and DAP, with the addition of varying concentrations of minerals and vitamins accordingly, but most of these are really from the yeast extracts and autolyzed yeast and not separative additives.Yeast Hulls and Other Non-nutrient AdditivesYeast hulls aid fermentation, but they aren't really a nutritional source for the yeast. Yeast hulls are primarily composed of polysaccharides with ß-(1-3, glucan being the most prevalent. More importantly, yeast hulls are a source of sterols for the yeast. They don't consume sterols for energy but rather use them to shore up their cell walls. This is crucial for ethanol tolerance. Yeast can make sterols on their own but only under aerobic conditions.Besides sterols, yeast hulls are known to absorb some of the yeast autotoxins, that are toxic to the yeast, produced during fermentation. This makes yeast hulls a particularly useful additive when dealing with sluggish or stuck fermentations.The TTB does regulate the total amount of yeast hulls that can be added. The current limit is three pounds per thousand gallons. This limit seems to be on the low side since one can hit this limit just adding complex nutrients. I'd like to see some winery petition the TTB to raise this limit to six or nine pounds per thousand gallons to give us all some leeway to deal with problem fermentations. It's interesting to note that as a food additive, FDA regulations would permit up to 400 pounds of "Brewer's Yeast" per thousand gallons of wine since dead yeast are "generally recognized as safe" (GRAS). One would hope that it wouldn't be too difficult to convince the TTB to raise the limit a bit.Inert or inactive yeast, unless autolyzed, are just dead yeast cells. One could probably assume that at most they were partially autolyzed when killed, but most of the yeast extractives won't be available to the fermentation in a timely fashion.Other non-nutritious components of yeast nutrients are purified granulated cellulose, bentonite, caseinate salts and tannin. Cellulose is a polysaccharide like those found in yeast hulls. It is added to yeast nutrients to perform a similar function of absorbing compounds toxic to yeast. Cellulose does not provide the sterols that yeast hulls do. Bentonite and caseinate salts are fining agents, but hydrolyzed caseinate is used as a protein source for the yeast. UC Davis postdoctoral fellow Kathryn Weiss and professor Linda Bisson have found that while juice fined with bentonite resulted in slight reductions in the maximum fermentation rate, this effect was not seen if the bentonite were added during fermentation (AJEV 53:1, 28-36). One would expect a similar result from caseinate salts. Tannin, of course, has important organoleptic qualities, but isn't considered a yeast nutrient.Oxygen: The Free NutrientYeast are simple things really; without oxygen they produce ethanol. If they have access to oxygen, they don't bother wasting their energy on ethanol and instead make things like cell walls and new yeast cells. In addition, if they have oxygen, yeast can use nitrogen sources like proline that they can't use while fermenting. To this end, aerating the must is sometimes the best "nutrient addition" one can make.When to Use Which NutrientsThe timing of yeast nutrition is fairly important. Nitrogen, especially inorganic nitrogen, should be added after the fermentation gets established, but before it proceeds too far. Vitamins, minerals and yeast hulls can be added earlier. Oxygen and yeast hulls are good things to add when the initial sugars are high and when fermentations are sluggish.How to Buy, What to ChooseMost wineries will probably need a combination of all of the above yeast nutritional types. Both yeast hulls and DAP are relatively inexpensive and store well. Wineries should always overstock these in preparation for harvest. The cost of carrying over some from year to year is trivial compared to the potential cost of not being able to properly feed fermentations. In a similar vein, it is a good idea to stock "too much" of your favorite complex yeast nutrient.Deciding which complex yeast nutrient to use is a bit problematic because the actual composition information is frequently not available. Finding the right one may take a bit of legwork and experimentation. Don't assume that they are all equivalent. Quite often the cheaper product is cheaper for a reason, frequently because the yeast are only partially autolyzed or even not autolyzed at all. As we noted above, un-autolyzed yeast won't surrender their nutrients fast enough to do any good for the fermentation. By the same token, the most expensive nutrient isn't necessarily the best. At least one product on the market appears to be nothing more than DAP sold at a significant markup. Suspect something if your vendor won't provide a detailed description as to the nutrients' contents.ConclusionThe most important part of yeast nutrition is knowing what the yeast assimable nitrogen (YAN) is before you start throwing food at the yeast. Beyond this, one needs to feed the yeast a balanced diet. It does no good, however, if the nutrition is locked up in dead yeast cells since they won't autolyze quickly enough to feed the living ones. wbm