Yeast Assimilable Nitrogen II
Posted May 23, 2012; updated November 2015
I received several comments via LinkedIn regarding the previous article when I first posted it. Part of my love for winemaking is discussing it with others. I also enjoy learning and spreading my knowledge.
Find a few of the comments in italics below with my responses after.
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If this is so, how come good wine is made without this technique? It may ensure that you don't have problems, but I add all nutrients in near inoculation time.
Maybe the practice is better for fruit forward and the high alcohol wines.
I've always loved the mantra that 'great wine is made in the vineyard'. As a winemaker, I don't believe in adding anything unnecessary; great wine can be made without the addition of anything (nutrients, enzymes, tannins, etc.) but requires excellent fruit quality. As indicated, adding all nutrients at the beginning of ferment is often not a problem and a very common practice. Adding in stages is a safer practice, particularly in the case of highly nitrogen deficient must (for instance, higher brix must).
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DAP is considered yeast junk food, it's pretty much like us eating pure sugar. Fermentation nutrients such as Yeastex and Superfood are better.
DAP can be equated to pure sugar consumption by humans; it is the easiest compound for yeast to derive necessary nitrogen. I always suggest using fermentation nutrients in conjunction with DAP (or in lieu of, depending on how nitrogen deficient must is). Bear in mind that these blends only supply a fraction of the YAN that DAP would; for example, 100 ppm of Superfood provides approximately 7.75 ppm YAN while the same weight of DAP provides 22.5.
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Unused DAP can result in the formation of the carcinogenic compound ethyl carbamate so it must all be added with caution and only during the first half of fermentation to ensure it is all used up.
The main precursor of ethyl carbamate (otherwise known as urethane) is actually urea, who's main precursor is arginine (very common amino acid found naturally in grapes). Urea is formed during early and middle stages of normal alcoholic fermentation, then utilized by yeast in later generations. 'Stop ferments' (fortified wines or wines stopped sweet) have a much higher propensity to lead to ethyl carbamate issues since they are often stopped when urea levels are still high. Urea production during fermentation is highly dependent on yeast strain.
The reason DAP has been indicated to lead to ethyl carbamate production is because adding too much DAP (particularly at the beginning of fermentation) will stop yeast from metabolizing free amino acids (including arginine). So, caution is definitely required.
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An early stinky ferment is best caught very early and aerated to provide sufficient oxygen for yeast to build sterols into their cell walls. Sterols are important for nitrogen transport and ensure plant-derived YAN is efficiently used.
Oxygen additions during fermentation can be very beneficial. As you indicated, oxygen is necessary for yeast to synthesize lipids and sterols to produce properly functioning cell membranes. Yeast are what is referred to as facultative organisms, they can operate aerobically and anaerobically. Oxygen is consumed very rapidly at the onset of fermentation (usually within the first few hours), so yeast switch from aerobic to anaerobic fermentation; once this occurs, yeast cannot synthesize these building blocks and must rely on those already in solution. Your addition of oxygen curing your stinky ferment is more likely helping alleviate a stressed yeast population that wasn't adequately prepared to switch from aerobic to anaerobic activity.
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Thanks for the discussion. Anyone with comments or questions can always feel free to contact me.
