Adding Nitrogen to Fermentations
Yeast requires nitrogen for fermentation. If there is not enough, yeast cells are stressed and produce excess H2S—something that gives fermentations an off-odor. The H2S starts converting to mercaptans almost immediately, and fermentations without adequate nitrogen will leave you with reduced sulfur compounds and off-smells in the finished wine. This is one of the most common problems for home winemakers using real grapes.
If you want to avoid problems, you must be prepared to add nitrogen to your fermentations.
What is YAN?
YAN stands for Yeast Available Nitrogen. It is a measure of the amount of nitrogen available to the yeast in grape juice. YAN is measured in parts per million (ppm) or milligrams per liter (mg/L), which is the same thing.
The YAN in grapes comes in two main forms. One is the assimilable nitrogen from the alpha amino acids in the grapes (this is sometimes called alpha amino or a-amino nitrogen). The other is Free Ammonia Nitrogen (sometimes abbreviated as FAN). Both sources are important. A-amino nitrogen plays a bigger role at the start of fermentation, while ammonia (FAN) usually supplies the bulk of the nitrogen required during the latter stages of the ferment. If you have just one figure for YAN, it includes both types; if you have separate figures you add them together to get the total YAN.
The natural YAN in grapes varies widely depending on the terroir, grape variety, and season. Sometimes grapes naturally contain more than enough nitrogen for a satisfactory wine fermentation. More often they do not, especially with the higher Brix typical of today’s harvests. Some grapes from some vineyards have notoriously low YAN levels almost every year. If you want a good ferment, you must be prepared to add nitrogen via Fermaid-K and DAP.
There are no general rules of thumb that help very much in deciding on the amount of the additions. The Central Coast grapes we are getting in 2008 have YAN levels that vary between 79 and 327. At 328 there is probably no need for any additions, and additions could hurt. At 79, you have an extremely low level of YAN. If you are not making heavy additions you are going to have real problems.
How much YAN do fermentations need?
As a practical rule, everyone agrees that you have to keep your nose in the fermentation to make adjustments as the fermentation proceeds. If you are getting a pronounced H2S smell from your ferment, your yeast is stressed from a lack of nitrogen and you need to do something right away. The best thing, however, is to know how much nitrogen to add to your fermentation and add it before problems emerge.
In general, the three things that most affect the total YAN a fermentation needs are (in order of importance):
- Yeast variety, and
- Fermentation temperature.
Brix (or sugar level) indicates how long and hard the yeast has to work—the higher the Brix, the more YAN required. These are the standard recommendations (which always seem to be given with no particular yeast mentioned).
|Brix||Average YAN Required||YAN Range Required|
Yeast descriptions commonly provide information on their nitrogen requirements. Technically, this is measured as the amount of nitrogen (in milligrams) needed by the yeast to produce CO2 (in grams). Here is a compilation of readily available information. The suggested multipliers are my own and will be used in YAN calculations below.
|Yeast Variety||mg N2 per G CO2||Usual way of describing yeast N requirements||Suggested multiplier|
Obviously, the yeasts we commonly use can vary a lot! BM-45 requires twice as much YAN as 71-B. Even yeasts commonly described as “low” or “high” in nitrogen requirements can vary by 25% or more.
The temperature of the fermentation also affects nitrogen requirements. Lower temperature fermentations (as are typical with white wines) require less nitrogen and also usually result in more efficient transformation of sugar to alcohol. For example, Vin-13 is specifically described by manufacturers as having low nitrogen requirements at low temperatures.
How to calculate your YAN requirements
Unless you are using a very high fermentation temperature (something people seldom do any more), you can largely ignore the temperature issue. Then the calculation of total YAN required for a fermentation is easy.
- Note the YAN required by your Brix level in Table 1 above. and multiply by the suggested multiplication ratio for the yeast you are using. For example, if you have Brix of 25, you need 325 ppm YAN., and if you are using
- Multiply this number by the suggested multiplication ratio in Table 2 for the yeast you are using. For example, if you are using BM4*4 yeast, you should multiply 325 by 1.4—this gives a total YAN requirement of 455. [This may be more generous than really necessary. BM4*4 is supposed to be an improvement over BM-45 in part to reduce the high nitrogen requirements.]
- If the grapes came with a YAN level of 225, you need to add 230 ppm YAN during fermentation.
It probably doesn’t hurt to be a bit more generous than this with your additions. But don’t overdo it. Research by Bruce Zoecklein and his Virginia Tech group suggests too much nitrogen in a ferment causes some of the same problems as too little.
You some times see YAN estimates from Brehm Vineyards and others cast only in the general language of "Low" or "Extremely Low" or such other rather vague terms. There is, as far as I know, no generally accepted language for these kinds of descriptions and obviously the interpretation of any particular YAN level depends, at the very least, on the Brix of the wine . However, Scott Labs uses these terms to describe YAN levels for musts at 22 Brix.
- HIGH—225 and above [required no supplements except Go-Ferm]
- MEDIUM—125-225 [add 93-104 ppm YAN]
- LOW—less than 125 [add 104-167 ppm YAN]
Gusmer provides these descriptions of Risk Levels with its recommendations for nitrogen additions:
- NO RISK—YAN of more than 250 (or 300 for 25+ Brix) [total added YAN=19]
- LOW RISK—YAN of 200-250 (or 250-300 for 25+ Brix) [total added YAN=37]
- MODERATE RISK—YAN of 150-200 (or 200-250 for 25+ Brix) [total added YAN=94]
- HIGH RISK RISK—YAN of 100-150 (or 150-200 for 25+ Brix) [total added YAN=143]
- VERY HIGH RISK—YAN less than 100 (or less than 150 for 25+ Brix) [total added YAN=172]
It seems to me that these recommendations from Scott and Labs and Gusmer are conservative, and don't really take yeast into account. If you follow the procedure recommend above for calculating YAN requirement, you will be adding more YAN than these people suggest, especially if you are using a yeast like BM-45.
What to add and how much?
You must have a good scale to measure the additions you require to increase your YAN level. And remember that nutrient additions should be dissolved in water before being added to your must.
The following chart shows the supplements that Nanaimo Winemakers typically use. NOTE that each contains a different proportion of nitrogen.
|Nitrogen Source||% N||Forms of N||Comment|
|Go-Ferm||3%||all alpha amino||Nitrogen is incidental|
|Superfood||7.6%||a-amino and ammonia||Contains other nutrients|
|Fermaid K||13%||a-amino and ammonia||Contains other nutrients|
|DAP||21%||all ammonia||Cost effective for nitrogen|
To calculate the nitrogen yield of a product in parts per million you need to realize that 1 g/L of something is 1000 mg/L or 1000 ppm. The nitrogen component is a simple multiplication of this using the above chart. Thus:
- 1 g/L of Fermaid-K (13% N) adds 130 ppm of nitrogen to a liter of juice
- 1 g/L of DAP (21% N) adds 210 ppm of nitrogen to a liter of juice
If we continue with the previous example, where we wanted to add 230 ppm YAN, and we decided to roughly split the additions between Fermaid K and DAP, some calculations would show that 0.6 g/L Fermaid K and 0.75 g/L of DAP will give you about 235 ppm additional YAN which is close to your target of 230 ppm. I tend to err on the high side and to ignore the small proportion of additional YAN that come from Go-Ferm.
You might find it convenient to set up a spreadsheet in Excel or a similar program to calculate your additions. The matrix could look like this:
|1||N source||g/L addition||* proportion N||*1000||=YAN ppm/L|
|4||Total ppm YAN added per L||235.5|
The YAN/L column (E) is simply the product of the three cells which precede it in the row (i.e., E2=B2*C2*D2). The total YAN addition per L is simply the product of these calculations (i.e., E4=E2+E3). You manipulate the calculations in the spreadsheet by substituting numbers for g/L in cells B2 and B3 until you can the mix of additives and the total YAN addition you want in cell E4.
REMEMBER that you still have to multiply your g/L additions by the number of liters of juice you are expecting, which is usually around 13L per 50 lbs (one pail) of red grapes.
NOTE: Willem Wyngaards has prepared a couple of Excel spreadsheet that do most of the work for you. The first version calculates just the Fermaid-K and DAP additions once you select Brix levels and your yeast. A more recent version of the spreadsheet (called version 2) also calculates recommended yeast and Go-Ferm additions (according to Scott Labs) and allocates your N additions over the usual three stages outlined in the next section. The most recent version of this calculator (October 2009) can be found here.
When to make your YAN additions
There is only one rule for sure here and that is: DON’T add any nitrogen after your ferment has reached about 12 Brix or a SG of .050.
Otherwise, people add nitrogen in different ways. Kit makers (and many wineries, who tend to use only DAP) add everything up front.
But the recommended strategy now is to make nitrogen additions in three stages. If the initial YAN level is really low, I would add a forth stage and put some DAP in even before yeast inoculation. Then I would proceed as follows:
- Stage 1: As the active fermentation gets under ways (after the lag phase)
- you can skip this addition if you are adding relatively little nitrogen in total, but this is an important feeding point if your must has low YAN naturally
- typically this addition is heavy on Fermaid-K and light on DAP
- this is typically the major feeding point, with both Fermaid-K and DAP being used
- this stage is typically DAP dominated
Remember, no matter what you do, the almost universal advice is no additions after the half-way point. I say "almost universal" because I have heard one professional winemaker say that if he had unexpectedly smelly fermentation he would add Fermaid-K even down to about a SG of 0.025.
To return to the example in the previous section, where we were adding 0.6 g/L Fermaid K and 0.75 g/L DAP, the schedule might be as follows:
|Stage 1 After ferment starts||0.2||0.1|
|Stage 2 At one-quarter sugar depletion||0.3||0.3|
|Stage 3 Before one-half sugar depeletion or SG 0.050||0.1||0.35|
Adapted from a presentation by Rod Church for a meeting of the Nanaimo Winemakers on October 2, 2007. The report borrows heavily from sources who also borrowed heavily. Many thanks to those who actually did the research that underpins the presentation.