Problems and Solutions Associated with High YAN Fermentations
By: Denise Gardner
A few questions have crossed my Inbox during the 2014 harvest season regarding what to do in high YAN (>250 mg N/L) situations throughout primary fermentation. These are fantastic inquiries, and I thought it may be a good idea to provide this information to the entire industry through a blog post.
If you have been following along through previous posts, you’ll recall that YAN is composed of inorganic (ammonium ion) and organic (amino acid) nitrogen components. Amino acids are brought into the yeast cell through transport across the cell membrane. The presence of alcohol and ammonium ions (i.e. DAP) inhibit amino acids from being brought into the cell. This is why winemakers are advised NOT to add DAP at inoculation or at the beginning of fermentation, as yeast can actively absorb organic nitrogen in the juice (aqueous) environment.
Once alcohol concentrations begin to increase, as a result of primary fermentation progression, transport of amino acids from the wine into the yeast cell will be inhibited. Therefore, the primary source of nitrogen will then come from inorganic sources, such as DAP. A more thorough summary of how nitrogen is utilized by yeast can be found here.
In general, winemakers can select from three different kinds of nitrogen-based products to add during fermentation:
- Hydration Nutrients (e.g. GoFerm, Nutriferm Arom, etc.)
- Complex Nutrients (e.g. Fermaid K, Nutiferm Advance, Superfood, etc.)
- Diammonium Phosphate or DAP
Use Acceptable Fermentation Aids
Commercial wineries should ensure that they are using commercially acceptable suppliers for nutrient additions. While there are several generic nutrient supplements on the market, commercial products are tested regularly for ingredient purity and efficacy. Additionally, technical support is easily available for most of these products. For example, wineries that have questions about GoFerm or Fermaid K could call Scott Labs or their Lallemand sales rep directly. Or wineries that use the Nutriferm product line could contact Enartis Vinquiry directly. Compared to secondary distributors, these employees will have the most knowledge and greatest understanding of their products.
Why Very High YAN’s can be a Fermentation Nightmare
A very high YAN concentration (>300 mg N/L) in the must can create a challenging situation for the winemaker. Due to the excess amount of available nutrients, yeast can grow and reproduce quickly, which often leads to very rapid fermentations. The speed of fermentation, of course, can affect the aromatics and quality of the wine (i.e. fast fermentations often lead to simpler aroma and flavor profiles). This may not be an issue with some fermentations, but for many white wine or fruit (other than grapes)-based fermentations, aromatic retention should be a priority by the winemaker.
Higher concentrations of the inorganic component of YAN can lead to a high initial biomass of yeast. This is a problem because the rapid increase in yeast populations can lead to starvation by the majority of the yeast by mid- to late-fermentation. Yeast starvation leads to yeast stress, and one of the stress responses by yeast is the production and release of hydrogen sulfide. This can obviously cause hydrogen sulfide issues in the wine by the time fermentation is complete.
Due to the fact the initial YAN is so high, all of the nitrogen contents may not be utilized by the yeast population by the end of fermentation, and could remain in suspension in the finished wine. This excess “food” could be available for other microorganisms (like acetic acid bacteria or Brettanomyces), which could potentially lead to spoilage problems if the wine is not properly stabilized. Such spoilage is detrimental to wine quality.
Finally, Cornell University recently reported higher initial YAN values may led to increased concentrations of ethyl carbamate. Ethyl carbamate is naturally produced by fermentation, but it is a mild carcinogenic compound. For this reason, many countries have legal maximum ethyl carbamate concentrations in wine. For more information on ethyl carbamate, please see this guide published by UC Davis or this Extension report from Virginia Tech’s Enology Grape Chemistry Group.
How to Manage High YAN’s
Nutrient management strategies are not a 100% guarantee that wines will not end up with hydrogen sulfide by the end of fermentation. However, proper nutrient management strategies do minimize the risk of getting hydrogen sulfide by the time fermentation is complete. For more information on what is included in nutrient management strategies or the various commercial products available for wineries, please see this Penn State Wine Made Easy fact sheet.
With high YAN situations, including those with musts that have YAN in the 300-500+ mg N/L range, Penn State Extension recommends that winemakers follow supplier protocols for use of their products. In most cases, this will include using a hydration nutrient at yeast inoculation.
Additionally, most suppliers will recommend a small complex nutrient addition about 24-48 hours after inoculation, and perhaps one-third of the way through fermentation. This addition is intended to minimize yeast stress. Many commercial grade complex nutrients contain a proprietary mix of components that aid in absorbing yeast inhibitors that may have accumulated during the initial stages of primary fermentation. Also, a small amount of DAP is provided in many complex nutrients, which will serve as a final nutrient supply through the end of fermentation (as yeast can no longer take in amino acids, or organic nitrogen sources, by later stages in fermentation due the formation of alcohol).
However, suppliers do not often recommend DAP additions for high YAN musts, as this can lead to hydrogen sulfide formation. Therefore, winemakers should head caution and avoid making DAP additions when initial YANs are very high. In fact, this is a good reason as to why measuring YAN is important. For those winemakers that make routine DAP additions to all fermentations, it can sometimes do more harm than good, as in the case with high YAN fermentations.
This may also be a good situation to use yeast strains with a high nutrient demand. Recall that some yeast have higher nutrient needs than others. Winemakers are encouraged to contact suppliers for yeast recommendations that may perform well in high YAN environments.