December 2014 Issue of Wines & Vines
 
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High-YAN Fermentations

Problems and solutions for managing nitrogen to achieve better fermentation results

 
by Denise Gardner
 
 
Adding hydration during yeast inoculation
 
Adding hydration nutrients during yeast inoculation is one way to achieve desired YAN. Photo by Denise Gardner

In order for proper fermentation to occur, yeast must have adequate nutrients available. One essential nutrient required for yeast health during the fermentation process is nitrogen: It contributes to the development of essential yeast molecules, which allow for healthy yeast growth and metabolism. The yeast assimilable (or available) nitrogen (YAN) content can be measured at harvest (in grape juice or must) and indicates the level of nitrogen (N) available at the start of fermentation. The YAN value for a given lot of grape must or juice directs winemakers to determine what nutrient additions need to be made to ensure a complete fermentation and minimize the potential for hydrogen sulfide (H2S) production.

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., diammonium phosphate or DAP) inhibits 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.

    HIGHLIGHTS
     

     
  • It is critical that winemakers test grapes for yeast assimilable nitrogen (YAN) before fermentation begins.
     
  • Careful observation of the fermentation and modification of nutrient additions can reduce H2S development.
     
  • Excess nitrogen not utilized by yeast during fermentation may be food for microorganisms and lead to spoilage.
     
  • DAP should not be added with yeast hydration, as it is toxic to the yeast and can inhibit primary fermentation.

High YAN levels can be a fermentation nightmare
A high-YAN concentration of more than 300mg of nitrogen per liter of must can create a challenging situation for the winemaker. Due to the excessive 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, and fast fermentations often lead to more simple 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 of 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, which can increase the incidence of stuck or sluggish fermentations. Another stress response by yeast is the production and release of hydrogen sulfide. This can obviously cause hydrogen sulfide issues in wine by the time fermentation is complete.

When the initial YAN is very high, the yeast population may not utilize all of the nitrogen contents by the end of fermentation, and nitrogen could remain suspended in the finished wine. This excess “food” could be available for other microorganisms such as acetic acid bacteria or Brettanomyces, creating potential spoilage problems if the wine is not properly stabilized. Such spoilage obviously is detrimental to wine quality.

High initial YAN values may also lead to increased concentrations of ethyl carbamate, according to a recent report from Cornell University. Ethyl carbamate is naturally produced by fermentation, but it is a mild carcinogenic compound. For this reason, many countries have established maximum legal levels for ethyl carbamate concentrations in wine.

How to manage high YANs
The first step toward managing YAN levels is to know what that level is. Winemakers should analyze for YAN during their normal pre-harvest assessment of their grapes one to two weeks before those grapes are harvested. When the fruit arrives at the winery, the grapes can be tested again, although it is not required to repeat the pre-harvest test. Whether using the pre-harvest numbers or those from delivered grapes, ammoniacal and amino acid-derived nitrogen nutrients can be modified, if necessary, to provide the best chance for a clean fermentation. These tests will avoid adding excessive amounts of either component to the fermentation. Unfortunately there is no common predictor for the YAN concentration, and consequently it should be measured in the fruit or must prior to primary fermentation to ensure that adequate nitrogen is available through the duration of fermentation.

There are several analytical kits that can perform these tests, some of which do not require expensive laboratory equipment. The simplest test, known as the formol titration technique, was modified by Dr. Barry Gump, professor emeritus of chemistry at California State University, Fresno, and currently professor of beverage management at Florida International University, to incorporate this procedure with the titratable acidity test that is often run at this time of year. (A description of this test is available at vitisresearch.com/gumptayan.html.) While YAN can be measured in the winery, wine analysis labs can also measure YAN for wineries throughout harvest.

Once the amount of YAN is determined, winemakers can select from three different sources for nitrogen-based products to add during fermentation:

• Hydration nutrients such as GoFerm or Nutriferm Arom

• Complex nutrients including Fermaid K, Nutriferm Advance and Superfood

• Diammonium phosphate or DAP

Each winery 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.

Nutrient management strategies are not a 100% guarantee that all fermentations will be H2S free. Any fermentation can end up with hydrogen sulfide by the end. Careful observation of the fermentation, with modification of the nutrient additions if necessary, can help inhibit H2S development and ensure more successful, completed fermentations. With high-YAN situations, including those with musts that have YAN in the range of 300mg to 500mg per liter, winemakers should carefully follow supplier protocols for use of their products. In most cases, this will include using a hydration nutrient at yeast inoculation.

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 absorption of 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. Winemakers should be cautious and avoid making DAP additions when initial YAN measurements are high. This is a good reason why measuring YAN is important. Those winemakers who make routine DAP additions to all fermentations can sometimes do more harm than good, especially in the case of high-YAN fermentations.

The table on this page lists several different suppliers of nitrogen-based products that contribute nitrogen during primary fermentation, and it is included here to emphasize the variation in nutrient products and how supplier recommendations differ. Each of the brands offered by a given supplier represents a line of products to be used for different YAN conditions. Fermaid, for example, has several options that winemakers can choose from based on their specific wine: Fermaid K, Fermaid O and Fermaid A.

Although there are differences in what nutrient suppliers consider a high-, medium and low-YAN reading, the general principle is the same: use the initial YAN content and Brix of the juice/must in order to determine proper nutrient supplementation. Some consideration may also need to be given to yeast strain selection, as some yeasts have higher nitrogen requirements than others and should be treated accordingly.

Most suppliers offer a chart that indicates how much nitrogen is contributed to the YAN value based on the dosage rate of the product. For example, at a 30 g/hL (grams per hectoliter) addition rate of GoFerm, the product contributes 10mg N/L (milligrams of nitrogen per liter) to the YAN concentration. If the addition rate was doubled to 60 g/hL, then 20mg N/L are contributed to the YAN value. Therefore, if the must’s starting YAN was 150mg N/L, a 30 g/hL addition of GoFerm will bring the YAN up to 160mg N/L. Most commercial nutrient products, including DAP (diammonium phosphate), have documented quantities of nitrogen that each of their products contributes based on standard addition rates.

Many wineries may opt to only add a rehydration nutrient such as GoFerm and make DAP additions, as this was encouraged when research on nutrient management was initiated. However, recent research indicates that reliance on DAP may not suffice in addressing nutrient needs for some fermentations. Furthermore, some evidence indicates that larger DAP additions, without complex nutrient additions, may simplify the aroma and flavor component of the wine by the end of fermentation.

It should be emphasized that DAP should never be added with yeast hydration, as it is toxic to the yeast and can inhibit primary fermentation from starting properly. Yeast hydration nutrients such as GoFerm or Nutriferm should be used for each inoculation, and complex nutrients like Fermaid K or complimentary products can be added prior to the use of DAP after primary fermentation has started. DAP is recommended to obtain that minimum YAN value when the yeast hydration nutrient and complex nutrient are not enough to reach the critical minimum. Although DAP is less expensive than many complex nutrients, it only contains ammonium, the inorganic form of nitrogen.

Conclusion
Research on YAN and its relationship with hybrid or native varieties is ongoing, especially in the Mid-Atlantic region. While following adequate nutrient strategies is not a 100% guarantee against hydrogen sulfide production, measuring YAN, understanding nutrient additions and adequately feeding the yeast during fermentation can minimize H2S production in the winery. While this task may seem overwhelming at first, especially during the harvest season, many wineries have found taking this preventative step to be less time consuming and better for wine quality than dealing with copper additions, copper trials or potentially not being able to adequately fix reductive wines.

 
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