By: Denise M. Gardner
The eastern U.S. growing seasons can be somewhat unpredictable. Late season rains or untimely hurricane events can be a recipe for disaster for local grape growers (http://www.pawinegrape.com/uploads/PDF%20files/Documents/Viticulture/Harvest/Rain%20at%20Harvest.pdf), and a few have been unprepared for such events in the past. These weather events can lead to higher incidences of the grey-rot form of Botrytis in addition to other rots, which may also be related to pest damage. Furthermore, these weather incidences and pest damage can ultimately impact picking decisions for growers and wineries (Osborne, 2017).
It is almost inevitable that wineries need to be prepared for end-of-season weather flops, and plan for the best possible ways to manage or maintain wine quality in light of above-average disease pressure.
One disease that winemakers can prepare for prior to harvest is Botrytis. For the purpose of this article, we’ll be using the term Botrytis to indicate the grey-mold or grey-rot form of the disease. Grey-mold, the form of Botrytis more commonly noticed in humid regions or during heavy-precipitation seasons, can ultimately affect wine quality. Peynaud (1984) has defined 4 ways in which the grey-mold can negatively affect wine quality:
- Deplete wine color (especially important in red varieties),
- Increase the risk of premature browning (through oxidative enzymes),
- Deplete varietal character (through degradation of grape skins), and
- Contribution to off-flavors developed by the mold’s presence on the fruit.
Based on a 1977 study by Loinger et al., guidelines pertaining to wine quality were developed with regards to a visual assessment of Botrytis incidence on incoming fruit:
- 5-10% Botrytis rot on clusters: noticeable reduction in wine quality; wine quality is still “good” (as opposed to very good with 0% rot on clusters)
- 20-40% Botrytis rot on clusters: marked reduction in wine quality; wine quality is “low”
- >80% Botrytis rot on clusters: wine is commercially unacceptable
With a noticeable sensory and chemical difference in Botrytis-infected clusters, it is best for wineries to develop a standard operating procedure (SOP) for assessing rot-infected fruit, as well as how the grapes should be handled and processed during production. While there is no one correct way to work with the wine, below are some suggestions or options that wineries can integrate when dealing with Botrytis-infected grapes. For a full list of possibilities, please visit: http://extension.psu.edu/food/enology/wine-production/producing-wine-with-sub-optimal-fruit/fermenting-with-botrytis-101
Some wineries will sort through all incoming grape clusters prior to the crushing/destemming process to assess for any cluster damage or presence of unwanted material. If your operation is not set up with this equipment, sorting can also take place in the vineyard. Depending on the concentration of disease and on the projected wine style or quality parameter the fruit will go towards, disease portions of clusters can be cut out in the vineyard. Or diseased fruit can be left in the vineyard to deal with after the harvest is complete. Sorting out diseased fruit from that of decent quality will reduce the impact of the mold on the wine’s aroma, flavor, and quality.
Limit Contact Time with Skins
Depending on the resource, there are various recommendations for how to handle diseased fruit. In whites, some recommend whole cluster pressing and tossing the first 10+ gallons, which are rich in Botrytis metabolites (Fugelsang and Edwards, 2007). Many recommend separating juice press fractions for white and rosé wines, as this will give the vintner more control over the chemical constituents (e.g., phenolics, enzymes, and disease-related off-flavors) in the final wine.
Depending on the desired outcome for a red wine, treating or limiting skin contact with diseased fruit may be ideal post -primary fermentation. This would include avoiding extended maceration processes. Due to the fact that the presence of Botrytis on red varieties reduces anthocyanin and phenolic extraction (Razungles, 2010) in addition to the varietal aromatics, excessive skin contact may not be ideal during primary fermentation. Whole berry fermentations, as opposed to a more aggressive crush and destem process, may help minimize extraction of Botrytis metabolites, which can also contribute to mouthfeel variations or off-flavors.
Tannin additions pre-fermentation may also be good considerations to compensate for phenolic losses associated with Botrytis infection. Pre-fermentation and post-fermentation additions may help rebuild the wine’s structure or provide constituents for color stabilization.
Flash pasteurization (i.e., flash détente) has been previously recommended for Botrysized fruit to inactive the laccase enzyme associated with Botrytis, enhance color stability in reds, as well as improve the aromatics and flavors associated with the final wine. Wines that undergo a thermovinification step tend to extract more anthocyanins and phenolics compared to traditionally fermented wines (Razungles, 2010). Additionally, this heat step helps to inactivate laccase, which can contribute to early browning or oxidation of young wines. However, commercial producers may not find this technological application easily accessible.
Therefore, in addition to minimizing skin contact time, winemakers will want to reduce contact time with the gross lees, and may also remove the wine from fine lees associated with the mold-infected fruit quickly. The integration and use of clean, fresh lees, however, is still encouraged. Removing the lees associated with mold-infected fruit can help reduce additional contact time with rot metabolites that have settled out with the lees. This inhibits further integration of those metabolites into the wine.
Inoculate with a Commercial Yeast Strain
The presence of rot is one incidence in which processing techniques (e.g., cold soak) that encourage native microflora to dominate the fermentation are probably not desired. Things like cold soak and native ferments allow ample opportunity for the mold to progress and contribute to the wine’s flavor.
Fruit that has rot or microflora issues is best inoculated with commercial yeast and malolactic bacteria strains to outcompete the native microflora (including those microorganisms that contribute to the rot), and to give the fermentation its best chance at completing the fermentation cleanly. Remember that proper yeast nutrition is important to support the yeasts’ growth and to reduce the risk of hydrogen sulfide development. For more information on determining the starting nitrogen concentrations (YAN) and how to properly treat your fermentation with added nutrients, please refer to:
Penn State Extension’s Wine Made Easy Fact Sheet: Nutrient Management During Fermentation
With high Botrytis concentrations, a more robust yeast strain may be preferred in order to quickly get through primary fermentation. A quicker fermentation may simplify the aromatics associated with the wine, but it will also ensure little opportunity for additional spoilage. Saccharomyces bayanus strains are often selected as more robust yeast strains.
Use of Sulfur Dioxide
Sulfur dioxide additions at crush will be determined based on the style of wine in which you are producing (e.g., white, rosé, red, etc.), but in general, the use of sulfur dioxide can help inhibit further spoilage of your product and retain antioxidant capacity. Sulfur dioxide additions in the juice stage will help minimize early browning, but primarily inactivate PPO.
In general, botrysized wines tend to require more sulfur dioxide as Botrytis metabolites bind with free sulfur dioxide (Goode, 2014). This is true even when processing wines with the noble rot version of Botrytis.
When primary fermentation, and malolactic fermentation (dependent on style), is complete it is a good idea to ensure that the wine has an adequate free sulfur dioxide content in order to retain its antimicrobial protection.
Some fining agents may also be applicable in the juice stage. For example, some producers find it helpful to fine juice with bentonite in order to reduce protein content, as well as help minimize rot-associated off-flavors or partially reduce laccase concentrations.
PVPP can be added to the juice to reduce potential browning pigments or their precursor forms (Van de Water, 1985).
In both of these scenarios, neither bentonite or PVPP is specific for rot-related constituents, but each could be helpful to avoid potential challenges later on in the production process.
The presence of Botrytis can also contribute glucans to the must/wine, which can cause filterability problems for heavily-infected wines. In this situation, many suppliers have beta-glucanase enzymes that can be applied either to the juice, wine, or both, to help breakdown the glucans and enhance ease of filterability.
A Word about Laccase
Both polyphenol oxidase (PPO) and laccase can cause early browning in grapes and wine. However, PPO is inhibited by the alcohol content that is developed during primary fermentation. Laccase, however, is not inhibited by the presence of alcohol, and can only be inactivated by a pasteurization step, heated to at least 60°C (140°F) (Wilker, 2010).
Grapes tend to be higher in laccase concentration when infected with Botrytis, and, thus, wines produced from grapes that had a high incidence rate of Botrytis can develop a brown hue post-primary fermentation. This oxidative activity can occur even in young wines.
If you are concerned about the prevalence of laccase in diseased-fruit, wineries can submit wine samples to a wine lab for a laccase test. Or, if you own a copy of “Monitoring the Winemaking Process from Grapes to Wine: Techniques and Concepts” by Patrick Iland et al., pg. 90 and 94 have 2 laccase test protocols that outline how wineries can assess oxidation by laccase. The results of these test will indicate if extreme treatments are required during production to avoid the rapid and early oxidation caused by laccase.
- Fermenting with Botrytis 101
- Management of Botrytis Infected Fruit
- Managing Botrytis Infected Fruit Fact Sheet
Goode, J. 2014. The Science of Wine: From Vine to Glass. (2nd Ed.) University of California Press: Berkley, California. 216 pg.
Fugelsang, K.C. and C.G. Edwards. 2007. Wine Microbiology: Practical Applications and Proceedings. (2nd Ed.) Springer: New York, NY. 393 pg.
Loinger, C., S. Cohen, N. Dror, and M.J. Berlinger. 1977. Effect of grape cluster rot on wine quality. AJEV. 28(4): 196-199.
Peynaud, E. 1984. Knowing and Making Wine. Wiley-Interscience: New York, NY. 391 pg.
Razungles, A. 2010. Extraction technologies and wine quality. In Managing Wine Quality, Vol. 2 Oenology and Wine Quality. Andrew G. Reynolds, Ed. Woodhead Publishing: Philadelphia, PA. 651 pg.
Van de Water, L. 1985. Fining Agents for Use in Wine. The Wine Lab.
Wilker, K.L. 2010. How should I treat a must from white grapes containing laccase? In Winemaking Problems Solved. CRC Press: Boca Raton, Florida. 398 pg.
By: Denise M. Gardner
If you are a wine producer in the northern hemisphere, harvest may feel quite far away. However, given that it is now the month of July, it will be here before we all know it.
The month of July is a great time to start preparing a few essential pre-harvest tasks including getting a bottling schedule ready, especially if bottling operations have not yet begun, and ordering harvest supplies. This blog post will focus on these two tasks.
Prepare and Enact a Bottling Schedule
New grapes are about to flood your winery with juice and future wine. Now is the time to review inventory within the cellar and determine what has to be moved and what has to be bottled before harvest begins.
Freeing up previous years’ inventory by moving it into bottle will free up tank, barrel and storage space for this year’s incoming fruit. It makes for a much easier transition if all of the wines that need bottling are bottled before harvest season starts. Bottling during harvest is not only chaotic, but it tires employees, pulls resources from the incoming product, and may lead to harvest decisions that may be regretted later.
Always make sure to get bottled wines properly stored and away from any “wet areas” on the production floor. If possible, bottled wines should have a separated storage area within an ideal environment that is physically separated from production. From there, stored wines can be moved into retail space when needed.
For more information on how to get wines prepared for bottling, please visit our previous posts:
Ordering Fermentation and Lab Supplies
Many suppliers and wine labs offer free shipping in July, which can especially be useful for wineries that are not geographically close to a winery supply store-front. Planning ahead and determining what fermentation supplies will be needed in August, could save extra money. Not to mention, having supplies on hand during the busy processing season can be a big stress relief.
Winemakers should also take the time to look at new fermentation products and assess the previous year’s needs in order to adequately supply for the up-and-coming harvest. Keeping an annual inventory of purchases can be helpful to isolate regular needs.
Things to consider purchasing include:
- Fermentation Nutrients
- Malolactic Bacteria
- Yeast Hulls
- Salts for Acid Adjustments
- Pectic Gums and/or Inactivated Yeast Products
- Fining Agents
- Oak Alternatives or Barrels
- Sanitizing Agents
While new yeasts are released frequently, being constructive about the production’s fermentation needs can help isolate what yeasts are needed for the upcoming harvest. I typically recommend that all vintners have at least 5 strains on hand for harvest: 2 reliable strains that will get through primary fermentation with little hassle, 1 strain that can be relied upon for sluggish or stuck fermentations, and 2 strains for specialty needs (e.g., sparkling or fruit wine/hard cider production) or experimental use.
Fermentation nutrients should be a must-have for all wineries to help minimize the risk of hydrogen sulfide. Always double check nutrient requirements for yeast strains purchased. In general, wineries will need hydration nutrients (e.g., GoFerm), complex nutrients (e.g., Fermaid K), and diammonium phosphate (DAP).
For more information on why YAN is important and how yeasts utilize nitrogen during primary fermentation, please visit the following blog posts:
- Reviewing YAN and Hydrogen Sulfide Part 1
- Reviewing YAN and Hydrogen Sulfide Part 2
- Yeast Selection and Hydrogen Sulfide
If you need further step-by-step instructions on how to determine adequate nutrient additions during primary fermentation, please visit our Penn State Extension fact sheet: Wine Made Easy Nutrient Management during Fermentation
Sometimes hydrogen sulfide will arise in a wine by the time primary fermentation ends despite all preventative care. Making sure there are adequate supplies on hand, such as copper sulfate and PVI/PVP can save time in the future. Also make plans for ways that the production can reserve fresh lees. PVI/PVP is a fining agent that can help reduce metals like residual copper, but fresh lees will also help reduce the perception of hydrogen sulfide aroma/flavor and residual copper in the wine. Having a plan for retaining and storing lees during harvest season can save time during challenging situations that develop through the end of harvest and into the winter’s storage season. A fact sheet on copper screens and addition trials can be found at the Penn State Extension fact sheet: Wine Made Easy Sulfur-Based Off-Odors in Wine.
I also like to make sure we have supplies on hand in case of heavy disease pressure come harvest. This includes things like Lysozyme, beta-gluconase, pectinase or other clarification enzymes, and fermentation tannins. Lysozyme can help reduce lactic acid bacteria levels while beta-gluconase can assist clarification problems associated with Botrysized wines. For further information on how to manage high-disease pressured fruit, please visit the Penn State Extension website on Fermenting with Botrytis or Managing Sour Rot in the Cellar.
Double check the storage requirements for all materials purchased before and after the product is opened. It’s important to store all of those supplies in the winery properly as it will ensure their efficacy by the time the product is needed.
By: Denise M. Gardner
It’s that time of year again: bottling time! The past year’s vintage is slowly starting to take up too much room in the cellar and now is the time for decision making in terms of preparing for the pending vintage. Finalizing a good bottling schedule before harvest starts is an essential good winemaking practice, but bottling comes with its own set of challenges.
It is not uncommon for winemakers to express feelings of “not being able to sleep at night” when wines get bottled, as they are worried about possible re-fermentation issues. As wine naturally changes through its maturity, it is easy to feel insecure about bottling wines, especially those wines that may have had challenges associated with it throughout production.
However, there are several analytical tests that winemakers can add to their record books every year to ensure they are bottling a sound product. The following briefly describes a series of analytical tests that provide information to the winemaker about stability and potential risks associated with the product when it goes in bottle.
Basic Wine Analysis Pre-Bottling:
This first list is the bare minimum data that should be measured and recorded for each wine getting bottled, regardless of the wine’s variety or style. Keeping accurate records of these chemistries is also helpful in case something goes wrong while the bottle is in storage or after it is purchased by a customer.
pH is essential to know as it gives an indication for the wine’s stability in relation to many chemical factors including sulfur dioxide, color, and tannin. For example, high pH (>3.70) wines provide an indication that more free sulfur dioxide is needed to obtain a 0.85 ppm molecular free sulfur dioxide content. At the 0.85 ppm molecular level, growth of any residual yeast and bacteria in the wine should be adequately inhibited.
High pH wines tend to have issues with color stability. At this point, color stability can be addressed by blending or with use of color concentrates (e.g., Mega Purple). Keep in mind that if the wine is blended with another wine, all chemical analyses, including pH, should be completed on the blend (as opposed to average individual parts) prior to bottling.
Free and Total Sulfur Dioxide Concentration
In the United States, total sulfur dioxide is regulated and must fall under 350 mg/L for all table wines (CFR: https://www.ecfr.gov/cgi-bin/text-idx?SID=eddaa2648775eb9b2423247641bf5758&mc=true&node=pt27.1.24&rgn=div5#sp27.1.24.a).
However, the free sulfur dioxide concentration provides an indication to the winemaker regarding antioxidant strength and perceived antimicrobial protection. To inhibit growth of yeast and bacteria during bottle storage, a 0.85 ppm molecular free sulfur dioxide concentration must be obtained. The free sulfur dioxide concentration required to meet the molecular level is dependent on pH. Therefore, free sulfur dioxide additions should be altered and based on a wine’s pH for optimal antimicrobial protection.
Analytically, it can be daunting to measure free sulfur dioxide as the wet chemistry set up looks intimidating. However, many small commercial wineries have benefited from the integration of a modified aeration-oxidation (AO) system, and with a little practice, have been relatively successful at monitoring free sulfur dioxide concentrations. A few wineries have worked to validate use of Vinmetrica’s analyzer (https://vinmetrica.com/), and found results comparable to those obtained by use of the AO system.
Residual (or Added) Sugar
Any remaining sugar in the bottle, whether through an arrested fermentation or direct addition, can pose a risk for re-fermentation post-bottling. This is especially true if the winery lacks good cleaning and sanitation practices. Nonetheless, it is a good idea to assess the sugar content pre-bottling to record a baseline value of the sugar concentration going into bottle. If bottles were to start re-fermenting, a sugar concentration could be analyzed and used to compare against the baseline value in order to assess the potential of yeast re-fermentation.
For wineries with minimal residual sugar concentrations, a glucose-fructose analysis (often abbreviated glu-fru) is often used to help determine accurate sugar content. For wines with added sugar an inverted glucose-fructose analysis may be required.
If you are concerned about potential risk for Brettanomyces (Brett) bloom post-bottling, it is usually encouraged to reduce the sugar content in the finished wine below 1% (<10 g/L sugar) in the bottle.
Malic Acid Concentration
While using paper chromatography to monitor malolactic fermentation (MLF) is useful, it does not give an accurate reflection of residual malic acid concentration. In fact, some winemakers find that a paper chromatogram may show a MLF has been “completed,” but would prefer to have lower residual malic acid concentrations remaining in the wine.
During my time at an analytical company, 0.3 g/L of malic acid and below was considered “dry.” This is typically a safe level of residual malic acid to avoid post-bottling MLF.
Volatile acidity (VA) is federally regulated, and levels are indicated in the Code of Federal Regulations (CFR: https://www.ecfr.gov/cgi-bin/text-idx?SID=eddaa2648775eb9b2423247641bf5758&mc=true&node=pt27.1.24&rgn=div5#sp27.1.24.a). For most states, with California as an exception, the maximum allowable VA for red wines is 1.40 g/L acetic acid (0.14 g/100 mL acetic acid) and for white wines is 1.20 g/L acetic acid (0.12 g/100 mL acetic acid).
Monitoring VA through production is a good indicator of acetic acid bacteria spoilage. At minimum, wineries should record VA
- immediately post-primary fermentation,
- periodically through storage (e.g., every 2-3 months) and
Whiling monitoring VA, sharp increases in VA should alarm the winemaker of some sort of contamination. Typically, these increases are caused by acetic acid bacteria, which can only grow with available oxygen.
As a general rule of thumb, knowing the final alcohol concentration is a good idea. Alcohol content helps determine a tax class for the wine and is required for the label.
Titratable Acidity (TA)
All wines are acidic in nature as they fall under the pH 7.00. However, titratable acidity (TA) acts as an indicator for the sour sensory perception associated with a given wine. For example, two wines, Wines 1 and 2, with a pH of 3.40 may have different TAs. If Wine 1 has a TA of 8.03 g/L tartaric acid while Wine 2 has a TA of 6.89 g/L tartaric acid, Wine 1 would likely taste more acidic (assuming all other variables are the same).
Cold stability tests are often recommended to ensure the wine is cold stable, and will, therefore, not pose a threat of precipitating tartrate crystals during its time in bottle. Not all wines require a cold stability process (e.g., seeding and chilling). Cold stability testing can be done prior to a cold stabilization step in order to avoid extraneous processing operations, saving time and money.
For more information on cold stability processes and testing, please visit Penn State Extension’s website: http://extension.psu.edu/food/enology/analytical-services/cold-stabilization-options-for-wineries
Additionally, haze formation is a potential risk post-bottling. While hazes do not typically offer any safety threat to wine consumers, they often look unappealing. Protein hazes tend to make the wine look cloudy. Some varieties are more prone to protein hazes then others, and running a protein stability trial could minimize the risk for a protein haze in-bottle.
It is important to remember that due to the fact protein stability is influenced by pH, cold stability production steps should take place before analyzing the wine for protein stability and before going through any necessary production steps to make the wine protein stable. This is due to the fact that cold stability processes ultimately alter the wine’s pH, and the chemical properties of proteins are influenced by the pH.
Analysis for Those that May Consider Bottling Unfiltered:
Yeast and Bacteria Cultures (Brett, Yeast, Lactic Acid Bacteria, Acetic Acid Bacteria)
Having a microscope in the winery can be a great reference point in terms of scanning for potential microbiological problems. However, if the winery does not have a microscope, but knows that some microbiological issues or risks may exist in a wine, having a lab set test the wine on culture plates is a good indicator for potential growth risks during the wine’s storage.
If the wine is going to be bottled using a sterile filtration step, keep in mind that wines are not bottled sterile. Assuming the absolute filtration method is working properly, the wine has potential to become re-contaminated with yeasts and bacteria from the point of which it exits the filter. In fact, it is not uncommon for wines to pick up yeast or bacteria contamination during the bottling process.
Managing free sulfur dioxide concentrations can help inhibit any potential growth from contamination microorganisms if the proper antimicrobial levels (0.85 ppm molecular) are obtained at that wine’s pH and retained during the bottle’s storage.
4-EP and 4-EG Concentrations for Reds
For wines that may have had a Brettanomyces (Brett) bloom, knowing the concentrations of 4-EP and 4-EG in the wine going into bottle is a good result to keep on file. If a Brett bloom occurs later in the bottle, it is likely (although, not guaranteed) that the volatile concentration of 4-EP and/or 4-EG may increase and confirm the problem.
Furthermore, evaluating a wine for 4-EP and 4-EG concentrations can also help isolate a possibility of Brett existence, especially if their concentrations are below threshold. However, it should be noted that both compounds can also exist in wines that are stored in wood, even without a Brett contamination.
Double Check: PCR for Reds
Brett can be a tricky yeast to isolate and identify. It is usually recommended to run multiple analytical tests related to Brett in order to confirm its existence or removal from a wine. While culture plating identifies living populations of microorganisms, PCR cannot typically differentiate between live and dead cells as it is measuring the presence of DNA. A microorganism’s DNA can get into a wine after yeast death and through autolysis. Therefore, a positive PCR result for Brettanomyces is hard to confirm if the result includes live cells, dead cells, or a combination of both.
Culture plating can help confirm the presence of active, live cells, but the success rate of growing Brettanomyces in culture plates is variable.
Nonetheless, scanning wines by PCR for Brett can help winemakers isolate a general presence and risk of Brett in their wines.
Still Worried About Your Wine Post-Bottling?
Bottle sterility testing is helpful, especially when a winemaker wants to ensure wines have been bottled cleanly. For this type of testing, it is best to sample a few bottles
- at the beginning of a bottling run,
- immediately before any breaks,
- immediately after any breaks, and
- at the end of a bottling run.
Bottles can, again, be evaluated under a microscope and evaluated for the presence of microorganisms. Bottles can also be sent to a lab for culture plating. The growth of yeasts or bacteria from culture plates at this stage indicates a failure of the sterile filtration system or contamination of the wine post-filtration. Clean wines, obviously, should help put a winemaker’s mind at ease as it matures in bottle.
Ensuring a wine’s stability post-bottling is a challenge. However, with proper cleaning and sanitation methods coupled with the right analytical records, winemakers can reduce their worry. For information on any of these topics, please visit:
- An Overview of Winery Sanitation by Patricia Howe: https://www.umpqua.edu/images/areas-of-study/career-technical/viticulture-enology/downloads/conferences/technical-symposia/2011-march-wine-flaws/2011-ts-howe-winery-sanitation.pdf
- Making Cleaning and Sanitation Practical for the Small Commercial Winery by Denise M. Gardner: http://bit.ly/PracticalWinerySanitation
- Minimizing Spoilage of Wines in Barrel by Denise M. Gardner: http://bit.ly/WineBarrelSanitation
- Bottling Line Cleaning Protocol by Scott Labs: http://www.scottlab.com/uploads/documents/technical-documents/1191/Bottling%20Line%20Cleaning%20Protocol.pdf
- Preparing Wines for Bottling by Enartis Vinquiry: http://www.enartis.com/upload/images/03_2016/160311011309.pdf
- Starting a Lab in a Small Commercial Winery: http://extension.psu.edu/food/enology/analytical-services/setting-up-your-winerys-lab
- Wine Analytical Labs: How Your Winery Can Use Them: http://extension.psu.edu/food/enology/analytical-services/wine-analytical-labs-how-your-winery-can-use-them
By: Denise M. Gardner
Yeast assimilible nitrogen (YAN) is the sum of the amino acid and ammonium concentrations available in the grape juice at the start of fermentation. Typically, the amino acid proline is not included in the reported amino acid content as it is not readily utilizable by yeast cells.
The amino acid component of YAN is often referred to as the “organic” YAN form. In contrast, the ammonium ion content is referred to as the “inorganic” YAN form and may be written in its ionic abbreviation: NH4+. Due to the fact that ammonium is only connected to a series of protons (H+ ions), it tends to be easier to move in and throughout the yeast cell to be consumed during fermentation (Mansfield, 2014). When these two components (organic + inorganic) are added together, the resultant value is the YAN, written with the units: mg N/L.
The winemaking challenge associated with YAN is the fact that it is quite variable, and current research has not identified ways to change the YAN, predictively, in fruit through the manipulation of vineyard practices. YAN varies by vintage year, grape variety, cultivar, and with the use of various vineyard management practices. In Penn State’s research vineyards, ~1 acre in size and containing 20 different wine grape varieties, YAN values ranged dramatically each vintage year amongst the various wine grape varieties. On any given vintage year YAN values ranged from low (<100 mg N/L) to high (>300 mg N/L) amongst the varieties grown in that one site.
The variability associated with YAN provides a secondary challenge to winemakers: the lack of predictability associated with hydrogen sulfide formation during primary fermentation due to unfulfilled nitrogen needs by wine yeasts.
What does YAN have to do with Hydrogen Sulfide?
Winemakers often talk about YAN in relation to hydrogen sulfide (H2S) as the two have been associated with one another throughout primary fermentation. Although there are several potential causes of hydrogen sulfide formation during wine production, some of which we will talk about in our Part 2 series, nitrogen imbalance has been one of the factors that winemakers can influence through production. Unfortunately, there is no way to ensure that a wine will not produce hydrogen sulfide by the end of fermentation, but treating wines with proper nutrient supplementation can help minimize the incidence of hydrogen sulfide production during primary fermentation.
Hydrogen sulfide is produced by the yeast cell via the sulfate reduction pathway (Figure 1). While I know this figure looks scientifically daunting, we can try to simplify its purpose to discuss how hydrogen sulfide is released into wine. Sulfate (SO42-), naturally abundant in grape juice (Eschenbruch 1974), is transported into the yeast cell for amino acid (cysteine and methionine) development, which are naturally lacking in concentration in grape juice (Bell and Henschke, 2005). Energy is used by the yeast (represented as ATP in Figure 1) to chemically alter the structure of sulfate in order to make it useable by the yeast cell. This useable form can be seen as sulfide (S2-) in the image below. Using nitrogen, which is required to make an amino acid, the sulfide content is depleted as cysteine and methionine amino acids get produced. Therefore, as sulfide reserves are depleted, cysteine and methionine contents generally increase to be used for building proteins that will be needed by the existing or new yeast cells.
Sulfur dioxide (SO2) plays a role in the sulfate reduction pathway in that it bypasses the transport mechanism required to bring sulfur into the yeast cell. It other words, it can diffuse across the cell membrane and into the internal parts of the yeast cell. Sulfur dioxide will get chemically altered to be made into the useable sulfide , S2-, form as well. Therefore, fermentations that contain a high concentration of sulfur dioxide at the start of fermentation have the potential to increase the utilization of sulfur dioxide during yeast metabolism.
These processes function normally until a depletion of nitrogen (from the nitrogen pool) or an accumulation of sulfide develops in the yeast cell.
If there is not enough nitrogen (low YAN fermentations) available to make the sulfur-containing amino acids (cysteine and methionine) then, eventually, the yeast cell will not be able to continue manufacturing these amino acids. In this situation, the sulfide concentration generally starts to increase within the yeast cell.
The chemical form sulfide, however, is toxic to the yeast cell and thus, the yeast will try to eliminate it from its internal structures. Therefore, when sulfide concentrations get too high, the yeast will diffuse this across its cell membrane into the surrounding media: the fermenting juice. When hydrogen sulfide concentrations get high enough in the fermenting juice, winemakers can often sense the rotten or hardboiled egg aroma associated with the compound.
What if there is too much nitrogen?
In contrast, too much nitrogen (high YAN fermentations) can also be problematic. Higher concentrations of the inorganic component of YAN can lead to a high initial biomass (population) of yeast. The rapid increase in yeast populations can lead to nutrient starvation by a majority of the yeast when the wine is about almost finished completing fermentation. With a large biomass of yeast incapable of obtaining the proper nutrient (nitrogen) content to grow and reproduce, hydrogen sulfide development can result. This is due to the fact that there is a large population of yeast in situations in which there is not enough nitrogen to support their growth (i.e., there is not a lot of food to go around for all of the yeast cells). With hydrogen sulfide development occurring late in primary fermentation, it is obvious that the winemaker would become concerned with hydrogen sulfide retention by the time fermentation is fully complete.
Too much nitrogen can also cause other quality problems. Due to the excess amount of available nutrients, yeast can grow and reproduce quickly, which often leads to very rapid or very hot 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 styles of wine, but for many white wine or fruit (other than grapes)-based fermentations, aromatic retention is often a priority by the winemaker.
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. As yeasts begin to autolyze, all of their inner components, including the remaining nitrogen content, will become available in the wine. The excess “food” could be available for other microorganisms (like acetic acid bacteria, lactic acid bacteria, or Brettanomyces), which could potentially lead to spoilage problems if the wine is not properly stabilized. Such spoilage is, obviously, detrimental to wine quality and undesirable by the winemaker. Alternatively, remaining nutrients could be utilized by malolactic bacteria or those wines that will be given tirage for sparkling production (Bell and Henschke, 2005).
Finally, higher YAN concentrations can lead to an increased risk of ethyl carbamate production in wine; ethyl carbamate is a known carcinogen that can give susceptible individuals headaches, or even respiratory illness. Ethyl carbamate is produced in a reaction between ethanol and urea (Bell and Henschke, 2005). The heavy use of DAP has also been linked to a higher potential risks of ethyl carbamate due to the fact that DAP inhibits the transport of amino acids into the yeast cells, and therefore, leaves a higher concentration of amino acids available that can potentially be altered into urea, a precursor for ethyl carbamate (Bell and Henschke, 2005).
The fact that excess nitrogen can be problematic during wine production should provide insight to winemakers to avoid over-supplementing their fermentations. Hence, it is often recommended to that winemakers measure and identify their starting concentration of YAN and supplement accordingly.
Nitrogen (nutrient) management and supplementation is not uncommon during primary fermentation as nutrients are an important component of yeast cell growth and metabolism. In the yeast cell, nitrogen is a required nutrient in the synthesis of amino acids and to build proteins that are used in the yeast cell walls and organelles, as discussed above. Without protein development, the yeast cell cannot live.
Winemakers can supplement their fermentations with nitrogen by adding nutrient supplements in the form of:
- Hydration nutrients (e.g., GoFerm, Nutriferm)
- Complex nutrients (e.g., Fermaid K, Nutriferm)
- Diammonium phosphate (DAP)
DAP is considered an inorganic form of nitrogen, while the complex nutrients may contain additional organic yeast components that contribute organic forms of nitrogen. Recall, above, that the inorganic form of nitrogen is more readily consumed by yeast, and it can be easily absorbed by yeast cells even as alcohol concentrations rise during primary fermentation. Amino acids, on the other hand, require energy expenditure in order to be brought into the cell through transport proteins located on the cell membrane. The presence of both alcohol and ammonium ions inhibit the transfer of amino acids from the juice into the yeast cell (Santos, 2014). Therefore, it is often recommended to avoid the addition DAP or products that contain DAP (i.e., Fermaid K, Nutriferm Advance) at inoculation and until after yeasts have the opportunity to best absorb amino acids. If you are looking for some guidance on when to add nutrients to your fermentation, please refer to our Wine Made Easy fact sheet on the Penn State Extension website.
Starting YAN Concentrations
Nonetheless, nutrient supplementation strategies are often based on starting YAN concentrations in the fruit. Due to the regular variability of YAN concentrations, winemakers are encouraged to measure YAN for each lot of grapes every year. This is often problematic for winemakers whom do not have the time to run the appropriate analyses associated with YAN or the financial resources to send samples to an analytical lab. Such challenges force many winemakers into a situation in which all fermentation lots are treated with the same repeated nutrient supplementation regardless of the starting concentration of YAN.
In previous Extension workshops, research from Cornell University on Riesling wine grapes found that they could accurately predict the harvest YAN when good field samples were taken within 2 weeks from harvest (Nisbet et al., 2013). In 2016, Cornell released a second publication that focused on YAN prediction models for Cabernet Franc, Chardonnay, Merlot, Noiret, Pinot Noir, Riesling, and Traminette. While the prediction models were not recommended for regions outside of the Finger Lakes (where the data was sourced from for this study), they found that in some cases, YAN data could be obtained within 5 weeks of harvest (Nisbet et al., 2014). This extra flexibility in time can aid in obtaining accurate YAN results before the grapes reach the crush pad, which ultimately helps winemakers prepare for nutrient supplementation before the start of fermentation.
Until further research can provide predictive modeling for other wine regions, it is generally accepted that winemakers should measure YAN at or as close to harvest as possible.
YAN can be measured using the following the analytical procedures:
- Enzymatic methods for both primary amino acids and ammonium.
- Probe for ammonium ions.
- Formol titration
While the Formol titration is often preferred by many small wineries due to the lower start-up investment, the use of formaldehyde, a known carcinogen and lung irritant, in this protocol does require some consideration for laboratory safety. Additionally, the proper disposal of formaldehyde, a hazardous substance, can be an issue for many wineries.
Enzymatic methods by spectrophotometer definitely require a bit of experience in order to become more efficient in their use, which can be problematic for those operations that find measuring YAN too timely. Additionally, enzymatic kits have to be purchased fresh and have a small shelf life. The advantage of investing in a spectrophotometer, however, is that other enzymatic kits can be purchased to measure additional wine components including residual sugar, malic acid, and acetic acid.
Nonetheless, measuring YAN should be a consideration for wineries that struggle with hydrogen sulfide aromas by the end of primary fermentation. It is through the starting numerical value that winemakers can better manage and adjust nutrient supplementation strategies to help minimize the reoccurrence of hydrogen sulfide at the end of fermentation.
Nutrient availability during primary fermentation is only one potential contributor to hydrogen sulfide formation in wines. In the next blog post, we’ll explore other potential causes of hydrogen sulfide formation and how to best mediate the problem when it exists.
Eschenbruch. R. 1974. Sulfite and sulfide formation during winemaking – a review. Am. J. Enol. Vitic. 25(3): 157-161.
Bell, S.-J. and P.A. Henschke. 2005. Implications of nitrogen nutrition for grapes, fermentation and wine. Aust. J. Grape and Wine Res. 11:242-295.
Mansfield, A.K. Are you feeding your yeast?: The importance of YAN in healthy fermentation. Webinar. Feb. 2014.
Nisbet, M.A., T.E. Martinson, and A.K. Mansfield. 2013. Preharvest prediction of yeast assimilable nitrogen in Finger Lakes Riesling using linear and multivariate modeling. Am. J. Enol. Vitic. 64(4): 485-494.
Nisbet, M.A., T.E. Martinson, and A.K. Mansfield. 2014. Accumulation and prediction of yeast assimilible nitrogen in New York winegrape cultivars. Am. J. Enol. Vitic. 65(3): 325-332.
Santos, J. Getting Ready for Harvest: Yeast Nutritional Needs. Workshop Seminar. July 2014.
Sparkling Wine Production Workshop Coming to Penn State Extension – An Applied Workshop for Wine and Hard Cider Producers
By: Denise M. Gardner
On March 7, 2017, Penn State Extension will host their first sparkling wine production workshop titled: Improving Bubblies in the Eastern U.S. at the Great Valley Penn State campus in Malvern, PA. (For more information on this program, please click on the title of the workshop.)
Sparkling wine and sparkling [hard] cider production has become a hot topic for many Eastern producers. Some are interested in traditional sparkling wine production methods, occasionally referred to as méthode champenoise, while others are integrating modern approaches into their processing facilities to incorporate bubbles in their wines. The use of pressurized tanks, bottling under a pressurized system, and completing fermentation while retaining carbonation in a tank are all processes I have seen during my recent travels throughout the state. While pét-nats will not be covered at this workshop, there is a previous blog post pertaining to pét-nat production, which you can find here.
Sparkling wine production is a specific form of wine production that incorporates and retains carbon dioxide in the finished wine. The traditional method, méthode champenoise, includes the production of a base wine to about 10-11% alcohol and is bottled with the liquor de triage: a combination of sugar, yeast, and yeast nutrient. The bottle is sealed and as the second fermentation progresses in the bottle, the carbonation produced by the yeast is retained. Once this second primary fermentation is complete, the bottles are riddled (Figure 2) to collect the dead yeast cells within the neck of the bottle.
Each bottle is then individually disgorged and the dosage is added to the wine for final sugar adjustment. Then, each bottle is sealed with a Champagne cork. Both Champagne and Cava are great examples of wines produced by the traditional method.
Other methods of producing and retaining carbon dioxide exist. In the Charmat method, once the base wine is finished fermenting, it is moved to a tank that can withstand pressure. The triage is mixed into the wine within the pressurized tank. When the second fermentation is complete, the spent yeast will settle at the bottom of the tank, and the wine must be racked under pressure to retain the carbonation produced by the second dose of yeast and sugar. The final dosage is added to the wine and then bottled under pressure. Italian Prosecco sparkling wines are great examples of the use of the Charmat process.
Others may utilize direct carbonation after the base wine has been completely finished. This can aid in creating a very fruit-forward style of sparkling wine or used to carbonate fruit wines or ciders.
This program will cover information for producers looking to get into sparkling wine or cider production or for those that would like to improve the quality of their products just a bit more.
We’ll cover basic harvest parameters (i.e., Brix, pH, TA and grape flavors) associated with traditional sparkling benchmark producers and discuss the general production and chemical composition of the base wine used to create sparkling products.
Additional speakers include Jerry Forest, the founder of Buckingham Valley Vineyards, Steve DiFrancesco, the winemaker at Glenora Wine Cellars in NY, and Megan Hereford from Scott Labs. As popular sparkling wine producers, Jerry and Steve will discuss their experiences with sparkling wine production throughout their winemaking careers. They will cover technical details pertaining to managing the second fermentation in the bottle for those attempting to produce a sparkling wine in the méthode champenoise style. Additionally, Steve will cover alternative methods for incorporating and maintaining carbonation in sparkling wines. Megan will also give a technical talk on how to stabilize sparkling wines, including the use of CMC in sparkling wines. This is a great session for those producers looking for practical tips on how to produce sparkling wine.
After a catered lunch, a panel of regional winemakers will share sparkling wines for all attendees to taste and discuss the processing techniques associated with those wines. This is an educationally-focused tasting so discussion is encouraged and expectorating all samples is mandatory.
While this program has a tasting component focused on sparkling wines, all of the techniques and information will be applicable to hard cider producers, as well.
Registration, the full agenda, location, and cost of the program can be found here: Sparkling Wine Production: Improving Bubblies in the Eastern U.S. We hope to see you there!
By: Denise M. Gardner
Based on the number of questions I have received this year about yeast assimilable nitrogen (YAN), it looks like more winemakers are taking it upon themselves to measure YAN on pre-harvested fruit or on incoming juice. This can be a great step in improving wine quality! Measuring YAN offers several benefits to winemakers, including:
- Minimizing the incidence of hydrogen sulfide development in the wine.
- Enhancing varietal character by producing cleaner wines with adequate and specific nitrogen supplementation throughout primary fermentation.
- Minimizing excessive nutrient supplementations, in which left-over nitrogen (after primary fermentation) may act as nutrient sources for spoilage yeast and bacteria.
- Reducing unnecessary work for your employees by minimizing problematic production situations (e., fixing wines with hydrogen sulfide). Such actions could have economic benefit (i.e., reduction in supplies, reduction in time/labor)
Below is a quick refresher for those that may have questions about YAN.
- YAN = Ammonia Concentration + Primary Amino Acid Concentration given in the units: mg N/L (read: milligrams of nitrogen per liter)
- Most suppliers (g., Lallemand, Scott Labs, Enartis, Laffort) will provide recommendations on what to add in low, medium, or high YAN situations. Make sure you consult your handbooks or supplier websites for their product-specific recommendations.
- At the start of fermentation, you want to avoid adding diammonium phosphate (DAP) or complex nutrient additions that contain DAP (g., Fermaid K) when hydrating your yeast. Use hydration-specific products like GoFerm or Nutriferm Energy.
- Most suppliers recommend making 2 additional nitrogen supplementation additions during primary fermentation and after inoculation. If only making 1 nutrient addition after inoculation is practical for you, add your nitrogen supplement at about 1/3 of the way through primary fermentation (e., 1/3 drop in sugar depletion).
A Review: Why to not add DAP at yeast hydration/inoculation
YAN is composed of inorganic (ammonium ion) and organic (primary 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 at the following pages:
- Wine Made Easy Nutrient Management Fact Sheet
- Cornell University’s FAQs Associated with YAN
- YAN Data Review Over 6 Vintages
- Variations in YAN in the same vineyard sites across multiple vintage years
In general, winemakers can select from three different kinds of nitrogen-based products to add during fermentation:
- Hydration Nutrients (g., GoFerm, Nutriferm Arom, etc.)
- Complex Nutrients (g., Fermaid K, Nutiferm Advance, Superfood, etc.)
- Diammonium Phosphate or DAP
Need more direction on when to add which nutrients? Look no further! We have a practical fact sheet waiting for you at the Penn State Extension website. As a general rule of thumb, remember to make your YAN additions based on the volume of wine that you are treating. For whites, roses, and some reds (e.g., hot pressed Concords), YAN additions will be made based on the juice volume. For most other reds, YAN additions should be based on the must volume.
Dealing with Low YAN Fermentations
Low YAN fermentations are defined as having less than 125 mg N/L in the must/juice at the start of fermentation. In these situations, it’s essential for the winemaker to provide enough “food” for all of the yeast during primary fermentation.
Depending on the reference, most scientific literature will recommend adding up to 200 – 250 mg N/L. This concentration of nitrogen should provide adequate supplementation for the entire biomass throughout the duration of fermentation.
Be aware that if you are using a HIGH NITROGEN DEMANDING YEAST strain (e.g., BM45, ICV-GRE, among others), however, you may be required to add additional supplementation. If you are starting with a low YAN situation and would like to use a high nitrogen requiring yeast strain, we recommend contacting your supplier for specific nutrient addition instructions.
Dealing with High YAN Fermentations
Many suppliers define a high YAN fermentation anywhere above 250 mg N/L. However, some YANs from Pennsylvania grown grapes are at concentrations greater than 400 mg N/L! This YAN concentration can create a challenging fermentation and processing situation for the winemaker.
Due to the excess amount of available nutrients in these situations, yeast can grow and reproduce quickly, which often leads to very rapid and hot fermentations. The speed and temperature of fermentation 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, rosé, 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, especially if there is not enough nutrition to fulfill all of the yeast during fermentation. Yeast starvation leads to yeast stress, and one of the stress responses by yeast is the production and release of hydrogen sulfide. Therefore, having a high YAN at the start of fermentation may cause hydrogen sulfide issues in the wine by the time fermentation is complete.
What should you do if you have a high YAN?
- First, always reference your supplier recommendations. Each year, suppliers publish current guidelines for how and when to add various nutrients during fermentation.
- I’ve found it helpful to document trends in high YAN fermentations. For example, if you notice that a variety with a routine high YAN year-to-year, note the years where hydrogen sulfide becomes an issue. Good record keeping during primary fermentation can remind you what you did during production. You may need to alter these practices for the following vintage year.
- If all else fails, refer to Penn State’s Wine Made Easy Fact Sheet on Nutrient Supplementation during Primary Fermentation
Additionally, high YAN concentrations may leave some nitrogen left over 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. In high YAN situations, it is especially important to ensure that the wine is stabilized with adequate sulfur dioxide additions and by minimizing other risk factors (e.g., temperature control of the wine).
It is also be researched that high starting 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.
Our Understanding of YAN is still Developing
Every year, YAN is a big topic of conversation amongst industry suppliers and academics. Current investigations include:
- The impact of primary amino acid uptake as a function of temperature, reported by Cornell University and discussed at the 2016 American Society of Enology and Viticulture (ASEV) – Eastern Section conference (Missouri) in a presentation by Scott Labs.
- YAN recommendations for hybrid varieties produced in the Mid-Atlantic, a topic discussed by Dr. Amanda Stewart from Virginia Tech University during the 2014 PA Wine Marketing & Research Board Symposium. This includes looking at other nutritional factors beyond nitrogen supplementation, which was also discussed at the ASEV-Eastern Conference in 2016 by Scott Labs.
- Optimal nutritional strategies for challenging fermentations, which is often reported in supplier catalogs like the Scott Labs 2016 Handbook