By: Denise M. Gardner
The Pennsylvania Wine Marketing and Research Board (PA WMRB) annually awards researchers and graduate students grants to explore pertinent topics to the Pennsylvania wine industry. For the 2016 – 2017 fiscal year, four projects were awarded industry-funded grants. Results from these four projects will be presented at the 2017 Symposium, co-hosted by the PA WMRB, Penn State Extension, and the Pennsylvania Winery Association (PWA).
Registration is being organized through the PWA, and can be found here:
This year’s Symposium, held on Wednesday, March 29th at the Nittany Lion Inn (University Park, PA) will only run in the morning and is packed with 5 sessions of information pertinent to both the enology and viticulture fields in Pennsylvania. At the close of the Symposium a lunch will be provided for all attendees.
Guest Speaker has Enology and Tannin Focus
The WMRB Symposium key guest speaker is Dr. Catherine Peyrot des Gachons, Winemaker Consultant at Chouette Collective. Dr. Peyrot des Gachons has assisted Pennsylvania wineries with enhancing their quality production for several years. She will be speaking towards her tannin and wine aroma matrix research that she has been working on at the Viticulture and Enology Department through the University of Montpellier (France).
Tannins: Modulation of wine structure and aroma
From environmental factors on tannin biosynthesis to human interventions to modulate tannin content in wine what do we know and what can we do to modulate wine structure. Can this tannin content impact wine aroma? The presentation will focus on few main points of interest with practical applications.
An additional enology-based presentation will feature Laurel Vernarelli, a graduate student in Dr. Ryan Elias’s lab within the Penn State Department of Food Science. Laurel’s presentation will be an extension from Dr. Gal Kreitman’s work that was presented last year on predicting reductive off-odors in wines. Laurel will explore the use of copper fining in wine production and the potential impact it may have on wine quality. Given the prevalence of reductive off-odors, including hydrogen sulfide, and heavy reliance on copper fining, this topic should be of considerable interest to most wineries.
Reconsidering copper fining in wine
This presentation will include a brief overview of copper fining, along with the impact of reductive thiols and recent findings describing the effect that copper has in wine. A method for using immobilized copper materials in place of copper fining is described. Depending on the result obtained, winemakers can make informed decisions for use of alternative fining techniques when dealing with reductive issues.
For those with an interest in viticulture, this year’s program promises to deliver some key updates. Bryan Hed, Research Technologist for the Department of Plant Pathology, will present his annual updates regarding disease management for Pennsylvania vineyards. For those that are frequent blog followers, Bryan is a lead contributor to the important seasonal reviews. These tend to be very popular posts for growers and his presentations are always informative and practical. If you missed the 2016 seasonal reviews, you can find them here:
- Looking back at the 2016 season
- Late summer/early fall disease control, 2016
- 2016 Post-bloom disease management review
- 2016 Pre-bloom disease management review
Bryan’s talk at this year’s Symposium is a continued study with results collected over 2 years, which helps initiate trends and suggestions useful towards growers.
Updates on Grape Disease Management Research
Fruit zone leaf removal can be a very beneficial practice in the management of harvest season bunch rot. Bryan will start his presentation by briefly reviewing the pros and cons of different timings of this practice. In addition, leaf removal by hand is very expensive and labor intensive, and with the increasing scarcity and rising cost of hand labor, mechanization is crucial to increasing cost effectiveness and adoption of this practice, no matter what the timing. Bryan will follow up with an in depth discussion of the progress made toward mechanizing an early, pre-bloom leaf removal and comparing its effectiveness over a variety of wine grape cultivars and training systems during the past two seasons.
Maria Smith, Ph.D. candidate in Dr. Michela Centinari’s lab, will discuss her research regarding early leaf removal in Gruner Veltliner vines. Maria and Dr. Centinari have previously written a blog post pertaining to leaf removal strategies for Mid-Atlantic vineyards, which could act as an excellent primer to Maria’s presentation in March. Her presentation will deliver two-years (2015, 2016) of data regarding the effects of early leaf removal and cluster thinning techniques on Gruner Veltliner vines.
Vine response and management costs of early leaf removal for yield regulation in V. vinifera L. Gruner Veltliner
Early leaf removal (ELR) and cluster thinning (CT) were applied and compared for yield regulation in Grüner Veltliner over the 2015 and 2016 growing seasons. Early leaf removal was performed at two different times, trace-bloom and fruit-set. We compared the effects of ELR and CT on grape quality, vine health, and economic costs to un-thinned vines.
Finally, Dr. Michela Centinari will follow up with further results regarding sprayable products to reduce frost damage in wine grape vineyards. Michela’s frost research has been a prominent topic at previous Symposiums, and is often featured here on the blog site. While the updated results that will be presented at the 2017 Symposium have not yet been reported through Penn State Extension, please see some of her past blog posts pertaining to frost control and freeze damage in the vineyard:
- Understanding and Preventing Spring Frost/Freeze Damage – Spring 2016 Updates | Wine & Grapes U.
- Updates on Freeze Injury in Grapevines
- Evaluate cost-effective methods to decrease crop losses due to frost injury
- An update to studies on frost injury, by Maria Smith
Spray-on materials: can they reduce frost damage to grapevines?
Dr. Centinari will present results of studies conducted to test the efficacy of sprayable products as a low-cost frost protection strategy. Two materials Potassium-Dextrose-Lac (KDL) and a seaweed extract of Ascophyllum nodosum, were tested for their cryo-protective activity using a controlled-freezing technique on several grapevine cultivars.
We hope to see you there!
By: Andrew Harner and Michela Centinari
As we move forward through the winter season, many growers have begun or are planning on beginning their annual dormant pruning within the coming weeks. Though a routine task within the vineyard, dormant pruning is essential to maintaining a balanced vineyard that produces quality fruit. With that in mind, this post will both review the basics of dormant pruning and present a series of important considerations to keep in mind when pruning and planning to prune.
With that being said, we will begin with the basics: What is dormant pruning?
In short, dormant pruning is the intentional removal of grapevine tissue, in the form of canes, cordons, trunks, etc., during the annual period of plant dormancy.
What are we trying to gain or change through dormant pruning?
In order to understand the rationale and goals behind dormant pruning, it is first important to understand the biology of grapevines and their physical characteristics that have evolved over thousands of years. Grapevines behave as lianas, or woody, lignified vines that lack a specific growth form on their own; instead, they use other means of support for their growth (i.e., trees or trellis wires). Moreover, grapevine shoots exhibit an indeterminate growth pattern and will continue to grow as long as growing conditions allow and are hospitable. This helps explain why the wild grapevine species endemic to North America tend to be sprawling masses of extensive shoots, and overall have a vigor that differentiates grapevines from many other fruit crops. As a whole, they will remain in this vegetative state until there is access to sufficient sunlight to induce floral development.
Both trellising and pruning are means of harnessing this inherent productivity, with the goal of transitioning it into reproductive growth and consistent, quality fruit yields. Dormant pruning is the primary tool used by grape growers to maintain vine shape, as defined by the training system, and to effectively regulate crop load (fruit mass/vine size) so that a vine’s bearing capacity matches its vegetative vigor capacity. This balance is especially important, as over- or underestimating a vine’s capacity to ripen a fruit crop may result in overly vigorous or overly cropped vines, both of which can have long- and short-term negative consequences. Additional crop load adjustments through shoot thinning and cluster thinning may also be necessary during the growing season to fine-tune grapevine crop load.
Principles of vine balance and essential considerations
Various efforts by researchers to quantify the effects of pruning on vine performance have resulted in the establishment of a few metrics that can be used to guide dormant pruning. Perhaps one of the most basic but important ways to measure vine size is through the collection and weighing of the canes removed by pruning (Figure 1). These numbers could be used to compare final vegetative biomass between vines of any given season, and when combined with the crop yield measurement—taken on the same vines at the previous harvest—can be used to calculate the ratio of fruit yield to vegetative mass. This ratio is the basis for the Ravaz index (yield/pruning weight), an early metric of vine balance first pioneered by the French viticulturist Louis Ravaz during the early 20th century.
Otherwise called crop load, optimal Ravaz index values vary by grapevine species and variety: research on Vitis vinifera has suggested that optimal crop load values fall between 5 and 10; a Ravaz index below 5 indicates that vines were potentially under-cropped (a small vine with a large crop), while a Ravaz index close or above 10 indicates that vines may have been over-cropped (a small vine with a large crop). American and Canadian studies have suggested that interspecific hybrid varieties, more of which are grown in Pennsylvania and other regions of the northeast and Midwest US, are capable of achieving higher crop load values without compromising fruit quality.
Caution is still necessary when thinking about crop load, however, as these general ranges fail to detail variations in a vine’s capacity to ripen its crop due to genotype, weather, soil, and management strategies. Exceptions can occur, and often do occur. While any variety may produce high yields with good fruit quality at one site – and subsequently attain high Ravaz index values – the same variety may not be able to ripen the same amount of fruit at less vigorous sites and under different weather conditions (i.e., locations with shorter growing seasons and/or with lower heat accumulation).
The concept of balanced pruning focuses on cropping vines at yields that are tailored to the vine’s vigor potential and size. The goal is to prevent under- or overcropping and ensure proper shoot maturity and winter-hardiness through a conscious approach to pruning: for example, more vigorous vines are allocated a greater number of buds so the vegetative growth potential is spread across a greater number of nodes, resulting in lower individual shoot vigor. In terms of managing weaker vines, fewer buds are to be retained so the remaining buds will produce more vigorous shoots.
Simple model equations have also been developed for balanced pruning that allocate specific numbers of nodes based on total pruning weights. A classic example is the equation for Concord vines: 30+10, where 30 nodes are retained for the initial pound of pruning weight, and 10 nodes retained for every additional pound of pruning weight thereafter. This specific formula is unsuitable for hybrid and V. vinifera vines, however, as Concord vines are typically cropped at higher levels. Other suggested formulas are based on cluster size, with large clustered varieties (e.g., Chancellor) at 20+10, small clustered varieties (e.g., Marechal Foch) at 20+10, and medium clustered varieties at 10+10. Varieties with large clusters and highly fruitful shoots tend to overcrop, so additional cluster and shoot thinning may be necessary for optimal balance; this counteracts any overcropping that could potentially occur if the formula is followed strictly. Again, these formulas are not the rule, and exceptions to them can and will occur. Instead, it is immensely important to use them as a guideline and tailor final node counts to the individual vine or variety, while keeping in mind site and variety vigor, climate, soil type, and training system.
What differences exist between cane and spur pruning?
Depending on the type of training system implemented and the variety being pruned, dormant pruning methodology consists of either cane pruning or spur pruning. The difference lies in the length of bearing unit, or the one-year old wood, retained: spurs are typically 2-3 nodes long, whereas canes are longer – usually between 8 to 15 buds.
With spur pruning, the one-year old fruiting canes are pruned back to spurs of 2-3 nodes, being the fruiting wood that will yield new shoots in the subsequent growing season. This allows for the retention of cordons and mature, wooden arms (Figure 3).
Conversely, cane pruning entails the removal of one-year old growth back to the head or crown of the vine, with the retention of two canes – one for each side of the trunk – for the bilateral training systems that are used in many vineyards in the eastern US (Figure 4). In sites with high vigor potential, growers may choose to leave four canes instead of two (Figure 2A & B, and Figure 5), which will help to accommodate more vigor and leave the vine more balanced. Furthermore, choosing canes of the right size is especially important—the preferred cane diameter is within a range of 3/8 to 1/2 inch, easily represented by the diameter of a pencil—as thick, excessively vigorous bull canes are as unsuitable as thin, spindly canes. Canes that are too thin or too thick will only yield shoots and fruit of inconsistent quality, whereas well-matured canes with diameters within the mentioned range will only help with maintaining full canopies and fruit-bearing capacity.
Choosing the right pruning system is dependent upon many factors: the variety being grown is especially important, as basal buds of some varieties (e.g., Sauvignon blanc, Nebbiolo) have low fruitfulness and are therefore unsuitable for spur pruning. Vine spacing, mechanization, available labor, and time availability may also affect the choice of a pruning technique. Cane pruning requires more labor in the form of tying down canes, but cane-pruned vines generally require less shoot thinning during the growing season. It is therefore important to select canes with equally spaced internodes, as this allows for equally spaced shoots and reduced shoot crowding.
If the vines are spur-pruned, retaining equally spaced spurs is crucial in order to obtain uniform canopy density and improved sunlight penetration, though shoot thinning during the growing season will likely be necessary as well. Regardless of the pruning method chosen, maintaining sunlight into the renewal zone is essential, as poor light penetration will inhibit bud fruitfulness with negative consequences for future yield and fruit quality.
How do I know when to prune? Pruning strategies for cold climate viticulture
Regardless of the seemingly obvious answer that dormant pruning should be implemented during the dormant, winter season, the timing of pruning could have major implications for the following season’s growth.
In grape-growing regions where there is risk of exposure to damaging cold events, such as Pennsylvania, pruning during the late winter is preferred. Low winter temperature events can damage buds and vascular tissues of mainly cold-tender vine varieties – all vines have limits to their cold-hardiness, however, and even very cold-hardy hybrids can sustain injury to buds and other tissues under exceptionally cold events.
Pruning in the late winter would allow growers a chance to assess vine injury and accordingly adjust the number of buds/nodes to retain. Yet due to labor and time constraints, it is often not possible to do all pruning in the late winter; instead, it is best to begin with the most cold-hardy varieties and leave more cold-susceptible varieties until later in the season. Moreover, in an instance where a damaging cold event does occur, various levels of additional buds are recommended for retention during pruning, depending on the percentage of bud necrosis (Table 1). A high level of bud injury might require differing pruning strategies, such as retraining new trunks and renewing larger parts of the vine, but these topics will not addressed within this post.
When forming a plan for pruning, it is equally important to consider the topographic and microclimatic variability of a vineyard site, as this has implications for air drainage. Vines, rows, or blocks that are at lower elevations than the rest of the vineyard may be more susceptible to cold temperature injury if dense, cold air drains to these low points and pools there. This creates pockets of air that will have lower temperatures than the ambient temperatures of any surrounding blocks, rows, etc. that are at higher elevations.
This is also a consideration worth keeping in mind as the season progresses to bud-break, as these same microsites are also more susceptible to damaging spring frosts and could have any early season growth quickly curtailed. In these cases, double-pruning can be a potential strategy when spur-pruning: canes are first pruned back to long spurs/canes of 5-8 buds, which will allow for terminal bud growth first and will suppress basal bud growth due to apical dominance. Once the risk of frost has passed, a final pruning cut should be made to cut the spurs back to 2-bud spurs. An alternative for cane-pruned vines would entail leaving long canes and extra canes until the threat of frost has passed, and then subsequently making a final pruning cut that leaves the canes at the desired bud number.
Through this post we have hoped to provide an overview of balanced pruning methodology, as well as emphasize considerations that are essential to successful pruning and maintaining balanced, fruitful vines. We realize that many growers tend to have their own styles and methods of pruning, however, and their own rationale for using specific strategies that may differ from the ones listed here. We would be happy to hear about their systems, and any strategies that have proven successful for their vineyards and vines, or adaptations they have implemented to handle specific circumstances or issues within the vineyard. Please feel free to contact us with your ideas and experiences regarding dormant pruning.
In addition to a number of books and publications that detail balanced pruning methodology, principles, and philosophy, there exists an extensive online reservoir of material that can be easily accessed for guidance related to pruning. More detailed explanations and even experiment findings related to pruning can be found in the Wine Grape Production Guide for Eastern North America, The Grapevine: From the Science to the Practice of Growing Vines for Wine, Sunlight Into Wine, and myriad other texts.
Many cooperative extension services (Penn State University, Washington State University, Cornell University, etc.) can be particularly helpful with providing information about pruning, and instructional presentations and in-field videos can easily be found online through a simple web-based browser search or through the aforementioned universities’ cooperative extension websites.
Iland, P., P. Dry, and T. Proffitt. 2011. The grapevine: From the science to the practice of growing vines for wine. Patrick Iland Wine Promotions Pty Ltd, Adelaide, Australia.
Jordan, T.D., R.M. Pool, T.J. Zabadal, and J.P. Tomkins. 1966. Cultural practices for commercial vineyards. Miscellaneous Bulletin 111. Cornell University, Ithaca, NY.
Reynolds, A.G., and T.K. Wolf. 2008. Pruning and training. In Wine Grape Production Guide for Eastern North America. Tony K. Wolf (ed.), pp. 98-109. NRAES, Ithaca, NY.
Willwerth, D., K. Ker, and D. Inglis. 2014. Best management practices for reducing winter injury in grapevines. Cool Climate Oenology & Viticulture Institute, Brock University, Ontario, Canada.
Andrew Harner is a graduate student at Penn State, where he is pursuing a MS degree as a member of the Plant Science Department and studying how climatic and environmental factors influence rotundone synthesis and concentration in Noiret wine grapes with Dr. Michela Centinari. He is currently funded by the John H. and Timothy R. Crouch Program Support Endowment, an endowment founded and funded by the Crouch brothers, original owners of Allegro Winery in Brogue, PA. Drew graduated from Cornell University in 2016, where he focused his studies on Horticulture and Viticulture/Enology and was first introduced to the world of grapevines and plant-based research. Following the conclusion of his MS program he hopes to continue onwards within academia and pursue a PhD, and spends much of his free time either reading, cooking, or outside exploring the natural areas and parks of the U.S.
By: Denise M. Gardner
In a previous post, we discussed ways in which nutrient management during primary fermentation can affect hydrogen sulfide formation and the overall “health” of the wine. This week, we’re going to explore how to mediate hydrogen sulfide aromas and flavors in a finished wine.
Sulfur-Containing Off Aromas
In general, many wine sensory scientists and wine experts will agree that is relatively a bad habit to use the term “sulfur” to describe off-odors associated with hydrogen sulfide or “stinky” aromas that are usually described by the term “reduced.” One of the main arguments for avoiding “sulfur” as a description term for an aroma is due to the fact that there are actually several forms of aromatic sulfur-containing compounds found in wine, and they can have very different aromas (smells, odors) associated with that one compound. The most common groups of aromatic sulfur-containing compounds in wine are:
- Sulfur dioxide (SO2)
- Hydrogen sulfide (H2S)
- Mercaptans or Thiols
Additionally, many sensory experts will advise further to avoid using the chemical names as descriptors for describing an aroma found in wine (e.g., using the term “hydrogen sulfide” to describe the hard-boiled or rotten egg aroma). It is typically recommended to use an actual descriptor when describing an aroma (e.g., using the term “rotten eggs” when that smell exists in wine).
Sulfur Dioxide (SO2)
Sulfur dioxide is an antioxidant and antimicrobial preservative frequently used in wine production. However, it is also produced by yeast during primary fermentation, which is why wines (and other fermented products) cannot be sulfur dioxide-free (commonly referred to as “sulfite free” in the mass media). The aromatic descriptor commonly associated with a high concentration of sulfur dioxide is termed “burned match,” but a high concentration of sulfur dioxide can also cause a nasal irritation that many will describe as nasal burning. For more information on sulfur dioxide and managing its concentration in wine, please refer to this Wine Made Easy Fact Sheet produced by Penn State Extension.
Hydrogen Sulfide (H2S)
Hydrogen sulfide is an aromatic compound that is commonly described as having a “rotten egg” or “hard-boiled egg” aroma. Like many sulfur-containing compounds, hydrogen sulfide has a low sensory threshold (<1 – 1 part per billion, ppb), indicating that about 50% of the population could sense this compound at that concentration without being able to identify it, specifically, as hydrogen sulfide.
As we saw in our previous post, hydrogen sulfide development can result as a component of poor nutrient management during primary fermentation. Residual elemental sulfur from pesticide sprays has also been linked to latent development of hydrogen sulfide in wines. In a 2016 edition of Appellation Cornell, Dr. Gavin Saks’ lab provided a detailed and practical report on how hydrogen sulfide can be a problem for winemakers post-bottling and the potential links to hydrogen sulfide development as a function of residual sulfur from the vineyard (Jastrzembski and Saks, 2016).
Occasionally, winemakers may also experience hydrogen sulfide formation during a sur lie aging period; a time in which the finished wine remains on the lees when lees are stirred in the wine. It is also common for sparkling wines, produced in the traditional method, to exhibit a small perception of hydrogen sulfide when the bottle is first opened.
Mercaptans/Thiols and Disulfides
Finally, mercaptans or thiols, sulfur-containing compounds that contain the functional group –SH, and disulfides, sulfur-containing compounds that contain a S-S bond, can also be problematic for winemakers when found at high concentrations.
The presence of sulfur-containing volatile compounds is not always considered detrimental to wine quality. For some wine grape varieties (e.g., Sauvignon Blanc), these classes of compounds can make up their varietal aroma. In very small concentrations, sulfur-containing compounds can also be aroma enhancers, indicating that their presence can actually make the wine smell fruitier than if they were not present in the wine. However, when at substantial concentrations, volatile sulfur-containing compounds can also produce various “stink” aromas that mask a wine’s fruitiness, freshness, and make the wine generally unappealing. This is phenomena is dependent on the concentration of the sulfur-containing compound and the chemical makeup of the solution (i.e., wine) it is in.
Mercaptans or thiols and disulfides have a variety of descriptors associated with them, and their perception is largely based on concentration. When we’re discussing the negatively-associated descriptors, common terms include: garlic, onion, canned asparagus, canned corn, cooked cabbage, putrefaction, burnt rubber, natural gas, and molasses amongst others.
Are There Sulfur-Containing Off-Aromas in Your Wine?
To identify if hydrogen sulfide, mercaptans/thiols, or disulfide-based off-odors exist in your wine, it may be best to use a copper screen as a bench trial. While analytical identification of these compounds is possible, it is often expensive and leaves the winemaker guessing on what to do next.
For a quick assessment of a wine’s aroma, winemakers can drop 1-2 pre-1985 copper pennies into a glass of wine to see if the aroma freshens. The freshening aroma is due to the fact that the copper from the penny is reacting with the sulfur-containing compounds in the wine and making them aromatically inactive.
A technical copper screen takes a bit more work and should be conducted in a quiet and aromatically-neutral environment. It is recommended to do this outside of the cellar.
Copper addition, in the form of copper sulfate, is often used to remediate aromas/flavors associated with hydrogen sulfide. One-percent and 10% copper sulfate solutions can be purchased through your local wine supplier. The basic protocol associated with a copper screen is as follows:
- Add 50 milliliters of wine to two glasses.
- Label one glass “control” and the other “copper addition” (see image below).
- Add 1 mL of 1% copper sulfate to the “copper addition” glass.
- Cap both glasses for 15 minutes. Sniff the aroma of each wine.
Sniff (smell only!) both glasses. Most people start with the “control” and smell the treated wine (wine containing copper sulfate) second. If the aroma/flavor of the “copper addition” glass has improved, or the hydrogen sulfide aroma has subsided, then a copper addition trial should follow to determine the exact concentration of hydrogen sulfide needed to clean up the wine in question. Remember that the legal limit for copper allowed in a finished wine is 0.5 ppm. For a full protocol on how to run a copper addition bench trial, please refer to this Penn State Extension Wine Made Easy Fact Sheet.
Treatment of Sulfur-Containing Compound Off-Aromas
Sulfur-containing compounds are quite reactive, which can make dealing with them fairly difficult. Many educators agree that the best way to treat sulfur-containing compounds, especially those that stink, is to prevent their existence as best as possible.
In the Appellation Cornell newsletter that focused on sulfur pesticide residues, Jastrzembski and Saks (2016) recommended that sulfur residue concentrations should not exceed 1 mg/kg at harvest in order to avoid latent hydrogen sulfide or sulfur-containing off-aromas later in processing and storage. Additionally, many experts recommend appropriately treating fermenting musts with nutrient management strategies based on the starting YAN concentration to minimize the incidence of hydrogen sulfide formation during primary fermentation. This topic was covered in a previous blog post.
As described above, winemakers may also opt to treat the wine with copper sulfate to try to reduce the perception of hydrogen sulfide or other sulfur-containing aromas. It should be noted that aromas caused by disulfides cannot be mediated with a copper sulfate addition.
There has been more conversation in the academic community regarding the reemergence of hydrogen sulfide or sulfur-containing off-aromas after a wine has been treated with copper and post-bottling. The theory around this appears to circulate around residual copper initiating reactions in the wine that lead to more sulfur-containing off-odors. This continues to be an ongoing discussion amongst researchers and will likely be a hot topic within with the wine industry. For now, it is important for winemakers to understand that there may be a risk of off-odors reemerging post-copper treatment and post-bottling. This topic will also be discussed to some degree at the 2017 PA Wine Marketing and Research Board Symposium on March 29, 2017 in State College, PA, and winemakers are encouraged to attend.
Some hydrogen sulfide or sulfur-containing off-odors can sometimes be mediated with use of fresh lees stirred in the wine or the addition yeast lees-like products. Winemaking products like Lallemand’s Reduless, yeast hulls, or some cellulose-based products can help reduce or eliminate the intensity of these off-odors. As with any other product additions, it is recommended that wineries always do bench trials first and before adding to the entire volume of wine. Additionally, Enartis USA (Vinquiry) has previously distributed a fact sheet to help winemakers troubleshoot reduced wines and determine how to best treat a problem wine.
The incidence of reduction, sulfur-containing off-odors, or hydrogen sulfide can be a frustrating circumstance for winemakers. However, adequate vineyard care and proper nutrient management during primary fermentation can help minimize the incidence rate of sulfur-containing off-odors from occurring in their wines. Of course, problems with wines do occur, and we hope that the recommendations above will help winemakers solve wine problems pertaining to sulfur-containing off-odors.
Jastrzembski, J. and G. Sacks. 2016. Sulfur Residues and Post-Bottling Formation of Hydrogen Sulfide. Appellation Cornell, 3a.
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.
By: Denise M. Gardner
Early in 2016, I was asked to create a “behind the scenes” event in late October to feature our research winemaking program and share this with alumni to introduce them to some of the things that Penn State offers in the fields of viticulture and enology. This was, by far, one of the most interesting events I have organized during my time with Penn State, and it ended up being a very rewarding experience, personally, to see the pride and talent that contributed to make the event a success.
The challenge: teach a group of adults about wine production… most of whom have probably very little knowledge about or experience in actual wine production.
As many of us know, making wine is not really the romantic ideal that is often portrayed and associated with the wine industry. We all know that we aren’t overlooking our vineyards with a glass of wine in hand 24-7.
It’s hard work. It’s dedication. And it’s farming.
When I introduced this event idea to the Extension Enology Advisory Committee – a group composed of 13 volunteers from Pennsylvania’s wine industry and several representatives from various academic communities – they all jumped on the idea of showcasing the Penn State Extension Enology presence and the impact it has had on the local industry in addition to Penn State’s research programs.
Starting in April 2016, I went to work on developing a short [film] script to organize and develop a small video that highlighted our research initiatives and student involvement around winemaking at Penn State. The hope was that this video would feature how students, faculty, and staff are getting involved with industry members via Penn State Extension’s programs while also explaining how wine is generally produced.
With this video, I ended up interviewing two faculty members from our research team, Dr. Michela Centinari from the Dept. of Plant Sciences and Dr. Ryan Elias from the Dept. of Food Science. We collected their perspectives and opinions on various activities that they have been involved in and related it back to the growth and development associated with Penn State offering educational and research experiences in viticulture, enology, and wine marketing.
Luckily, one of the media specialists within the College of Agricultural Sciences, Jon Cofer, had a collection of footage that we had shot during wine processing days just in case we ever needed video footage for anything. As luck would have it, we did need the media footage! Jon sifted through hours of film to find the best footage, which we then tied back into the explanation on how research wines are generally processed at Penn State.
During our travels around the state, whether it was to check in on research trials or visit with industry members during Regional Winery Visits, Michela, a group of dedicated graduate students, and I collected video footage in commercial vineyards in an attempt to highlight what goes on during the growing season. And finally, I met with some recent graduates that experienced educational opportunities through Penn State and Extension, and who both work in Pennsylvania’s wine industry today. I have to admit, one of the most awarding experiences in being Penn State’s Extension Enologist is that I have watched several “students” graduate and find full-time job placement within our state’s wine industry. It is an absolute joy to see these young adults exceed in a growing industry.
The result of this event couldn’t have been better received. Instead of making wine with a group of non-winemakers, we set up three educational stations to teach about:
- wine grape properties and vineyard management by highlighting how to conduct a berry sensory analysis, explaining berry physiological differences, and teaching how to read a refractometer.
- the chemistry behind fermentation and sensory training associated with wine tasting through analytical demonstrations and “aroma guessing” with aroma standards.
- and evaluating the end result (finished wine!) of some of our best research wines and commercial winery collaborators.
The educational portion of this program was a big success. Attendees learned about native and wine grape varieties grown in Pennsylvania, and how those grapes compare to table grapes that people see in grocery stores. At the fermentation booth, participants learned how to measure Brix to determine potential alcohol and how a temperature-controlled stainless steel tank can be useful in wine production. Additionally, our graduate students put guests’ nose-sniffing skills to the greatest test in seeing if they could guess various wine aromas without peaking at the answers! It was enlightening to see our students teach the importance of these skills to develop a career in the wine industry.
The Penn State research wines that are made at University Park were also a big hit. Explaining the purpose of research wines can be a slight challenge, as most of our wines are never finished. This means that in order to emphasize a vineyard or winemaking treatment, fining, stabilizing, and finishing treatments (like oak aging) are kept to an absolute minimum or completely avoided. In many cases, bottled wines will never see any oak or fining other than getting racked off of their lees.
Our primary display was on the Noiret wines, which was a project funded by the PA Wine Marketing and Research Board to determine if vineyard management treatments affected the concentration and perception of rotundone, the primary aroma compound associated with the Noiret variety that exudes a black pepper aroma. The rosé wine, also made from Noiret, was an excellent contrast to the red wines produced from the same variety. Pairing the wines with various cheeses produced by Berkey Creamery was an excellent way to also talk about wine styles produced in Pennsylvania and the importance of food and wine pairing with many of the local wines.
If you are interested in tasting many of our wine trials, please join us at the annual PA Wine Marketing and Research Board Symposium. The 2017 Symposium will be held in University Park on March 29th! (More details on this conference will be released soon!)
But what happened to that video?! If you are still interested in evaluating our winemaking program, curious about what we have been up to for the past few years, please feel free to enjoy our short 12 minute video that highlights a small portion of our efforts to work with industry and participate in viticulture and enology research. While the program is young, we have truly been fortunate to work with some pretty amazing people: commercial growers and producers that are interested in research, students developing expertise, and other academic colleagues that have been willing to collaborate with us as we build our programs.
We truly hope that you have seen or experienced some of the benefits of our programs, but if you would like to know more about what we do, please do not hesitate to contact us! Our email addresses are readily available and we also try to document our regular activities on Facebook. We honestly couldn’t do it without the support of people like YOU!
Enjoy the video! We think it is fairly entertaining, a lot of work went into it, and it showcases a small fraction of the things we are trying to do at Penn State to help progress and educate the local wine industry:
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