Category: Enology

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Renovating the OSU Research Winery

A vital component of the Oregon Wine Research Institute’s “vine to wine” research approach is the ability to conduct a large number of controlled, research scale fermentations. This has allowed studies such as “Managing Nitrogen in the Vineyard or the Winery” to be performed or the 2020 project where eighty-four micro-fermentations of smoke impacted grapes from all across Oregon were conducted to provide information on the impact of smoke exposure in different regions and on different grape varieties. While the current research winery has served us admirably in the past, it is undersized and underequipped to meet our current and future needs. This hinders the ability for OSU to conduct research to address current and emerging industry priorities, such as mitigating the impact of smoke exposure from wildfires and recruiting and training the next generation of winemakers and viticulturists. Therefore, the primary need of the OSU enology program is a well-equipped winery with increased capacity for experimental winemaking.

An exciting opportunity to expand and update the winery has recently presented itself. The building the research winery is currently housed in, Withycombe Hall, will soon undergo a $55 million, fully funded renovation. This renovation will increase available space for the winery from approximately 800 sq feet to over 2,000 sq feet. With the infrastructure needs taken care of, all we need to do is fill that space with equipment essential for enology research and teaching. This will include the addition of a large number of small capacity research fermenters and appropriately scaled processing equipment, as well as a winery laboratory fully equipped to conduct essential grape and wine analysis. Investment in the new winemaking facility will provide the OSU enology program with a state-of-the-art winery with outstanding capabilities comparable to our peers at other Universities. In addition, larger-scale equipment for teaching purposes will strengthen training of current students along with providing improved work force training through extension courses.

The Erath Family Foundation has generously kicked off our $531,000 fundraising campaign with a gift of $100,000 and a promise to fund another $100,000 if the remaining $331,000 is raised by December 2022. Sam Tannahill and Cheryl Francis also committed to a $100,000 gift for this opportunity leaving $231,000 left on our goal. There are opportunities at many different levels to contribute as we fundraise towards this goal. If you are interested in finding out more about the winery renovation, please contact James Osborne at james.osborne@oregonstate.edu or call 541-737-6494. He would be happy to talk with you about the plans for the winery and how this facility will have a profound impact on our ability to conduct enology and viticulture research and train the next generation of winemakers and viticulturists. You can also contact Amy Crumley (amy.crumley@osufoundation.org), the Senior Director of Development for the College of Agricultural Sciences, for more details on how to partner with us on this project.

Winery processing room.
Winery processing room.
Fermentation room with temperature-controlled tanks.
Fermentation room with plastic fermenters waiting to be filled with grapes.

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Impact of Grapevine Red Blotch Virus on Pinot noir grape and wine composition

Dr. Michael C. Qian, Professor (food chemistry), Oregon State University

Grapevine Red Blotch Virus (GRBV) is a single-stranded circular DNA virus that can cause Grapevine Red Blotch Disease (GRBD). The virus was first identified in Cabernet Sauvignon grapes in 2008 in California, and now the disease is known to be widespread in many wine grape-growing regions in North America. The leaves of infected grapevines turn red, and the fruit does not ripen, typically having reduced Brix and color (anthocyanin). Specifically, GRBV inhibits grape ripening pathways by altering transcription factors and hormone networks, disrupting normal grape berry development.

To better understand the impacts of GRBD on grape and wine quality – and potentially remedy the issue – we examined wine aroma composition of wines produced from GRBV positive vines that had undergone two different leaf removal treatments. This work was done in conjunction with Dr. Patty Skinkis (Professor and Viticulture Extension Specialist) and Dr. James Osborne (Professor and Enology Extension Specialist), both of OSU. The leaf removal trial was implemented in 2018-2020 growing seasons with 100% cluster zone leaf removal applied pre-bloom and compared with east-side cluster zone leaf removal by machine at fruit-set (industry-standard method). The result showed that earlier and more complete leaf removal increased monomeric anthocyanin and phenolic compounds in wines. The early 100% leaf removal led to higher levels of bound form grape-derived aroma compounds in wines compared to the standard practice (E side only leaf removal at fruit set by machine). While leaf removal increased bound grape-derived aroma compounds, it did not impact fermentation-derived volatiles as there were no significant differences in these compounds between treatments. This study suggests early leaf removal may lessen the effect of red blotch disease on grape anthocyanin content and potentially improve aroma composition.

This research was funded by industry donations granted to the Oregon Wine Research Institute.

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Can you trust your pH meter?

Dr. James Osborne, Associate Professor and Enology Extension Specialist, Dept. Food Science & Technology, OSU

The pH meter is arguable the most important piece of laboratory equipment you have in your winery. It is used for many routine analyses such as pH, titratable acidity, and as part of the analysis of volatile acidity. Accurately knowing your grape and wine pH is also critical in the management of microbial stability. Spoilage microorganisms such as Pediococcus and Brettanomyces are less acid-tolerant than beneficial microorganisms such as Saccharomyces cerevisiae and Oenococcus oeni. Lower pH, lower microbial spoilage risk; higher pH, higher microbial spoilage risk. Furthermore, there is a key relationship between pH and free SO2. At lower pH, a greater proportion of free SO2 is present as molecular SO2, the most effective antimicrobial form of SO2. Because of this, free SO2 concentrations should always be evaluated in conjunction with pH when considering target SO2 levels. A pH meter will provide you with a pH value, but how do you know that the value is accurate? The consequences of using a pH meter that is not functioning properly can be significant. For example, an inaccurate pH meter may lead to over or underestimation of SO2 additions needed to achieve a target molecular SO2 concentration and/or what acid adjustments are required pre- or post-fermentation. 

So how can you ensure you are getting accurate and reliable results from your pH meter? Firstly, make sure you are using an appropriate pH electrode probe for grapes and wine. Not all pH electrode probes are suited to the unique physical and chemical composition of grape juice and wine. Many manufacturers have pH probes specific for grape and wine analysis, ensuring you are using the right tools for the job. Secondly, instigate a regularly scheduled calibration schedule in the winery lab. It should be standard practice to perform a calibration the first time the pH meter is used for the day. A logbook kept by the pH meter can be used to keep track of when calibration has been conducted and the result. If you have multiple people using the pH meter, they can quickly see when the last calibration occurred and know whether calibration is needed. Keeping track of the calibration results can help indicate if the probe may need to be cleaned or other reoccurring issues. Typically, you will have a small vial of each buffer for the calibration. These vials of buffer should be regularly changed out for fresh buffer solutions bi-weekly or monthly, depending on how often you use them. Make sure the buffers you are using match the built-in calibration set points for your pH meter. Most commonly, you will be using pH 4.0 and 7.01 set points, but some pH meters use pH 3.0 and 7.01. A buffer of 10.01 can also be used as a third calibration point, although in wine, you will be mainly measuring between pH 3.0 and pH 4.0, so this is where the greatest accuracy is required. The slope should be within ± 5% of the ideal (100%), while ± 10% or greater is considered out of range. Make sure you store the probe in an electrode storage solution (typically a KCL solution). A dried-out probe slows the exchange of ions between the probe and the solution you are measuring and results in false readings. Do not store in water or pH 7.01 buffer as this will result in leaching of the electrolyte solution from the pH probe. Some pH probes allow re-filling of the electrolyte solution, so keep an eye on this level if this is the case and re-fill when necessary (see manufacturers recommendations).

If you are starting to see pH drift and/or calibrations are challenging to conduct or have low accuracy, your pH probe may need cleaning. The build-up of grape and wine deposits on the outside of the pH probe bulb will cause fouling of the membrane and interfere with the interaction of ions in your juice/wine and the electrolyte solution. During heavy use, it may be necessary to clean your pH probe weekly or bi-weekly. This involves soaking the probe in a probe cleaning solution (often provided by probe manufacturers) for 30-60 minutes, followed by rinsing with DI water. This will improve the accuracy of your data as well as extend the life of your pH probe. Keep track of when cleanings occur in your pH meter logbook. By following a regular calibration and cleaning schedule for your pH meter/probe, you can ensure reliable and accurate pH data and improve the lifetime of the probe.

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What’s New with Malolactic Fermentation

Dr. James Osborne, Associate Professor and Enology Extension Specialist, OSU

The malolactic fermentation (MLF) is a vital step in the production of cool climate red wines as well as some white wines. But despite its importance, MLF often gets taken for granted and just considered a step to reduce wine acidity. However, MLF is much more than just a biological de-acidification process and can have a number of other impacts on wine quality. Our lab has been conducting a number of projects over recent years investigating various aspects of MLF. One project is investigating interactions between Oenococcus oeni and the spoilage yeast Brettanomyces bruxellensis. An interesting result from this study was discovering that some O. oeni strains were capable of increasing the concentration of the volatile phenol precursors p-coumaric acid and ferulic acid. These pre-cursor compounds are found in grapes and wine mainly bound to a tartaric acid and in this form are not utilized by Brettanomyces. However, some O. oeni strains can remove the tartaric acid through the action of an enzyme, cinnamic esterase, and release free p-coumaric and ferulic acid that Brettanomyces can then metabolize to 4-ethylphenol and 4-ethyl guaiacol. This finding has led to the labelling of many commercial O. oeni strains as either cinnamic esterase (+) or (-) with the recommendation being to avoid use of cinnamic esterase (+) strains in situations where the wine may be at risk for Brettanomyces spoilage.

An additional area of research has been determining the effect of MLF on red wine color. We know that MLF changes wine pH which can cause a shift in red color, but were there other impacts on color due to MLF? Our lab demonstrated that independent of pH change, MLF results in a loss of color and lower formation of polymeric pigments. Results from a number of studies showed that this color loss was likely due to the metabolism of acetaldehyde by O. oeni. Acetaldehyde plays a key role in the development of polymeric pigments and so metabolism of acetaldehyde during MLF reduced formation of these color compounds. Delaying MLF was shown to help mitigate this color loss but delaying MLF for long periods is risky from a microbial spoilage point of view, as SO2 cannot be added to the wine until MLF is complete. Additional strategies to mitigate color loss due to MLF are currently being explored. One such strategy is the use of ML bacteria that do not metabolize acetaldehyde. To date, all O. oeni strains screened can metabolize acetaldehyde but other lactic acid bacterial species such as Lactobacillus look more promising. There has been renewed interest in using certain Lactobacillus species and strains to conduct MLF. In particular, homofermentative species of Lactobacillus have been studied as potential ML starter cultures. These bacteria do not produce acetic acid from glucose metabolism and so could be used for conducting concurrent alcoholic and malolactic fermentations without the risk of increased acetic acid. Currently, there are commercially produced L. platarum cultures available outside of the USA for use in winemaking. However, at this time these cultures are not available for winemaking use in the USA. The use of concurrent alcoholic and malolactic fermentation is one final area our lab has been studying. While there are obvious time advantages to conducting alcoholic and malolactic fermentation at the same time, there are still some concerns over the impact on wine quality, particularly for red wines. We recently completed a study investigating how the timing of MLF impacts Chardonnay aroma and mouthfeel and will be continuing work in this area focused on concurrent fermentations of red wines. As we continue to study malolactic bacteria, we are gaining a better appreciation for the impact they can have on wine quality and potential new strategies for their use. For additional information on any of the studies we have conducted on MLF please contact me at: james.osborne@oregonstate.edu

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Red Blotch and Wine Quality

James Osborne, Enology Extension Specialist, OSU, Oregon Wine Research Institute

The impact of Grapevine red blotch associated virus (GRBaV, commonly referred to as red blotch) on wine quality is largely unknown, with most of the information available focused on fruit composition. A recent study on how GRBaV interferes with grape ripening at the molecular level (Blanco-Ulate et al., 2017) has been published, which may provide insights on how to mitigate the impact of the virus on fruit development in the vineyard. There are very few peer reviewed publications that have reported on winegrape compositional changes due to red blotch and most information regarding the impact on wine quality is anecdotal. A number of studies are currently being conducted in the US to determine the impact of red blotch on wine composition but results from these experiments have not yet been published. Early data from other studies suggest that the impact of red blotch is affected by site and year more than cultivar by cultivar, indicating that impact needs to be evaluated over multiple growing seasons. Based on the few published reports the two main effects on fruit quality have been:

  • A decrease in sugar accumulation leading to reduced Brix levels in grapes at harvest compared to grapes from non-infected vines. The reduction in Brix has been reported to range from 1 to as high as 5 with some varietal differences being noted (Poojari et al 2013), though in this publication the vines were co-infected with Grapevine fanleaf virus. To date the sample size is too small to make any conclusive statements about consistent differences between varietals but early reports indicate this may be the case. Other anecdotal information suggests site and season are more important than cultivar in the degree of impact GRBaV has on grape quality.
  • Lower anthocyanin concentration in grapes from red blotch infected fruit (Poojari et al 2013). Early results from studies being performed in Washington State and California also indicate lower Brix in fruit from red blotch infected vines as well as higher titratable acidity and lower anthocyanins.

While it would be expected that lower Brix will lead to wines with lower alcohol, the impact on other wine parameters such as flavor, aroma, mouthfeel, color, and sensory is relatively unknown. An upcoming presentation by Anita Oberholster (UC Davis) at the OWRI Grape Day will discuss results from some of the trials she has been conducting in California. This includes data regarding changes in wine anthocyanins and tannins as well as sensory attributes. This type of information will be vital for the development of strategies to manage this issue in the winery. If the only significant impact of GRBaV is lower Brix and higher acidity then that can be amended in the winery. However, if red blotch significantly impacts concentrations of tannins and flavor and aroma compounds then red blotch fruit will be more challenging to manage in the winery.  Sensory studies also need to be conducted to determine the specific sensory impact across different wines as well as what percentage of red blotch fruit can be used before sensory impacts become noticeable. It is likely that the percentage of red blotch fruit needed before sensory differences are noted will vary between different red wines as is seen with other taints/faults such as Brettanomyces taint where higher concentrations of volatile phenols are required in a Cab. sauvignon compared to a Pinot noir to be noticeable. We are really only at the very starting line when it comes to understanding both the specific effects of red blotch on wine quality and how these could be managed at the winery.   

Literature cited:

Blanco‐Ulate, B., Hopfer, H., Figueroa‐Balderas, R., Ye, Z., Rivero, R.M., Albacete, A., Perez-Alocea, F., Koyama, R., Anderson, M.M., Smith, R.J., Ebeler, S.E. and Cantu, D. 2017. Red blotch disease alters grape berry development and metabolism by interfering with the transcriptional and hormonal regulation of ripening. J. Exp. Bot. 68:1225-1238.  doi:10.1093/jxb/erw506

Poojari, S., Alabi, O.J., Fofanov, V.Y., and Naidu, R.A. 2016. A leafhopper-transmissible DNA virus with novel evolutionary lineage in the family Geminiviridae implicated in grapevine redleaf disease by next generation sequencing. Plos One. 8(6): e64194. doi:10.1371/journal.pone.0064194

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OWRI winemaker sensory panel update

Dr. Elizabeth Tomasino, Assistant Professor

The OWRI Winemaker Sensory Panel successfully met in summer of 2013 to determine which tests are most appropriate for future sensory evaluation of wines from the Statewide Crop Load Project led by Dr. Patty Skinkis, OSU Viticulture Extension Specialist. Panelists participated in sorting, ranking, preference and descriptive analysis tasks with Pinot noir wines. From the tastings, we determined the best array of sensory tests to answer the question: “how does crop load impact wine quality?” Panelists used descriptive analysis to characterize differences among the wines. They provided information on aroma and mouth-feel parameters and how these parameters relate to quality. Panelists also participated in a range of tasks evaluating Merlot wines from a collaborative project between the Deluc and Tomasino Labs.

The OWRI Winemaker Sensory Panel will meet once every two months from December to September each year but not during harvest from September to November. In the coming year, the Panel will evaluate wines from the Statewide Crop Load Project, develop thresholds for terpenes in wine, and evaluate wines from other research trials from the OWRI. The dates for 2014 will be provided in December 2013. Please contact me if you have any questions (email: Elizabeth.tomasino@oregonstate.edu, phone: 541-737-4866).

For those of you who are unable to participate in the OWRI Winemaker Sensory Panel, you can participate in other sensory analyses investigating regional differences in Oregon Pinot noir during winter and spring from 2014 to 2016. Stay tuned for more opportunities to be involved with wine sensory analysis!

 

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Application of Multidimensional Gas Chromatography (MDGC) technology for wine research

Dr. Elizabeth Tomasino, Assistant Professor

You may often wonder how one determines the complex composition of wine. There are various technologies developed to allow researchers to break up the various compounds and investigate each individually. One of the common techniques to determine aroma composition of wine is known as head space solid phase micro extraction gas chromatography mass spectrometry (HS-SPME-GCMS).

Aroma compounds that can volatilize are absorbed onto a fiber and then injected into a gas chromatography mass spectrometer (GCMS). High temperatures are applied to the fiber and remove the volatile compounds which move through a column that separates out each individual compound based on temperature, molecular weight, polarity, and other factors. Once the compounds reach the mass spectrometer, a unique spectrum is produced for each compound. This is similar to an person’s fingerprint (Figure 1).

Figure 1. An example of a “fingerprint” of limonene.
Figure 1. An example of a “fingerprint” of limonene.

Depending on the research question, it is possible to obtain both qualitative and quantitative information using a GCMS. However, there are limitations to this equipment as some compounds cannot be properly identified because they come out at the same time and do not separate, requiring other separation techniques. A technology that has emerged to provide greater separation is the multidimensional gas chromatography (MDGC). This technology was first developed in 1989 and has been used extensively in the petrochemical industry, and only recently has this been applied to wine science. When comparing the two methods, GC can identify about 150-200 compounds with one dimension of separation while up to 400 compounds can be identified and measured using MDGC with two dimensions of separation.

Multidimensional gas chromatography allows researchers to fine-tune compound separation by “cutting” areas that may consist of multiple compounds. The instrument consists of a GC connected to a GCMS by a heated transfer line (Figure 2).

Figure 2. The Tomasino Lab at OSU is equipped with an MDGC instrument.
Figure 2. The Tomasino Lab at OSU is equipped with an MDGC instrument.

Within my research lab at Oregon State University, I have a MDGC that can perform “heart-cutting,” where only specific portions of the compound spectrum (or chromatogram) are cut and transferred to a second GC. Flavor and fragrance analysis is commonly done using “heart-cut” MDGC. I used this technology during my PhD studies in New Zealand, and I am excited to apply it to a number of projects here. I will be focusing on correlating the new analytical information of specific compounds generated from MDGC to wine sensory data. Despite significant advancements in the determining of wine composition, our understanding of how individual compounds impact the sensory properties of a wine is still limited.

Currently we are using MDGC to measure chiral terpenes present in aromatic white wines. Terpenes are a class of aroma compounds responsible for floral, pine, and citrus-fruit aromas that are found in many plant essential oils. Terpenes can have significant impact on wine aroma, but they are difficult to measure since the various terpenes are closely related. The main issue in identification is due to the fact that these are chiral compounds that have the same atomic formula but a different three-dimensional arrangement of atoms that form mirror images that are not superimposable. Your left and right hands are examples of non-superimposable mirror images. Why do we care about chiral compounds? Well, these compounds may smell differently and be perceived at different concentrations. For example, limonene is a terpene that is found in the rind of citrus fruit. The isomers of limonene have different aroma activities; R-(+)-limonene, smells like fresh oranges and the odor threshold is 200 ppb. S-(-)-limonene, smells like turpentine and lemon with an odor threshold of 500 ppb (Figure 3). (Boelens et. al 1993, Friedman & Miller, 1971).

Figure 3. Limonene has chiral isomers that are mirror images of each other but are not superimposable, resulting in different aromas and sensory
Figure 3. Limonene has chiral isomers that are mirror images of each other but are not superimposable, resulting in different aromas and sensory

Depending on the amount and type of different isomers present, the wine may smell very different. A study is being conducted to measure a range of different chiral terpenes in wine to determine if different varieties, place of origin, or other winemaking processes impact the ratio of chiral terpenes. These data will be paired with sensory trials to determine concentration thresholds for compounds impacting aroma.

This MDGC technology is being used in a number of studies measuring wine volatile compounds and linking them to sensory impacts. I collaborated with Dr. Laurent DeLuc’s lab to determine the effects of berry variability at harvest on Merlot wine quality. The MDGC was also used in collaboration with an entomology project with Dr. Vaughn Walton to measure the volatile compounds associated with Brown Marmorated Stink Bug taint in wine. This method is being used in conjunction with winemaking and sensory research to determine threshold levels of Brown Marmorated Stink Bug taint. We will also look at the processing steps in winemaking that impact the taint expression.

Another study that is being conducted involves understanding the role of important volatile aroma compounds in Pinot noir. The MDGC technology is well-suited for this project, as Pinot noir aroma is difficult to characterize due to many closely-related compounds which impart specific aromas but are present at very low concentrations. In spring 2014, we will investigate the impact of two key norisoprenioids, ß-ionone and ß-damascenone, on Pinot noir aroma in Oregon wines. Future work will attempt to tie Oregon’s regional Pinot noir wine styles to chemical composition and sensory data. This equipment, combined with the already extensive analytical equipment available in various labs at the OWRI, will serve as another tool to increase the knowledge of wine science for the Oregon winegrape industry.

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Development of sulfur off-odors post-fermentation

Dr. James Osborne, Associate Professor & Extension Enologist

One ongoing concern during winemaking is the formation of volatile sulfur compounds (VSCs) that may negatively impact wine aroma. These compounds are either produced during primary fermentation or during wine aging. The most common of the VSCs produced during wine production is hydrogen sulfide (H2S) which imparts a distinctive “rotten egg” character to the wine and is a product of yeast sulfur metabolism (Rauhut 1993). Hydrogen sulfide may be produced by Saccharomyces cerevisiae during fermentation by a number of mechanisms including degradation of sulfur containing amino acids as nitrogen sources, reduction of elemental sulfur used as an antifungal treatment on grapes, and/or reduction of sulfate or sulfite present in the juice (Guidici and Kunkee 1994, Moreira et al. 2002). Due to the potential negative impact of H2S on wine quality and the fact that H2S formation occurs relatively frequently, a number of research studies have focused on understanding the formation of H2S and ways to prevent its formation. Many of the major factors influencing H2S formation are better understood thanks to these studies. Grape nutrient deficiencies and yeast strain are two of the dominant influences (Rauhut 1993, Spiropoulos et al. 2000).

While H2S formation occurs mainly during primary fermentation, additional VSCs can be formed at later stages during winemaking. The formation of these compounds can be difficult to predict, and their formation is not necessarily related to H2S issues during the primary fermentation. This may mean that even though sulfur off-odors were not noticed during primary fermentation, there may still be problems with sulfur compounds during barrel aging. The VSCs involved include mercaptans and disulfides that have distinctive aromas such as skunky, rubbery, garlic, onion, or cabbage-like. Often the problems occur soon after wine is placed in barrel. Some of this is triggered by the wine environment becoming increasingly reductive as it ages, particularly at the bottom of the barrel and in the wine lees. In addition, problems with sulfur off-odors may be more prevalent in one year versus another or in grapes from one vineyard block but not the adjacent block, despite identical vineyard management practices. Many factors contributing to this problem are not well understood and make developing strategies to prevent VSC formation difficult.

To understand this further, my laboratory began collaborating with Dr. Michael Qian’s flavor chemistry lab to conduct a research project investigating factors impacting VSC formation during post-fermentation. Our initial goal is to understand the link between grape composition, wine lees composition, and the development of VSC during aging. It is currently known that these sulfur off-odors often arise from degradation of sulfur-containing compounds in the yeast lees or from the re-release of chemically-bound sulfide during aging (Rauhut 1993, Moreira et al. 2002). For this reason, lees management may play a role in minimizing the formation of sulfur off-odors. In particular, wine should be removed from heavy lees as early as possible. Heavy lees are defined as those that precipitate within 24 to 48 hours after the completion of the primary fermentation. Wines should be separated from these lees as they can promote the production of sulfides and mercaptans. It is advised that you smell and taste your wine and lees frequently as sulfur off-odors may occur rapidly, and this will allow you to take quick action. Be sure to obtain a sample of your lees from the bottom of the barrel and monitor for the formation of sulfur off-odors. The lees may sometimes smell bad but the wine is not yet affected. The earlier the detection, the greater ability you have to take appropriate action to minimize further damage.

You may be wondering about the appropriate actions to treat sulfur-off odors in your wine. Well, while large amounts of H2S may be produced during fermentation, much of this H2S is usually volatilized from the wine along with CO2 during active fermentation. However, residual H2S can pose a problem due to its low sensory threshold and its potential reactivity. In particular, the formation of mercaptans and disulfides during cellar aging can be very problematic as these compounds are more difficult to remove. After fermentation, when H2S alone is present, aeration and splashing may dissipate the odor. If H2S aromas persist, then it may be necessary to treat the wine with copper sulfate. Treatment of wines with copper sulfate is a common practice used to remove H2S and mercaptans. Copper ions combine with H2S and mercaptans to form complexes with no offensive smell. After treatment with copper, the wine can then be racked off the lees. Copper sulfate is normally added to the wine, but bench top trials MUST be conducted to determine the appropriate dose. Results from lab-scale trials do not always transfer directly to larger volumes of wine, so you will need to reevaluate the wines after treatment and before conducting further cellar activities. Keep in mind that reactions may take longer to occur in the cellar than in the lab set-up, so allow extra time before determining whether sufficient copper has been added or whether additional additions should be made. Concentrations of between 0.05 and 0.3 mg/L of copper are commonly added. It is important to be careful with the amount of copper added to your wine, as TTB regulations allows additions of up to 6.0mg/L copper and residual levels of no more than 0.5mg/L. Copper should not be added to the wine until the fermentation is complete and the amount of yeast material is reduced by racking. Yeast cells can bind with copper and reduce effectiveness. Also, addition of copper during fermentation may promote H2S production by yeast.

The formation of disulfides during wine aging can be more problematic, mainly because they are more difficult to remove. They will not be removed by copper. If you aerate wine to remove sulfide aromas, you may oxidize mercaptans present to disulfides. Initially, you will notice a loss of the offensive mercaptan aromas as disulfides have a much higher sensory threshold than mercaptans and may not be detected even with the disulfides still present. When conditions in the wine become more reductive (during barrel aging or in the bottle) the disulfides can be reduced back to mercaptans resulting in a reappearance of sulfide aromas. Sulfide aromas may also reappear even after a copper treatment initially seemed to remove them; this is due to the presence of disulfides that were not removed by copper being reduced back to mercaptans. Since disulfides are difficult to remove from wine, the best approach is taking early preventative measures to minimize the production of H2S during fermentation and the formation of mercaptans. These measures include providing sufficient yeast nutrients for a healthy fermentation, using low H2S producing yeast strains, early removal of wine from heavy lees, and monitoring wine lees for sulfur off-odors during barrel aging. These strategies will help minimize the formation of the more troublesome mercaptans and disulfides.

Literature Cited

Giudici, P. and R.E. Kunkee. 1994. The effect of nitrogen deficiency and sulfur-containing amino acids on the reduction of sulfate to hydrogen sulfide by wine yeasts. Am. J. Enol. Vitic. 45:107-12.

Moreira, N., F. Mendes, O. Pereira, P. Guedes de Pinho, T. Hogg, and I. Vasconcelos. 2002. Volatile sulphur compounds in wine related to yeast metabolism and nitrogen composition of grape musts. Anal. Chim. Acta 458:157-167.

Rauhut, D. 1993. “Yeasts – production of sulfur compounds” in Wine Microbiology and Biotechnology, ed. G.H Fleet. Harwood Academic Publishers, Switzerland. 183-223.

Spiropoulos, A., J. Tanaka, I. Flerianos, and L.F. Bisson. 2000. Characterization of hydrogen sulfide formation in commercial and natural wine isolates of Saccharomyces. Am. J. Enol. Vitic. 51:233-248.