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!

 

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.

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.